Most of the body’s movements result from muscle contractions. Between 40 – 50% of our body weight is composed of muscle mass. 1Muscle Characteristics: ∙ Excitability: capacity to respond to a stimulus ∙ Contractility: ability to shorten forcefully (resulting in movement) ∙ Extensibility: ability to stretch beyond normal resting length, yet still contract ∙ Elasticity: ability to recoil to original length Muscle Functions ∙ Movement: muscle contraction (voluntary & involuntary) o Body movements: walking, manipulating objects with hands o Constriction of organs & vessels digestive tract propels & mixes food & water vessels regulates blood flow ∙ Posture maintenance: constant muscle tone ∙ Stabilize joints: some muscles keep joints from falling apart when other muscles cause those joints to move. This is just as important as the movement itself. Ex: rotator cuff muscles in the shoulder keep the ballandsocket joint of the should from dislocating every time we throw a ball or raise an arm. ∙ Production of body heat: (thermogenesis) muscle contraction work byproduct: heat (~85%) Section 9.2 – A skeletal muscle is made up of muscle fibers, nerves, blood vessels, and connective tissues. Structure: (Fig. 9.1) A whole skeletal muscle is considered an organ of the muscular system because it’s composed of skeletal muscle tissue, connective tissue, nervous tissue & vascular tissue Skeletal muscle Table 9.1 summarizes the levels of skeletal muscle organization Nerve and Blood Supply In general, one nerve, one artery, and one or more veins serve each muscle. Skeletal muscle has a RICH blood supply because muscle requires an almost constant supply of oxygen and nutrients. Muscles give off large amounts of metabolic wastes that must be removed by veins. Muscle capillaries, the body’s smallest blood vessels, allow for exchange of oxygen / carbon dioxide and nutrients / metabolic wastes, among other exchanges between the blood and muscle tissues. Connective Tissue Sheaths – Fig. 9.1 2 *Epimysium connective tissue sheath surrounding the entire muscle Perimysium and fascicles layer of connective tissue that surrounds each fasciculus (bundle) Endomysium layer of connective tissue that surrounds each individual muscle fiber Attachments When a muscle contracts, the movable bone (the muscle’s insertion), moves toward the immovable bone (the muscle’s origin). Muscle attachments may be direct or indirect: o Direct attachment: the epimyseium of the muscle is fused to the periosteum of a bone or perichondrium of a cartilage. o Indirect attachment: the muscle’s connective tissue wrappings extend beyond the muscle either as a: Tendon: a ropelike connection between the muscle and bone, or an Aponeurosis: a sheetlike covering of a bone or cartilage to the fascia of other muscles. Section 9.3 – Skeletal muscle fibers contain calciumregulated molecular motors Terminology 1) myo = muscle 2) sarco = flesh 3) muscle fiber = muscle cell 4) sarcoplasm = cytoplasm 5) sarcolemma = muscle cell membrane 6) sarcoplasmic reticulum = endoplasmic reticulum 7) glycosomes = granules of stored glycogen that provide glucose during muscle cell activity for ATP production 8) myoglobin = a red pigment that stores oxygen Skeletal muscle anatomy hierarchy (Fig. 9.2) Each fasciculus is composed of numerous muscle fibers (muscle cells) Muscle fibers consist of myofibrils and organelles (esp. mitochondria, sarcoplasmic reticulum) Myofibrils are composed of myofilaments o Thick filaments – containing myosin o Thin filaments – containing actin Actin myofilaments (thin myofilaments) Composed of 3 different proteins o Fibrous actin (F actin): look like 2 strands of pearls twisted around each other 3∙ Composed of ~ 200 small globular proteins (Globular actin or G actin), each with a myosin binding site o Tropomyosin: elongated protein that winds along the groove of the F actin double helix o Troponin: composed of 3 subunits ∙ one binds to actin ∙ one binds to tropomyosin ∙ one binds to Ca2+ Tropomyosin & troponin make up the TT Complex that lies over the actin myofilament when the muscle is relaxed, thus blocking the myosin binding sites on the actin Myosin myofilaments (thick myofilaments) Composed of a protein called myosin shaped like 2 golf clubs twisted together so that there are 2 head portions attached to the rod portion of the molecule o the head regions extend towards the actin myofilaments and bind to the myosin binding sites on the G actin proteins forming CrossBridges which are important in muscle contraction o the heads are attached to the rod portion by a hinge region that can bend & straighten during contraction o the heads have ATPase activity (breaks down ATP), releasing energy used during contraction Sarcoplasmic Reticulum and T Tubules – Fig. 9.5 Skeletal muscle fibers contain two sets of intracellular tubules that help regulate muscle contraction: ∙ Sarcoplasmic Reticulum: highlyspecialized, smooth endoplasmic reticulum that contains high [Ca2+] when the muscle fiber is relaxed ∙ T tubules: tunnellike folds along the sarcolemma surface that extend deep into the muscle fiber and lie near the sarcoplasmic reticulum; conduct impulses from the cell surface down to the SR Sliding Filament Model of muscle contraction in a nutshell: ∙ Actin and myosin myofilaments don’t change in length during contractions; the actin slides over the myosin; the position of the myosin remains the same, but the position of the actin changes. ∙ When sarcomeres shorten myofibrils also shorten muscle fibers shorten muscle bundles shorten muscle contacts So, how does that work? Let’s start with the anatomy of the sarcomere . . . Section 9.4 – Motor neurons stimulate skeletal muscle fibers to contract Sarcomeres (Figs. 9.3 and 9.6) 4∙ Units of organization found in skeletal and cardiac muscle, making these types of muscle appear striated. Each sarcomere lies between two Z disks (Z lines). ∙ Depending on the distribution and interconnection of myofilaments a number of "bands" and "lines" can be distinguished in the sarcomeres: o Z disk (Z line): a filamentous network of proteins forming a disklike structure for the attachment of actin myofilaments o I band (isotropic band): includes a Z disk and contains mainly actin filaments o A band (anisotropic band): contains mainly myosin filaments in center (H zone), with overlapped myosin/actin at either end o M line: in the middle of the H zone; consists of filaments that attach to the center of the myosin myofilaments & help hold them in place 7. Action potentials across the neuromuscular junction (Fig. 9.7) ∙ Membrane potentials o Plasma membranes are polarized there’s a voltage or electrical difference across the plasma membrane