CHAPTER 13 & 14

Peripheral Nervous System and Autonomic Nervous System


I. The peripheral nervous system (PNS) is made up of the nervous system structures outside of the brain and spinal cord. It allows the CNS to communicate with the outside world.

II. The PNS is made up of 3 general structural components.

A) Sensory receptors, motor endings, and nerves and ganglia.

1)   Sensory receptors detect environmental and bodily changes and send impulses to CNS.

2)   Motor endings innervate effector organs (muscle fibers and glands). Note that your text makes the distinction that these areas are separate from synapses. However, we will consider them as synapses.

3)   Nerves and ganglia. Nerves generally house both sensory and motor axons, although some carry only one or the other.


B) The sensory receptors: Detection of the environment.

1) Can be divided into 2 categories:

a) Having dendritic endings of sensory neurons.

b) Can be entirely a receptor cell.

2) Can be classified by 3 factors: location, stimulus they detect, and by structure.

a) Classified by location.

1)   Exteroreceptors are near the body surface. They detect touch, pain, pressure, temperature.

2)   Interoreceptors (visceroreceptors) are in the visceral organs and detect chemical changes, taste, tissue stretch, temperature, pain, etc.

3)   Proprioceptors are in the musculoskeletal organs (muscles, tendons, joints, fascia, etc). They detect stretch, velocity, joint position, etc. to detect body movement.

b) Classified by stimulus they detect.

1)   Mechanoreceptors respond to mechanical factors such as touch, pressure, vibration, stretch, etc.

2)   Thermoreceptors detect temperature changes.

3)   Chemoreceptors detect chemical changes.

4)   Photoreceptors detect light (in the eye).

5)   Nocioreceptors detect noxious, painful stimuli.

c) Classification by structure (primarily general sensory receptors only).

1)   Free dendritic endings. These are primarily pain and temperature sensors.

2)   Merkel discs. Detect light touch.

3)   Root hair plexuses. Detect hair movement.

4)   Encapsulated dendritic endings. Most or all are mechanoreceptors.

5)   Meissner's corpuscles. Beneath the skin. Sensitive to light touch.

6)   End bulbs. In mucous membranes. Sensitive to light touch.

7)   Pacinian corpuscles. In deep connective tissue. Sensitive to strong pressure and vibration.

8)   Ruffini's corpuscles. In the dermis. Sensitive to continual pressure, touch.

9)   Proprioceptors: muscle spindles, Golgi tendon organs, and joint kinesthetic receptors. You are responsible for everything in this section in text:


C) The peripheral motor endings. These endings innervate and activate the effector organs.

1)   Innervation of skeletal muscle starts at the neuromuscular junction (this is a synapse!). a) One neuromuscular junction per muscle fiber.

2)   Each neuron branches to innervate a number of muscle fibers. The neuron and all the fibers it innervates is called a motor unit.

3)   You are responsible for figure 13.3 in text.

4)   Innervation of visceral muscle and glands.

5)   Simpler and slower than innervation of skeletal muscle.

D) The nerves and ganglia.

1)   Cranial nerves.

      You are responsible for Figure 13.5b. That is, to know all the cranial nerves by name and number, and their sensory and motor function. Do not be concerned if they have parasympathetic fibers or not.

2)   Spinal nerves.

      31 pairs, named after the area they arise from in the spinal cord. They are very short: at the medial aspect (near spinal cord) they form the dorsal and ventral roots, and their lateral aspects branch to form the dorsal and ventral rami (see Figure 13.7b in text).

      The dorsal and ventral roots form from the spinal nerves and the roots attach directly to the spinal cord by dorsal and ventral rootlets.

      Dorsal roots are sensory fibers, ventral roots are motor fibers.

      The rami are where the spinal nerves branch. They contain both sensory and motor neurons.

      Nerve plexuses. Sacral, lumbar, brachial, and cervical.

      Area where ventral (i.e., motor) rami join one another allowing fibers to crisscross one another and redistribute.

      This allows muscles to receive nervous supply via more that one pathway, thus acting a backup system in case one route is damaged.


IV. The autonomic nervous system (ANS)


A) A functional segment of the PNS.

1) Generally considered part of the motor division of the PNS, but also includes the visceral sensory division.

B) Controls automatic functioning such as heart rate, blood pressure, digestion, urination.

C) Major divisions of the ANS are the sympathetic and parasympathetic divisions. These divisions generally have opposing effects (text pages 393-396).

1) Sympathetic.

a)    Called the 'fight or flight' regulator.

b)   Controls arousal such as increasing heart rate, blood pressure, ventilation rate, pupil dilation, etc.

c)    It has a general effect to vasoconstrict the blood vessels to increase blood pressure. However, some of the needed muscles, such as to run, have their supplying blood vessels vasodilated.

2) Parasympathetic.

a)    This system is active at rest (compare to sympathetic) to conserve energy (e.g., slow heart rate and ventilation rate).

b)   Parasympathetic also aids in digestion, elimination, and general maintaining of body functioning.