Nervous System Structure and Function PDF

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...

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  Central nervous system (CNS) Peripheral nervous system (PNS) Brain and spinal cord Cranial nerves and spinal nerves Integrative and control centers Communication lines between the CNS and the rest of the body Sensory (afferent) division Motor (efferent) division Somatic and visceral sensory Motor nerve fibers nerve fibers Conducts impulses from the CNS Conducts impulses from to effectors (muscles and glands) receptors to the CNS Somatic sensory Somatic nervous Autonomic nervous fiber system system (ANS) Skin Somatic motor Visceral motor (voluntary) (involuntary) Conducts impulses Conducts impulses from the CNS to from the CNS to skeletal muscles cardiac muscles, Visceral sensory fiber smooth muscles, Stomach and glands Skeletal muscle Motor fiber of somatic nervous system Sympathetic division Parasympathetic Mobilizes body division systems during activity Conserves energy Promotes house- keeping functions during rest Sympathetic motor fiber of ANS Heart Structure Function Sensory (afferent) division of PNS Parasympathetic motor fiber of ANS Bladder Motor (efferent) division of PNS 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 1A Schwann cell cytoplasm envelopes an axon. Axon Schwann cell nucleus 2 The Schwann cell then rotates around the axon, wrapping its plasma membrane loosely around it in successive layers. Neurilemma 3 The Schwann cell Myelin sheath cytoplasm is forced from between the membranes. The tight membrane wrappings surrounding (a) Myelination of a nerve the axon form the myelin fiber (axon) 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 Ground electrode membrane 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 Membrane potential (mV) 2 Depolarization 3 Repolarization 3 4 Hyperpolarization 2 Action potential Threshold 1 1 4 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 ◻ Some K+ channels remain open, allowing excessive K+ efflux ◻ This causes after-hyperpolarization of the membrane (undershoot)  The AP is caused by permeability changes in the plasma membrane Relative membrane permeability Membrane potential (mV) 3 Action potential 2 Na+ permeability K+ permeability 1 1 4 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 ◻ Largediameter, 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 Cell body (soma) of (b) postsynaptic neuron Figure 11.16  Chemical synapses transmit signals from one neuron to another using neurotransmitters. Presynaptic neuron Presynaptic Postsynaptic neuron neuron 1 Action potential arrives at axon terminal. 2 Voltage-gated Ca2+ channels open and Ca2+ Mitochondrion enters the axon terminal. Ca2+ Ca2+ Ca2+ Ca2+ 3 Ca2+ entry causes Synaptic neurotransmitter- cleft containing synaptic Axon terminal Synaptic vesicles to release their vesicles contents by exocytosis. 4 Neurotransmitter diffuses across the synaptic cleft and binds to specific Postsynaptic receptors on the neuron postsynaptic membrane. Ion movement Enzymatic Graded potential degradation 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) An EPSP is a local depolarization of the postsynaptic membrane that brings the neuron closer to AP threshold. Neurotransmitter binding opens chemically gated Threshold ion channels, allowing the simultaneous pas- sage of Na+ and K+. Stimulus Time (ms) (a) Excitatory postsynaptic potential (EPSP) Figure 11.18a Membrane potential (mV) 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. Threshold 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 Open ion (1st messenger) binds Adenylate cyclase channel channel and activates receptor. Receptor G protein 5a cAMP changes membrane permeability by opening or closing ion channels. 5c cAMP activates specific genes. GDP 5b cAMP activates enzymes. 2 Receptor 3 G protein 4 Adenylate activates G activates cyclase converts protein. adenylate ATP to cAMP cyclase. (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 Causes blurred vision myelin destroyed in numb legs or arms various parts of CNS can lead to paralysis hard scars (scleroses) form nerve impulses Treatments blocked no cure muscles do not bone marrow transplant receive innervation interferon (anti-viral drug) may be related to a hormones virus 10-29