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Terms in this set (46)

1) Ion channels
The permeability of membranes, and their electrical signaling properties, are
controlled by ion channels. Ion Channels are:
a) Ion specific
b) Regulatable - i.e. the ion flow thru the channel pore can be controlled:
Ligand gated - channel is opened only in the presence of a specific chemical.
Voltage gated - channel is opened by a change in the electrical properties (voltage across) of the membrane. (membrane potential) Mechanically gated - channel is opened by physical forces
2) The Signal "Spike" or "Action Potential"
a) Various agents can alter membrane permeability and thus change the
resting membrane potential. Two basic patterns are seen:
Hyperpolarization - an increase in the cell's negative charge.
Depolarization - a decrease in the cell's negative charge toward
zero
b) The Threshold potential - a depolarization of sufficient size to activate
(open) the voltage gated Na+ channels.
c) The Increased Na+ permeability seen upon reaching threshold will allow
ion movement across the membrane in response to; the inward Na concentration gradient
the internal negative charge
d) The local positive (Na+) ion inflow will
open more voltage gated Na+ channels - amplification of the
depolarization
cause a reversal of the membrane potential from the (negative) K+
to the Na+ (+) equilibrium potential - the spike or action potential
e) The spike is stopped and reversed by:
Na+ voltage gated channels close automatically
Build-up of internal (+) opposes continued Na+ entry
The opening of K+ channels increases K+ permeability
f) Since the gated channels are either open or closed production of a spike is an all or none phenomenon and can be considered an on/off or yes/no signal
This system is composed of the nerves supplying the internal body organs. It shares many afferent (sensory) fibers with the somatic system.

The efferent path is separate and usually consists of two neurons between the CNS and the effector organ. Usually these two neurons interact at a peripheral ganglion. This system functions independently of conscious effort.

Parasympathetic Division

This division is composed of the Cranial and Sacral nerves. The major neurotransmitter released by this division at the level of the effector organ is Acetylcholine. This neurotransmitter is involved in the stimulation and maintenance of all necessary physiologic functions of a person at rest. Overall it's activity promotes the production and conservation of energy. Thus it is often referred to as the "resting and digesting' system.

Sympathetic Division

This division is composed of Thoracic and Lumbar Spinal nerves. The major neurotransmitter it releases at the effector organ is Noradrenaline. This neurotransmitter is involved in preparing the body for energy-expending, stressful or emergency, "fight or flight" situations. By and large this division up-regulates the systems that are depressed by the parasympathetic and down-regulates those that are stimulated.

Note that the Parasympathetic and Sympathetic divisions both innervate all the same organs. They use different neurotransmitters, however, and therefore have opposite effects on each organ; one stimulating and one inhibiting organ activity. Thus balanced output from these divisions will keep all organ activity at the proper level for the existing conditions i.e. they maintain physiologic homeostasis.