Chapter 21 – Peripheral Nervous System (BIO 105)

Question-and-answer flashcards covering spinal nerves, plexuses, dermatomes, cranial nerves, and key somatic reflexes from Chapter 21 lecture notes.

How many pairs of spinal nerves exit the vertebral column?

31 pairs

Why are spinal nerves referred to as “mixed” nerves?

Because each carries both afferent (sensory) fibers to the spinal cord and efferent (motor) fibers away from it.

Through which opening do spinal nerves leave the vertebral canal?

The intervertebral foramen.

Give the breakdown of spinal‐nerve pairs by region (cervical, thoracic, lumbar, sacral, coccygeal).

8 cervical (C1–C8), 12 thoracic (T1–T12), 5 lumbar (L1–L5), 5 sacral (S1–S5), 1 coccygeal (Co1).

Where is spinal nerve C1 located?

Between the occipital bone and the atlas (C1 vertebra).

Between which vertebrae does spinal nerve C8 emerge?

Between C7 and T1.

What is a nerve plexus?

A complex network of ventral rami that merge and re‐split so damage to one spinal nerve does not fully paralyze a region.

What is the cauda equina?

The bundle of lumbar, sacral, and coccygeal nerve roots that extend past the spinal cord’s end (conus medullaris) around L2.

Which spinal-cord root carries motor neurons?

Ventral (anterior) root.

Which spinal-cord root carries sensory neurons and contains a ganglion?

Dorsal (posterior) root; the dorsal root ganglion houses sensory cell bodies.

What does the dorsal ramus innervate?

Skin and muscles of the posterior head, neck, and trunk (somatic motor and sensory).

List the fibers carried by a ventral ramus.

Somatic motor, somatic sensory, and autonomic (sympathetic) fibers.

Name the four major nerve plexuses.

Cervical, Brachial, Lumbar, Sacral/Coccygeal.

Which spinal‐nerve rami form the cervical plexus?

C1–C4 and a branch of C5.

What is the chief nerve of the cervical plexus and its function?

Phrenic nerve (C3–C5); motor supply to the diaphragm (“C3, 4, and 5 keep the diaphragm alive”).

Which rami create the brachial plexus?

C5–T1 ventral rami.

What general body region is served by the brachial plexus?

Shoulders and upper limbs (motor and sensory).

List the five terminal branches of the brachial plexus.

Axillary, Musculocutaneous, Radial, Median, Ulnar nerves.

Axillary nerve: give key motor and sensory targets.

Motor to deltoid & teres minor; sensory to skin over deltoid/superior posterior arm.

Musculocutaneous nerve: primary motor supply?

Coracobrachialis, biceps brachii, brachialis.

Radial nerve: major motor functions?

Triceps brachii, anconeus, brachioradialis, and extensor muscles of forearm.

Median nerve: what forearm muscles does it NOT supply?

Flexor carpi ulnaris (and medial half of flexor digitorum profundus).

Ulnar nerve: which digits receive its sensory innervation?

Digits 4 and 5 and the anteromedial forearm.

Which plexus supplies the quadriceps via the femoral nerve?

Lumbar plexus (L2–L4).

What is the largest nerve of the body and from which plexus does it arise?

Sciatic nerve; arises from the sacral plexus (L4–S3).

Name the two divisions of the sciatic nerve.

Common peroneal (fibular) and tibial nerves.

Which nerve supplies sensation to the dorsum of the foot?

Common peroneal (fibular) division of the sciatic nerve.

Define a dermatome.

A skin area supplied by sensory fibers of a single spinal nerve root.

What dermatome corresponds to the tip of the middle finger?

C7.

Which dermatome is at the umbilicus level?

T10.

Give the dermatome for the medial ankle.

L4.

Which dermatome covers the posterior-lateral leg?

S1.

How many pairs of cranial nerves exist?

Twelve pairs.

Provide the mnemonic for cranial‐nerve names.

“On Old Olympus’ Towering Tops A Friendly Viking Grows Vines And Hops.”

Provide the mnemonic for cranial‐nerve functions (sensory/motor/both).

“Some Say Marry Money But My Brother Says Big Business Matters More.”

Cranial Nerve I and its primary function?

Olfactory nerve; sense of smell.

Which cranial nerve constricts the pupil?

Oculomotor (III).

Which cranial nerve supplies facial expression muscles?

Facial nerve (VII).

What are the two functional parts of Cranial Nerve VIII?

Vestibular (balance) and Cochlear (hearing).

Which cranial nerve slows heart rate via parasympathetic fibers?

Vagus nerve (X).

What is the primary action tested by Cranial Nerve XI?

Shoulder shrug via trapezius and sternocleidomastoid (accessory nerve).

Which neurotransmitter stimulates skeletal muscle in the somatic motor pathway?

Acetylcholine.

Define a reflex.

A predictable, involuntary response resulting from impulses passing over a reflex arc.

Where is the integration center for a spinal reflex located?

In gray matter of the spinal cord.

What clinical reflex indicates corticospinal tract damage if present in adults?

Positive Babinski sign (extension of great toe during plantar reflex).

Which nerves mediate the corneal blink reflex?

Sensory: Ophthalmic branch of Trigeminal (CN V); Motor: Facial (CN VII).

Absence of the abdominal reflex may suggest what?

An upper motor neuron lesion (though it can be absent in pregnancy or without pathology).