Transcript The Peripheral Nervous System: Efferent Division

The Peripheral Nervous
System: Efferent Division
Chapter 7
• The autonomic nervous system (ANS)
innervates cardiac and smooth muscle,
most exocrine glands and some
endocrine glands.
• The ANS releases only two different
neurotransmitters on the structures it
controls: acetylcholine and
norepinephrine.
• An autonomic nerve pathway has two
neurons: a preganglionic fiber and
postganglionic fiber.
The two subdivisions of the ANS are
the sympathetic and parasympathetic
nervous systems.
• Sympathetic nerve fibers originate from the thoracic and lumbar
levels of the spinal cord.
– Its preganglionic fibers are short. Its postganglionic fibers are long,
terminating at effectors.
• Parasympathetic nerve fibers originate from the cranial and
sacral levels of the CNS.
– Its preganglionic fibers are long. Its postganglionic fibers are short,
terminating at effectors. There are terminal ganglia near effector
organs.
An autonomic ganglion is between the
preganglionic and postganglionic fibers of an
autonomic pathway. The preganglionic fibers of
both branches of the ANS release acetylcholine
(ACh).
• Parasympathetic
postganglionic fibers release
ACh. They are cholinergic
fibers.
• Sympathetic postganglionic
fibers release noradrenalin
(norepinephrine). They are
adrenergic fibers.
The ANS controls the involuntary
responses of visceral organs.
• These responses range from
circulation and digestion to
sweating and urination.
• The visceral organs receive dual
innervation. They are signaled
by both subdivisions of the ANS.
• Usually both subdivisions are
active in controlling the activity
of an internal organ. This
ongoing activity is called
sympathetic or parasympathetic
tone activity.
The sympathetic dominates during
the fight-or-flight response. Examples
of these responses are:
–
–
–
–
–
–
–
pupils of the eye dilate
sweating is promoted
heart rate increase
bronchioles dilate
motility for digestion is inhibited
liver: conversion of glycogen into glucose
urination is inhibited
• These responses prepare the body for
emergency or stressful situations.
The parasympathetic
dominates during quiet,
relaxed situations.
• The responses signaled by this
subdivision is usually the opposite (e.g.,
decreased heart rate) or none (no
activity of glucose production from the
liver).
• Dual, reciprocal innervation allows the
precise control over the activity of a
visceral organ.
There are several exceptions
to the general rule of dual,
reciprocal innervation.
• Most innervated blood vessels (arterioles and
veins) receive only sympathetic nerve fibers.
• Most sweat glands are innervated only by
sympathetic nerves.
• Both ANS subdivisions stimulate the activity
of salivary glands.
• As another exception, the adrenal medulla is
a gland that evolved from postganglionic
neurons.
Each autonomic
neurotransmitter binds to
several different receptors.
• Cholinergic receptors bind to ACh.
– Nicotinic receptors are found on the postganglionic cells bodies of
all autonomic ganglia. These ganglia are found where the
preganglionic neuron signals the postganglionic neuron.
– Muscarinic receptors bine to ACh from parasympathetic
postganglionic nerve fibers.
• Adrenergic receptors bind to norepinephrine and epinephrine
from sympathetic postganglionic fibers.
– Activation of alpha 1 receptors usually produces an excitatory
response in the effector. Activation of alpha 2 receptors usually
produce an inhibitory response
– Beta 1 receptors are found mainly in the heart. They cause an
excitatory response. The response to beta 2 receptor activation is
general inhibitory.
Certain drugs selectively
enhance or mimic autonomic
responses. They are
agonists.
• Other drugs block autonomic
responses. They are antagonists.
• Autonomic activities are controlled by
different areas of the CNS. They
include the spinal cord, medulla,
hypothalamus, and prefrontal
association cortex.
The somatic nervous system signals
skeletal muscles. One axon in its
nerves from the spinal cord terminate
on skeletal muscles.
• Acetylcholine is the terminal neurotransmitter at the
neuromuscular junction. This junction is where the axon of a
somatic efferent neuron signals a skeletal muscle fiber (cell).
– The axon terminates at a terminal button. The motor end plate is
the specialized part of the muscle cell membrane facing the
terminal button.
• Somatic motor neurons are the final common pathway. Skeletal
muscle activity can be affected only by acting on these motor
neurons.
– See Fig. 7-5
Acetylcholine is released at
the neuromuscular junction.
• An action potential at the axon terminal opens voltage-gated
calcium channels in the terminal button. Calcium ions diffuse
into the terminal button from the ECF.
• By this calcium influx, acetylcholine is released by exocytosis
from vesicles in the terminal button.
• Acetylcholine diffuses through the space between the nerve cell
and muscle cell.
• This neurotransmitter binds to receptor sites on the motor end
plate of the muscle cell membrane.
• This binding opens channels for sodium influx into the ICF of the
muscle cell.
• This produces a local current flow that opens adjacent sodium
channels in the motor end plate.
• An action potential is initiated through the muscle fiber.
– See Figure 7-6
Acetylcholinesterase destroys
acetylcholine. This terminates
the action potential and
response of the muscle cell.
• The neuromuscular junction is vulnerable to
several chemical agents and diseases.
• Examples are:
– Two toxins, black widow spider venom and
botulinum toxin, alter the release of acetylcholine.
– Curare blocks the effect of released acetylcholine.