Resting membrane potential: inner surface of p.m. is negatively charged, outer surface is positively charged (ion concentration differences) Action potentials (electrical signals): reversals of the resting membrane potentials inner surface of the p.m. becomes positively charged compared to the outer surface o due to diffusion of ions through ion channels in the membrane ∙ Action potentials carried by motor neurons cause action potentials to be produced in muscle fibers because of events that occur in the neuromuscular junction (Focus Fig. 9.1) o Presynaptic terminal: the axon terminal Contains synaptic vesicles (ACh) and numerous mitochondria ∙ Acetylcholine (ACh): a neurotransmitter composed of acetic acid & choline o Neurotransmitter: substance that stimulates or inhibits production of an action potential o Synaptic cleft: the space between the axon terminal and the muscle fiber o Motor End Plate (postsynaptic membrane): the sarcolemma of the muscle fiber in the area of the junction Contains ACh receptors A. Mechanism of neuromuscular transmission of action potential (Focus Fig. 9.1) 1) The action potential (impulse) reaches the presynaptic terminal of the axon 2) Voltagegated Ca 2+ channels open 3) Ca 2+ enter the presynaptic terminal from interstitial fluid initiate release of ACh from synaptic vesicles (exocytosis) 4) ACh diffuses across synaptic cleft and binds to ACh receptors at motor end plate increases permeability of ligandgated Na+ channels 55) Increased Na+ permeability Na+ diffuse into the cell results in depolarization of Postsynaptic membrane, resulting in an action potential in the muscle fiber Now the action potential needs to get deep into the muscle fiber, to the myofilaments, in order for contraction to occur. 8. ExcitationContraction Coupling (Focus Fig. 9.2) ∙ The mechanism by which an action potential causes contraction of a muscle fiber ∙ Involves the sarcolemma, T tubules, sarcoplasmic reticulum, Ca2+ , troponin, tropomyosin Mechanism (Focus Fig. 9.3) 1) The impulse (action potential) is transferred from the neuron to the sarcolemma of a muscle fiber 2) The impulse travels along the sarcolemma, down the TTubules to the sarcoplasmic reticulum 3) As the impulse travels along the SR, the Ca2+ gates in the SR membrane open Ca2+ diffuses out of the SR and among the myofilaments 4) Ca2+ binds on the troponin Ca2+ binding sites alters shape & position of troponin troponin releases G actin causes tropomyosin to move, thus exposing myosinbinding sites on G actin 5) Myosin heads bind to active sites on G actin form crossbridges ∙ Initially, the crossbridge is extended; ADP and inorganic phosphate are attached to myosin 6) The myosin head then bends, thereby creating force and sliding the actin myofilament past the myosin the power stroke ∙ During the power stroke, myosin releases the ADP and Pi 7) ATP then binds to myosin the myosin releases the actin myofilament ATP gets split into ADP and Pi ∙ The energy from this split (at the bond between the 2nd & 3rd P) resets the myosin head to its original position recovery stroke 8) The process is repeated, and the muscle shortens (contracts) 9) After the action potential has passed, the Ca2+ gates close Ca2+ pumps on SR pump Ca2+ into SR Ca2+ ions dissociate from troponin 10) Troponin returns to original shape tropomyosin covers myosin binding sites on G actin 11) Muscle relaxes Summary 1) During relaxation: a) There is a low concentration of Ca2+ in the sarcoplasm, a high concentration in the SR b) The myosin binding sites on G actin are covered by the TT complex, so no contact exists between actin and myosin 2) During contraction: a) Actin and myosin myofilaments stay the same length b) Actin slides past myosin myofilament, but myosin remains in the same position c) Z disks come closer to each other 6 d) Sarcomeres get shorter 3) Muscle relaxation: skeletal muscle relaxes when the nervous impulse (action potential) stops a) At the synapse (neuromuscular junction), the ACh that diffuses across the synaptic cleft and binds to the ACh receptors on the motor plate end is quickly broken down by acetylcholinesterase (AChE) (so that it doesn’t act as a constant stimulus) to acetic acid and choline b) Choline molecules are reabsorbed by the presynaptic terminal and then combined with the acetic acid produced within the cell to form ACh that’s packaged into vesicles, ready for another action potential c) Ca2+ gates on the SR membrane close, Ca2+ pumped back in to SR, Ca2+ move from troponin Section 9.5 – Wave summation and motor unit recruitment allow smooth, graded skeletal muscle contractions. Motor neurons (specialized nerve cells) stimulate muscles to contract o Motor Unit: motor neuron + muscle fibers that it controls When an electrical impulse travels down the axon, all muscle cells innervated by the motor neuron contract simultaneously Motor units vary in size according to location and movement ∙ In locations where delicate & precise movement is needed (eyes, fingers), 1 neuron may control only 2 to 3 muscle fibers ∙ In locations where muscles perform more powerful but less precise contractions (leg), 1 neuron may control as many as 2000 fibers Physiology of Skeletal Muscle When a muscle fiber is stimulated, whether or not it will contract depends on the strength of the stimulus. When it does contract, the cell goes through a full contraction it follows the all or none principle (see below) o The stimulus must be at or above threshold to respond by contracting o Threshold: the weakest strength of a stimulus that causes a contraction o Subthreshold: below threshold – no response occur o Latent (lag) phase: the time between application of the stimulus to the motor neuron and the beginning of the contraction 7∙ ~ 2 milliseconds; during this time the Ca2+ is being released by the SR Muscle Twitch (Fig. 9.11): the contraction of a muscle in response to a stimulus that causes an action potential in one or more muscle fibers, followed by relaxation; most normal activities involve sustained muscle contraction Motor Units: respond in an allornone fashion o all the muscle fibers within a given motor unit contract or relax nearly simultaneously; that is, it’s not possible for some of the muscle fibers of a motor unit to contract while others in the same motor unit relax o The allornone law of skeletal muscle contraction: if the muscle fibers of a motor unit are activated by the nerve sufficiently to contract (at or above threshold), those fibers will contract maximally Whole Muscles: respond in a graded fashion o Strength of contractions can vary from weak to strong, depending on the number of motor units stimulated to contract multiple motor unit summation