Cranial Nerves Overview

Cranial Nerves I, II, and VIII

• The olfactory nerve (I) is responsible for the sense of smell.
• The optic nerve (II) is responsible for vision. It is not a true nerve but a brain tract.
• The vestibulocochlear nerve (VIII) is responsible for hearing and equilibrium. It has a somatic motor component that regulates balance.

Cranial Nerves III, IV, VI, XI, and XII

• The oculomotor nerve (III) controls four extrinsic eye muscles, which control eye movement. It also has parasympathetic functions affecting pupil constriction and lens accommodation.
• The trochlear nerve (IV) controls one extrinsic eye muscle, the superior oblique muscle, which rotates the eye.
• The abducens nerve (VI) controls one extrinsic eye muscle, the lateral rectus muscle, which moves the eye laterally.
• The accessory nerve (XI) controls movements of the head and neck. It is primarily a somatic motor nerve.
• The hypoglossal nerve (XII) controls tongue movements. It is a somatic motor nerve.

Cranial Nerves V, VII, IX, and X

• The trigeminal nerve (V) is responsible for general sensation in the face, the mouth, and the anterior scalp. It also has a somatic motor component innervating muscles of mastication.
• The facial nerve (VII) is responsible for facial expressions. It also has somatic motor control over some muscles responsible for swallowing and stapes control. It has parasympathetic functions affecting tear production, salivary gland secretion, and taste.
• The glossopharyngeal nerve (IX) is responsible for general sensation and taste in the posterior tongue and the pharynx. It also has a somatic motor component controlling swallowing. It has parasympathetic functions affecting salivary gland secretion and taste.
• The vagus nerve (X) controls the movement of the soft palate, pharynx, and larynx. It also has parasympathetic functions, including heart rate, breathing, blood pressure, and digestive function.

Location of the Brain Stem

• The pons lies below the insula and the thalamus and above the medulla.
• The medulla oblongata lies inferior to the pons and superior to the spinal cord.

Structures in the Midbrain

• The thalamus acts as a relay center for sensory information traveling to the cerebral cortex. It is a part of the diencephalon, which also includes the hypothalamus, subthalamus, and epithalamus.
• The hypothalamus controls autonomic functions, such as temperature regulation, hunger, thirst, and sleep.
• The corpora quadrigemina, located on the posterior surface of the midbrain, are involved in visual and auditory reflexes.
• The choroid plexus produces cerebrospinal fluid (CSF). It lines the ventricles of the brain.
• The anterior commissure is a bundle of nerve fibers connecting the two hemispheres of the brain.
• The septum pellucidum is a thin membrane that separates the two lateral ventricles.
• The fornix is a bundle of white matter that connects the hippocampus to the hypothalamus.
• The mammillary bodies are part of a pathway involved in memory.

Structures in the Cerebellum

• The vermis is the narrow median portion of the cerebellum that connects the two cerebellar hemispheres.
• The arbor vitae is the white matter within the cerebellum that appears tree-like.

The Olfactory Bulb

• The olfactory bulb is located just above the cribriform plate, which is a bone in the skull. It receives signals from olfactory receptors in the nose.

Cervical Plexus

• The cervical plexus is a network of nerves that emerge from the cervical spinal cord, supplying the neck, shoulder, and diaphragm.
• Cutaneous branches (superficial):
  • The lesser occipital nerve supplies the skin on the posterolateral neck.
  • The greater auricular nerve supplies the skin of the ear and parotid gland.
  • The transverse cervical nerve supplies the skin on the anterior and lateral neck.
  • The supraclavicular nerves supply the skin of the shoulder and clavicular region.
• Motor branches (deep):
  • The ansa cervicalis, comprised of the superior and inferior roots, innervates infrahyoid muscles of the neck (omohyoid, sternohyoid, and sternothyroid).
  • Segmental and other muscular branches innervate deep muscles of the neck, including the geniohyoid and thyrohyoid. They also supply portions of the scalenes, levator scapulae, trapezius, and sternocleidomastoid muscles.
  • The phrenic nerve innervates the diaphragm.

Brachial Plexus

• The brachial plexus is a network of nerves that emerge from the cervical and thoracic spinal cord, supplying the shoulder, arm, forearm, and hand.
• Musculocutaneous nerve:
  • Supplies the anterior arm muscles (biceps brachii, brachialis, and coracobrachialis) and the skin on the lateral forearm.
• Median nerve:
  • Supplies most flexor muscles of the arm, and intrinsic muscles of the lateral palm (most thenar muscles and two lumbricals). It innervates the skin of the lateral two-thirds of the hand on the ventral side and the dorsum of fingers 2 and 3.

