Chapters 12 and 13 Notes; Anatomy 1
Chapters 12 and 13 Notes; Anatomy 1 BSC 215
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BSC 215 Tuesday, December 29, y Chapter 12: Nervous System Nervous System Process Sense organs receive information about changes in the body and the external environment, and transmit coded messages to the spinal cord and the brain (sensory input) (afferent) brain and spinal cord processes this info, relate it to past experiences, and determine what response is appropriate to the circumstances (integration) brain and spinal cord issue commands to muscles and gland cells to carry out such a response (motor output)(efferent) Organization Central Nervous System (CNS) • brain and spinal cord enclosed in bony coverings • enclosed by cranium and vertebral column Peripheral Nervous System (PNS) • all the nervous system except the brain and spinal cord; composed of nerves and ganglia • nerve a bundle of nerve fibers (axons) wrapped in fibrous connective tissue • ganglion a knotlike swelling in a nerve where neuron cell bodies are concentrated PNS Sensory (afferent) division carries sensory signals from various receptors to the CNS • informas the CNS of stimuli within or around the body 1 BSC 215 Tuesday, December 29, y • somatic sensory division carries signals from receptors in the skin, muscles, bones, and joints • visceral sensory division carries signals from the viscera of the thoracic and abdominal cavities heart, lungs, stomach, and urinary bladder Motor (efferent) division carries signals FROM the CNS to and and muscle cells that carry out the body’s response • effectors cells and organs that respond to commands from the CNS • somatic motor division carries signals to skeletal muscles output… • visceral motor divison (autonomic nervous system) carries signals to glands, cardiac muscle, and smooth muscle involuntary, and responses of this system and its receptors are known as visceral reflexes sympathetic division • tends to arouse body for action • accelerates heart beat and respiration, while inhibiting digestive and urinary systems parasympathetic division • tends to have a calming effect • slows heart rate and breathing • stimulates digestive and urinary systems Neurons 2 BSC 215 Tuesday, December 29, y Properties of Neurons • Excitability (irritability) respond to environmental changes called stimuli • Conductivity neurons respond to stimuli by producing electrical signals that are quickly conducted to other cells at distant locations • Secretion when electrical signal reaches end of nerve fiber, a chemical neurotransmitter is secreted that crosses the gap and stimulates the next cell Functional Classification • sensory (afferent) neuron specialized to detect stimuli transmit information about stimuli to the CNS • BEGIN in almost every organ in the body and END in the CNS • afferent conducting signals toward CNS • interneurons (association neurons) lie entirely within the CNS receive signals from many afferent neurons and carry out the integrative function • process, store, and retrieve information and “make decisions” that determine how the body will respond to stimuli 90% of all neurons are interneurons lie between and interconnect the incoming sensory pathways and the outgoing motor pathways of the CNS 3 BSC 215 Tuesday, December 29, y • motor (efferent) neuron send signals out to muscles and gland cells (the effectors) • motor because most of them lead to muscles • efferent neurons conduct signals away from the CNS Structure • neurons= structural unit fo the nervous system conduct nervous impulses longlived do not undergo mitosis high metabolic rate (require constant oxygen and glucose) cell body (soma)= control center dendrites • short; many branches • possess organelles • receptive to signals • dendritic spines contact other neurons • convey incoming signals toward cell body (graded potentials) axons • axon hillock is where the axon attaches • every neuron can only have one axon and thus one axon hillock • long axons are known as nerve fibers 4 BSC 215 Tuesday, December 29, y • neurolemma/myelin sheath made of Schwann cells that are wrapped around the axon the spaces between Schwann cells are called Nodes of Ranvier • conducts and sends signals AWAY from the cell body Unlike a graded response (which can go up and down slowly), axons have a threshold energy that is needed to send a signal. It is an all or nothing response. • axons lack machinery for protein synthesis. therefore, they rely on the cell body to produce proteins and then the axons will/can transport