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Chapter 11

ANP1105 Chapter 11 (p. 387-414) The Nervous System (nervous tissue, membrane potentials and the synapse)

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Anatomy and Physiology
Jacqueline Carnegie

(pg. 390-393) Neurons: • Neurons are structural units of the nervous system • Typically large, highly specialized cells • Conduct messages in the form of nerve pulses from one part of the body to another • 3 special properties: o Extreme longevity: if adequately nourished can function 100 years+ o Amitotic: lose their ability to divide, therefore, if neurons are destroyed they cannot be replaced (exception: hippocampus stem cells can create new neurons) o High metabolic rate: require continuous and abundant supplies of oxygen. Cannot survive for more than a few minutes without oxygen • All have a cell body and one or more slender processes Three functional regions: • Receptive region • Conductive region • Secretory region Neuron cell body: • Also called soma • Consists of spherical nucleus and nucleolus surrounded by cytoplasm • Major biosynthetic center of the body and contains the organelles needed to synthesize proteins • Protein-membrane machinery, consisting of free ribosome clusters and extensive rough ER, is most active and developed in the body • Rough ER is also called Nissle bodies • Golgi apparatus: well developed and circles around nucleus • Contains mitochondria • Contains microtubules and neurofibrils, which are responsible for maintaining cell shape; form a network throughout cell body • PM also acts as receptive region that receives information from other neurons • Most are located in CNS (protected by skull and bones) • clusters of cell bodies in CNS are called nuclei, in PNS are called ganglia Neuron processes: • Arm like projections • In CNS contain: cell bodies and processes, in PNS contain: mostly processes • Bundles of neuron processes in CNS: tracts, in PNS: nerves • 2 types of processes: dendrites and axons (differ in structure and PM) Dendrites: • Short, tapering, diffusely branching extensions • Hundreds of dendrites per cell body • Main receptive regions- provide enormous surface area for receiving signals from other neurons • Conduct impulses toward cell body • Electrical signals are usually not action potentials, but short-distance signals called graded potentials Axon: Structure: • Arises from axon hillock, then narrows to form a slender process • Rate of conduction increases with axon diameter • Axons can be short or long; long axons are called nerve fibers • Each neuron has single axon, but axons can have branches called axon collaterals • Branches profusely at its end (terminus) with 10,000+ terminal branches • Endings of terminal branches are called axon terminals Function: 1. Axon generates nerve impulses and transmits them away from cell body, along PM (or axolemma) 2. In motor neurons, impulse is generated at junction between axon hillock and axon, called the trigger zone, and conducted along axon to axon terminals (secretory region) 3. Impulse reaches axon terminals and causes neurotransmitters (signally chemicals stored in vesicles) to be released into extracellular space 4. Neurotransmitters excite or inhibit surrounding neurons • Conducting region • Contains same organelles as cell body and dendrites, but lacks rough ER, so quickly decays if cut or damaged Transport along axon: • Through motor proteins and cytoskeletal elements (microtubules and actin filaments), substances travel continuously along axon • Movement away from cell body is called anterograde movement, ex: mitochondria, cytoskeletal elements, membrane components used to renew axon PM and enzymes needed to resynthesize neurotransmitters • Movement to the cell body is called retrograde movement, ex: organelles returning to cell body to be degraded and a means of intracellular communication *Clinical note: viruses such as polio and rabies reach cell body through retrograde movement. Researchers are investigating using retrograde transport to treat genetic diseases by introducing viruses containing “corrected” genes to suppress defective genes Classification of neurons: • Classified structurally and functionally Structural: • Grouped according to number of processes extending from cell body • Three major neuron groups: multipolar, bipolar and unipolar Multipolar: • 3 or more processes- 1 axon and the rest are dendrites • Most common neuron types (99%) • Major type in CNS Bipolar: • 2 processes- 1 axon and 1 dendrite- extending from opposite sides of cell body • Rare: found in some special sense organs (ex: retina of the eye) Unipolar: • 1 short process that divides T-like into proximal and distal branches • Distal, or peripheral process, is associated with a sensory receptor • Proximal, or central process, enters the CNS • Originate as bipolar neurons, then in embryonic development, the two processes fuse • Found chiefly in ganglia of PNS (function as sensory neurons) Functional: • Groups according to direction impulse travels relative to CNS • Three types: sensory, motor and interneurons Sensory: • Afferent • Transmit impulses toward CNS • Almost all unipolar and located outside CNS Motor: • Efferent • Transmit impulses away from CNS • Most are multipolar, and located in CNS Interneuron: • Lie between motor and sensory neurons • Shuttle signals through the CNS • Make up over 99% of the body’s neurons • Almost all multipolar, but vary in size and shape (pg. 395-404) Membrane Potential: Situations in which there are separated electrical charges of opposite sign have potential energy Voltage: potential energy due to separation (PM) of oppositely-charged particles (ions) • (-70 mV for many neurons) • Greater difference in charge between two points = higher voltage (higher potential) Current: flow of electrical charge from one point to another Resistance: the hindrance to charge flow provided by substances through which the current must pass (ex: PM) • Substances with high resistance are called insulators, and substances with low resistance are called conductors In the body, electrical currents reflect flow of ions across membranes. There is a difference in numbers of negatively and positively charged ions on the two sides of PM, so there is potential across the membranes. PM provides resistance. Role of