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Nervous System Structure and Function Functions Sensory input Integration Motor output Classification Central Nervous System CNS Peripheral Nervous System PNS Divisions of Peripheral Nervous System Sensory Division Motor Division muscles and glands Divisions of the Motor Somatic Autonomic Peripheral nervous system (PNS) Central nervous system (CNS) Cranial nerves and spinal nerves Communication lines between the CNS and the rest of the body Brain and spinal cord Integrative and control centers Sensory (afferent) division Somatic and visceral sensory nerve fibers Conducts impulses from receptors to the CNS Somatic sensory fiber Motor (efferent) division Motor nerve fibers Conducts impulses from the CNS to effectors (muscles and glands) Somatic nervous system Somatic motor (voluntary) Conducts impulses from the CNS to skeletal muscles Skin Visceral sensory fiber Stomach Skeletal muscle Motor fiber of somatic nervous system Sympathetic division Mobilizes body systems during activity Sympathetic motor fiber of ANS Structure Function Sensory (afferent) division of PNS Motor (efferent) division of PNS Parasympathetic motor fiber of ANS Autonomic nervous system (ANS) Visceral motor (involuntary) Conducts impulses from the CNS to cardiac muscles, smooth muscles, and glands Parasympathetic division Conserves energy Promotes housekeeping functions during rest Heart Bladder Figure 11.2 How it works Neurons = nerve cells Long lived, no mitosis, Cell body- developed Golgi Extensions outside the cell body Dendrites Axons Axonal terminals contain vesicles with neurotransmitters Axonal terminals are separated from by a gap Synapse Nerve Coverings Myelin- Lipid/Protein Schwann cells Nodes of Ranvier Schwann cell plasma membrane Schwann cell cytoplasm Axon 1 A Schwann cell envelopes an axon. Schwann cell nucleus 2 The Schwann cell then rotates around the axon, wrapping its plasma membrane loosely around it in successive layers. Neurilemma Myelin sheath (a) Myelination of a nerve fiber (axon) 3 The Schwann cell cytoplasm is forced from between the membranes. The tight membrane wrappings surrounding the axon form the myelin sheath. Figure 11.5a Classification of Neurons Multipolar neurons Bipolar Unipolar Classification Cont.. Sensory Neurons afferent most are unipolar some are bipolar Interneurons multipolar in CNS Motor Neurons multipolar carry impulses to effectors, muscle Table 11.1 (2 of 3) Neuroglial Cells: Support Cells Schwann Cells-PNS Oligodendrocytes-CNS Microglia-CNS Astrocytes- CNS Ependyma-CNS Regeneration of Injury (if possible) Principles of Electricity Opposite charges attract each other Energy is required to separate opposite charges across a membrane Energy is liberated when the charges move toward one another If opposite charges are separated, the system has potential energy Definitions Voltage (V): measure of potential energy generated by separated charge Potential difference: voltage measured between two points Current (I): the flow of electrical charge (ions) between two points Definitions Resistance (R): hindrance to charge flow (provided by the plasma membrane) Insulator: substance with high electrical resistance Conductor: substance with low electrical resistance Role of Membrane Ion Channels 1. Leakage (nongated) channels—always open 2. Gated channels (three types): Chemically gated (ligand-gated) Voltage-gated channels Mechanically gated channels Generating a Nerve Impulse polarized membrane: inside is negative relative to the outside under resting conditions -70 mV Voltmeter Plasma membrane Ground electrode outside cell Microelectrode inside cell Axon Neuron Figure 11.7 Action Potential (AP) Brief reversal of membrane potential with a total amplitude of ~100 mV Occurs in muscle cells and axons of neurons Does not decrease in magnitude over distance Principal means of long-distance neural communication The big picture 1 Resting state 3 Repolarization Membrane potential (mV) 2 Depolarization 3 4 Hyperpolarization 2 Action potential Threshold 1 4 1 Time (ms) Figure 11.11 (1 of 5) Generation of an Action Potential Resting state Only leakage channels for Na+ and K+ are open All gated Na+ and K+ channels are closed Depolarizing Phase Na+ influx causes more depolarization At threshold (–55 to –50 mV) positive feedback leads to opening of all Na+ channels, and a reversal of membrane polarity to +30mV (spike of action potential) Repolarizing Phase Repolarizing phase Na+ channel slow inactivation gates close Membrane permeability to Na+ declines to resting levels Slow voltage-sensitive K+ gates open K+ exits the cell and internal negativity is restored Hyperpolarization Hyperpolarization K+ channels remain open, allowing excessive K+ efflux This causes after-hyperpolarization of the membrane (undershoot) Some 3 2 Action potential Na+ permeability K+ permeability 1 4 1 Relative membrane permeability Membrane potential (mV) The AP is caused by permeability changes in the plasma membrane Time (ms) Figure 