o Recruitment: as the stimulus strength increases between threshold and maximum values, the number of motor units responding to the stimuli increases and the force of contraction produced by the muscle increases in a graded fashion Tetanus: 2 conditions a normal occurrence and a pathological condition Normal Occurrence Fig. 9.12 A complete contraction of a muscle that occurs when the impulses are fired very close together and the SR does not have the time to allow for uptake of the Ca2+ in between each impulse sustained contraction Tetanus pathology (no relationship to normal response mentioned above) 1) Caused by a neurotoxin released by the bacterium Clostridium tetani (often found in soil); it’s one of the most potent known toxins 2) The bacterium is an anaerobe that thrives in low oxygen environments; can get in through even a tiny pinprick or scratch, but deep puncture wounds or cuts are especially susceptible to infection with tetanus 3) The toxin is spread throughout the body via the blood & lymph vessels 4) The toxin causes the blocking of neurotransmitters from inhibitory motor neurons (which normally prevent overstimulation of the skeletal muscles) skeletal muscles enter a state of constant contraction and spasm 5) The first area that experiences the contractions is usually the mandible; therefore, tetanus is also referred to as “lockjaw” and is especially dangerous if swallowing and breathing muscles are affected 6) Incubation is ~ 2 weeks; complete recovery may take months 7) Immunization (tetanus antitoxin) is available 8Section 9.6 – ATP for muscle contraction is produced aerobically or anaerobically Energy Sources (Fig. 9.16) A. ATP, the energy molecule used by living cells, provides the energy needed for muscle contractions; as long as adequate amounts of ATP are present, muscles can contract repeatedly for a long time ∙ ATP synthesis = ATP breakdown The energy required to produce ATP comes from 3 sources: 1) Creatine Phosphate a) During rest, creatine phosphate is produced and accumulates in the muscle cells (stores energy) b) When ATP levels fall, ADP reacts with creatine phosphate to produce ATP & creatine c) This reaction occurs rapidly and will maintain ATP levels as long as creatine phosphate is available; but these levels are depleted rapidly d) ATP and creatine phosphate provide enough energy to maintain muscle contraction for 8 – 10 sec. 2) Anaerobic Respiration a) The breakdown of glucose in the absence of oxygen b) 1 glucose molecule 2 ATP + lactic acid c) Not as efficient as aerobic respiration, but occurs faster, producing ATP in a short time d) During intense exercise (ex. sprinting) anaerobic resp. combined w/ the breakdown of creatine phosphate can provide the muscles with enough ATP for 2 – 3 minutes e) Limited by lactic acid buildup in muscle fibers 3) Aerobic Respiration a) The breakdown of organic molecules (including glucose, fatty acids, amino acids) in the presence of oxygen b) 1 glucose (up to ) 38 ATP + CO2 + H2O (much more efficient) c) As the muscles continue to contract, more blood carrying oxygen will go to them d) The process is more complex and takes longer than anaerobic (citric acid cycle & electron transport chain) e) Resting muscles, or those undergoing longterm exercise (longdistance running or other endurance exercises) depend primarily on aerobic respiration for ATP synthesis B. Oxygen Debt ∙ The additional oxygen that must be taken into the body after vigorous exercise to restore all systems to their normal states Muscle Fatigue ∙ Results from ATP depletion ∙ Without adequate ATP levels in muscle fibers, crossbridges cannot function normally 9Physiologic Contracture: a condition resulting from extreme muscular fatigue where, due to a lavk of ATP within the muscle fibers, the formed crossbridges cannot release between the myosin & actin myofilaments Section 9.7 – The force, velocity, and duration of skeletal muscle contractions are determined by a variety of factors Types of Fibers in Skeletal Muscle Table 9.2 Not all skeletal muscles have identical functional capabilities; they differ in many ways, including muscle fibers that contain different forms of myosin A. Slow Oxidative Fibers = Type I Muscle Fibers 1) Large number of mitochondria, many blood capillaries and myoglobin (a dark pigment, similar to hemoglobin, which binds Oxygen and acts as a reservoir for it when the blood does not supply an adequate amount) 2) Contract slowly, but can sustain a contraction for a long time 3) Primary source of ATP synthesis = aerobic respiration 4) Examples: neck and postural muscles B. Fast Oxidative Fibers = Type II Muscle Fibers 1) Lower concentration of myoglobin, fewer & smaller mitochondria, fewer blood vessels 2) Contract quickly but fatigue quickly 3) Have large deposits of glycogen & are well adapted to perform anaerobic respiration 4) Examples: eye muscles C. Fast Glycolytic Fibers 1) Intermediate type of fibers 2) Less common than the other two types 3) Contract quickly but are oxygen dependent NOTE: Even though some muscles are all one type and other muscles are a combination or more than one type, each MOTOR UNIT must be of JUST ONE TYPE. Section 9.8 – How does skeletal muscle respond to exercise? Exercise Effects ∙ A muscle increases in size (hypertrophies), and increases in strength and endurance in response to exercise o Increase in metabolic enzymes, myofibrils, sarcomeres & mitochondria; more capillaries ∙ A muscle that is not used decreases in size (atrophies) 10o Decrease in metabolic enzymes, myofibrils, sarcomeres & mitochondria; less capillaries o Severe atrophy (elderly, limited mobility) involves irreversible decrease in # of muscle fibers can lead to paralysis A Closer Look: Athletes Looking Good and Doing Better with Anabolic Steroids? Anabolic Steroids and Muscles A. Synthetic hormones taken to increase the size & strength of muscles (hypertrophy) 1) Pharmaceutical form of testosterone, altered so that the reproductive effects are minimized, but the effects on skeletal muscle are maintained 2) Introduced in the 1950s to treat muscle atrophy diseases 3) People who take large doses exhibit an increase in body weight and total skeletal muscle mass 4) Harmful side effects: testicular atrophy, sterility, cardiovascular disease (heart attack, stroke), abnormal liver function leading to liver cancer, aggressive behavior ∙ When used by females: increase in muscle definition & body hair; decrease in pitch of voice Regeneration of Muscle Fibers A. Skeletal muscle fibers 1) Do not go through mitosis after birth 2) Increase in size of skeletal muscle due to hypertrophy of existing cells B. Cardiac muscle fibers 1) Cardiac muscle was thought