The Department of Human anatomy

Peripheral nervous system.The Cranial Nerves Part - 1

Plan

The peripheral nervous system- Structural Organization

spinal nerves cranial nerves ganglia

The peripheral nervous system (PNS) consists of the nerves and ganglia outside of the brain and spinal cord. The main function of the PNS is to connect the central nervous system (CNS) to the limbs and organs. Unlike the CNS, the PNS is not protected by the bone of spine and skull, or by the blood–brain barrier, leaving it exposed to toxins and mechanical injuries. The peripheral nervous system is divided into the somatic nervous system and the autonomic nervous system.

Nervous System: Structural Organization

Structural subdivisions of the nervous system: Central nervous system (CNS)

brain and spinal cord Peripheral nervous system (PNS)

cranial nerves (nerves that extend from the brain) spinal nerves (nerves that extend from the spinal

cord) ganglia (clusters of neuron cell bodies (somas)

located outside the CNS)

Functional Organization of the PNS

Figure 14.1

Spinal Nerves 31 pairs – contain thousands of nerve

fibers Connect to the spinal cord

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)

Spinal Nerves Posterior View

Figure 14.9

Spinal Nerves Posterior root– contains

sensory fibers Anterior root– contains

motor fibers Just outside the spinal

cord there is a spinal ganglion consisting of nerve cells

Anterior and posterior roots join to form mixed spinal nerve

Outside the intervertebral foramen the spinal nerve divides into :1. Ramus communicans (white and gray)2. Ramus dorsalis3. Ramus ventralis

4. Ramus meningeus

Innervation of the Skin: Dermatomes Dermatome – an area of skin

Innervated by cutaneous branches of a single spinal nerve

Upper limb – skin is supplied by nerves of the brachial plexus

Lower limb Lumbar nerves – anterior surfaceSacral nerves – posterior surface

Map of Dermatomes – Anterior View

Figure 14.17a

Map of Dermatomes – Posterior View

Figure 14.17b

The plexusesForms by ventral rami

Cervical plexus Brachial plexus Lumbar plexus Sacral plexus Coccygeal plexus

Thoracic ventral rami do not form nerve plexuses

Cervical plexus (C1-C4) innervates the muscles and skin of the neck and shoulder

most important:Its phrenic nerve* (C3-C5) is the sole motor supply of diaphragm: one reason why neck injuries are so dangerous – can be lethal (respiratory arrest = stop breathing)

Brachial plexus Serves upper limbs

and shoulder girdle Arises primarily from

C5-T1 Main nerves:

Musculocutaneous – to arm flexors

Median – anterior forearm muscles and lateral palm

Ulnar – anteromedial muscles of forearm and medial hand

Axillary – to deltoid and teres minor

Radial – to posterior part of limb

Musculo-cutaneous

Median

Ulnar

Axillary

Radial

Lumbar plexus

L1-L4 Lies within the psoas major muscle Innervates anterior and medial muscles of

thigh through femoral and obturator nerves respectively

Femoral nerve also innervates skin on anterior thigh (including quads) and medial leg

Sacral plexus L4-S4 Supplies muscles

and skin of posterior thigh and almost all of the leg

Main branch is the large sciatic nerve,

Coccygeal plexus

This plexus is formed by the fifth sacral nerve (with a contribution from S4) and the coccygeal nerve. It gives rise to the anococcygeal nerve (provides sensory innervation to the skin over the coccyx)

Names of cranial nerves

Ⅰ Olfactory nerve Ⅱ Optic nerve Ⅲ Oculomotor nerve Ⅳ Trochlear nerve Ⅴ Trigeminal nerve Ⅵ Abducent nerve Ⅶ Facial nerve Ⅷ Vestibulocochlear nerve Ⅸ Glossopharyngeal nerve Ⅹ Vagus nerve Ⅺ Accessory nerve Ⅻ Hypoglossal nerve

Classification of cranial nerves Sensory cranial nerves: contain only afferent (sensory) fibers

ⅠOlfactory nerve ⅡOptic nerve Ⅷ Vestibulocochlear nerve

Motor cranial nerves: contain only efferent (motor) fibers Ⅲ Oculomotor nerve Ⅳ Trochlear nerve ⅥAbducent nerve Ⅺ Accessory nerv Ⅻ Hypoglossal nerve

Mixed nerves: contain both sensory and motor fibers--- ⅤTrigeminal nerve, Ⅶ Facial nerve, ⅨGlossopharyngeal nerve ⅩVagus nerve

Cranial Nerve I: Olfactory Arises from the olfactory epithelium Passes through the cribriform plate of the ethmoid bone Fibers run through the olfactory bulb and terminate in the

primary olfactory cortex (uncus) Functions solely by carrying afferent impulses for the sense of

smell Lesions result in ANOSMIA

Cranial Nerve II: Optic Arises from the retina of the

eye Optic nerves pass through the

optic canals and converge at the optic chiasm

They continue to the thalamus where they synapse

From there, the optic radiation fibers run to the visual cortex

Functions solely by carrying afferent impulses for vision

Cranial Nerve III: Oculomotor Fibers extend from the ventral midbrain, pass

through the superior orbital fissure, and go to the extrinsic eye muscles

Functions in raising the eyelid, directing the eyeball, constricting the iris, and controlling lens shape

