Physiology of the Neuron.doc

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Physiology of the Neuron
-also called nerve cells; they are the structural unit of the nervous system
-highly specialized cells that conduct messages in the form of nerve impulses from one part
of the body to another
-the 3 functional regions are the receptive region, conducting region, and the secretory
Special Characteristics:
1) Extreme Longevity
-with good nutrition, neurons can function optimally for a lifetime
2) Amitotic
-neurons lose their ability to divide as they assume their roles as communicating links of
the nervous system
3) High Metabolic Rate
-requires abundant supplies of oxygen and glucose and cannot survive more than a few
minutes without oxygen
Neuron Cell Body
-consists of a spherical nucleus with a nucleolus surrounded by cytoplasm
-also called perikaryon or soma
-biosynthetic centre of a neuron
-rough ER is known as Nissl bodies or chromatophilic substance
-mitochondria are scattered among the other organelles
-elaborate Golgi
-cell body is the focal point for outgrowth of neuron processes
nuclei- clusters of cell bodes in the CNS
ganglia- cell bodies that lie along the nerves in the PNS
-armlike processes extend from the cell body of all neurons
-the PNS contains mostly of neuron processes
-bundles of processes are called tracts in the CNS and nerves in the PNS
-the 2 types of processes are dendrites and axons
-the main receptive or input regions
-provide enormous surface area for receiving signals from other neurons
-convey incoming messages toward the cell body
-carry graded short-distance signals or graded potentials
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-a neuron has a single axon
-the initial region of the axon arises from a cone-shaped area of the cell body called the
axon hillock and then narrows to form the process
nerve fibre- a long axon (i.e.: axons controlling the skeletal muscles of your foot from the
lumbar region)
axon collaterals- occasional branches from an axon
-axons branch profusely at its end and these are called terminal branches
-the knoblike distal endings of terminal branches are call axon terminals / synaptic knobs
-the axon is the conducting region of the neuron; it generates nerve impulses and transmits
them away from the cell body, along the plasma membrane (axolemma)
-the nerve impulse is generated at the junction of the axon hillock and the axon and
conducted along the axon to the axon terminals which are the secretory region of the
-when the impulse reaches the axon terminals, it causes NTs to be released to be released
into extracellular space and they either excite or inhibit neurons
-axons lack Nissl bodies and golgi apparatus which are involved in protein synthesis and
-the axon depends on its cell body to renew the necessary proteins and membrane
components and depends on the efficient transport mechanisms to distribute them
anterograde movement- transport of molecules and nutrients toward the axon terminal;
these substances include mitochondria, cytoskeletal elements, membrane components used
to renew axon plasma membrane, and enzymes needed for synthesis of certain NTs
retrograde movement- transport of molecules and nutrients away from axon terminals;
these substances are mostly organelles being returned to the cell body for degradation or
Myelin Sheath and Neurilemma
-nerve fibres that are long or large in diameter are covered with a whitish, fatty, segmented
myelin sheath which increase the speed if transmission of nerve impulses
-myelin sheath protects and insulates fibres
-myelinated fibres conduct nerve impulses rapidly
-unmyelinated fibres conduct impulses slowly
-myelinated sheaths are associated only with axons and dendrites are always
-myelinated sheaths in the PNS are formed by Schwann cells
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neurilemma- the portion of the Schwann cell which includes the exposed part of its plasma
nodes of Ranvier- gaps between adjacent Schwann cells along an axon
Membrane Potentials
-when a neuron is adequately stimulated, an electrical impulse is generated and conducted
along the length of its axon; this response is called the action potential
-there are areas in the body where one type of charge predominates, making those regions
positively or negatively charged
-energy must be used to separate them
-the coming together of opposite charges liberates energy that can be used to do work
-situations where there are seperated electrical charges of opposite sign have potential
voltage- electrical potential energy due to separation of oppositely charged particles
Resting Membrane Potential
-the potential different in a resting neuron; the membrane is polarized
- the value of resting membrane potential varies from -40 mV to -90 mV in different types
of neurons (for our purposes we will use -70 mV)
-at rest, the membrane is impermeable to large anionic cytoplasmic proteins, slightly
permeable to sodium, and 75x more permeable to potassium than to sodium, and also quite
freely permeable to chloride ions
-this reflects the properties of the leakage ion channels in the membrane
-potassium ions diffuse out of the cell along their concentration gradient much
more easily than sodium ions can enter the cell
-K+ flowing out of the cell causes the cell to become more negative inside
-the sodium-potassium pump first ejects 3 Na+ from the cell and then transports
two K+ back into the cell; this stabilizes the resting membrane potential by
maintaining the concentration gradients for sodium and potassium
-neurons use changes in their membrane potential as communication signals for receiving,
integrating, and sending information
-a change in membrane potential can be produced by:
1) anything that alters ion concentrations on the 2 sides of the membrane
2) anything that changes membrane permeability to any ion
-changes in membrane potential can produce 2 types of signals:
1) graded potentials- incoming signals operating over short distances
2) action potentials- long distance signals of axons
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