to be postmitotic tissue; when there is injury to the heart wall (as in a myocardial infarction – heart attack), noncontracting fibrous connective tissue replaces the cardiac muscle ∙ Recent studies show regeneration promise: undifferentiated stem cells were directly injected into muscle tissue and they differentiated into heart muscle cells C. Smooth muscle fibers 1) Retains the greatest ability to regenerate, but still limited compared to other mitotically active tissue ∙ In the uterus, mitosis occurs when needed during pregnancy Homeostatic Imbalance 9.1 – Myasthenia gravis Homeostatic Imbalance 9.2 – Rigor mortis Homeostatic Imbalance 9.3 – Disuse atrophy Homeostatic Imbalance 9.4 – Muscular dystrophy CHAPTER 10: THE MUSCULAR SYSTEM This chapter deals with the description of the major named skeletal muscles and will be studied in detail in A & P II. 11Chapter Objectives: 10.1 – Describe the functions of prime movers, antagonists, and synergists. Explain how a muscle’s position relative to a joint affects its action. 10.2 – List the criteria used in naming muscles. Provide an example to illustrate the use of each criterion. Most skeletal muscles extend from one bone to another & cross at least one joint. Muscles contract pull on one of the bones toward the other bone across a movable joint movement Section 10.1 – For any movement, muscles can act in one of three ways. Origin (head): the stationary end of the muscle some muscles have multiple origins (e.g. biceps & triceps brachii) Insertion: the end of the muscle attached to the movable bone(s). Belly: the part of the muscle between origin & insertion (the bulging part of the muscle) Muscles can be classified into three functional groups: 1. Prime mover (or agonist): the muscle (among a group of synergists) that’s mainly responsible for a movement. 2. Antagonist: a muscle acting in opposition to an agonist (Focus Figure 10.1 a,b) 3. Synergists: muscles working together to produce a movement Section 10.2 – How are skeletal muscles named? Skeletal muscles are named according to the following criteria: ∙ Muscle location ∙ Muscle shape ∙ Muscle size ∙ Direction of muscle fibers ∙ Number of origins ∙ Location of the attachments ∙ Muscle action – see Figs. 8.5 & 8.6 CHAPTER 11: FUNDAMENTALS OF THE NERVOUS SYTEM AND NERVOUS TISSUE 12Chapter Objectives: 11.1 – List the basic functions of the nervous system. Explain the structural and functional divisions of the nervous system. 11.2 – List the types of neuroglia and cite their functions. 11.3 – Define neuron, describe its important structural components, and relate each to a functional role. Differentiate between (1) a nerve and a tract, and (2) a nucleus and a ganglion. Explain the importance of the myelin sheath and describe how it is formed in the central and peripheral nervous systems. Classify neurons by structure and by function. 11.4 – Describe the relationship between current, voltage, and resistance. Identify different types of membrane ion channels. Define resting membrane potential and describe its electrochemical basis. 11.5 – Describe graded potentials and name several examples. 11.6 – Compare and contrast graded potentials and action potentials. Explain how action potentials are generated and propagated along neurons. Define absolute and relative refractory periods. Define salutatory conduction and explain how it differs from continuous conduction. 11.7 – Define synapse. Distinguish between electrical and chemical synapses by structure and by the way they transmit information. Section 11.1 – The nervous system integrates, and res ponds to information A. Major Functions of the Nervous System include: Fig. 11.1 1. Sensory Input: internal & external sensory receptors monitor stimuli at the conscious & unconscious level ∙ Vision, hearing, taste, smell, touch, pain, body position, temp ∙ Blood pH, blood gases, blood pressure 2. Integration: the nervous system analyzes sensory information & either produces an immediate response, stores the info (as a memory) or just ignores it 3. Motor Output: the nervous system activates effector organs – muscle and glands – to cause a response. B. Division of the Nervous System (Fig. 11.2) 1. Central Nervous System (CNS) a. Brain b. Spinal Cord 2. Peripheral Nervous System (PNS) a. Consists of: 1) Sensory receptors: nerve cell endings or specialized cells that detect temperature, pain, touch, pressure, light, etc. 13 2) Nerves (cranial & spinal): bundles of axons & their protective sheaths; connect CNS to sensory receptors, muscles & glands 3) Ganglia: collections of neuron cell bodies located outside the CNS 4) Plexuses (“braids”): network of axons (& some cell bodies) located outside the CNS b. Subcategories 1) Afferent (Sensory) Division a) Transmits action potentials (AP) from sensory receptors to CNS 2) Efferent (Motor) Division a) Transmits action potentials from CNS to effector organs (e.g. muscles, glands) i. Somatic Nervous System: transmits AP from CNS to skeletal muscles ii. Autonomic Nervous System: transmits AP from CNS to smooth & cardiac muscles and to certain glands o Sympathetic Division: most active during physical activity o Parasympathetic Division: regulates resting & vegetative functions (e.g. emptying urine bladder) o Enteric Nervous System: extensive plexus system along digestive tract local reflexes within these plexuses; provide most of the nervous control of the GI tract Section 11.2 – Neuroglia support and maintain neurons Neuroglia (Glial Cells): supporting cells of the CNS; do not conduct impulses; more numerous than neurons (> ½ brain’s weight) Types of Neuroglia Fig. 11.4 Central Nervous System – Fig. 11.4 a,b,c,d Astrocytes: starshaped with cytoplasmic extensions (“feet”) that cover surfaces of blood vessels & neurons ∙ Play a role in forming the bloodbrain barrier (crucial in what substances pass from the blood into the brain & spinal cord tissue) ∙ They also synthesize, absorb & recycle neurotransmitters, thus regulating ion & gas concentrations Ependymal cells: ciliated glial cells lining the ventricles (cavities) of the CNS that aid in the movement of the cerebrospinal fluid (CSF) that is secreted by specialized ependymal cells & blood vessels that form the choroid plexuses Microglia: specialized macrophages that become mobile phagocytic cells in response to inflammation, thus protecting the CNS from microbes & foreign substances. Oligodendrocytes: most common glial cells in the CNS ∙ Have cytoplasmic extensions that wrap around axons myelin sheaths o A single cell can form myelin sheaths around portions of several axons Periopheral Nervous System – Fig. 11.4 e Schwann cells: found in the PNS; wrap around axons, forming myelin