11.11 (2 of 5) Voltage at 0 ms Recording electrode (a) Time = 0 ms. Action potential has not yet reached the recording electrode. Resting potential Peak of action potential Hyperpolarization Figure 11.12a Voltage at 2 ms (b) Time = 2 ms. Action potential peak is at the recording electrode. Figure 11.12b Voltage at 4 ms (c) Time = 4 ms. Action potential peak is past the recording electrode. Membrane at the recording electrode is still hyperpolarized. PLAY A&P Flix™: Propagation of an Action Potential Figure 11.12c Impulse Conduction Coding for Stimulus Intensity All action potentials are alike and are independent of stimulus intensity How does the CNS tell the difference between a weak stimulus and a strong one? Strong stimuli can generate action potentials more often than weaker stimuli The CNS determines stimulus intensity by the frequency of impulses Saltatory Conduction Appear the jump from node to node. Speed of impulses is much faster on myelinated nerves then unmyelinated ones. Speed also increases with increase in diameter. Ex.) 120m/s skeletal muscle .5m/s skin. Conduction Velocity Conduction velocities of neurons vary widely Effect of axon diameter Effect of myelination Myelin sheaths insulate and prevent leakage of charge Saltatory conduction in myelinated axons is about 30 times faster Nerve Fiber Classification Group A fibers Large diameter, myelinated somatic sensory and motor fibers Group B fibers Intermediate diameter, lightly myelinated ANS fibers Group C fibers Smallest diameter, unmyelinated ANS fibers The Synapse Presynaptic neuron—conducts impulses toward the synapse Postsynaptic neuron—transmits impulses away from the synapse Axodendritic Axosomatic Some electrical, most chemical Cleft = gap Axodendritic synapses Dendrites Axosomatic synapses Cell body Axoaxonic synapses (a) Axon Axon Axosomatic synapses (b) Cell body (soma) of postsynaptic neuron Figure 11.16 Chemical synapses transmit signals from one neuron to another using neurotransmitters. Presynaptic neuron Presynaptic neuron Postsynaptic neuron 1 Action potential arrives at axon terminal. 2 Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal. Mitochondrion Ca2+ Ca2+ Ca2+ 3 Ca2+ entry causes neurotransmittercontaining synaptic vesicles to release their contents by exocytosis. Axon terminal Ca2+ Synaptic cleft Synaptic vesicles 4 Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane. Postsynaptic neuron Ion movement Enzymatic degradation Graded potential Reuptake Diffusion away from synapse 5 Binding of neurotransmitter opens ion channels, resulting in graded potentials. 6 Neurotransmitter effects are terminated by reuptake through transport proteins, enzymatic degradation, or diffusion away from the synapse. Figure 11.17 Membrane potential (mV) Threshold An EPSP is a local depolarization of the postsynaptic membrane that brings the neuron closer to AP threshold. Neurotransmitter binding opens chemically gated ion channels, allowing the simultaneous passage of Na+ and K+. Stimulus Time (ms) (a) Excitatory postsynaptic potential (EPSP) Figure 11.18a Membrane potential (mV) Threshold An IPSP is a local hyperpolarization of the postsynaptic membrane and drives the neuron away from AP threshold. Neurotransmitter binding opens K+ or Cl– channels. Stimulus Time (ms) (b) Inhibitory postsynaptic potential (IPSP) Figure 11.18b Integration: Summation A single EPSP cannot induce an action potential EPSPs can summate to reach threshold IPSPs can also summate with EPSPs, canceling each other out Neurotransmitters Most neurons make two or more neurotransmitters, which are released at different stimulation frequencies 50 or more neurotransmitters have been identified Classified by chemical structure and by function Some excite and some inhibit Can be nucleotides, gas, protein, amino acid, lipoprotein Neurotransmitters Ion flow blocked Ions flow Ligand Closed ion channel Open ion channel (a) Channel-linked receptors open in response to binding of ligand (ACh in this case). Figure 11.20a 1 Neurotransmitter Closed ion channel Adenylate cyclase (1st messenger) binds and activates receptor. Open ion channel Receptor G protein 5a cAMP changes membrane permeability by opening or closing ion channels. 5c cAMP activates specific genes. 5b GDP 2 Receptor activates G protein. 3 G protein activates adenylate cyclase. 4 Adenylate cAMP activates enzymes. cyclase converts ATP to cAMP (2nd messenger). Nucleus Active enzyme (b) G-protein linked receptors cause formation of an intracellular second messenger (cyclic AMP in this case) that brings about the cell’s response. Figure 11.17b Figure 11.22a Figure 11.22b Figure 11.22c, d Clinical Application. Multiple Sclerosis Symptoms • blurred vision • numb legs or arms • can lead to paralysis Treatments • no cure • bone marrow transplant • interferon (anti-viral drug) • hormones Causes • myelin destroyed in various parts of CNS • hard scars (scleroses) form • nerve impulses blocked • muscles do not receive innervation • may be related to a virus 10-29