Chapter 6 - Human Physiology
Chapter 6 - Human Physiology BIOL 2213
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Chapter 6 – Neuronal Signaling and the Structure of the Nervous System Divisions of the Nervous System – The two main divisions are the central nervous system (CNS) which is composed of the brain and spinal cord, and the peripheral nervous system (PNS) which is composed of the nerves that connect the brain or spinal cord to the body’s muscles, glands, and sense organs. 1. Nuclei – clusters of neurons in the CNS 2. Ganglia – clusters of neurons in the PNS Structures of a Neuron – Most neurons contain a cell body and two processes – the axons and dendrites. 1. Cell Body – this is also known as the soma, and contains the nucleus, ribosomes, mitochondria, and the genetic information necessary for protein synthesis. 2. Dendrites – a series of highly branched outgrowths of the cell body. The dendrites receive input from other neurons, and their function is to increase surface area for this to occur. 3. Dendritic Spines – these increase the surface area of the dendrites even further. They also contain ribosomes, which allow them to synthesize proteins so they can remodel their shape in response to variation in synaptic activity. 4. Axon – a long process that extends from the cell body and carries output to its target cells. The axon contains an initial segment, which is a region of the axon that arises from the cell body. Electrical signals are generated at the initial segment. The end of the axon splits into branches, called collaterals that continue branching until they end in axon terminals. Axon terminals are responsible for releasing neurotransmitters. Glial Cells – Glial cells are found in both the CNS and PNS. However, different glial cells are found in each. 1. Glial Cells of the CNS a. Astrocytes – helps regulate the composition of the extracellular fluid by removing K and neurotransmitters around synapses. They also stimulate the formation of tight junctions between the cells that make up the walls of capillaries found in the central nervous system, forming the bloodbrain barrier. They also provide glucose to neurons and remove ammonia. Finally, they have neuronlike characteristics and are thought to take part in information signaling of the brain. b. Microglia – these are specialized macrophage cells that perform immune functions in the CNS c. Ependymal Cells – these cells line fluidfilled cavities of the brain d. Oligodendrocytes – the myelinforming cells of the CNS. Oligodendrocytes may branch to form myelin on other axons. 2. Glial Cells of the PNS a. Satellite Cells – these surround neural bodies in the PNS b. Schwann Cells the myelinforming cells of the PNS. These form myelin sheath segments at regular intervals. The spaces in between the myelin are called the nodes of Ranvier. The nodes of Ranvier are exposed portions of the axon’s plasma membrane to extracellular fluid. Axonal Transport – this describes the movement of organelles and other materials between the soma and the axon terminals. 1. Kinesins and Dyneins – these are known as “motor proteins.” These are double headed proteins that bind to their cellular cargo and microtubule rails. They use the energy from the hydrolysis of ATP to move along the microtubules. Kinesins move from the soma to the axon terminal (anterograde). Dyneins move from the axon terminal to the soma (retrograde). Functional Classes of Neurons – Neurons are divided into 3 functional classes. It is important to note that a nerve fiber is a term used to refer to a single neuron and axon, while a nerve is a term used to describe a bundle of neurons bound together with connective tissue, and is therefore an organ. 1. Afferent Neurons – transmit information into the central nervous system from receptors (not proteins) at their peripheral endings. Cell bodies are located in the PNS while the axon enters the CNS. 2. Efferent Neurons – convey information away from the central nervous system to effector cells like muscles, glands, or other nerve cells. Cell bodies are located in the CNS while their axon is in the PNS. 3. Interneurons – connect neurons within the central nervous system. The entire neuron, including the cell body, is located in the CNS. Synapse – a synapse is an anatomically specialized region between two neurons where one neuron alters the electrical and chemical activity of another neuron. Signals are transmitted across synapses by neurotransmitters. Neurotransmitters can also affect the effector cells (muscles, glands, etc.). Mostly, synapses occur between