CHAPTER 13-THE PERIPHERAL NERVOUS SYSTEM AND REFLEX ACTIVITY
I. THE PERIPHERAL NERVOUS SYSTEM-includes all neural structures outside of the brain
and spinal cord.
A. The PNS contains sensory and motor neurons that function by transmitting information
to and from the central nervous system.
B. Sensory Receptors-of the PNS, are neurons that are specialized to respond to changes in the
body’s environment. Types of Sensory Receptors:
1. Mechanoreceptors
2. Thermoreceptors
3. Photoreceptors
4. Chemoreceptors
5. Nociceptors
II. NERVES-cordlike organs that are part of the PNS.
A. Nerves are composed of numerous nerve fibers that are organized into bundles known as
fasicles.
B. Connective Coverings Around Nerve Fibers
1. The Epineurium-surrounds and protects the entire nerve.
2. The Perineurium-surrounds bundles of nerve fibers (fasicles).
3. The Endoneurium-surrounds individual nerve fibers.
C. Ganglia-collections of neuron cell bodies associated with nerves in the PNS.
D. In general, nerves cannot repair themselves if damaged.
E. Recall that neurons are either sensory or motor in function. Nerves contain numerous neurons.
Some nerves are composed of both sensory and motor neurons. These are referred to as mixed
nerves. Other nerves are either sensory or motor in their function.
F. Cranial Nerves-12 pairs of these. They are associated with the brain and they pass through the
various foramina of the skull as they leave the brain. The 12 Cranial Nerves include the following:
1. Olfactory Nerve (I)-sensory nerve that carries impulses for smell to the brain.
2. Optic Nerve (II)-sensory nerve that carries impulses for vision to the brain.
3. Oculomotor Nerve (III)-motor nerve that carries impulses to the extrinsic eye muscles
which help direct the position of the eyeball. This nerve also carries impulses to the
muscles that regulate the size of the pupil.
4. Trochlear Nerve (IV)-motor nerve that carries impulses to one extrinsic eye muscle (the
superior oblique muscle). Once again, this muscle helps regulate the position of the eyeball.
5. Trigeminal Nerve (V)-a mixed nerve. The sensory fibers of this nerve carry impulses for
touch, temperature and pain associated with the face, teeth, lips and eyelids. The motor
fibers of this nerve carry impulses to some of the mastication muscles of the face.
6. Abducens Nerve (VI)-a mixed nerve, but primarily a motor nerve. This nerve carries
impulses to the lateral rectus muscle of the eye. This muscle is an extrinsic eye muscle
which is involved in positioning the eyeball.
7. Facial Nerve (VII)-a mixed nerve. The sensory fibers of this nerve carry touch, temperature,
pressure and pain sensations from the face to the brain. The motor fibers of this nerve carry
impulses to many of the muscles of the face and they carry impulses to the lacrimal glands.
8. Vestibulocochlear Nerve (VIII)-a sensory nerve that caries impulses for hearing and
equilibrium from the ear to the brain.
9. Glossopharyngeal Nerve (IX)-a mixed nerve. The sensory fibers of this nerve carry basic
sensory information and taste sensations from the pharynx and tongue to the brain. The
motor fibers of this nerve carry impulses associated with swallowing to the tongue and
pharynx.
10. Vagus Nerve (X)-a mixed nerve. The sensory fibers of this nerve carry impulses from the
pharynx, larynx and some internal organs to the brain. The motor fibers of this nerve carry
impulses to some internal organs and to the skeletal muscles of the larynx and
pharynx.
11. Accessory Nerve (XI)-a mixed nerve, but primarily motor. Carries impulses to
muscles of the larynx, pharynx and neck.
12. Hypoglossal Nerve (XII)-primarily a motor nerve. This nerve carries impulses to the
muscles that move and position the tongue.
G. Spinal Nerves-31 pairs of these. They arise from the spinal cord and carry impulses to the body
(excluding the head and neck region of the body). These are all mixed nerves.
1. These attach to the spinal cord by two roots: the dorsal root and the ventral root. The
ventral roots contain motor fibers and the dorsal roots contain sensory fibers.
2. Some of the more important spinal nerves include:
a. The Intercostal nerves
b. The Phrenic nerve
c. The Ulnar and Radial nerves
d. The Sciatic nerve
III. REFLEXES-rapid, predictable motor response to a stimulus. Most are involuntary.
A. Reflexes are usually regulated and integrated by the spinal cord.
B. 2 Types of Reflexes:
1. Somatic reflexes-involve skeletal muscle contractions.
2. Autonomic reflexes-involve smooth muscle and cardiac muscles activity and
glandular activity.
C. Reflex Arcs-the pathways traveled by reflexes in the body. Components of a reflex arc include:
1. The Receptor-is the site of the stimulus action.
2. The Sensory Neuron-detects the stimulation and sends impulses to the spinal cord and
brain.
3. The Integration Center-makes decisions relating to the reflex. This is either the brain or
the spinal cord (depending on the specific reflex).
4. The Motor Neuron-conducts impulses from the integrating center to the muscles and
glands of the body.
5. The Effector-is the muscle or gland that responds to the impulses from the motor neuron.
D. Important Human Reflexes
1. Patellar Reflex-involves knee extension. This is used as an indicator of nerve damage in the
mid-lumbar region of the back.
2. Achilles Reflex-involves extension of the big toe. This reflex may be absent in diabetics
and alcoholics.
3. Babinski Sign-extension of the great toe with fanning of the other toes. This reflex usually
disappears around the age of 2. In adults, this becomes the plantar flexion reflex which
involves curling of the toes.
IV. RELATED CLINICAL TERMS
A. Analgesia-reduced ability to feel pain. Analgesics are pain relieving drugs.
B. Neuralgia-pain along the course of one or more nerves. This pain is usually the result of inflammation.
C. Neuritis-inflammation of a nerve.
V. SENSATION-the awareness of external or internal stimuli.
A. Sensations include: temperature change sensations, hearing, balance, smell, pain.
B. Components of a Sensation
1. Stimulation
a. Stimulus-a change in the environment that can activate certain sensory neurons
in the body.
b. Sensory neurons detect stimuli. These particular neurons convert the stimulus into
an impulse (electrical signal).
2. Transduction-the actual conversion of a stimulus into an impulse. Once again, this is carried
out by sensory neurons.
3. Conduction-the movement of an impulse from the body to the CNS.
4. Integration-the conversion of an impulse into a sensation. This occurs in the brain
(primarily in the cerebrum).
VI. SENSORY NEURONS (SENSORY RECEPTORS)
A. These types of neurons are described as being selective-they respond to only one type of
stimulus.
B. Classification of Sensory Neurons based on their Location
1. Exteroceptors
2. Interoceptors
C. Classification of Sensory Neurons Based on the stimulus they Detect:
1. Mechanoreceptors
2. Thermoreceptors
3. Nociceptors
4. Photoreceptors
5. Chemoreceptors
VII. 2 MAJOR CATEGORIES OF SENSES IN THE HUMAN BODY
A. General or Somatic Senses-involve sensations associated with the skin and body.
B. Special Senses-includes taste, smell, sight, hearing, balance, smell.