Peripheral Nervous System: Cranial and Spinal Nerves, Plexuses, and Reflexes

Cranial Nerves

Overview of Cranial Nerves

The cranial nerves are a set of twelve paired nerves that arise directly from the brain, primarily supplying the head and neck regions (except the vagus nerve, which extends to thoracic and abdominal organs).

• 12 pairs of cranial nerves, numbered I to XII in a rostral to caudal sequence using Roman numerals.

• Names reflect their structure or function.

• Types:

  • Sensory (S): Cell bodies in sense organs or cranial sensory ganglia.

  • Motor (M): Cell bodies in brainstem nuclei.

  • Both (B): Most cranial nerves are mixed, carrying both sensory and motor fibers.

Spinal Nerves

General Features of Spinal Nerves

Spinal nerves are mixed nerves that emerge from the spinal cord and supply all body regions except the head and part of the neck.

• 31 pairs of spinal nerves:

  • 8 pairs of cervical nerves (C1–C8)

  • 12 pairs of thoracic nerves (T1–T12)

  • 5 pairs of lumbar nerves (L1–L5)

  • 5 pairs of sacral nerves (S1–S5)

  • 1 pair of coccygeal nerves (Co1)

• Named for the point of issue from the spinal cord.

• Each spinal nerve is connected to the spinal cord via two roots:

  • Ventral roots: Contain motor (efferent) fibers from anterior horn motor neurons.

  • Dorsal roots: Contain sensory (afferent) fibers from sensory neurons in dorsal root ganglia.

Spinal Nerve Numbering and Exit Points

• 7 cervical vertebrae give rise to 8 pairs of cervical spinal nerves.

• First 7 pairs (C1–C7) exit superior to their corresponding vertebrae; C8 exits inferior to C7.

• All other spinal nerves exit inferior to the vertebra for which they are named.

Innervation of Specific Body Regions

Spinal Nerve Branches and Plexuses

Spinal nerve rami and their branches supply somatic regions of the body. The ventral rami (except T2–T12) form nerve plexuses in cervical, brachial, lumbar, and sacral regions.

• Nerve plexuses: Interlacing networks of nerves formed by ventral rami.

• Fibers from different spinal nerves crisscross and are redistributed, so each branch contains fibers from several spinal nerves.

• Damage to one spinal nerve does not cause total paralysis of a limb.

Cervical Plexus and the Neck

• Formed by ventral rami of C1–C4.

• Major nerve: Phrenic nerve (C3–C5), which innervates the diaphragm and is essential for breathing.

Anterolateral Thorax and Abdominal Wall

• Ventral rami of T1–T12 are intercostal nerves (no plexus), supplying muscles of ribs, thorax, and abdominal wall.

• Give off cutaneous branches to skin along their course.

Brachial Plexus and Upper Limb

• Formed by ventral rami of C5–C8 and T1.

• Major terminal branches:

  • Axillary: Innervates deltoid and teres minor.

  • Musculocutaneous: Innervates biceps and more.

  • Median: Innervates most flexors, forearm pronators, wrist and finger flexors, thumb opposition muscles.

  • Ulnar: Supplies flexor carpi ulnaris, part of flexor digitorum profundus, most intrinsic hand muscles, skin of medial hand.

  • Radial: Innervates essentially all extensor muscles, supinators, and posterior skin of limb.

Major terminal branches (peripheral nerves)	Cords	Divisions	Trunks	Roots (ventral rami)
Musculocutaneous	Lateral	Anterior	Upper	C5, C6
Median	Lateral/Medial	Anterior	Upper/Middle	C5–T1
Ulnar	Medial	Anterior	Lower	C8, T1
Radial	Posterior	Posterior	All	C5–T1
Axillary	Posterior	Posterior	Upper	C5, C6

Lumbar Plexus

• Arises from L1 to L4.

• Femoral nerve: Innervates quadriceps and skin of anterior thigh and medial surface of leg.

• Obturator nerve: Passes through obturator foramen to innervate adductor muscles.

Sacral Plexus

• Arises from L4 to S4.

• Serves buttock, lower limb, pelvic structures, and perineum.