an axon terminal of a presynaptic neuron and a dendrite of a postsynaptic neuron. The neurotransmitters released from the presynaptic neuron bind with specific protein receptors on the membrane of the postsynaptic neuron. Neural Growth – The nervous system develops from undifferentiated stem cells that develop into neurons or glia. Once fully differentiated, a growth cone forms at the tip of each extending axon and is involved in finding the correct route to its final target. As the axon grows, it is guided along by glial cells. Neurotrophic factors are growth factors for neural tissue in the extracellular fluid surround the growth cone or the distant target. As the nervous system is refined, neurons undergo preprogrammed cell death in a process called apoptosis. Furthermore, the term plasticity refers to the phenomenon in which the developing brain has much greater potential for remodeling in response to stimulation or injury than in the adult brain. Neural Injuries – Injuries can occur in several ways. 1. PNS Axonal Severance Injury – If axons are severed, they can repair themselves in the PNS as long as the cell body is not damaged. The severed axon degenerates and a new growth cone forms from the cell body which grows out towards the effector. Regrowth of the axon only occurs at 1 mm per day so injury repair is very slow. 2. CNS Spinal Injury – Spinal injuries normally crush the axons, rather than sever them. When this happens, the nearby oligodendrocytes undergo apoptosis, damaging the myelin coat so axons cannot transmit signals efficiently. If the axons are severed, no significant regeneration occurs in the CNS. a. Research attempts to repair nervous tissue – researchers are creating tubes to support regrowth of severed axons, and redirecting the axons to regions of the spinal cord that lack growthinhibiting factors. This prevents apoptosis of the oligodendrocytes so that myelin is maintained. In addition, neurotrophic factors are injected to support recovery and regrowth. Basics of Electricity – Terms for the basics of electricity 1. Electric Potential – separated electrical charges of opposite sign that have the potential to do work if they are allowed to come together 2. Potential Difference – the amount of charge between two points a. Referred to as potential 3. Current – the movement of electrical charge 4. Resistance – the hindrance to electrical charge movement I= V V=IR 5. Ohm’s Law: R or Resting Membrane Potential – all cells in resting conditions have a potential difference across the plasma membrane, with the inside of the cell being negatively charged. In neurons, the resting potential exists because of a small excess of negative ions inside the cell and an excess of positive ions outside. The charges, being attracted, form a tight shell around the plasma membrane. A neuron has a resting potential of 40 to 90 mV. The magnitude of the resting potential is determined by 2 factors: 1. Differences in specific ion concentrations in the intracellular and extracellular fluids 2. Differences in membrane permeabilities to the different ions, which reflects the number of open channels for the different ions in the plasma membrane. Equilibrium Potential – There are 2 factors that can cause ions to move across a membrane: chemical concentration and electrical potential. So, if 2 positively charged ions are on either side of a membrane, but only 1 specific ion channel is open, then that free ion will move down its chemical concentration gradient, making one compartment more positive that the other creating an electric potential. However, at a point, this will stop because the like charges will start to repel each other. When the flux due to chemical concentration gradient is equal to the flux due to the membrane electric potential, this is called equilibrium potential for the specific free ion. In + neurons, the membrane potential is mostly influenced by the movement of K out of the cell until equilibrium potential is reached. We have been talking about ion channels so now we will be more specific of the types involved in neurons: 1. Leak K Channel – These are a type of nonmediated transport ion channels, specific to + potassium. This channel allows for a net movement of K out of the cell. It is important to note that there are much more K inside the cell, but some escape to the extracellular fluid due to equilibrium potential. This is why the resting potential is negative. + 2. VoltageGated Na Channel – these are a type of nonmediated transport ion channels, specific to sodium. They open only at a certain