sheath o A single cell can form a myelin sheath around a portion of only one axon Satellite cells: found in the PNS within ganglia, providing support & nutrients to these 14 collections of neuronal cell bodies Section 11.3 – Neurons are the structural units of the nervous system Neurons: receive stimuli & transmit AP to other neurons or to effector organs; perform the functions of the nervous system Structure (Fig. 11.5) 1) Cell body (perikaryon, soma): nucleus, nucleolus, mitochondria, ER, golgi, lipid droplets… 2) Processes a) Axons (nerve fibers): carry information away from the cell body ∙ These are efferent processes ∙ Most neurons have one axon that arises from a coneshaped area of the soma called the axon hillock ∙ The axon can remain a single structure or can branch into collateral axons ∙ Constant diameter, length to > 1 meter ∙ At the end, the axons branch to form presynaptic terminals (terminal boutons) which contain many vesicles containing neurotransmitters b) Dendrites (trees): the input part of the neuron; receive info from other neurons (through synapses with axons of those neurons) ∙ These are afferent processes ∙ Short, highly branched, tapered at tips ∙ Usually not myelinated Myelination (Fig. 11.6) * There are 2 types of glial cells that are involved with myelination: oligodendrocytes in CNS & Schwann cells in PNS Cytoplasmic extensions of these cells surround axons, thus protecting & electrically insulating them from one another Myelinated axons surrounded by layers of tightly wrapped membranes rich in phospholipids (the cytoplasmic extensions) == myelin sheath o Myelin sheath is not continuous; it’s interrupted by the Nodes of Ranvier o Action potentials travel along myelinated axons more rapidly than along unmyelinated axons Types of Neurons 1) Functional Classification (direction of Action Potential) a) Sensory (Afferent) Neurons: AP toward CNS b) Motor (Efferent) Neurons: AP away from CNS (towards muscles & glands) c) Interneurons (Associated Neurons): AP between neurons within the CNS 15 2) Structural Classification: # of processes (Table 11.1) a) Multipolar: one axon + many dendrites ∙ Most of the neurons within the CNS & the motor neurons b) Bipolar: 2 processes 1 dendrite + 1 axon ∙ Sensory organs (e.g. retina of eye) c) Unipolar: a single process (extending from the cell body) that divides into 2 branches, both functioning as a single axon ∙ Sensory that deliver sensory information from the PNS to the spinal cord Section 11.4 – The resting membrane potential depends on differences in ion concentration and permeability Neurophysiology / Electric Signals Humans depend on electric signals called action potentials [APs] (impulses) to communicate & process information; this is how cells transfer information from one area to another. Stimuli perceived by sensory cells are conducted (as action potentials) from these cells to the spinal cord & brain; APs originating in the brain & spinal cord are conducted to other neurons, muscles & glands Nervous tissue & muscle tissue have the ability to conduct a current on their membranes (an Action Potential is a type of current) o Current: flow of electrical charges In living systems, a current is established by the flow of ions (charged atoms) across the cell membrane (through ion channels) 1. Membrane Channels: 2 basic types a. Nongated Ion Channels (Leak Channels) ∙ Always open ∙ Each channel specific for ion type ∙ More K+ & Cl than Na+ b. Gated Ion Channels ∙ Open & close in response to stimuli o Chemicallygated ion channels – Fig. 11.7 Ligand (molecule that binds to a receptor) Open or close in response to ligand binding to a receptor o Voltagegated ion channels – Fig. 11.7 Open & close in response to small voltage changes across the cell membrane o Othergated ion channels Touch receptors (mechanical stimulation), temperature receptors 162. Impulse Propagation (Action Potential) There are 3 steps to an action potential being established on a neuron: resting membrane potential, depolarization & repolarization. a. Resting Membrane Potential (Fig. 11.8) ∙ There is no AP on the membrane ∙ Results from the permeability characteristics of the resting plasma membrane and the difference in [ion] between the intracellular & extracellular fluids o The type & concentration of ions inside and outside of the neuron is different Higher [Na+] outside the cell Higher [K+] inside the cell ∙ There is also a difference in the internal and external charge along the cell membrane o Inside = negative; Outside = positive o Even though there is a high [K+] inside the cell, there are also many negatively charged substances within the cell Proteins, phosphates (PO4), sulfates (SO4) o K+ diffuse out of the cell through nongated K+ channels (along their [ ] gradient) o The negativelycharged molecules remain inside the cell It’s very important that during the resting potential the membrane is Polarized. FOCUS FIGURE 11.1 – RESTING MEMBRANE POTENTIAL – Generating a resting membrane potential depends on (1) differences in K+ and Na+ concentrations inside and outside cells, and (2) differences in permeability of the plasma membrane to these ions. There will be a flow of Na+ to the inside and K+ to the outside, following their concentration gradients (by simple diffusion), through the nongated channels. There is a mechanism within the membranes called the Na+K+ Pump; this mechanism will take the Na+ that has diffused into the cell and put it on the outside again while pumping the K+ that has diffused out of the cell and place it inside again. No equilibrium can occur and membrane polarization is preserved. Recall that the Na+K+ pump actively transports (3) Na+ out of the cell and (2) K+ into the cell, using Energy (ATP). Let’s write out the important information to know about the sodium potassium pump: What ions are pumped OUT of the cell each time by each protein pump? _____________________ How many ions each time? _____________________ What charge is on those ions? _____________________ What ions are pumped IN to the cell by each time by each protein pump? _____________________ 17How many ions each time? _____________________ What charge is one those ions? _____________________ What is the purpose of the sodiumpotassium pump? ___________________________________________________________________________________ Depolarization – Fig. 11.9 a ∙ When a stimulus above threshold occurs, changes in the permeability of the neuron’s membrane take place and depolarization occurs; this is the first part of the AP ∙ Depolarization: a change in the electric charge difference across the cell membrane that causes the difference to be smaller or closer to 0mV (less polar) 