• Sciatic nerve: Longest and thickest nerve of the body, composed of tibial and common fibular nerves; innervates hamstring muscles, adductor magnus, and most muscles in leg and foot.

Innervation of Skin: Dermatomes

• Dermatome: Area of skin innervated by cutaneous branches of a single spinal nerve.

• All spinal nerves except C1 participate in dermatomes.

• Most dermatomes overlap; destruction of a single spinal nerve will not cause complete numbness.

Innervation of Joints

• Hilton's law: Any nerve serving a muscle that produces movement at a joint also innervates that joint and the skin over it.

Reflexes

General Features of Reflexes

Reflexes are rapid, predictable motor responses to stimuli. They can be intrinsic (inborn) or learned (acquired).

• Intrinsic reflexes: Inborn, automatic responses (e.g., withdrawal from pain).

• Learned reflexes: Acquired through experience (e.g., driving).

Reflex Arc

The reflex arc is the neural pathway that mediates a reflex action.

1. Receptor: Site of stimulus action.

2. Sensory neuron: Transmits afferent impulses to CNS.

3. Integration center: May be monosynaptic or polysynaptic region within CNS.

4. Motor neuron: Conducts efferent impulses from integration center to effector organ.

5. Effector: Muscle fiber or gland cell that responds to efferent impulses.

Spinal Reflexes: Somatic Reflexes

• Occur without direct involvement of higher brain centers.

• Brain is advised of reflex activity and can influence reflexes.

• Stretch and tendon reflexes: Help maintain skeletal muscle activity.

• Key sensory receptors:

  • Muscle spindles: Detect muscle length.

  • Tendon organs: Detect muscle tension.

Stretch Reflex: Muscle Spindle

• Muscle spindles are composed of 3–10 modified skeletal muscle fibers (intrafusal fibers) enclosed in a connective tissue capsule.

• Central regions of intrafusal fibers lack myofilaments and serve as receptive surfaces.

• Regular effector fibers are called extrafusal muscle fibers.

• Two types of afferent endings in muscle spindle send sensory inputs to CNS:

  • Annulospiral endings: Primary sensory endings, wrap around center of spindle.

  • Flower spray endings: Secondary sensory endings, stimulated by degree of stretch only.

Example: Knee-Jerk Reflex

• Stretch reflex that keeps knees from buckling when standing upright.

• Helps maintain muscle tone in large postural muscles and adjusts reflexively.

• All stretch reflexes are monosynaptic and ipsilateral (motor activity is on same side of body).

• Positive reflex reactions provide information about muscle and nerve connections between muscle and spinal cord.

Tendon Reflex

• Involves polysynaptic reflexes.

• Helps prevent damage due to excessive stretch.

• Important for smooth onset and termination of muscle contraction.

Flexor Reflex and Crossed Extensor Reflex

• Flexor (withdrawal) reflex: Initiated by painful stimulus; causes automatic withdrawal of threatened body part.

• Is ipsilateral and polysynaptic.

• Brain can override reflex (e.g., during blood tests).

• Crossed extensor reflex: Occurs with flexor reflexes in weight-bearing limbs to maintain balance; consists of ipsilateral withdrawal reflex and contralateral extensor reflex.

• Example: Stepping barefoot on broken glass causes damaged leg to withdraw and opposite leg to extend to support weight.

Superficial Reflexes: Cutaneous Stimulation

• Plantar reflex: Tests integrity of spinal cord from L4–S2.

  • Test: Stroke lateral aspect of sole of foot.

  • Normal response: Downward flexion of toes.

  • Damage to motor cortex or corticospinal tracts causes Babinski's sign (dorsiflexion of big toe and fanning of other toes).

• Abdominal reflex: Tests integrity of cord T8–T12.

  • Test: Stroke skin of lateral abdomen above, below umbilical region.

  • Response: Contraction of abdominal muscles and movement toward stimulus.

  • Absence indicates lesion in corticospinal tract.

Additional info: These notes cover the structure and function of peripheral nerves, their organization into plexuses, and the mechanisms of somatic reflexes, which are essential for understanding the nervous system in Anatomy & Physiology.