voltage, so that sodium can rush in. Graded and Action Potential Terms: 1. Depolarization – the membrane potential becomes less negative 2. Overshoot – the membrane potential reverses and the inside of the cell becomes positive 3. Repolarization – the membrane potential returns toward resting value 4. Hyperpolarization – the membrane potential is more negative than resting value Graded Potentials – These are important in shortdistance signaling. The magnitude of the potential can change. It has no threshold or refractory period. Depending on the stimulus, graded potentials can either depolarize or hyperpolarize the membrane at the site. Steps are as follows: 1. Chemical Signal 2. Cationchannels (voltage gated channels) open so cations rush in the cell. a. Extracellular Na flow from the nearby positive portion towards the protein + b. Intracellular K flow away from the protein channel towards the nearby negative portion (so once the channels open, K and Na flow away from the channel) 3. A local charge flow (current) forms that decreases with increasing distance from the site of depolarization. This is referred to as being decremental. Introduction to Action Potentials – These are large alterations in the membrane potential. Many cells are capable of producing action potentials like neurons, muscle cells, endocrine, immune, and reproductive cells. Membranes on these cells that are capable of producing action potentials are called excitable membranes, and the ability to produce action potentials is called excitability. Action potentials are generated within 14 milliseconds. Voltage Gated Ion Channels – Ligandgated ion channels and mechanicallygated ion channels serve as the initial stimulus for an action potential. However, voltagegated channels give the + membrane its ability to undergo action potentials. K voltagegated ion channels are slower to open and close than Na voltagegated ion channels. Action Potential Mechanism – The following 7 steps outline an action potential: + + 1. The membrane is at steady resting potential. K leak channels are open while the Na + channels are closed. A neurotransmitter binds to a receptor, allowing Na to flow in the neuron. + 2. Local membrane is brought to threshold voltage by a depolarizing stimulus. Na entry causes depolarization, which opens more voltage gated Na channels. + + 3. More Na channels open to propagate the action potential, causing the localized area in the cell to become positive. + + 4. Voltagegated K channels (different from leak channels) open and Na channels are inactivated by the inactivation gate, causing the depolarization to slow down and come to a halt. + 5. Outflow of K ions repolarizes the membrane back to a negative potential. The return to negative potential caused the Na voltagegated channels to close. + 6. Persistent current through K channels hyperpolarizes the membrane (behaves more like + + the K equilibrium potential) because the K voltage gated channels are slow to close. 7. K channels close and the membrane potential is brought back to resting potential by + + Na /K ATPase. Threshold Potential – the membrane must be depolarized to about 55 mV in order to initiate an action potential. Once this is reached, the action potential delivers an “allornone” response. Local Anesthetics – These drugs prevent action potentials. They block voltage gated Na + channels, preventing their opening in response to depolarization. Without action potentials, graded signals generated in the periphery do not reach the brain, and pain is not felt. Refractory Period – During the action potential, a second stimulus will not produce an action potential. That certain region of the membrane is said to be in its absolute refractory period. This occurs during the period when the voltagegated Na channels are either already open or have preceded to the inactivated state during the first action potential. Therefore, absolute refractory period is the point up to the minimum on the action potential curve (hyperpolarization). Following this, there is an interval in which a second action potential can be produced only if the stimulus is strong enough. This interval is called the refractory period. Most neurons can deliver 100 action potentials per second. Finally, refractory periods are also the key in determining the direction of the electrical signal down the axon. Action Potential Propagation – Action potentials can only go forwards in the axon, since the area directly behind is in absolute refractory period. The velocity of action potentials depends on the thickness of the fiber and