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Peripheral Nervous System (PNS)
-- all nerves connecting the CNS to the ‘rest of the body’ make up the PNS.
(eg. motor nerves to muscles, internal organs/glands (viscera), and sensory
neurons from the periphery or viscera to the CNS).
-- to that end, peripheral nerves include:
i. Sensory Nerves -- receive sensory input and send impulses to the CNS
-- possess long dendrites and shorter axons
-- cell bodies form the dorsal-root ganglion
ii. Motor Nerves -- carry ‘response’ impulses away from the CNS to the
appropriate effectors
-- possess short dendrites and longer axons
-- cell bodies congregate within the CNS (‘ventral-root
ganglion’)
iii. Mixed Nerves – a combination of both types wrapped together.
Organization of the Human Nervous System
HUMAN NERVOUS SYSTEM
CENTRAL N.S.
BRAIN
PERIPHERAL N.S.
SPINAL CORD
MOTOR
NERVES
AUTONOMIC N.S.
SYMPATHETIC N.S.
SENSORY
NERVES
SOMATIC N.S. VISCERA
PARASYMPATHETIC N.S.
Somatic Nervous System: Skeletal Muscles
-- sensory nerves of the somatic nervous system gather stimuli from the
periphery of the body (from sensory receptors in skin/muscle cells) and
transmit the information received to the CNS for integration.
-- motor nerves of this nervous system synapse on skeletal muscle and often
control voluntary movement.
-- reflex arcs that result in physical movement are controlled by the Somatic
NS; thus, an involuntary reflex is generated in muscles that tend to be
voluntarily controlled – this, so we do not have to actually think about
certain stimuli, we can simply react in a quicker, involuntary manner.
Autonomic Nervous System: Viscera, Smooth Muscle, & Cardiac Muscle
-- motor nerves of the autonomic nervous system control involuntary
responses by glands, organs, smooth muscle, and/or cardiac muscle through
synaptic connections (eg. regulation of blood pressure, breathing rate, heart
rate, etc.).
-- sensory nerves that signal the autonomic nervous system possess sensory
receptors on or within internal organs/glands, or on smooth/cardiac muscle.
-- this sensory information is transmitted to the interneurons of the
autonomic nervous system where it is then relayed to the medulla
oblongata or to the hypothalamus for assessment and subsequent response
via a motor neuron emerging from a particular portion of the brain (vagus
nerve) or spinal cord.
** Relevant Information:
Brain
Cranial Nerves
Cervical Nerves
Thoracic Nerves
Lumbar Nerves
Sacral Nerves
Two Divisions of the Autonomic Nervous System
a. Sympathetic Nervous System
b. Parasympathetic Nervous System
* each of these systems’ motor nerves synapse with the same targets but
produce opposite effects.
* as well, each of these systems only possess a motor nerve division.
Similarities between the Sympathetic N.S. and the Parasympathetic N.S.
1. Both function automatically/subconsciously (ie. we do not need to ‘think’
about utilizing them).
2. Innervate all internal organs.
3. Utilize two motor neurons (bundled into nerves) and one ganglion for
each impulse:
The two motor neurons are:
i. Preganglionic Axon -- this is the ‘first’ motor neuron; its cell body
is located within the CNS.
ii. Postganglionic Axon -- this is the ‘second’ motor neuron; its
cell body is located within the ganglion.
Sympathetic Nervous System: ‘Fight or Flight’
Key Points:
i. Important during emergency/stressful situations (ie. associated with ‘fight
or flight’). For example: causes an increase in breathing rate and heart rate,
dilation of pupils, increased blood flow (flow shunting – arteriole sphincter
muscles) to skeletal muscles/heart etc., decreased blood flow to digestive
tract (intestines), stimulation of adrenal medulla gland for release of
hormone adrenalin.
ii. Preganglionic nerve fibers originate from the thoracic/lumbar (middle)
portion of the spinal cord.
iii. Preganglionic nerve fiber is much shorter than the postganglionic fiber
(fig. 17.9 p. 330 + pics on p. 331).
eg.
iv. The neurotransmitter released by the postganglionic axon bulbs is
norepinephrine (noradrenaline), which causes desired effects in effector,
such as those described above.
Parasympathetic Nervous System: ‘Housekeeper’ - Maintains Normalcy
Key Points:
i. Promotes all internal responses that are associated with a relaxed state.
For example: causes a decrease in breathing rate and heart rate, contracts
pupil, increases blood flow to digestive system (intestines), decreases blood
flow to skeletal muscles/heart etc, inhibits stimulation of adrenal medulla
gland (shuts off release of adrenalin).
ii. Preganglionic nerve fibers originate from the cranial (vagus nerve) and
sacral regions of the spinal cord (top and bottom).
iii. Preganglionic nerve fiber is much longer than the postganglionic fiber
since the ganglia themselves lie near or within the organ (fig. 17.9 p. 330 +
pics on p. 331).
eg.
iv. The neurotransmitter released by the postganglionic axons is
acetylcholine, which promotes aforementioned effects in effectors.
*Analyze fig. 17.9 p. 330 closely as a class and individually.
Refer to table 17.1 p. 331 for summary of Sympathetic NS and
Parasympathetic NS.