1) Voltagegated Na+ channels open and Na+ diffuse into the cell o This small patch of the neuron, for that fraction of a second, is said to be depolarized the outside is now negative compared to the inside that is positive o The depolarization causes additional voltagegated Na+ channels to open more Na+ diffuse into cell greater depolarization … 2) Some voltagegated K+ channels open, but these open more slowly 3) Voltagegated Na+ channels close Repolarization 1) More voltagegated K+ channels open 2) K+ diffuse out of cell into extracellular fluid o This results in this small patch of membrane being positive on the outside and negative on the inside again 3) Voltagegated K+ channels close At the end of repolarization, the charges have been reestablished (positive outside, negative inside), but the distribution of the ions has not; these will be reestablished by the Na+K+ pump (so that there will be a higher [Na+] outside and a higher [K+] inside. Hyperpolarization – Fig. 11.9 b 1) An increase in membrane potential. 2) This occurs because some K+ channels remain open longer than Na+ channels during repolarization, causing an “overshoot” of the 70mv resting membrane potential. 3) Under normal circumstances, the cell membrane must recover back to 70mv resting membrane potential before another action potential can move through the membrane. *** Local Anesthetics: drugs like procaine (Novocain), lidocaine and other local anesthetics are used to block pain in a specific area while the patient remains awake. This medication will prevent the Na+ gated channels from opening, thus preventing depolarization. If there are no impulses sent to the brain for interpretation of the stimulus, there will be no pain. It actually prolongs the resting potential state. 18Section 11.6 – Action potentials are brief, longdistance signals within a neuron Propagation of Action Potentials Fig. 11.10, Focus Figure 11.2, Fig. 11.11 and Fig. 11.14 An action potential does not affect the entire membrane at one time; it propagates (spreads) An AP produced at one location in the cell membrane can stimulate the production of an AP at an adjacent area of the cell membrane it’s a SelfPropagating Effect (like toppling dominos) In nonmyelinated fibers: o APs are produced in every patch of membrane; the next AP is generated immediately adjacent to the previous AP o Conduction is relatively SLOW In myelinated fibers: o The myelinated sheath provides insulation to the fiber and prevents Na+ and K+ from diffusing o The action potentials can only occur at the Nodes of Ranvier high [ ]s of voltagegated Na+ channels o Conduction of the impulse is much faster This is called Saltatory Conduction (saltare – “to leap”) – like a jumping grasshopper (Fig.11.14) o Myelinated fibers conduct the impulse 25x faster than unmyelinated The speed of AP conduction is also affected by the thickness of the myelin sheath ∙ Thicker more rapid Intensity of the Stimulus – Figs. 11.12 and 11.13 When a stimulus occurs, it will follow the All or None Principle: it will be strong enough for an AP to form, or it will not be strong enough, and there will be no AP formation The stimulus must be above Threshold Essentially, all action potentials are the same size The key is the number or frequency of action potentials that a stimulus causes to occur and that reach the brain for interpretation o A light touch generates a lower frequency of impulses than a firmer touch o A firm pressure stimulates more neurons than a light touch Section 11.7 – Synapses transmit signals between neurons 19Synapse Transmission of Action Potential A synapse is a functional junction between 2 neurons, a neuron and a muscle or a neuron and a gland where APs in one cell can cause the production of APs in another cell. (review neuromuscular junction from Ch 9; here we will look at neuronneuron synapses) 2 types of synapses exist between neuron and neuron: a. Electrical Synapse: ionic current spreads directly from 1 neuron to the next through Gap Junctions o They allow for the fastest type of transmission of an action potential o (Coordinated contractions in heart and digestive system) b. Chemical Synapse: (Focus Figure 11.3) the action potential travels from a presynaptic neuron to a postsynaptic neuron across a synaptic cleft o Impulses do not jump this space, but are transmitted from presynaptic to post synaptic by specialized chemicals called neurotransmitters Transmission of an Action Potential Across a Chemical Synapse Between Neurons 1. Action potential arrives at axon terminal. 2. Voltagegated Ca2+ channels open and Ca2+ enters the axon terminal. 3. Ca2+ entry causes synaptic vesicles to release neurotransmitter by exocytosis. 4. Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane. 5. Binding of neurotransmitter opens ion channels, creating graded potentials. 6. Neurotransmitter effects are terminated. Many times, neurotransmitters will be actively transported back into the neuron that released them by membrane proteins called neurotransmitter transporters o One of the reasons why cocaine produces feelings of euphoria is that it blocks the transporters that uptake the neurotransmitter dopamine in the area of the brain that interprets pleasure o This allows the stimulus (dopamine) to linger within the synaptic cleft and stimulate more receptors longer o Pharmaceutical companies are very interested in neurotransmitters in the treatment of emotional disorders, mostly targeting Serotonin, a brain neurotransmitter that is involved with moods o The antidepressants Prozac and Zoloft inhibits Serotonin uptake Regeneration of Nervous Tissue 20 At about 6 months of age, the neuronal cell body loses its mitotic apparatus and is no longer able to divide A neuron in which the cell body is destroyed will die and not be replaced In the PNS, if there is damage to the axon or dendrites, but the cell body and the Schwann cells are O.K., the processes may regenerate In the CNS, little or no repair occurs, and injuries to the CNS may be devastating Neuroglia maintain mitotic ability throughout life Pathology: 1. Multiple Sclerosis Unknown cause, possibly autoimmune response to viral infection Progressive destruction of the myelin sheaths around CNS neurons o The soft tissue hardens into scleroses areas of hardened scars o This destruction slows or stops the conduction of the impulses (APs) o Paralysis may occur o The person may experience symptomatic periods separated by periods of apparent remission; however, with each recurrence, many neurons are permanently damaged o Progressive symptoms: exaggerated reflexes, tremor, speech defects, rhythmic oscillation of the eyes (nystagmus) o Drug therapy may slow progression, but no cure @ this time 2. Gliomas: brain tumors that arise from glial cells; they are highly malignant & grow very quickly CHAPTER 12: THE CENTRAL NERVOUS SYSTEM Chapter Objectives: 12.10 – Describe the gross and microscopic structure of the spinal cord. Distinguish between flaccid and spastic paralysis, and between paralysis and paraesthesia. 