proteins and organelles along a twoway path Polarity • multipolar neuron one axon and multiple dendrites most common most neurons in the brain and spinal cord • bipolar neuron one axon and one dendrite olfactory cells, retina, inner ear • unipolar neuron single process leading away from the soma sensory from skin and organs to spinal they start out as bipolar neurons but when they develop, they become unipolar • anaxonic neuron 5 BSC 215 Tuesday, December 29, y many dendrites but not axon help in visual processes only receive signals, not send Axonal Transport many proteins made in soma must be transported to axon and axon terminal • to repair axolemma, serve as gated ion channel proteins, as enzymes or neurotransmitters axonal transport twoway passage of proteins, organelles, and other material along an axon • anterograde transport movement down the axon away from soma • retrograde transport movement up the axon toward the soma microtubules guide materials along axon • motor proteins (kinesin ad dynein) carry materials “on their backs” while they crawl along microtubules kinesin motor protein in anterograde transport dynein motor protein in retrograde transport fast axonal transport occurs at a rate of 20400 mm/day • fast anterograde transport (up to 400 mm/day) organelles, enzymes, synaptic vesicles, and small molecules • fast retrograde transport for recycled materials and pathogens rabies, herpes simplex, tetanus, and polio viruses delay between infection and symptoms is time needed for transport up the axon • slow axonal transport (0.510mm/day) 6 BSC 215 Tuesday, December 29, y always anterograde move enzymes, cytoskeletal components, and new axoplasm down the axon during repair and regeneration of damaged axons damaged nerve fibers regenerate at a speed governed by slow axonal transport Supportive Cells Neuroglia of CNS • neuroglia cells that help neurons do their thing; more abundant than neurons! • astrocytes anchor neurons to capillaries facilitate neuronal migration; when we have to restructure and move neurons assist in synapse formation recycle neurotransmitters can release neurotransmitters influence neuron function Main Function: maintain homeostasis of neurotransmitters • microglia monitor neuron health detect microorganism presence detect cell death transform into phagocytic cells/ macrophages Main Function: defense cells 7 BSC 215 Tuesday, December 29, y • ependymal cells line cavities of brain and spinal cord produce cerebrospinal fluid (CSF) permeable barrier between CSF and CNS cells • oligodendrocytes wraps neuron fibers from the outside—> in responsible for myelin sheath (CNS) the “Schwann cells” of the CNS can coil around many neurons simultaneously Nodes of Ranvier more widely spaces • White Matter collections of myelinated fibers (fatty tissue=oligodendrocytes) MS demylination in the CNS • Grey Matter nerve cell bodies; unmyelinated fibers …of PNS • Schwann cells “neurillemocytes” form myelin sheaths around nerve fibers of PNS assist in PNS neuron regeneration Schwann cells indent to receive axon and then wrap tightly around the axon, generating the myelin sheath; wrap from inside—> out 8 BSC 215 Tuesday, December 29, y • nucleus and cytoplasm of Schwann cells in external bulge (neurilemma) • adjacent schwann cells do not touch, leaving gaps along axon • nodes of ranvier exposed portions of axon, evenly spaces • satellite cells envelop neuron cell bodies provide electrical insulation around soma • Myelin Sheath fatty protective layer that insulates nerve fibers (axons) increases the speed that nervous impulses travel along axon • smaller axons unmyelinated fibers conduct nervous impulses slowly Conduction Speed of Nerve Fibers Speed at which a nerve signal travels along a nerve fiber depends on two factors • Diameter of fiber • Presence or absence of myelin Signal conduction occurs along the surface of a fiber • Larger fibers have more surface area and conduct signals more rapidly • Myelin further speeds signal conduction Conduction Speed • smal, unmyelinated fibers: 0.52.0 m/s • small myelinated fibers: 315.0 m/s 9 BSC 215 Tuesday, December 29, y • large, myelnated fibers: up to 120 m/s • slow signals supply the stomach and dilate pupil where speed is less of an issue • fast signals supply skeletal muscles and transport sensory signals for vision and balance Regeneration of Nerve Fibers regeneration of a damaged peripheral nerve fiber can occur IF: • it’s soma is intact • at least some neurilemma remains fibers distal to the injury cannot survive and degenerates because it has