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Chapter 10 Nervous System I Divisions of the Nervous System • The organs of the nervous system are divided into two major groups: – Central Nervous System (CNS) = brain & spinal cord – Peripheral Nervous System (PNS) = nerves that extend from the brain (cranial nerves) and spinal cord (spinal nerves) Divisions of PNS • Sensory Division – picks up sensory information and delivers it to the CNS • Motor Division – carries information to muscles and glands • Divisions of the Motor Division – Somatic – carries information to skeletal muscle – Autonomic – carries information to smooth muscle, cardiac muscle, and glands Three Major Functions of the Nervous System Functions of the Nervous System • Sensory Function – sensory receptors (located at the ends of peripheral neurons) gather information – information is carried to the CNS – A sensory impulse is carried on a sensory neuron Functions of the Nervous System • Motor Function – decisions are acted upon – impulses are carried to effectors – Motor impulses are carried from CNS to responsive body parts called effectors – A motor impulse is carried on a motor neuron – Effectors = 2 types: • muscles (that contract) • glands (that secrete a hormone) Functions of the Nervous System • Integrative Function – Can involve CNS or PNS – sensory information used to create • • • • sensations memory thoughts decisions Neuron Structure • Each neuron is composed of a cell body and many extensions from the cell body called neuron processes or nerve fibers • Cell Body = central portion of neuron – contains usual organelles, except centrioles • identify: nucleus, prominent nucleolus, and many Nissl bodies = RER • Neuron Processes/ Nerve Fibers = extensions from cell body Neuron Structure cont. • Dendrites: – many per neuron – short and branched – receptive portion of a neuron – carry impulses toward cell body • Axons: – one per neuron – long, thin process – carry impulses away from cell body Figure 10.01 Neuron Structure cont. • Note terminations of axon branch = axonal terminals (synaptic knobs) • Axons in PNS: • Large axons are surrounded by a myelin sheath produced by many layers of Schwann Cells (neuroglial cell) – "myelinated nerve fiber" – myelin = lipoprotein – Interruptions in the myelin sheath between Schwann cells = Nodes of Ranvier • Small axons do not have a myelin sheath – "unmyelinated nerve fibers" – however all axons (in PNS) are associated with Schwann cells Figure 10.04c Neuron Structure cont. • Neuron = the structural & functional unit of the nervous system – a nerve cell • Neuron Structure – – – – Nerve Fibers Axons (continued) Axons in CNS (i.e. in brain & spinal cord) Myelin is produced by an oligodendrocyte rather than Schwann Cells • A bundle of myelinated nerve fibers = "White Matter" • This is in contrast to CNS "Gray Matter" = A bundle of cell bodies (or unmyelinated nerve fibers) Structural Classification • Bipolar Neurons – two extensions – one fused dendrite leads toward cell body, one axon leads away from cell body • Unipolar Neurons – one process from cell body – forms central and peripheral processes – only distal ends are dendrites • Multipolar Neurons – many extensions – many dendrites lead toward cell body, one axon leads away from cell body Functional Classification • Sensory neurons – afferent neurons – carry sensory impulses from sensory receptors to CNS – input information to CNS – Location of receptors = skin & sense organs • Interneurons (Association) – CNS – link other neurons together (i.e. sensory neuron to interneuron to motor neuron) Functional Classification cont. • Motor Neurons – efferent neurons – carry motor impulses away from CNS and to effectors – output information from CNS – Effectors = muscles & glands Neuroglial Cells • Neuroglial Cells = accessory cells of nervous system form supporting network for neurons PNS = 2 Types • Schwann cells – produces myelin (in the PNS) • Satellite Cells – support clusters of neuron cell bodies (ganglia) Neuroglial Cells cont. CNS =4 types: – provide bulk of brain and spinal cord tissue • Astrocyte – scar tissue – star-shaped Function: – mop up excess ions, etc – induce synapse formation – connect neurons to blood vessels • Oligodendrocyte – looks like eyeball – Function: produces myelin • Microglia – looks like spider – Function: phagocytosis • Ependyma – epithelial-like layer – Function: lines spaces in CNS – brain = ventricles Nerve Repair • Regeneration of Nerve Axons – Cell body injury = death of neuron – Damage to an axon may allow for regeneration The Synapse •Nerve impulses pass from neuron to neuron at synapses Synaptic Transmission •Neurotransmitter s are released when impulse reaches synaptic knob Distribution of Ions Potential Difference • A resting neuron's cell membrane is said to be polarized = electrically charged: – Consequently, a potential difference (PD) exists across this resting cell membrane • Potential Difference (PD) = the difference in electrical charge between 2 points (i.e. across a cell membrane) Resting Membrane Potential • The resting membrane potential (RMP) of a neuron results from the distribution of ions across the cell membrane – K+= high inside – Na+= high outside – Cl-= high outside – Negatively charged proteins or Anions-; high inside • Recall that these ion concentrations are maintained by active transport mechanisms – mainly the Na+/K+ pump Resting Membrane Potential cont. • Resting Potential – The RMP of a nerve cell is measured to be -70 mV or millivolts (inside / outside) – As long as the RMP in a nerve cell is undisturbed, it remains polarized. – In order for a nerve impulse to be started or propagated in a nerve cell, this resting potential must be disturbed Local Potential Changes • caused by various stimuli • temperature changes • light • pressure • environmental changes affect the membrane potential by opening a gated ion channel Development of resting membrane potential Slide number: 1 Nerve axon – Na+ Na+ Na+ K+ Na+ High Na+ Low – – Na+ – Na+/K+ pump – Na+ – K+ – Na+ Na+ K+ diffusion – Na+ – Na+ – + High Na+ Low Na+ – Na+ Na+ – – Na+ – K+ Na+ – Na+ K+ – Na+ – – Na+/K+ pump K+ Na+ – K+ diffusion Na+ – Na+ – Na+ – Na+ Na+ – K+ Cell membrane K+ Intracellular fluid – – K+ High K+ K+ – A – + Na+ – Na+ Na+ – Na+ Na+ – – K+ K+ Na Low Na+ Na+/K+ + Na diffusion – pump K+ High Na+ Low K+ – Na+ K+ Na+ Na+ – – – K+ K+ – – – diffusion Na+ – Na+ K+ K+ – – – Extracellular fluid Na+ diffusion K+ Na+ – – – Na+ Na+ Na+ – – Na+ – B Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. – – Na+ K+ K+ K+ – – – Na+ K+ Na+ + High Na+ Low K+ – Na+ – K+ – K+ K+ Na+/K+ Na+ diffusion pump K+ – Na+ – – K+ Low Na+ K+ K+ – Na+ diffusion – High K+ Na+ – – – Na+ Na+ K+ diffusion – – – – Na+ – Na+ – Development of resting membrane potential Slide number: 2 Nerve axon – Na+ Na+ Na+ – – K+ – Na+ Na+ Na+ – – Na+ – – Na+ – – Na+ Na+ – Na+ Cell membrane K+ – – K+ K+ – K+ – Na+ – – Na+ Na+ Extracellular fluid – Na+ – Na+ – – K+ K+ – – Na+ – – K+ K+ Na+ Na+ – – K+ – – – – Na+ Intracellular fluid K+ K+ Na+ A Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. K+ Na+ Na+ – – – Na+ Development of resting membrane potential Slide number: 3 Nerve axon – Na+ Na+ Na+ High Na+ – – – K+ – Na+ Na+ Na+ – Na+ – – Na+ – – Na+ Na+ – Na+ Cell membrane K+ – – K+ – Na+ High – – Na+ Na+ Na+ – Na+ – – – – K+ K+ – – Na+ – – K+ K+ Na+ Na+ – – K+ Na+ K+ K+ – – – – Na+ Low Na+ Intracellular fluid Extracellular fluid K+ K+ Na+ A Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. K+ Na+ Na+ – – – Na+ Development of resting membrane potential Slide number: 4 Nerve axon – Na+ Na+ Na+ High Na+ – – – K+ – Na+ Na+ Na+ – Na+ – – Na+ – – Na+ Na+ – Na+ Cell membrane K+ – – K+ – – Na+ – High – Na+ diffusion – Na+ – – K+ K+ – – – Na+ – – K+ K+ – – Na+ – – K+ K+ Na+ Na+ – – K+ Na+ Na+ Na+ K+ Na+ Low Na+ Intracellular fluid Extracellular fluid Na+ diffusion K+ Na+ A Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. K+ Na+ Na+ – – – Na+ Development of resting membrane potential Slide number: 5 Nerve axon – Na+ High Na+ – Na+ Na+ Low K+ – K+ – Na+ – – Na+ – – Na+ Na+ Na+ – Na+ – Na+ – Na+ Cell membrane K+ Intracellular fluid – – High K+ K+ – High – Low Na+ Na+ Na+ Low Na+ – – K+ Na+ Na+ – – K+ K+ – – Na+ – – K+ K+ Na+ Na+ – – K+ Na+ – – – – K+ K+ Na+ – – Extracellular fluid K+ K+ Na+ A Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. K+ Na+ Na+ – – – Na+ Development of resting membrane potential Slide number: 6 Nerve axon – Na+ High Na+ – Na+ Na+ Low K+ – K+ – Na+ Na+ – K+ diffusion – Na+ Na+ Na+ – Na+ – – – Na+ – Na+ Cell membrane K+ Intracellular fluid – – High