Brachial Plexus & Nerves

• The brachial plexus originates from spinal nerves C5-T1.
• It innervates the muscles and skin of the shoulder, arm, forearm and hand.
• The brachial plexus branches anteriorly into the medial and lateral cords, and posteriorly into the posterior cord. 
• The flexor muscles of the arm and forearm, and most intrinsic muscles of the hand are innervated by the medial cord.
• The posterior arm and forearm muscles, and extensor muscles are innervated by the posterior cord. 

Brachial Plexus - Ulnar Nerve

• Origin: C8 and T1 levels
• Innervates:
  • flexor carpi ulnaris and medial half of flexor digitorum profundus muscles in the anterior forearm
  • hypothenar muscles, all interossei, medial two lumbricals and adductor pollicis in the hand.
• Cutaneous: skin of the medial third of the hand, both anterior and posterior aspects.

Brachial Plexus - Radial Nerve

• Origin: C5 - C8 and T1 levels
• Innervates:
  • Posterior muscles of the arm and forearm, including the triceps brachii, anconeus, supinator, brachioradialis, extensors carpi radialis longus and brevis, extensor carpi ulnaris, extensor digitorum.
• Cutaneous: skin of the posterolateral surface of the entire limb, except dorsum of fingers 2 and 3. 

Brachial Plexus - Axillary Nerve

• Origin: Posterior cord of the brachial plexus, C5 and C6 levels
• Innervates: deltoid and teres minor muscles.
• Cutaneous: shoulder joint and skin covering inferior half of deltoid muscle.

Brachial Plexus - Dorsal Scapular Nerve

• Origin: Branches of C5 rami
• Innervates: rhomboid muscle and levator scapulae.

Brachial Plexus - Long Thoracic Nerve

• Origin: Branches of C5-C7 rami
• Innervates: serratus anterior muscle

Brachial Plexus - Subscapular Nerve

• Origin: Posterior cord of the brachial plexus, C5 & C6 levels
• Innervates: teres major and subscapularis muscles

Brachial Plexus - Thoracodorsal Nerve

• Origin: Branches of C6-C8 levels
• Innervates: latissimus dorsi muscle

Brachial Plexus - Suprascapular Nerve

• Origin: Upper trunk of the brachial plexus, C5 and C6 levels
• Innervates: shoulder joint, supraspinatus and infraspinatus muscles

Brachial Plexus - Pectoral Nerves

• Origin: Branches of lateral (C5-T1) and medial (C5-T1) cords of the brachial plexus
• Innervates: pectoralis major and minor muscles

Lumbar Plexus

• Origin: Ventral rami of L2-L4 spinal nerves
• Innervates: anterior compartment of the thigh:
  • quadriceps muscle
  • sartorius muscle
  • pectineus muscle
  • iliacus muscle

Sacral Plexus

• The sacral plexus is a network of nerves that arise from the ventral rami of spinal nerves L4-S4.
• The sacral plexus innervates the posterior thigh, leg, and foot, as well as the pelvis and perineum.

Nerves of the Sacral Plexus

• Sciatic Nerve:
  • The largest nerve in the body.
  • Formed by the ventral rami of L4-S3.
  • Composed of two nerves: the tibial nerve and the common fibular nerve.
  • Supplies the motor and sensory innervation of the posterior thigh, leg, and foot.
• Superior Gluteal Nerve:
  • Formed by the ventral rami of L4-L5 and S1.
  • Supplies the muscles of the gluteal region: gluteus medius, gluteus minimus, and tensor fascia latae.
• Inferior Gluteal Nerve:
  • Formed by the ventral rami of L5-S2.
  • Supplies the gluteus maximus muscle.
• Posterior Femoral Cutaneous Nerve:
  • Formed by the ventral rami of S1-S3.
  • Provides sensory innervation to the posterior thigh, popliteal region, and part of the calf and heel.
• Pudendal Nerve:
  • Formed by the ventral rami of S2-S4.
  • Provides sensory and motor innervation to the perineum, including the external genitalia and anus.,
  • Supplies muscles like the external anal sphincter.

Dermatomes

• Dermatomes are areas of skin that are innervated by sensory fibers from a single spinal nerve root.
• Dermatomes are used to assess the level of spinal nerve injury.
• Each spinal nerve root supplies a specific area of skin.

Nerve roots and dermatomes

• L4: Supplies the anterior thigh and medial aspect of the leg.
• L5: Supplies the lateral aspect of the thigh, leg, and foot.
• S1: Supplies the posterior thigh, leg, and foot.
• S2: Supplies the posterior aspect of the thigh, leg, and foot
• S3: Supplies the perineum.
• S4: Supplies the perineum.

Clinical Significance

• Sciatica: Damage to the sciatic nerve can cause pain and weakness in the leg.
• Foot drop: Damage to the common fibular nerve can cause difficulty in extending the foot and toes.
• Perineal nerve damage: Damage to the pudendal nerve can lead to problems with bladder and bowel control, as well as sexual dysfunction.