been cut off from its supply of nutrients • macrophages clean up tissue debris at the point of injury and beyond soma swells, ER breaks up, and nucleus moves off center • due to loss of nerve growth factor from neuron’s target cell (the effector cells; such as muscle cells) axon stump sprouts multiple growth processes • severed distal end continues to degenerate regeneration tube formed by Schwann cells, basal lamina (organized connective tissue; interacts with the neurilemma), and neurilemma • a sprout will find itself in the regeneration tube and continue to grow towards the original target cells and reestablishes synaptic contact. the others disintegrate. nucleus returns to normal shape regeneration of damaged nerve fibers in the CNS cannot occur at all 10 BSC 215 Tuesday, December 29, y denervation atrophy of muscle due to loss of nerve contact by damaged nerve; nerves and muscles communicate with each other to grow without this connection, the muscle cells start dying. Action Potential In the case of the human body, electrical currents are generated by the flow of ions across cellular membranes electrophysiology cellular mechanisms for producing electrical potentials and currents • basis for neural communication and muscle contraction electric potential a difference in the concentration of charged p articles between one point and another electrical current a flow of charged particles from one point to another • in the body, currents are movements of ions, such as Na+ or K+ ,through gated channels in the plasma membrane • gated channels are opened or closed by various stimuli • enables cell to turn electrical currents on and off living cells are polarized • usually more negative inside than they are outside because we tend to concentrate solutes within the cell and they are typically larger in charge than the positive charged particles within the cell. Positively charged particles are also usually waller and can thus cross the membrane very easily. resting membrane potential (RMP) charge difference across the plasma membrane • about 70 mV in a resting, unstimulated neuron 11 BSC 215 Tuesday, December 29, y • negative value means there are more negatively charged particles on the inside of the membrane than on the outside • stimulated neuron has about 90 mV • RMP exists because of unequal electrolyte distribution between extracellular fluid (ECF) and intracellular fluid (ICF) • RMP results from the combined effect of three factors Ions diffuse down their concentration gradient through the membrane plasma membrane is selectively permeable and allows some ions to pass easier than others electrical attraction of cations and anions to each other potassium ions (K+) have the greatest influence on RMP plasma membrane is more permeable to K+ than any other ion leaks out until electrical charge of cytoplasmic anions attracts it back in and equilibrium is reached and net diffusion of K+ stops K+ is about 40 times as concentrated in the ICF as in the ECF cytoplasmic anions (negatively charged) cannot escape due to size or charge (phosphates, sulfates, small organic acids, proteins, ATP, and RNA) membrane much less permeable to high concentration of sodium (Na+) found outside the cell • some leaks and diffuses into the cell down its concentration gradient • Na+ is about 12 times as concentrated in the ECF as in the ICF • resting membrane is much less permeable to Na+ than to K+ Na+/K+ pumps out 3 Na+ for every 2 K+ it brings in 12 BSC 215 Tuesday, December 29, y • works continuously to compensate for Na+ and K+ leakage and requires great deal of ATP 70% of the energy requirement of the nervous system • necessitates glucose and oxygen be supplied to nerve tissue (energy needed to create the resting potential) • pump contributes about 3 mV to the cell’s resting membrane potential of 70 mV Beyond Resting Membrane Potential • chnages in membrane potentials —> communciation signals • how do you change membrane potentials? open channel and ions will flow across their concentration gradient change membrane permeability to ions alter ion concentrations on either side of the membrane • local potential local disturbance in membrane potential; act over short distance When neuron is stimulated by chemicals, light, heat, or mechanical disturbance • opens the Na+ gates and allows Na+ to rush into the cell • Na+ inflow neutralizes some of the internal negative charge • voltage measured across the membrane drifts toward zero • depolarization case in which membrane voltage shifts to a less negative value • Na+ diffuses for short