whether or not the fiber is myelinated. The larger the fiber is, the faster the current moves, since resistance is reduced. Action potentials only occur along the nodes of Ranvier, in a “jumping” process known as saltatory conduction. The myelin sheaths that create the nodes of Ranvier are helpful for numerous reasons: 1. Membrane pumps do not have to restore as many ions. 2. Myelin coated neurons are more metabolically efficient because they use less ATP. 3. Myelin sheaths allow neurons to be thinner, saving space in the nervous system. Generation of Action Potentials – In afferent neurons, the initial depolarization to threshold is achieved by a graded potential, called a receptor potential. Receptor potentials occur at the peripheral ends of neurons. In other neurons, the depolarization to threshold is accomplished by a graded potential, synaptic potential, or pacemaker potential. Synapses – Synapses are junctions between 2 neurons. There are 2 types of synapses: 1. Electrical Synapses – the plasma membrane of the presynaptic neuron and the postsynaptic neuron are connected via gap junctions. The local currents from an action potential allow to flow directly across from one neuron to another. 2. Chemical Synapses – the axon of the presynaptic neuron ends in a slight swelling called the axon terminal, which hold synaptic vesicles that contain various neurotransmitters. The postsynaptic membrane has an area called the postsynaptic density, which contains a high concentration of membrane receptor proteins that bind to neurotransmitters. Between the presynaptic neuron and postsynaptic neuron is the 10 to 20 nm space called the synaptic cleft. Neurotransmitters are chemical released by the axon terminals that diffuse across the synaptic cleft. If more than one neurotransmitter is released, the addition chemical is called a cotransmitter. Activation of Presynaptic Cell – the following steps outline the mechanism for neurotransmitter release in the presynaptic cell: 1. An action potential reaches the terminal of the presynaptic membrane. 2+ 2. Depolarization causes Ca voltagegated channels to open, so calcium influxes inside the axon terminal. 3. Calcium ions activate the fusion of docked vesicles with the synaptic terminal membrane. a. Vesicles are docked in the active zones by SNARE proteins. When calcium enters, Ca binds to vesicle proteins called synaptotagmins, which pulls SNARE proteins, resulting in membrane fusion and neurotransmitter release. Activation of Postsynaptic Cell – the following steps outline the mechanism for the activation of the postsynaptic cell via neurotransmitters: 1. Neurotransmitters diffuse across the synaptic cleft. 2. Neurotransmitters bind to protein receptors on the plasma membrane. There are 2 types of receptors here: a. Ionotropic Receptors – Ion channels (Ligandgated channel receptor) b. Metabotropic Receptors – G Protein Coupled Receptors 3. Ion channels release their neurotransmitters when the surrounding concentration of free floating neurotransmitters decreases. 4. Unbound neurotransmitters are removed from the synaptic cleft by the following steps: a. Diffusion away from the receptor site b. Are enzymatically transformed into inactive substances c. Active transport back into the axon terminal in a process called reuptake. Types of Chemical Synapses: 1. Excitatory – the postsynaptic response to the neurotransmitter is a depolarization (a peak), bringing the membrane closer to threshold, so that an action potential is more likely. The excitatory change is called the EPSP. 2. Inhibitory – the postsynaptic response to the neurotransmitter is a hyperpolarization (a valley), bringing the membrane farther from threshold, so that an action potential is less likely. The inhibitory change is called the IPSP. Synaptic Integration – the membrane potential of the postsynaptic neuron is a result of all the synaptic activity affecting it at that precise moment. Therefore, the membrane potential is an additive effect of all EPSPs and IPSPs. Temporal summation is defined as the process by which input signals arrive from the same presynaptic cell at different times, resulting in the opening of more ion channels causing a greater flow of positive ions into the cell. Neuromodulators – These substances modify the presynaptic and postsynaptic cell’s response to neurotransmitters, by either amplifying or dampening the effectiveness of synaptic activity. Both neurotransmitters and neuromodulators have receptors. Receptors for neurotransmitters affect the condition of ion channels. Receptors for neuromodulators