K+ K+ – High – Low Na+ Na+ Na+ Low Na+ – – K+ Na+ Na+ – – Na+ Na+ K+ – K+ – – K+ – K+ K+ – K+ Na+ – – – – K+ K+ Na+ – – Extracellular fluid K+ K+ diffusion Na+ – Na+ A Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. K+ Na+ Na+ – – – Na+ Development of resting membrane potential Slide number: 7 Nerve axon – Na+ Na+ High Na+ – Low K+ – – Na+/K+ pump Na+ Na+ – Na+ Na+ – K+ – Na+ – Na+ Na+ – – Na+ – Na+ Cell membrane K+ Intracellular fluid – – High K+ K+ – High – Low Na+ Na+ Na+ – Low Na+ – – Na+ Na+ – – K+ K+ – – Na+/K+ pump K+ Na+ K+ – Na+ – – Extracellular fluid K+ K+ K+ K+ – – Na+ – – K+ K+ – Na+ Na+ – Na+ A Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. K+ Na+ Na+ – – – Na+ Development of resting membrane potential Slide number: 8 Na+ + – – Na+ High Na+ Low K+ – Na+ Na+ – Na+ K+ – Na+ – Na+ K+ High K+ – Na+ diffusion – – Na+ + High – Na+ Na+ – Na+ K+ Low K+ Na+ – – Na+ K+ Na+ – K+ – Na+ K+ Na+ Na+/K+ pump K+ K+ K+ – – – – – K+ Na+ – – Na+ – – K+ Low Na+ K+ – Na+ – Na+ Na+/K+ – + pump Na+ K diffusion – – Na+ diffusion – – Na+ Na+ Na+ K+ K+ diffusion – – – Na+ – – B Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Na+ – Local Potential Changes (Graded Potentials) • The RMP of - 70 mV can be disrupted or changed in one of two directions: • more negative = "hyperpolarization" • less negative (i.e. towards zero) = "depolarization“ • summation can lead to threshold stimulus that starts an action potential Figure 10.15 Action Potential • When the resting membrane potential (RMP) of a neuron is depolarized to -55mV, threshold potential is reached – The threshold potential for a neuron is -55mV – Therefore, a threshold stimulus = +15 mV • When threshold potential is reached, the rapid opening of Na+ channels results in rapid depolarization (and even reversal of the membrane potential [MP] to +30mV) – This event is called the action potential – The action potential represents the start of the nerve impulse on a neuron. Action Potential cont. • Then K+ channels open, (while Na+ channels close), and repolarization occurs = recovery of the RMP to -70mV • This all occurs very quickly = 1/1000 sec • An action potential represents the start of a nerve impulse in one small portion of the neuron's membrane Figure 10.15 Action potential Na+ K+ Na+ K+ Na+ Na+ Na+ Na+ Na+ Na+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ Na+ Na+ Na+ Na+ K+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ Threshold stimulus K+ K+ Na+ K+ Na+ A Na+ K+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ B Region of depolarization K+ K+ Na+ K+ K+ Na+ Na+ Na+ K+ K+ K+ K+ K+ K+ Na+ Na+ Na+ K+ K+ K+ K+ K+ K+ K+ K+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ C Region of repolarization Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Slide number: 1 Action potential Na+ K+ A Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Slide number: 2 Action potential Na+ K+ Na+ K+ Na+ Na+ Na+ Na+ Na+ Na+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ Na+ Na+ Na+ Na+ K+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ Na+ K+ Na+ A Na+ K+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ B Region of depolarization Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Slide number: 3 Action potential Na+ K+ Na+ K+ Na+ Na+ Na+ Na+ Na+ Na+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ Na+ Na+ Na+ Na+ K+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ Threshold stimulus K+ K+ Na+ K+ Na+ A Na+ K+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ B Region of depolarization Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Slide number: 4 Action potential Slide number: 5 Na+ K+ Na+ Na+ K+ Na+ Na+ Na+ Na+ Na+ Na+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ Na+ K+ Threshold stimulus K+ K+ Na+ Na+ K+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ B Region of depolarization K+ K+ Na+ K+ K+ Na+ Na+ Na+ K+ K+ K+ K+ K+ K+ Na+ Na+ Na+ K+ K+ K+ K+ K+ K+ K+ K+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ C Region of repolarization Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Action Potential cont. • Nerve Impulse (NI) = the propagation of action potentials (AP) along a nerve fiber; (i.e. the entire length of the neuron) – The NI is an electrical impulse • An NI is similar to a row of dominos falling (i.e. once the first domino