distance on the inside of the plasma membrane producing a current that travels toward the cell’s trigger zone; this shortrange change in voltage is called a local potential Differences of local potentials from action potentials 13 BSC 215 Tuesday, December 29, y • graded: vary in magnitude with stimulus strength; stronger stimuli opens mores Na+ gates • decremental: get weaker the farther they spread from the point of stimulation voltage shift caused by Na+ inflow diminishes rapidly with distance • reversible: when stimulation ceases, K+ diffusion out of cell returns the cell to its normal resting potential • either excitatory or inhibitory: some neurotransmitters (glycine) make the membrane potential more negative hyperpolarize it so it becomes less sensitive and less likely to produce an action potential • action potential longerterm change in membrane potentials; act over long distances more dramatic change produced by voltageregulated ion gates in the plasma membrane only occur where there is a high enough density of voltageregulated genes soma (5075 gates per micrometer squared); cannot generate an action potential trigger zone (350 to 500 gates per micrometer squared); where action potential is generated • if excitatory, local potential spreads all the way to trigger zone and is still strong enough when it arrives, it can open these gates and generate an action potential only nerve and muscle cells (excitable membranes) can generate action potentials action potential: brief reversal of membrane potential with total amplitude of 100mV characteristics of action potential versus a local potential • follows an all or none law 14 BSC 215 Tuesday, December 29, y if a threshold is reached, the neuron fires at its maximum voltage. when it fires, it fires at 100mV only, nothing more and nothing less if threshold is not reached, it does not fire • nondecrememntal does not get weaker with distance • irreversible once started, goes to completion and cannot be stopped depolarization—> repolarization—> hyperpolarization—> resting 35mV is the threshold so on a graph, up until the potential is equivalent to 35mV, it is local potential resting—> all channels are closed—> axon membrane (axon hillock) depolarized by local currents—> voltagegated Na+ channels open and Na+ pours into the cell—> membrane becomes more depolarized, more VG Na+ channels open causing even more depolarization (positive feedback loop)—> once depolarization reaches threshold, more VGNa+ channels open…more depolarization…VGNa+ channels are open—> membrane permeability to Na+ has skyrocket and membrane potential overshoots to 30+mV—> inactivation gates of Na+ close after about 1 millisecond—> membrane permeability to Na+ decreases…action potential “spike” stops rising—> VGK+ channels open and K+ pours out of the cell along its gradient—> is die the neuron becomes more negative= repolarization —> takes a while for VGK+ channels to close resulting in hyperpolarization—> Na+/K+ pump restores ion distribution across the neuron membrane back to resting The Refractory Period • during an action potential and for a few milliseconds after, it is difficult or impossible to stimulate that region of a neuron to fire again • refractory period the period of resistance to stimulation when the charges are being reversed • Two Phases absolute refractory period 15 BSC 215 Tuesday, December 29, y • no stimulus of any strength will trigger action potential • as long as Na+ gates are open • from action potential to RMP relative refractory period • after the absolute refractory period • only especially strong stimulus will trigger new action potential K+ gatrs are still open and any effect of incoming Na+ is opposed by the outgoing K+ • the refractory period refers only to a small patch of the neuron’s membrane at one time • other parts… Signal Conduction in Unmyelinated Nerve Fibers for communication to occur, the nerve signal must travel to the end of the axon unmyelinated fiber has voltageregulated ion gates along its length action potential from the trigger zone causes Na+ to enter the axon and diffuse into adjacent regions beneath the membrane the depolarization excites voltageregulated gates immediately distal to the action potential Na+ and K+ gates open and close producing a new action potential by repetition, the membrane distal to that is excited chain reaction continues to end of axon Signal Conduction in Myelinated Nerve Fibers 16 BSC 215 Tuesday, December 29, y voltagegated channels needed for action potentials • fewer than 25 per square micrometer in