bring about changes in the metabolic process of neurons which can affect enzyme activity or protein (receptor) synthesis. Classes of Neurotransmitters: 1. Acetylcholine – This is found in both the CNS and PNS, especially at neuromuscular junctions. Neurons that release ACh are called cholinergic neurons. Acetylcholine acts on the receptors muscarinic and nicotinic, which is found at neuromuscular junctions. ACh is formed from the enzymatic reaction of acetyl CoA and choline and is broken down in the synaptic cleft by acetylcholinesterase to produce acetate and choline. Alzheimer’s disease is associated with the degeneration of cholinergic neurons and a decrease in ACh. 2. Biogenic Amines – These neurotransmitters are made from amino acids. Biogenic amines act on the receptor, Adrenergic, which is a GCoupled protein receptor. Biogenic amines consist of the following: a. Catecholamine b. TyrosineBased i. Dopamine ii. Epinephrine – found in the PNS iii. Norepinephrine – found in both the CNS and PNS c. Serotonin (made from tryptophan) – this is found in the CNS, especially the brainstem. It is responsible for sleep, emotions, cell growth, smooth muscle contraction, mood and anxiety. d. Histamine (made from histidine) – this is found in the CNS, especially the hypothalamus, and modulates sleep. It can sometimes be found in the PNS where it is involved in allegoric reactions, nerve sensitization, and acid production in the stomach. 3. Amino Acid Neurotransmitters – Excitatory AA neurotransmitters are aspartate and glutamate. Inhibitory AA neurotransmitters are glycine and GABA. Glutamate is estimated to be the primary neurotransmitter at 50% of excitatory synapses in the CNS. GABA is the main inhibitory neurotransmitter in the brain. 4. Neuropeptides – these are short chains of amino acids with peptide bonds. An example is the endogenous opioids that regulate pain. Morphine and codeine mimic the effects of these neuropeptides. 5. Gas Neurotransmitters – These are produced by enzymes in the axon terminal. They diffuse from their sites of origin across the synaptic cleft and into the intracellular fluid of effector cells. Examples are nitric oxide and carbon monoxide. Neuroeffector Junctions – These junctions occur between an efferent neuron and muscle fibers or glands. They act much in the same way as normal synapses. The major neurotransmitters released at Neuroeffector junctions are acetylcholine and norepinephrine. Divisions of the Nervous System – The two major physical divisions are the afferent division and efferent division. These can be broken down even further. 1. Afferent Division – consists of the somatic sensory, visceral sensory, and special sensory system. 2. Efferent Division – consists of the somatic motor and autonomic motor (consisting of the sympathetic, parasympathetic, and enteric systems). Anatomical Structures of the Brain – Memorize Figure 638 1. Forebrain a. Cerebral Hemispheres – this has an outer shell of gray matter (cell bodies) and an inner shell of white matter (myelinated fiber tracts). The diencephalon is the central core of the forebrain. The longitudinal fissure separates the 2 hemispheres. Within the cerebral hemispheres is the corpus callosum, which is a bundle of nerves that connects the 2 hemispheres. Each of the halves of the cerebral cortex can be subdivided into 4 lobes: frontal, parietal, occipital, and temporal i. Functions include perception, generation of skilled movement, reasoning, learning, and memory. Subcortical nuclei participate in the coordination of skeletal muscle. ii. There are 2 types of cells: pyramidal cells and nonpyramidal cells. Pyramidal cells are the output cells, sending their axons to other parts of the cortex and other parts of the CNS. Nonpyramidal cells receive inputs into the cortex b. Thalamus i. Acts as a synaptic relay station for sensory pathways on their way to the cerebral cortex ii. Participates in control of skeletal muscle c. Hypothalamus i. Regulates a variety of life sustaining and homeostatic behaviors d. Limbic System i. Participates in generation of emotions and emotional behavior ii. Plays essential role in most kinds of learning 2. Cerebellum a. Receives information from muscles and joints as well as skin, eyes, ears, and viscera b. Coordinates movements – posture and balance c. Participates in some forms of learning 3. Brainstem a. Contains all fibers passing between the spinal cord, forebrain, and cerebellum b. Contain the reticular formation – cardiovascular and respiratory activity c. Contains nuclei for cranial nerves III through XII