Peripheral Nervous System (PNS)
-- all nerves that connecting the CNS to the ‘rest of the body’ make up the PNS. (eg. motor nerves, sensory nerves, mixed nerves). Recall:
i. Sensory Nerves -- receive sensory input and send impulses tothe CNS;
-- possess long dendrites and shorter axons;
-- cell bodies congregate just outside the CNS (dorsal- root ganglion)
ii. Motor Nerves -- carry ‘response’ impulses away fromthe 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.PERIPHERAL N.S.
BRAINSPINAL CORDMOTORSENSORY
NERVESNERVES
AUTONOMIC N.S. SOMATIC N.S. VISCERA
SYMPATHETIC N.S.PARASYMPATHETICN.S.
Somatic Nervous System
-- 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 to the CNS for integration.
-- motor nerves of this nervous system synapse on skeletal muscle andcontrol voluntary movement.
-- reflex arcs that result in physical movement are controlled by the Somatic NS.
-- thus, an involuntaryreflex 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
-- motor nerves of the autonomic nervous system control involuntary responses by the viscera (glands/organs), smooth muscle, and/or cardiac muscle (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.
The 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 oppositeeffects.
* 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 aganglion.
Sympathetic Nervous System: ‘Fight or Flight’
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).
iv. The neurotransmitter released by the postganglionic axon bulbs is norepinephrine (noradrenaline), which causes excitation in effectors, such as those described above.
Parasympathetic Nervous System: ‘Housekeeper’ - Maintains Normalcy
i. Promotes all internal responses that are associated with a relaxed state.
For example:causes a decreasein breathing rate and heart rate, contracts pupil, increases blood flow to digestive system, 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).
iv. The neurotransmitter released by the postganglionic axons is acetylcholine, which promotes inhibition in effectors.
*Analyze fig. 17.9 p. 330 closely.
Refer to table 17.1 p. 331 for summary of SympatheticNS and ParasympatheticNS.