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Organ System #4 Nervous System Chapters 11, 12, 13

The Nervous System Functions are Monitor changes in environment Process & interpret information Effect a response Maintain body homeostasis Complement endocrine system

Figure 7.1 The nervous system’s functions. © 2000  The Benjamin/Cummings Publishing Company

Nervous System Organization Structural level of organization Central nervous system(CNS) Brain Spinal cord Peripheral nervous system (PNS) Cranial nerves Spinal nerves

Nervous System Organization Functional level of organization Afferent (sensory) division Somatic and visceral Efferent (motor) division Somatic (somewhat voluntary) Autonomic (involuntary) (ANS)

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Nervous Tissue Two types of cells Neurons--specialized to conduct information Neuroglia--specilaized to support and defend neurons

Neuron Structure Longlived, amitotic, high BMR Large cells--may be 3’ long Large central cell body--soma Many small processes which branch--dendrites One or few long processes which end on target cell--axon

Cell body (Soma) Contains large nucleus Contains most of cytoplasm Large number of dark structures called Nissl bodies (Ribosomes) Extra cytoskelton No centrioles (amitotic)

Dendrites Many small, highly branched processes which extend from the soma In contact with other cells and bring information into the cell, act as receivers of the cell Most of cell membrane surface is in the dendrites

Axons Usually single long process  Many covered with myelin sheath Ends in many branches (telodendria) with axonal terminals (synaptic knobs) Terminals filled with vesicles containing neurotransmitter Carry information away--transmit

Axonal Terminals Swollen knob of cytoplasm containing many vesicles and mitochondria Vesicles contain neurotransmitter Membrane separated from next cell by synaptic cleft--synapse is two membranes and space between

Myelin Sheath Myelin--white, fatty material Many axons surrounded by myelin sheath, carry signal faster PNS, Schwann cell, jelly roll cover, most of cytoplasm in outermost layer Sarcolemma, nodes of Ranvier CNS, oligodendrocytes, many per cell, nodes widely spaced

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Neuroglia Supporting cells of system, play role of other tissues elsewhere CNS neuroglia Astrocytes Oligodendrocytes Ependymal cells Microglia

Neuroglia, cont PNS neuroglia Schwann cells Satellite cells

Classification of Neurons Functional classification Sensory (Afferent) neurons Soma in ganglia outside CNS Receptors may be specialized Motor (Efferent) neurons Soma in CNS--Nuclei Interneurons between sensory and motor neurons soma in CNS

Classification of Neurons Structural classification Based on number of processes Multipolar Unipolar Bipolar Axons in CNS called tracts Axons in PNS called nerves

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Nerve Physiology Neurons are excitable and conductive Neurons have a resting potential of     -70 mV Neurons can be depolarized by stimili Action potential carried down axon Causes release of neurotransmitter into synaptic cleft

Nerve Physiology, cont Neurotransmitter diffuses to target cell membrane Binds with receptor on target cell membrane Stimulates some response from target cell-stimulate or inhibit

Nerve Physiology, cont Nerve must repolarize before another signal can be sent Neurotransmitter degraded in synapse and more must be released for another signal to be sent Neuronal activity is an electro-chemical event

Basic electricity Human body electrically neutral Separation of opposite charges requires energy, creates potential Potential measured in volts/mV Current is flow of charges Resistance is hinderance to flow

Electricity in body Current is ion flow, not electrons Membrane has protein channels, specific to ions (Na+, K+, Ca++, etc) Passive (leakage) channels Active channels--gated open or closed Chemical (physical) gates Voltage  (electrical) gates

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Resting Membrane Potential In neurons, average -70mV Potential only across membrane based on differential leakage of sodium and potassium vs pump When gates open, ions flow across electrochemical gradient, create current and change magnitude and direction of potential

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Signalling Neurons communicate by changing membrane potential graded potential-short distance action potential-long distance depolarization--potential closer to 0, (more excitable) hyperpolarization--potential becomes more negative (inhibited)

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Graded Potentials Short lived, local events de- or hyper- polarizations current decreases with distance stimulus opens gated channel, but depolarization due more to ion rearrangement than flow

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Action potentials Brief reversal of potential to +30mV Maintains strength over distance Only occurs on axon (electrical gates) Graded potential reaches axon hillock and converts to action potential

Generating an AP Resting state-active channels closed Depolarization-- elec Na+ gates open Repolarization--elec Na+ gates close, K+ gates open slowly, then close Self propagating Threshold and all or none Refractory periods

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Conduction velocities Factors--diameter and myelinated Speeds 1 mps to 150 mps Myelin-saltatory conduction fiber classification--A, B, or C Inhibition of conduction Multiple Sclerosis