Membrane ion channels: • PM is peppered with ion channels • Channels are selective to types of ions that pass Chemically gated: open when the appropriate chemical (neurotransmitters and hormones) binds Voltage-gated: open and close in response to changes in membrane potential Mechanically gated: open in response to physical deformation of receptor (sensory receptors- pressure) • Ions move along concentration gradients when they diffuse passively from an area of high concentration to an area of low concentration • Ions move along electrical gradients when they move toward an area of opposite electrical charge • Together create the electrochemical gradients • When gated channels open, ions diffuse quickly across the PM following their electrochemical gradients, creating changes in current and voltage Resting Membrane Potential: • Avoltmeter is used to measure potential difference between the cell and extracellular fluid • RMP for neuron is -40 to -90mV • Minus sign indicates that the cytoplasmic side (inside) is negatively charged relative to the outside • Potential difference in a resting neuron is called resting membrane potential and membrane is said to be polarized • Generating a RMP depends on (1) differences in K+ and Na+ concentrations inside and outside the cell, and (2) differences in permeability of the PM to these ions (1) Differences in ionic composition: + + • Cell contains less NA and more K than extracellular fluid • Negatively charged proteins help balance the positive charges of intracellular K + + - • In extracellular fluid, the positive charges of Na are balanced by Cl • K plays most important role in generating membrane potential (2) Differences in PM permeability: • PM is 25x more permeable to K than Na , and quite permeable to Cl ions - • resting permeabilities reflect properties of leakage channels • K ions diffuse out of the cell along their concentration gradient more easily than Na + diffuses in, causing the cell to become more negative • Na makes the cell slightly more positive than if only K flowed • Therefore, at resting membrane potential, the negative cell interior is due to a much greater ability for K to diffuse out, than for Na to diffuse in • Since K+ and Na+ are always diffusing in and out of the cell, their concentrations do not + necessarily ever even out, becauseATP-driven sodium-potassium pump ejects 3 Na from the cell, and transport 2 K back in. • Sodium potassium pump (or Na -K ATPase) stabilizes the RMP by maintaining concentration gradients for sodium and potassium Membrane Potentials act as signals: • Neurons use changes in membrane potential as communication signals to receive and send information • Achange in membrane potential can be produced by (1) altering ion concentration, and (2) changing PM ion permeability • Only permeability changes are important for transferring information • Can produce two types of signals: graded potentials and action potentials • Depolarization: decrease in membrane potential: inside of membrane becomes less negative (moves closer to zero) than resting potential- also includes events in which membrane potential becomes positive • Hyperpolarization: increase in membrane potential: inside of membrane potential becomes more negative (further from zero) than resting potential • Depolarization increases probability of producing nerve impulses, hyperpolarization decreases probability Graded Potentials: • Short-lived changes in membrane potential • Either depolarization or hyperpolarization • Current decreases in magnitude with increased distance • Graded because magnitude determined by stimulus strength (stronger stimulus = farther the current flows) • Triggered by change in neuron environment which open gated ion channels • When the receptor of a sensory neuron is excited by energy (ex: heat), it is called generator potential • When the stimulus is a neurotransmitter released from another neuron, it is called postsynaptic potential (neurotransmitter is released into synapse and influences neuron beyond synapse) 1) Depolarization: a small patch of membrane depolarizes 2) Depolarization spreads: opposite charges attract each other. This creates currents that depolarize adjacent membrane areas, spreading the wave of depolarization 3) Membrane potential decays with distance: because current is lost through “leaky” PM, the voltage declines with distance from the stimulus (decremental). Consequently, graded potentials are short-distance signals. Action Potentials: • Only cells with excitable membranes (neurons and muscle cells) can generate action potentials; in neurons only axons • AP: a brief reversal of membrane potential (from -70mV to +30mV) • Do not decay over distance • AP is also called nerve impulse • Neuron generates nerve impulse when there is stimulus, stimulus changes permeability of PM by opening specific voltage-gated channels on axon • Channels open and close in response to changes in membrane potential, activated by graded potentials that spread toward axon Generation of AP: I. Resting state: • All gated K+ and Na+ channels are closed • Leakage channels are open • Each Na+ channel has two gates: an activation gate (closed at rest and opens at depolarization) and an inactivation gate (blocks channel once it is open) • Depolarization opens and then inactivates sodium channels • Both gates must be open for Na+ flow. II. Depolarization: • Na+ channels open • Na+ rushes into cell and depolarizes local patch of membrane further, opening more Na+ channels (inside of cell becomes less negative) • Voltage-gated Na+ channels are fast • Depolarization reaches threshold (between -50 to -55mV) and becomes self- generating, urged on by ionic current of Na+ (positive feedback) • As more Na+ enters, all Na+ channels are opened • Na+ permeability = 1000x that of resting neuron • Membrane potential becomes +30mV, and the rush creates a “spike” ofAP III. Repolarization: • Na+ channels are inactivating, and K+ channels open • RisingAP lasts 1 millisecond, and then Na+ inactivation gates close • Na+ permeability declines to resting state • AP spike stops rising • Voltage-gated K+ channels open and K+ rushes out of cell • Restores internal negativity (repolarization) • Decline of Na+ and increase in K+ contribute to repolarization IV. Hyperpolarization
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