NERVOUS SYSTEM

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NERVOUS SYSTEM
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NERVOUS SYSTEM COMPONENTS

• Brain
• Cranial nerves
• Spinal cord
• Spinal nerves
• Ganglia - cell bodies of neurons outside
• brain spinal cord
• enteric plexuses
• Sensory receptors

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NERVOUS SYSTEM ORGANIZATION

• Central Nervous System - thought, emotion,
• brain memory
• spinal cord
• Peripheral Nervous System (PNS)
• Somatic (SNS) - voluntary
• Autonomic (ANS) - involuntary

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Subdivisions of the Nervous System
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AUTONOMIC NERVOUS SYSTEM

• Sensory
• Motor
• - sympathetic
• - parasympathetic

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Subdivisions of Nervous System
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NERVOUS SYSTEM FUNCTIONS

• Sensory
• Integrative
• Motor
• Concerned with homeostatic regulation

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NERVOUS TISSUE

• Neurons - conduct impulses
• Neuroglia - do not conduct
• impulses

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NEURON DIVERSITY

• Sensory (afferent) neurons
• Transmit information from receptors to the brain
  or spinal cord (CNS)
• Interneurons (association neurons)
• Located in the CNS
• process, store retrieve information
• Motor (efferent) neuron
• send signals from the CNS to muscles gland
  cells (effectors)

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INTERNEURONS(90 of neurons)

• Located only in the CNS
• Purkinje cells in cerebellum
• Renshaw cells in spinal cord
• Pyramid cells in brain

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NEURON DIVERSITY

• Multipolar has several dendrites
• Bipolar has one main dendrite one axon
• Unipolar - axon dendrite fuse into a single
  process
• Anaxonic has several dendrites but no axon

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Neuron Diversity
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NEUROGLIA DIVERSITY

• Astrocytes in CNS contribute to blood brain
  barrier regulate composition of brain tissue
  fluid
• Oligodendrocytes - in CNS - produce myelin
• Microglia (macrophages) - in CNS
• Ependymal cells - in CNS - line cavities
  produce cerebrospinal fluid
• Schwann cells - in PNS - produce myelin
• Satellite cells - in PNS - uncertain function

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Neuroglial Cells of CNS
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DEFINITIONS

• Dendrites - receiving or input portions of neuron
• Cell body Soma Perikaryon
• Nissl bodies - clusters of rough ER
• Neurofibrils bundles of actin filaments
  (cytoskeleton)

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DEFINITIONS

• Axon - long, thin, cylindrical, impulse
  propagative portion of neuron
• Myelin sheath - insulating layer around a nerve
  fiber
• Synapse - junction between two neurons or between
  neuron and effector

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PROPERTIES OF NEURONS

• Electrically excitable
• Communicate via electrical signals.
• Generate action potentials for short long
  distance communication
• Generate local (graded) potentials only for
  short distance communication
• Limited regenerative ability

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PROPERTIES OF NEURON MEMBRANES

• Polarized - have resting potential (- 70 mV) -
  inside of a nerve cell membrane is negative
  charge relative to the outside
• Specific ion channels - allow flow of ions
• - Leakage channels (always open)
• - Gated channels (open close)

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RESTING MEMBRANE POTENTIAL

• Due to unequal distribution of ions across
  membrane
• Relative permeability of membrane to sodium
  potassium ions - (50 to 100 times greater to
  potassium than to sodium in a resting neuron)
• Electrochemical gradient exists.

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Ionic Basis of Resting Membrane Potential

• Na concentrated in extracellular fluid (ECF)
• K concentrated in intracellular fluid cell (ICF)

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GATED ION CHANNELS

• Voltage - open in response to change in membrane
  potential (voltage)
• Ligand - open close in response to
• specific chemical stimulation e.g.
  transmitters, hormones, ions
• Mechanical - open close in response to
  mechanical stimulation e.g. vibration, pressure,
  stretching

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LIGAND-GATED CHANNELS

• Ligand molecules may bind directly with a channel
  protein cause channel opening or closure.
• Ligand molecules e.g. hormones may act indirectly
  via a membrane G protein that activates a
  messenger in the cytosol to operate the channel
  gate.

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ACTION POTENTIALS

• All-or-none events - occur if depolarization
  reaches a certain threshold (about -55 mV)
• Always have the same amplitude
• Depolarizing phase Repolarizing phase
• After-hyperpolarization
• Refractory period - absolute relative - action
  potential cannot be generated

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IMPULSE CONDUCTION

• Continuous - in muscle fibers
• unmyelinated axons
• Saltatory - in myelinated axons
• impulses jump quickly from one node of Ranvier
  to the next

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Impulse Conduction in Unmyelinated Fibers
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CONDUCTION ACROSS SYNAPSES

• Electrical synapses - connexons (tubular
  proteins) allow ionic current to spread directly
  between adjacent cells through gap junctions
• Chemical synapses - neurotransmitters allows
  impulse conduction between neurons

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POSTSYNAPTIC POTENTIALS

• Excitatory - (EPSP) - occurs if postsynaptic
  membrane becomes depolarized (less negative) -
  glutamate aspartate are excitatory
  neurotransmitters
• Inhibitory - (IPSP) - occurs if postsynaptic
  membrane becomes hyperpolarized (more negative) -
  glycine ?-aminobutyric acid (GABA) are
  inhibitory neurotransmitters

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EVENTS AFTER SYNAPTIC CONDUCTION

• Diffusion of transmitter from synaptic cleft
• Active uptake of transmitter substance by
  neurotransmitter transporters in neuron
• Spatial temporal summation

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Chemical Synapse Structure

• Presynaptic neurons have synaptic vesicles with
  neurotransmitter and postsynaptic have receptors

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NEUROTRANSMITTERS

• Acetylcholine
• Amino acid neurotransmitters
• Monoamines
• catecholamines (epinephrine, norepinephrine
  dopamine)
• indolamines (serotonin histamine)
• Neuropeptides

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SUMMATION

• Temporal summation - occurs when a single synapse
  receives many EPSPs in a short period of time
• Spatial summation - occurs when a single synapse
  receives many EPSPs from many presynaptic cells