12.11 – List key characteristics of neuronal pathways. Identify the major ascending and descending pathways. Section 12.10 – The spinal cord is a reflex center and conduction pathway The spinal cord provides the link between the brain and the rest of the body: (Fig. 12.27) ∙ sensory information to the brain from the PNS ∙ motor information from the brain to the PNS 21A. Characteristics: 1. Continuous with the brain; extends from foramen magnum to 2nd lumbar vertebra 2. Protection a. Vertebral column b. Cerebrospinal fluid (CSF) i. Shock absorber ii. Produced in brain c. Meninges: (Fig. 12.29) sheaths of connective tissue that surround spinal cord & brain i. Dura Mater (“tough mother”): outermost, thickest membrane (tough, fibrous dense irregular connective tissue) ii. Arachnoid (“spiderlike”) Mater: middle membrane (collagen & elastic fibers) iii. Pia (“tender”) Mater: deepest, most delicate, highly vascular membrane that is attached to the surface of the spinal cord Spaces between the membranes: a. Epidural space: between the dura mater and the vertebral periosteum i. Filled with adipose tissue, blood vessels & spinal nerve roots Epidural block: the most popular method of pain relief during childbirth Antiinflammatory steroidal epidural injections also administered to relieve back and neck pain resulting from osteoarthritis, rheumatoid arthritis, etc. b. Subdural space: between dura mater and arachnoid mater i. Contains a small amount of serous fluid c. Subarachnoid space: between arachnoid mater and pia mater i. Contains cerebrospinal fluid (CSF) CSF can be tested for the diagnosis of various neurological diseases; typically done between L3L4 or L4L5 (avoids damage to spinal cord) = Lumbar Puncture (Spinal Tap) o Analyze pressure, cell count, levels of glucose & protein Spinal Block: similar to epidural, but injected into subarachnoid space 3. Gross Anatomy (Fig. 12.27) a. Cylindrical, ~ 17 inches long, ~ 2 inches wide EXCEPT for 2 swollen regions where large #s of nerves enter/leave the spinal cord i. Cervical Enlargement: supply the upper limbs ii. Lumbar (lumbosacral) Enlargement: supply the lower limbs b. Conus Medullaris: tapered region just inferior to the lumbar enlargement c. Filum Terminale (coccygeal ligament): thin cord of pia mater that extends from the conus medullaris and secures the spinal cord to the coccyx d. Cauda Equina (“horse’s tail”): the collection of lumbar and sacral nerves emerging from the conus medullaris and continuing through the vertebral canal e. Spinal (Peripheral) Nerves 22o 31 Pairs; most exit through the intervertebral foramina (sacral nerves exit from the sacral foramina, 1st pair [C1] exits above the atlas) o Formed by the dorsal & ventral roots mixed nerves (sensory & motor neurons) 4. CrossSection of the Spinal Cord (Figs. 12.29, 12,30 and 12.31) a. Peripheral white matter, H or butterflyshaped inner gray matter i. White Matter: myelinated axons (Nerve Fibers) called Nerve Tracts or Nerve Pathways o lipids contribute to color; few or no cell bodies or dendrites Anterior (ventral), posterior (dorsal) and lateral columns a. Each column contains Ascending & Descending nerve tracts – Fig. 12.31 i. Ascending Tracts: sensory axons; bring info to the brain ii. Descending Tracts: motor axons; bring info to effector organs ii. Gray Matter: neuron cell bodies (in the CNS called a nucleus), dendrites, unmyelinated axons & neuroglia The central space at the crossbar is the central canal; runs the length of the spinal cord & contains CSF The spinal cord gives rise to 31 pairs of spinal nerves, most of which exit through the intervertebral & sacral foramina. b. The spinal nerves are formed by the joining of a ventral (anterior) root and a dorsal (posterior) root lateral to the spinal cord – Fig. 12.29 c. Each dorsal root contains a nodule called the Dorsal Root Ganglion Fi. 12.29 Ganglion: a collection of neuron cell bodies in the PNS o Clusters of sensory (unipolar one axon, no dendrites) neuron cell bodies [afferent] o Fig. 12.30: When sensory information enters the spinal cord, it will enter the dorsal root ganglion and then pass through the dorsal root into the posterior horn of the gray matter where the axons will either o Synapse with interneurons (accessory neurons) OR o Pass into the white matter & ascend or descend in the spinal cord via a nerve tract d. The ventral (anterior) root contains the motor axons that transmit information to the effector organs o Spinal nerves are Mixed Nerves: they are composed of both sensory & motor axons o Spinal nerves are part of the Peripheral Nervous System Spinal Cord Trauma and Disorders: Flaccid paralysis ∙ Occurs when the spinal cord or ventral roots are injured ∙ Nerve impulses do not reach the affected muscles ∙ Muscles cannot move either voluntarily or involuntarily and atrophy 23Spastic paralysis ∙ Occurs only if the upper motor neurons of the primary motor cortex are damaged. ∙ Muscles remain healthy longer but movements are no longer under voluntary control ∙ Muscles tend to shorten permanently Transection (crosssectioning) of the spinal cord ∙ Paraplegia o If transection occurs between T1 and L1 vertebrae, the lower limbs are affected o Loss of both sensory and motor function in the lower body region and limbs ∙ Quadriplegia o If transection occurs in the cervical region, all four limbs are affected o Loss of both sensory and motor function in the body and limbs below the site of injury. ∙ Hemiplegia o Paralysis of one side of the body o Usually reflects brain injury rather than spinal cord injury Spinal Shock ∙ A transient period of functional loss that follows an injury to the spinal cord ∙ Immediately depresses all reflex activity below the site of injury to the spinal cord ∙ Neural function usually returns within a few hours following injury ∙ If function does not resume within 48 hours, paralysis is permanent in most cases. Poliomyelitis ∙ polio = gray matter; myelitis = inflammation of the spinal cord ∙ results from the polio virus which typically enters the body via fecescontaminated water and destroys the ventral horn motor neurons. ∙ Early symptoms: fever, headache, muscle pain and weakness, loss of certain somatic reflexes. ∙ Later symptoms: paralysis and muscle atrophy; death from paralyzed respiratory muscles. ∙ Vaccines have nearly eliminated this disease, although it