The Synapse Junctions to transfer information Most comon are axodendritic or axosomatic, others poorly known Pre and post-synaptic neurons Post-synaptic cell-effector cell Electrical and chemical synapses

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Electrical synapse Correspond to gap junctions electrically couples cells rapid and bi-directional flow synchronize activity of neurons some in adult brain, more common in embryo, but replaced

Chemical synapse Release and receive neurotrx Neurotrx open chemical gates, affect membrane potential 3-part structure: presynaptic, cleft, postsynaptic Unidirectional communication based on neurotrx diffusion across synaptic cleft

Sequence of events 1.  Ca++ enters axon terminal 2.  Vesicles exocytose neurotrx 3.  Neurotrx diffuses across cleft 4.  Neurotrx binds to receptor protein 5.  Channel opens, ions flow 6.  Effect terminated:  degradation, reuptake, or diffusion out of synapse

Post-synaptic effects Receptor is chemical gate only Excitatory Postsynaptic potentials (EPSP) are depolarizing, make neuron more sensitive, may generate action potential at axon hillock Inhibitory Postsyanptic potentials (IPSP) are hyperpolarizing, decrease neuron sensitivity, less likely AP

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Integration of events Summation, temporal or spatial Axon hillock keeps score of all EPSP and IPSPs generated Synaptic potentiation-use Presynaptic Inhibition Neuromodulation

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Neurotransmitters Chemical communicators, >50+ Must be in terminal, released on stimulation, have experimental effect, and be naturally removed. Most cells make and respond to more than one neurotrx (NT) Classified by chemical structure

NT classification Acetylcholine Biogenic amines: dopamine, , E/NE, seratonin, histamine

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NT classification Acetylcholine Biogenic amines: dopamine, , E/NE, seratonin, histamine Amino acids: GABA, glycine, glutamate, aspartate, others? Peptides: Substance  P, endorphins, enkephlins, gut-brain peptides Novel messengers: ATP, NO, CO

Functional classification Effects: excitatory, inhibitory (some are receptor dependent) Mechanism: direct or indirect Receptor effects:  channel receptor-ion gate  G-protein-metabotropic

Neural Integration Neuronal pools: functional groups of neurons Example incoming neuron branches to several other neurons excitation stimulates some and facilitates others discharge or facilitation zones

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Types of circuits Diverging circuits

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Types of circuits Diverging circuits Converging circuits

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Types of circuits Diverging circuits Converging circuits Reverberating/oscillating circuits

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Types of circuits Diverging circuits Converging circuits Reverberating/oscillating circuits Parallel after-discharge circuits

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Processing Patterns Serial processing single pathway to destination reflexes Parallel processing several simultaneous pathways way the brain works

Reflex Arc Reflex is a rapid, involuntary, stereotyped response to stimulus Neural pathways which carry are called reflex arcs Afferent neuron, efferent neuron, and (usually) an interneuron Pre-programmed, for survival

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Reflexes Somatic reflexes--skeletal muscle Autonomic reflexes--viscera and glands Some only involve spinal cord; others involve brain integration Knee jerk, withdrawal reflexes

Anatomy of CNS Central Nervous System (CNS)  Hollow tube, folded anteriorly Formed of two parts, joined and function together Brain Spinal Cord

Brain anatomy Five anatomical regions Telencephalon, diencephalon, mesencephalon, metencephalon, & myelencephalon AKA Cerebrum, diencephalon, midbrain, cerebellum, pons, and medulla oblongata Hollow--4 ventricles

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Anatomy of Cerebrum Twin hemispheres, R & L Hollow, ventricles 1 and 2 Wrinkled, with sulci (grooves) & gyri (ridges) on surface Divided by fissures Regional lobes named for skull bones overlying each region

Cerebral lobes Frontal lobe somatic sensory area Parietal lobe somatic motor area Frontal lobe Thought, intellectual reasoning

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Cerebral lobes, cont Temporal lobe Auditory, olfactory in insula Occipital lobe Visual area Each lobe also has association areas which process multiple information from other lobes

Anatomy of Cerebrum Brain regionalized internally into gray and white matter Gray matter in outer layer (cortex) and inner clusters (basal nuclei) White matter consists of tracts connecting the cortex and nuclei; corpus callosum connects cerebral hemispheres

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Diencephalon Like a room with small open space (3rd ventricle), covered by cerebrum Roof (epithalamus) choriod plexus and pineal body Walls (thalamus) Floor (hypothalamus) Pituitary gland attached to hypothalamus by stalk (infundibulum)