Neurophysiology



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Lecture: Neurophysiology
I. Overview of Nervous System Organization
A. Central Nervous System (CNS) - brain and spinal cord
B. Peripheral Nervous System (PNS) - spinal/cranial nerves
1. Sensory (Afferent) Division - TO the CNS
a. somatic afferents - from skin, muscle, joints

b. visceral afferents - from membranes & organs


2. Motor (Efferent) Division - FROM the CNS
a. Somatic Nervous System (Voluntary) - to skeletal muscles

b. Autonomic Nervous System (Involuntary) - to organs & glands

i. Sympathetic Division

ii. Parasympathetic Division



II. The Structure of a Neuron (Nerve Cell)
A. neuron - special cells of nervous system that carry messages in the form of electrical impulses

B. Supporting Cells of Neurons
1. Support Cells of the CNS (Glial Cells)
a. astrocytes - regulate environment around neurons and selective transport from capillaries

b. microglia - eat infectious microbes of CNS

c. ependymal cells - line cavities of brain and spinal cord, flushing cerebrospinal fluid

d. oligodendrocytes - form "myelin sheaths" around axons of CNS; increase speed of impulses


2. Support Cells of the PNS
a. Schwann cells - form "myelin sheaths" around axons; also assist in regeneration of axon

b. satellite cells - control chemical environment




C. Special Characteristics of Neurons
1. amitotic - "not mitotic"; they cannot reproduce or regenerate after certain point in life

2. longevity - neurons can survive entire lifetime

3. high metabolic rate - require OXYGEN and GLUCOSE at all times
D. Neuron Cell Body (soma; perikaryon)
1. major part from which the processes (axons and dendrites) project; 5-140 micron diameter

2. single large spherical nucleus with nucleolus

3. Nissl Bodies - Rough Endoplasmic Reticulum (RER); make proteins and plasma membrane

4. nucleus - a collection of cell bodies in the CNS

5. ganglion - a collection of cell bodies in the PNS
E. Typical Neuron Processes (Dendrites & Axon)
1. dendrites - branching, rootlike extensions off the cell body
a. receptive/input component of the neuron; incoming signals are forwarded to the cell body

b. signals of dendrites are NOT all-or-none action potentials, but are graded potentials that result from summation of inputs


2. axon - extension that carries an all-or-nothing action potential from the cell body to the target; conducting component of the neuron connecting it to other cells or neurons
a. tract - a bundle of axons in the CNS

b. nerve - a bundle of axons in the PNS

c. axolemma - plasma membrane of neuron

d. axon hillock - the cone-shaped region of attachment of the axon to the cell body; site where action potential is triggered

e. axon collaterals - rare branches of an axon

f. telodendria - typical terminal branches of an axon which may number up to 15,000

g. synaptic knobs/ boutons/ axon terminals - at the end of each telodendria, abut the target tissue to secrete a chemical neurotransmitter; secretory component of the neuron

h. axon depends upon the cell body for everything: organelles, proteins, and enzymes for synthesis of neurotransmitter


i. anterograde transport - movement of material from cell body to synaptic knobs

ii. retrograde transport - movement of material from synapse to cell body


3. myelin sheath - wrap of Scwhann cells (PNS) and oligodendricytes (CNS) around the axon
a. increases speed of action potential signal [myelinated (150 m/s); unmyelinated (1 m/s)]

b. nodes of Ranvier - gaps between myelin cells at regular intervals on axon

c. white matter of brain - areas with myelinated axons

d. gray matter of brain - areas with cell bodies and unmyelinated cell processes


F. Structural Classification of Neurons
1. multipolar neuron - has three or more cell processes; typically many dendrites and one axon (throughout the CNS)
2. bipolar neuron - have two (bi) processes: one dendrite and one axon, each extending from opposite sides of the cell body (retina of the eye)
3. unipolar neuron - one long process attached to the cell body by a "T" like extension

a. peripheral process - the part that starts at the sensory receptor (eg. skin)

b. central process - the part that terminates in the CNS (eg. spinal cord)
G. Functional Classification of Neurons
1. sensory (afferent) neuron - transmit impulses from sensory receptors TOWARD the CNS

a. almost all are unipolar and located just outside the spinal column

i. Dorsal Root Ganglion of the spinal cord (sensory info from body)
2. motor (efferent) neuron - transmit impulses AWAY FROM the CNS to the target tissue

a. almost all are multipolar, with cell bodies in the CNS


3. association neuron (interneuron) - between sensory and motor neurons
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