myelincovered regions (internodes) • up to 12,000 per square micrometer in nodes of Ranvier fast Na+ diffusion occurs between nodes • Signal weakens under myelin sheath, but still strong enough to stimulate an action potential at next node • Saltatory conduction—nerve signal seems to jump from node to node Synapses (Section 12.5) Junction between neurons (or between neuron and effector cell) A nerve signal can go no further when it reaches the end of the axon • triggers the release of a neurotransmitter • stimulates a new wave of electrical activity in the next cell across from the synapse Synapse between two neurons • First neuron in the signal path is the presynaptic neuron releases neurontransmitter • Second neuron is the postsynaptic neuron responds to the neurotransmitter Presynaptic neurons can attach to soma, dendrite, or axon of postsynaptic neuron creating an axodendritic, axosomatic, or axoaxonic synapses A neuron can have an enormous number of synapses • spinal motor neuron covered by about 10,000 synaptic knobs from other neurons 8,000 ending on its dendrites 17 BSC 215 Tuesday, December 29, y 2,000 ending on its soma • in the cerebellum of brain, one neuron can have as many as 100,000 synapses synaptic cleft fluidfilled space between presynaptic and postsynaptic neurons not always a continuous electrical signal; can use chemicals to cross synaptic cleft Synaptic knob of presynaptic neuron contains synaptic vesicles containing neurotransmitter • Many docked on release sites on plasma membrane Ready to release neurotransmitter on demand • A reserve pool of synaptic vesicles located further away from membrane • Postsynaptic neuron membrane contains proteins that function as receptors and liganregulated ion gates Neurotransmitters and Related Messengerss • more than 100 neurotransmitters have been identified • fall into four major categories according to chemical composition Acetylcholine • formed from acetic acid and choline • one way to stop the signal is to break the acetylcholine into acetic acid and choline Amino Acid Neurontransmitters • include glycine, glutamate, aspartate, and gammaaminobutyric acid (GABA) Monoamines • synthesized form amino acids by removal of the COOH (carboxyl) group • retaining the NH 2(amino) group 18 BSC 215 Tuesday, December 29, y • major monoamines epinephrine, norepinephrine, dopamine= (catecholamines) histamine and serotonin Neuropeptides • some we will talk about later= neuromodulators, which typically have a more longterm effect compared to neurotransmitters Synaptic Transmission • neurotransmitter synthesized by the presynaptic neuron in the soma released in response to stimulation (action potential) bind to specific receptors on the postsynaptic cell alter the physiology of that cell by allowing inflow of some ion diverse in their action • some excitatory; some inhibitory • some the effect depends on what kind of receptor the postsynaptic cell has • some open ligandregulated ion gates while some act through second messenger systems • A given neurotransmitter does not have the same effect everywhere in the body; different parts of the body might produce different types of responders • multiple receptor types exist for a particular neurotransmitter different nerves can have different receptors that respond to the same neurotransmitter and create different responses to it (alter internal physiology, alter external physiology, pass the signal down, etc) 14 receptor types for serotonin 19 BSC 215 Tuesday, December 29, y • receptor governs the effect the neurotransmitter has on the target cell • synaptic delay time from the arrival of a signal at the axon terminal of a presynaptic cell to the beginning of an action potential in the postsynaptic cell 0.5 ms for all the complex sequence of events to occur • Three kinds of synapses with different modes of action excitatory cholinergic synapse • employs acetylcholine (ACh) as its neurotransmitter ACh excites some postsynaptic cells • skeletal muscle inhibits others 1. Arrival of nerve signal opens voltagegated Ca channels 2. Ca enters knob and triggers release of ACh 3. Empty vesicles reload ACh 4. ACh diffuses into synaptic cleft and bind to ligandgated channels on the + + postsynaptic neuron (Na enters cell, K leaves) + 5. As Na enters, depolarization occurs creating a local potential called the postsynaptic potential 6. If the local potential is strong enough to reach the axon hillock, neuron fires. inhibitory GABAergic synapse • GABAergic synapse employs gammaamenobutyric acid as its neurotransmitter • nerve signal