Peripheral Nervous System

Chapter 14

Spinal Nerves

• 31 pairs of spinal nerves are connected to the spinal cord– Numbered according to the portion of the vertebral

column at which they exit• 8 cervical nerve pairs• 12 thoracic nerve pairs• 5 lumbar nerve pairs• 5 sacral nerve pairs• 1 coccygeal nerve pair

– All spinal nerves carry both motor and sensory fibers so they are designated as mixed nerves

Structure of Spinal Nerves• Ventral and dorsal roots join to form spinal

nerves• Each spinal nerve branches into a dorsal

ramus and ventral ramus. (plural = rami)

Nerve Plexuses• The ventral rami subdivide to form complex

networks call plexuses– All spinal nerves except T2-T12

• Each nerve that emerges from a plexus contains all the nerve fibers that innervate a particular region of the body

• Four major subdivisions (Table 14-1):– Cervical plexus (C1-C4)– Brachial plexus (C5-T1)– Lumbosacral plexus (L1-S4)– Coccygeal plexus (S5 & coccygeal nerve)

Dermatomes & Myotomes

Dermatomes• Dermatome: a skin surface area supplied by

sensory fibers of a given spinal nerve

Myotome• Myotome: a skeletal muscle or group of

muscles that receives motor axons from a given spinal nerve

Cranial Nerves

Cranial Nerves• Olfactory Nerve (I)

– Sensory– Sense of smell

• Optic Nerve (II)– Sensory– vision

• Oculomotor Nerve (III)– Motor– Eye movements, regulation of pupil size

• Trochlear Nerve (IV)– Motor– Eye movements

Cranial Nerves• Trigeminal Nerve (V)

– Mixed– Sensations of head and face, proprioception– Chewing movements

• Abducens Nerve (VI)– Motor– Abduction of eye

• Facial Nerve (VII)– Mixed– Taste– Facial expressions, secretion of saliva & tears

• Vestibulocochlear Nerve (VIII)– Sensory– Balance, equilibrium, hearing

Cranial Nerves• Glossopharyngeal Nerve (IX)

– Mixed– Sensations of tongue, reflex control of blood pressure & respiration– Swallowing movements, secretion of saliva

• Vagus Nerve (X)– Mixed– Sensations and movements of organs supplied (ex: slows heart,

increases peristalsis, contracts muscles for voice production)• Accessory Nerve (XI)

– Motor– Shoulder movements, turning head movements, movement of

viscera, voice production• Hypoglossal Nerve (XII)

– Motor– Tongue movements

Cranial Nerves Acronyms

• Names:On Old Olympus’ Tiny Tops, A Friendly Viking Grew Vines And Hops

• Functional Classification:Some Say ‘Marry Money’; But My Brother Says ‘Bad Business, Marry Money’

Reflexes

• A reflex is the action that results from a nerve impulse passing over a reflex arc– A predictable response to a stimulus– Conscious or unconscious– Somatic Reflexes• Skeletal muscle contraction

– Autonomic Reflexes• Contraction of smooth or cardiac muscle; glandular

secretion

Somatic Reflexes of Clinical Importance

• In certain diseases or after trauma to the nervous system certain reflexes may be abnormal

• Testing of reflexes is a valuable diagnostic tool

Stretch Reflexes

• Knee jerk or patellar reflex– Extension of lower leg in response to tapping

patellar tendon– L2-L4 segments of the spinal cord

• Ankle jerk or Achilles reflex– Plantar flexion of the foot in response to tapping

the Achilles tendon– S1-S2 segments of the spinal cord

Stretch Reflexes

• Biceps Reflex– Flexion at the elbow in response to tapping the

brachii tendon– C5-C6 segments of the spinal cord

• Triceps Reflex– Extension at the elbow in response to tapping

proximal to the elbow– C6-C7 segments of the spinal cord

Cutaneous Reflexes• Cutaneous reflexes result from stimulation of the

skin receptors• Babinski Reflex– Reflex in response to stimulation of the outer portion of

the sole of the foot (make a ‘J’ from the heel along the lateral edge through the ball of the foot)

– Infant (to 1 ½ yrs): extension and fanning of toes– Children & adults: plantar flexion– Change in response due to corticospinal tract becoming

fully myelinated– A + babinski’s in an adult means destruction to the

corticospinal tract

Cutaneous Reflexes

• Abdominal Reflex– Drawing in of the abdominal wall in response to

stroking the side of the abdomen– T9-T12 spinal nerves and segments of the spinal

cord

Quiz yourself on a sheet of paperMatch the following cranial nerve with the correct

function (write it out)• Olfactory A. taste, facial expression• Vagus B. hearing, balance• Facial C. sensation of head/face;

chewing movements

• Vestibulocochlear D. sense of smell• Trigeminal E. sensation/movement of

organs

Quiz yourself on a sheet of paper Warm Up:1. What are the 3 parts of the diencephalon? Name one

function for each part.2. Differentiate between a dermatome and myotome.3. What are the 5 sections of spinal nerves? How many

spinal nerve pairs are in each section?