The latter 2 functions are parasympathetically controlled

Parasympathetic cell bodies are in the ciliary ganglia

Cranial Nerve III: Oculomotor

Figure III from Table 13.2

Cranial Nerve IV: Trochlear

Fibers emerge from the dorsal midbrain and enter the orbits via the superior orbital fissures; innervate the superior oblique muscle

Primarily a motor nerve that directs the eyeball

Cranial Nerve IV: Trochlear

Figure IV from Table 13.2

Cranial Nerve V: Trigeminal Composed of three divisions

Ophthalmic (V1) Maxillary (V2) Mandibular (V3)

Fibers run from the face to the pons via the superior orbital fissure (V1), the foramen rotundum (V2), and the foramen ovale (V3)

Ophthalmic nerve Sensory from the eyes,

conjunctiva and orbital contents including the lacrimal gland

Maxillary nerve Sensory branches from

anterior and middle cranial fossa, nasopharynx, palate, nasal cavity, teeth of the upper jaw, maxillary sinus, skin of the side of the nose, lower eyelid, cheek, upper lip

Mandibular nerve Motor fibers innervate

muscles of mastication, tensor tympani, anterior belly of digastric

Sensory fibres from the skin of the lower face, cheek, lower lip, ear, external auditory meatus and temporal region, anterior two thirds of the tongue, teeth of the lower jaw, mastoid air cells, mucous membrane and dura in the middle cranial fossa

Lesion involves loss of sensation, weakness in chewing, Jaw deviation towards the affected side

(Tic douloureux) or trigeminal neuralgia

- Most excruciating pain known

- Caused by inflammation of nerve

- In severe cases, nerve is cut; relieves agony but results in loss of sensation on that side of the face

Cranial Nerve VI: Abducens Fibers leave the inferior pons and enter the orbit

via the superior orbital fissure Primarily a motor nerve innervating the lateral

rectus muscle (abducts the eye; thus the name abducens)

Cranial Nerve VII: Facial

Fibers leave the pons, travel through the internal acoustic meatus, and emerge through the stylomastoid foramen to the lateral aspect of the face

Motor functions include; Facial expression Transmittal of parasympathetic impulses to

lacrimal and salivary glands (submandibular and sublingual glands)

Sensory function is taste from taste buds of anterior two-thirds of the tongue

Cranial Nerve VII: Facial

Figure VII from Table 13.2

Facial Nerve (CN VII) Bell’s palsy: paralysis of facial

muscles on affected side and loss of taste sensation

Caused by herpes simplex I virus Lower eyelid droops Corner of mouth sags Tears drip continuously and eye

cannot be completely closed (dry eye may occur)

Condition may disappear spontaneously without treatment

Cranial Nerve VIII: Vestibulocochlear Fibers arise from the hearing and equilibrium

apparatus of the inner ear, pass through the internal acoustic meatus, and enter the brainstem at the pons-medulla border

Two divisions – cochlear (hearing) and vestibular (balance)

Functions are solely sensory – equilibrium and hearing

Cranial Nerve VIII: Vestibulocochlear

Figure VIII from Table 13.2

Cranial Nerve IX: Glossopharyngeal Fibers emerge from the medulla, leave the skull

via the jugular foramen, and run to the throat Nerve IX is a mixed nerve with motor and

sensory functions Motor – innervates posterior 1/3 of the tongue

and pharynx, and provides motor fibers to the parotid salivary gland

Sensory – fibers conduct taste and general sensory impulses from the tongue and pharynx

Cranial Nerve IX: Glossopharyngeal

Figure IX from Table 13.2

Cranial Nerve X: Vagus

The only cranial nerve that extends beyond the head and neck

Fibers emerge from the medulla via the jugular foramen

The vagus is a mixed nerve Most motor fibers are parasympathetic fibers to

the heart, lungs, and visceral organs Paralysis leads to hoarseness Total destruction incompatible with life

Cranial Nerve X: Vagus

Cranial Nerve XI: Accessory Formed from a cranial root emerging from the

medulla and a spinal root arising from the superior region of the spinal cord

The spinal root passes upward into the cranium via the foramen magnum

The accessory nerve leaves the cranium via the jugular foramen Innervates the trapezius and sternocleidomastoid,

which move the head and neck

Cranial Nerve XI: Accessory

Figure XI from Table 13.2

Cranial Nerve XII: Hypoglossal

Fibers arise from the medulla and exit the skull via the hypoglossal canal

Innervates both extrinsic and intrinsic muscles of the tongue, which contribute to swallowing and speech

If damaged, difficulties in speech and swallowing; inability to protrude tongue

Cranial Nerve XII: Hypoglossal

Figure XII from Table 13.2