triggers release of GABA into synaptic cell • GABA receptors are chloride channels 20 BSC 215 Tuesday, December 29, y • Cl enters cell and makes the inside more negative than the resting membrane potential • postsynaptic neuron is inhibited, and less likely to fire excitatory adrenergic synapse • Adrenergic synapse employs norepinephrine (NE) aslo called noradrenaline • NE and other monoamines, and neuropeptides, act through secondary messenger systems such as cyclic AMP (cAMP) need to know that when AMP is changed to cAMP, it is a chemical change that turns it into a messenger • receptor is not an ion gate, but a transmembrane protein associated with a G protein • slower to respond than cholinergic and GABAergic synapses • has advantage of enzyme amplification— single molecule of NE can produce vast numbers of product molecules in the cell 1. The unstimulated NE receptor is bound to a G protein 2. Binding of NE to the receptor causes G protein to dissociate 3. G protein binds and activates adenylate cyclase to convert ATP to cAMP 4. cAMP can produce 3 effects i. can produce a logan that binds and opens ion channels (Na+) from the inside= depolarizes cell ii. enzyme activation that leads to metabolic changes (ex: release of glucose by liver; converting glycogen into glucose and releasing it into blood stream) iii. genetic transcription= new enzymes for metabolic functions • Cessation of Signal 21 BSC 215 Tuesday, December 29, y Mechanisms to turn off stimulation to keep postsynaptic neuron from firing indefinitely (would lead to nerve breakdown) • Neurotransmitter molecule binds to its reeptor for only a millisecond or so then dissociates from it • if presynaptic cell continues to release neurotransmitter, one molecule is quickly replaced by another and the neuron is restimulated • astrocytes clean up excess neurotransmitters Stop adding neurotransmitter • achieved simply by stopping signals to presynaptic nerve Get rid of neurotransmitter already there (3 ways) • Diffusion neurotransmitter escapes the synapse into the nearby ECF astrocytes in CNS absorb it and return it to neurons • Reuptake synaptic knob reabsorbs amino acids and monoamines by endocytosis neurotransmitters broken down in synaptic knob by the enzyme monoamine oxidase (MAO) • Degradation in the synaptic cleft enzyme acetylcholinesterase (AChE) in synaptic cleft degrades ACh into acetate and choline choline reabsorbed by synaptic knob 22 Tuesday, December 29, y Chapter 13: The Spinal Cord, Spinal Nerves, and Somatic Reflexes Functions Conduction nerve fibers that conduct info up and down the cord, connecting different levels of the trunk with each other and the brain Neural integration input from multiple sources integrated and executed output Locomotion repetitive, coordinated contractions of several muscle groups in the limbs (controlled by CNS) Reflexes involuntary stereotyped responses to stimuli Somatic Reflexes reflexes quick, involuntary, stereotyped reactions of glands or muscle to stimulation Four Important Properties • reflexes require stimulation; not spontaneous actions, but responses to sensory input • reflexes are quick; involve few if any interneurons and minimum synaptic delay • reflexes are involuntary; occur without intent and difficult to suppress • reflexes are stereotyped; occur essentially the same way every time Reflexes include glandular secretion and contraction of all three types of muscle • include some learned responses: conditioned reflexes (Pavlov’s dogs) Here, we are dealing with unlearned skeletal muscle reflexes mediated by the brainstem and spinal cord called somatic reflexes Pathway of Reflex Arc 1 Tuesday, December 29, y • somatic receptors in skin, muscles, or tendons • afferent nerve fibers carry information from receptors to posterior horn of spinal cord or the brain stem • integrating center a point of synaptic contact between neurons in gray matter (axon material) of spinal cord or brainstem determines whether efferent neurons issue signal to muscles • efferent nerve fibers carry motor impulses to skeletal muscle • effectors the somatic effectors that carry out a response • Muscle Spindles many somatic reflexes involve stretch receptors called muscle spindles (stretch receptors embedded in skeletal muscles) proprioceptors specialized sense organs to monitor the position and movement of body parts muscle spindles inform the brain of muscle length and body movement enables brain to send motor commands back to the muscles that control coordinated movement, corrective reflexes, muscle tone… **only thing you need to know is that they know when muscle is being stretched or not Spindles are abundant in fine control muscles • hands and feet have approx. 