Anatomy and Physiology I Chapter 12 Nervous Tissue

2 Divisions Central Nervous System (CNS) Brain and spinal cord Peripheral Nervous System (PNS) Everything else Composed of nerves and ganglia Nerves- carry signals to and from CNS Ganglia- swelling in nerve

PNS Sensory Division (afferent) Signals from receptors to CNS Informs CNS of stimuli Somatic sensory- signals from skin, muscle, bones, joints Visceral sensory- signals from viscera

PNS Motor Division (efferent) Signals from CNS to glands or muscles Effectors Somatic motor- signals to skeletal muscles voluntary cx and reflexes Autonomic (visceral) motor- signals to glands, cardiac and smooth muscle Involuntary actions 2 divisions

PNS Autonomic Motor Division Sympathetic Parasympathetic Arouse body for action Parasympathetic Calming effect

Nervous System Electrical and chemical 3 steps Sensory CNS processes Receives info about environment Transmits to CNS CNS processes Determine response Commands issued Muscles, glands

Properties of Neurons Excitability Conductivity Secretion Respond to stimuli Conductivity Produce electrical signals Secretion Neurotransmitter

Neuron Classes Sensory (afferent) Neurons Interneurons Detect stimuli Transmits info to CNS Interneurons Entirely within CNS Receive signals, integrate signals, determine reaction 90% Motor (efferent) Neurons Signals to muscles or glands Carry out response to stimuli (effectors)

Neuron Structure Soma- control center (cell body) Central nucleus Nissl bodies Dendrites- receive signals from other neurons Axon hillock- axon originates (mound) Axon- rapid conduction of nerve signals Away from soma Nodes of Ranvier in myelinated fibers neurilemma Synaptic knob- swelling that forms a junction At end of axon Synaptic vesicles- neurotransmitters

Neuroglia Supportive cells Protect and help function 6 types of neuroglia Oligodendrocytes Ependymal cells Microglia Astrocytes Schwann cells Satellite cells

Oligodendrocytes Form myelin in CNS Arm-like processes Spirals around nerve fiber Myelin sheath- insulates nerve fiber

Ependymal Cells Lines cavities of CNS Produce and circulates CSF Cilia

Microglia Phagocytize and destroy Wander through CNS Multiple times a day Concentrated in areas of infection, trauma, stroke

Astrocytes Most abundant- 90% Found everywhere in CNS Supportive framework Blood-brain-barrier Nourish neurons Promote neuron growth Communicate with neurons Absorb excess neurotransmitters Form scar tissue

Schwann Cells Only PNS Envelop nerve fibers Myelin sheath Regeneration of damaged fibers

Satellite Cells Only PNS Surround somas Electrical insulation Regulate chemical environment

Myelin Myelin sheath- insulating layer around nerve fiber Oligodendrocytes- CNS Schwann cells- PNS Myelination- production Dietary fat important Myelin sheath is segmented Nodes of Ranvier- gaps in myelin sheath Speeds signal conduction

Conduction Speed of conduction Fiber diameter Presence or absence of myelin Large fibers- fast More surface area Myelin- fast (skeletal muscles, sensory signals) Unmyelinated- slow (secrete stomach acid, pupil dilation)

Nerve Fiber Regeneration PNS- soma must be intact 1. normal nerve fiber 2. local trauma Fiber distal to injury can’t survive Macrophages clean up 3. Soma swells Axon sprouts growth processes

Nerve Fiber Regeneration 4. Schwann cells form regeneration tube 5. Regeneration tube guides growing sprout to target cell 6. Reestablishes synapse Soma shrinks Not perfect Functional deficit post-injury

Electrical Potentials Difference in the concentration of charged particles b/t one point and another Produce current- flow of charged particles RMP- charged difference across PM Unstimulated neuron

RMP Electrolytes distributed unequally b/t ICF and ECF 3 factors Concentration gradient Selective permeability Electrical attraction NaK Pump- 70% of energy requirement Pumps 3 Na out for every 2 K it brings in equilibrium