appears to be making a comeback due to some families choosing not to vaccinate their children. ∙ Some survivors of the great polio epidemic of the late 1940’s and 1950’s (before the vaccine was developed) have developed what is now called the postpolio syndrome o sharp, burning pains in their muscles, progressive muscle weakness and atrophy o unknown cause o likely explanation is that as these persons age, they have already drawn on their remaining healthy neurons to replace those wearing out when they were fighting off the polio virus as a young person. Now they don’t have that extra reserve to draw upon and their bodies are suffering as a result. Amyotrophic Lateral Schlerosis (ALS) ∙ Also known as Lou Gehrig’s disease ∙ Devastating neuromuscular condition that progressively destroys ventral horn motor neurons an fibers of the pyramidal tracts (major descending motor pathways) ∙ As the disease progresses, the person loses the ability to speak, swallow and break. ∙ Death usually occurs within 5 years, although there are a few individuals that live longer. 24∙ Environmental and genetic factors interact to cause ALS o 10% of cases – mutations are inherited; 90% probably spontaneous mutations ∙ Only currently known treatment is Riluzole, a drug that interferes with glutamate signaling, the mechanism believed to be involved in the progression of ALS. Section 12.11 – Neuronal pathways carry sensory and motor information to and from the brain. Neuronal Pathways – All major spinal tracts are part of multineuron pathways that connect the brain to the rest of the body. There are four key points in regard to spinal tracts and pathways: 1. Decussation – Most pathways cross from one side of the CNS to the other (decussate) at some point along their journey. 2. Relay – Most pathways consist of a chain of two or three neurons (a relay) that contribute to successive tracts of the pathway. 3. Somatotopy – Most pathways exhibit somatotophy, a precise spatial relationship among the tract fibers that reflect the orderly mapping of the body. 4. Symmetry – All pathways and tracts are paired symmetrically (right and left), with a member of the pair present on each side of the spinal cord or brain. Ascending Pathways – Fig. 12.32 and Table 12.2 ∙ Spinocerebellar pathway transmits proprioceptive only to the cerebellum and is unconscious. ∙ Dorsal column – medial lemniscal pathway transmits touch and soncious p roprioception signals to the cerebral cortex ∙ Spinothalamic pathway – transmits pain and temperature to the cerebral cortex. Descending Pathways – Fig. 12.33 and Table 12.3 ∙ Pyramidial (lateral and ventral corticospial) pathways control skilled voluntary movements ∙ Rubrospinal tract – regulates muscle tone Reflex Arc (Fig. 11.22) from Chapter 11 Humans have 3 types of neurons: 1. Sensory neurons: long axons, transmit impulses from sensory receptors all over the body to the CNS 2. Motor neurons: long axons, transmit impulses from the CNS to effectors (muscles, glands) all over the body 3. Interneurons (association or relay neurons): exclusively within the brain and spinal cord; transmit impulses from sensory to motor neurons These 3 types of neurons are arranged in circuits, the most basic of which is the Reflex Arc. 25Reflex: an involuntary motor response to a stimulus produced by a reflex arc. 5 Basic Components of the reflex arc: 1. Sensory Receptor: specialized cell that responds to a specific stimulus and converts it to an action potential (AP) 2. Sensory Neuron: brings the AP to the CNS 3. Interneuron: carries the AP from sensory to motor neuron 4. Motor Neuron: AP to effector organ 5. Effector Organ: responds to the motor neuron impulse (e.g. muscle or gland) Reflexes are homeostatic. Some (somatic reflexes) will remove the body from painful stimuli or keep the body from falling. Others (autonomic reflexes) maintain constant blood pressure & water intake, regulate the amount of light coming into the eyes, the CO2 levels in the blood, etc. The integration center for spinal reflexes is the gray matter of the spinal cord; the brain is not involved with directing the effector, but some information may be sent to the cortex of the brain. Some reflexes are excitatory (eg muscle contraction), others are inhibitory (eg muscle relaxation). Monosynaptic reflexes: don’t involve interneurons Polysynaptic reflexes: include one or more interneuronal synapse Stretch Reflex (eg Patellar or KneeJerk Reflex): the simplest monosynaptic reflex in which muscles contract in response to a stimulus. 1. Tap on knee pulls on tendon of quadriceps femoris 2. Muscle stretches in response to pull on tendon 3. Afferent sensory neurons send info to spinal cord & brain 4. Sensory neurons in spinal cord act directly on motor neurons contraction of quadriceps femoris 5. Extension o This reflex prevents injury from overstretching and plays a central role in maintaining balance o This reflex is tested to determine the functionality of the higher CNS centers (since information is also sent to areas of the brain concerned with movement); slow response defect in nerve conduction D. Pathology (Clinical Impact, pp. 413. 416. 417, 419, 420) and (Diseases and Disorders, p. 425) a. Disrupt ascending tracts loss of sensation b. Disrupt descending tracts loss of motor function o Carpal Tunnel Syndrome: compression of the median nerve (originates in the Brachial Plexus) in the carpal tunnel (at the wrist) due to increased pressure in the tunnel resulting from inflammation or an increase in tendon size. Clinical Impact, p. 419 26o Sciatica (Ischiadica): inflammation of (neuritis), or damage to, the sciatic nerve leading to severe spasms of throbbing or stabbing pain radiating down the back of the thigh & leg; most common cause = herniated lumbar disk pressure on spinal nerves at the lumbar plexus Clinical Impact, p. 420 o Herpes Zoster (Shingles): Sensory skin pathology that involves the dermatomes a. A skin rash that develops on half of the body (unilateral), in a beltlike pattern; but, can occur on any part of the body b. Caused by the Varicella Zoster (herpes) Virus Table 12.1, p. 425 o This virus causes chicken pox (usually in the young); after the chicken pox heals, the virus remains dormant in the dorsal root ganglia and becomes active when the person’s immune system becomes weakened o Once active, it begins to multiply within the dorsal root ganglia damage & swelling occurs o The virus then moves along the nerve to the dermatome, causing swelling & damage as it goes; when it reaches the skin, it causes the telltale rash o This is a very painful condition that can last 3 to 6 months (or longer) 27
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