100 per gram of muscle tissue • only a few in large muscles like quadriceps (coarse movements) • none in the middleear muscles Intrafusal fibers muscle fibers within the spindle 2 Tuesday, December 29, y • inform the brain about muscle length, and speed of change in length 9the brain can then provide quick reflexes to maintain balance) Extrafusal fibers all other “normal” muscle fibers doing work Nerve fibers in muscle spindle • primary afferent fiber sensitive to small changes in length • secondary afferent fibers inform brain of length • gamma motor neurons adjust sensitivity of the spindle Stretch Reflex stretch (myotatic) reflex when a muscle is stretched, it “fights back” and contracts, maintaining tonus and making it stiffer than unstretched muscle • helps maintain equilibrium and posture head starts to tip forward muscles contract to raise the head • **stabilize joints by balance tension in extensors and flexors smoothing muscle actions so it’s not jerky stretch reflex is mediated primarily by the brain • not strictly a spinal reflex tendon reflex reflexive contraction of a muscle when its tendon is tapped • kneejerk (patellar) reflex is a monosynaptic reflex one synapse between the afferent and efferent neuron • testing somatic reflexes helps diagnose many diseases 3 Tuesday, December 29, y reciprocal inhibition reflex phenomenon that prevents muscles from working against each other by inhibiting the antagonist (telling them to relax and not fight against the reaction) patellar tendon reflex arc • tap on patellar ligament excites nerve endings of muscle spindle in quadriceps femoris • stretch signals travel to spinal cord via primary afferent fiber and dorsal root • primary afferent neuron stimulates alpha motor neuron in the spinal cord • efferent signals in alpha motor nerve fiber stimulate quadriceps to contract, producing knee jerk (alpha neuron attaches to skeletal muscle) • at same time, a branch of the afferent nerve fiber stimulates inhibitory motor neuron in spinal cord • that neuron inhibits alpha motor neuron that supplies hamstring muscles • hamstring contraction is inhibited so hamstrings (knee flexors) do not antagonize quadriceps (knee extensor) flexor reflex the quick contraction of flexor muscles resulting in the withdrawal of a limb from injurious stimulus • polysynaptic reflex arc pathway in which signals travel over many synapses on their way back to the muscle • stimulus alerts sensory neuron which activates multiple interneurons • ipsilateral (same side of body) motor neurons to flexor excited • ipsilateral flexion contracts • contralateral (crosses to other side of body) motor neurons to extensor excited to contract • … 4 Tuesday, December 29, y tendon organs proprioceptors in a tendon near its junction with a muscle • golgi tendon organ: 1mm long; nerve fibres entwined in collage fibers of the tendon • rubber bands side by side; neurons in between bands; when bands are pulled taut, they press against the neural ends which causes it to send messages telling you to stop tendon reflex in response to excessive tension on the tendon • inhibits muscle from contracting strongly • moderates muscle contraction before it tears a tendon or pulls it loose from the muscle or bone Reflex Testing Important diagnostic for assessing the condition of the nervous system distotrted, exaggerated, or absent reflexes may indicate degeneration or pathology of portions of the nervous system, often before other signs are apparent if the spinal cord is damaged, then reflex tests can help determine the area of injury. For example, motor nerves above and injured area may be unaffected, whereas those below the damaged area may be unable to perform the usual reflex activities. Patellar Tendon Reflex • Have patient sit with legs hanging • lightly tap patellar tendon with Taylor hammer (below patella) • record response 0: absent reflex 1+: trace, or seen only with reinforcement 2+: normal 3+: brisk 5 Tuesday, December 29, y 4+: nonsustained clonus (i.e. repetitive vibratory movements) 5+: sustained clonus (knee extended and does not go back) Normal= 13 unless they are asymmetric or there is a dramatic difference between the arms and the legs. Calcaneal tendon (Achilles) reflex • tests sacral nerves S1S2 sciatic nerve pathology delay in hypothyroidism • have patient sit with foot dorsiflexed • tap calcaneal tendon… **only need to know up to reflex arcs…. 6