falls, the entire row will fall) • A nerve impulse begins on a dendrite (or cell body of a neuron), runs toward the cell body, through the cell body, and then down the axon Characteristics of a Nerve Impulse (NI) • All or Nothing Response = if a nerve cell responds at all, it responds completely. – sub threshold stimulus (5mV) = no AP/no NI – threshold stimulus (15mV) = yes AP/yes NI • > threshold stimulus (20mV) = yes AP – yes NI, but no greater intensity than above • Refractory Period = the period following a NI when a threshold stimulus cannot produce another NI – The RMP has to be restored before it can be depolarized again – (i.e. dominos must be set up in order to be knocked down again) Impulse Conduction Review Characteristics NI cont. • Impulse Conduction = the manner in which the NI runs down the neuron/nerve fiber • unmyelinated nerve fibers: NI must travel the length of the nerve fiber – slow • myelinated nerve fiber: "Saltatory Conduction" – NI jumps from node of Ranvier to node of Ranvier – Very fast transmission THE SYNAPSE Nerve impulses are transferred from one neuron to the next through synaptic transmission. The Synapse • Synapse = the junction between two neurons where a nerve impulse is transmitted • occurs between the axon of one neuron and dendrite or cell body of a second neuron • Note that the two neurons do not touch There is a gap between them = synaptic cleft Synaptic Transmission • NI reaches axonal terminal of pre-synaptic neuron causing depolarization of synaptic knob – Ca++ channels open and calcium ions rush into axonal terminal – synaptic vesicles (filled with neurotransmitter/NT) to release NT via exocytosis into the synaptic cleft – NT diffuses across synaptic cleft and depolarizes the post-synaptic neuron's membrane. – An action potential (AP) is triggered and a NI begins in the post-synaptic neuron Neurotransmitters Synaptic Potentials EPSP • excitatory postsynaptic potential • graded • depolarizes membrane of postsynaptic neuron • action potential of postsynaptic neuron becomes more likely IPSP • inhibitory postsynaptic potential • graded • hyperpolarizes membrane of postsynaptic neuron • action potential of postsynaptic neuron becomes less likely Summation of EPSPs and IPSPs EPSPs and IPSPs are added together in a process called summation • More EPSPs lead to greater probability of action potential • Neurotransmitters (NT) • at least 30 different produced by CNS • some neurons produce/release only one while release many; • Most typical NT is Acetylcholine (ACh) – ACh is released by all motor neurons (i.e. those that stimulate skeletal muscle) – some CNS neurons Other NTs • monoamines (modified amino acids) • are widely distributed in the brain where they play a role in: – emotional behavior and – circadian rhythm • are present in some motor neurons of the ANS. • include: – – – – • • • • • epinephrine norepinephrine dopamine serotonin histamine unmodified amino acids glutamate aspartate GABA (gamma aminobutyric acid) • glycine Fate of Neurotransmitter in Synaptic Cleft • Destruction of Neurotransmitter: – Enzymes that are present in the synaptic cleft destroy NT – For example, acetylcholinesterase destroys ACh • Reuptake of Neurotransmitter: – NT is transported back into pre-synaptic knob • Both of the above processes prevent continual stimulation of the post-synaptic membrane! Neuropeptides • synthesized by CNS neurons • act as NTs or neuromodulators that either: – alter a neuron's response to a NT – block the release of a NT • include enkephalins: – synthesis is increased during painful stress – bind to the same receptors in the brain as the narcotic morphine – relieve pain • include endorphines: – same as above, but with a more potent and longer lasting effect IMPULSE PROCESSING Impulse Processing • Neuronal Pools – neurons that synapse and work together – Working together results in facilitation – a general excitation that makes stimulation easier to achieve • Convergence – many neurons come together on fewer neurons (summation occurs) – typical of motor pathways – many inputs from brain, but usually only one motor response Impulse Processing cont. • Divergence – fewer neurons spread out to signal many neurons (signal amplifies) – typical of sensory pathways – reason that a stimulus (i.e. odor) can cause many responses