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Action Potentials Rapid up and down shift in membrane voltage Resting neuron- polarized 1. Sodium enters cell Depolarization of the membrane 2. Must meet the threshold Minimum needed to open gates 3. Neuron produces action potential Further depolarizes membrane

Action Potentials 4. Voltage peaks Positive inside Negative outside 5. Potassium leaves cell and repolarization occurs 6. Potassium gates stay open longer Membrane potential drops slightly more negative than original RMP- hyperpolarization

Action Potential

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Action Potential All-or-None Law Nondecremental Irreversible If threshold reached- neuron fires at max voltage If threshold not reached- neuron doesn’t fire Stronger stimulus does not produce stronger action potentials Nondecremental Do not get weaker with distance Irreversible Threshold reached- action potential completes Can’t be stopped

Refractory Period Period of resistance to restimulation Absolute Impossible or difficult to stimulate same region Absolute No stimulus will trigger new action potential Lasts from start of AP until membrane returns to resting potential Relative Unusually strong stimulus will trigger potential Lasts until hyperpolarization ends

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Signal Conduction Unmyelinated fibers Voltage-regulated gates along entire length Action potentials triggers new action potential distally Continues until reaches axon end Stimulates next one Can’t go backwards Slower Nondecremental

Signal Conduction Myelinated fibers Saltatory Conduction- Voltage-regulated gates are scarce Conduction is decremental Recharges at nodes of Ranvier Creates new action potential Saltatory Conduction- propagation of nerve signal that jumps from node to node Fast

Synapses Presynaptic neuron- releases neurotransmitter Postsynaptic neuron- responds to neurotransmitter Synaptic cleft- gap b/t neurons

Neurotransmitters 1. Synthesized by presynaptic neuron 2. Released in response to stimulation 3. Bind to specific receptors on postsynaptic cell 4. Alter physiology of that cell Excitatory/ Inhibitory

Excitatory Cholinergic Synapse Acetylcholine (ACh) 1. nerve signal arrives at synaptic knob Opens Ca gates 2. Ca enter knob- triggers vesicles Ach released 3. Ach diffuses across synaptic cleft Bind to gates on postsynaptic neuron Gates open: Na enter, K leave 4. Na enters cell, depolarizes it, triggers postsynaptic potential

http://www.youtube.com/watch?v=LT3VKAr4roo&NR=1

Cessation of Signal Turn off stimulus Prevents postsynaptic cell from firing indefinitely 1. Stop new neurotransmitters Cessation of signal 2. Get rid of old Diffusion- astrocytes absorb Reuptake- synaptic knob reabsorbs, breaks down Degradation in synaptic cleft- AChE breaks down ACh

Neural Integration Ability of neurons to process information, store and recall it, and make decisions Based on postsynaptic potentials Excitatory postsynaptic potential (EPSP)- voltage change that makes neuron more likely to fire Inhibitory postsynaptic potential (IPSP)- makes neuron less likely to fire Summation- process of adding up postsynaptic potentials and responding to their net effect

Postsynaptic Potentials Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. –20 mV –40 Threshold –60 EPSP Resting membrane potential Repolarization –80 Depolarization (a) Stimulus Time –20 mV –40 Threshold –60 Resting membrane potential IPSP Figure 12.24 –80 Hyperpolarization (b) Stimulus Time

Summation Temporal summation Spatial summation ESPSs generated so quickly that a new one is generated before the old one fades Spatial summation EPSPs from several synapses add up to threshold

Summation of EPSPs +40 +20 Action potential mV –20 –40 Threshold –60 Action potential mV –20 –40 Threshold –60 EPSPs Resting membrane potential –80 Stimuli Time

Memory Physical basis of memory 3 kinds Memory trace- pathway through the brain Synapses formed to make transmission easier Added, taken away, modified 3 kinds Immediate memory Short-term memory Long- term memory

Immediate Memory Few seconds Flow of events and sense of present Read

Short-term Memory Few seconds to few hours Quickly forgotten if stop mentally reciting it, distracted, or new

Long-term Memory Lifetime Declarative memory- retention of events and facts Procedural memory- retention of motor skills