Cranial Nerves - Peripheral Nervous System Part A

Peripheral Nervous System (PNS)

• The PNS includes all neural structures outside the brain and spinal cord.
• It includes sensory receptors, peripheral nerves, associated ganglia, and motor endings.
• The PNS provides links to and from the external environment.

Structure of a Nerve

• A nerve is a cordlike organ of the PNS, consisting of peripheral axons enclosed by connective tissue.
• Connective tissue coverings in a nerve:
  • Endoneurium: loose connective tissue surrounding axons.
  • Perineurium: coarse connective tissue bundling fibers into fascicles.
  • Epineurium: tough fibrous sheath around a nerve.

Classification of Nerves

• Nerves are classified based on their function:
  • Sensory or afferent: carry impulses to the CNS.
  • Motor or efferent: carry impulses from the CNS.
  • Mixed: carry sensory and motor fibers to and from the CNS; most common type of nerve.

Peripheral Nerves

• Mixed nerves carry both somatic and autonomic (visceral) impulses.
• Types of mixed nerves:
  • Somatic afferent and somatic efferent: involved in voluntary movement and sensory information.
  • Visceral afferent and visceral efferent: involved in involuntary bodily functions and sensory information from internal organs.
• Peripheral nerves originate from the brain or spinal column.

Cranial Nerves

• Twelve pairs of cranial nerves arise from the brain.
• Functions: sensory, motor, or both.
• Each nerve is identified by a number (I through XII) and a name.
• Four cranial nerves carry parasympathetic fibers that serve muscles and glands.

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.
• Sole function: carrying afferent impulses for the sense of smell.

Cranial Nerve II: Optic

• Arises from the retina of the eye.
• Optie nerves pass through the optic canals and converge at the optic chiasm, the X-shaped structure formed at the point below the brain where the two optic nerves cross over each other.
• They continue to the thalamus where they synapse.
• From there, the optic radiation fibers run to the visual cortex.
• Sole function: 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:
  • Raising the eyelid.
  • Directing the eyeball.
  • Constriction of the iris.
  • Controlling lens shape.
• Parasympathetic cell bodies are in the ciliary ganglia.

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 V: Trigeminal

• Composed of three divisions: ophthalmic (V1), maxillary (V2), and mandibular (V3).
• Fibers run from the face to the pons via the:
  • Superior orbital fissure (V1).
  • Foramen rotundum (V2).
  • Foramen ovale (V3).
• Conveying sensory impulses from various areas of the face (V1) and (V2), and supplies motor fibers (V3) for mastication.

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, which allows for outward movement of the eye.

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.
• Mixed nerve with five major branches.
• Motor functions:
  • Facial expression.
  • Transmittal of autonomic impulses to lacrimal and salivary glands.
• Sensory function: taste from the anterior two-thirds of the tongue.

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).
• Solely sensory: equilibrium and hearing.

Cranial Nerve IX: Glossopharyngeal

• Fibers emerge from the medulla, leave the skull via the jugular foramen, and run to the throat.
• Mixed nerve with motor and sensory functions.
• Motor:
  • Innervates part of the tongue and pharynx.
  • Provides motor fibers to the parotid salivary gland.
• Sensory:
  • Fibers conduct taste and general sensory impulses from the tongue and pharynx.

Cranial Nerve X: Vagus

• The only cranial nerve that extends beyond the head and neck.
• Fibers emerge from the medulla via the jugular foramen.
• Mixed nerve.
• Most motor fibers are parasympathetic fibers to the heart, lungs, and visceral organs.
• Sensory function: taste.

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.
• Leaves the cranium via the jugular foramen.
• Primarily a motor nerve:
  • Supplies fibers to the larynx, pharynx, and soft palate.
  • Innervates the trapezius and sternocleidomastoid, which move the head and neck.

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.

Peripheral Nervous System (PNS)

• The PNS includes all nerve structures outside of the brain and spinal cord.
• It is composed of sensory receptors, peripheral nerves, associated ganglia, and motor endings.
• The PNS transmits information to and from the external environment.

Structure of a Nerve

• A nerve is a cord-like organ of the PNS composed of peripheral axons enclosed in connective tissue.
• Connective tissue coverings include the endoneurium (surrounding axons), perineurium (bundling fibers into fascicles), and epineurium (tough fibrous sheath around the nerve).

Classification of Nerves

• Nerves are classified based on their functions:
  • Sensory (afferent): Carry impulses to the CNS.
  • Motor (efferent): Carry impulses from the CNS.
  • Mixed: Carry both sensory and motor fibers, the most common type.

Peripheral Nerves

• Peripheral nerves are mixed nerves that carry both somatic and autonomic (visceral) impulses. There are four types: somatic afferent, somatic efferent, visceral afferent, and visceral efferent.
• Peripheral nerves originate from the brain or spinal column.

Cranial Nerves

• There are twelve pairs of cranial nerves arising from the brain.
• Each cranial nerve has a unique number (I-XII) and name.
• These nerves can have sensory, motor, or both sensory and motor functions.
• Four of the cranial nerves carry parasympathetic fibers that serve muscles and glands.

Cranial Nerve I: Olfactory

• This nerve originates from the olfactory epithelium and passes through the cribriform plate of the ethmoid bone.
• Its fibers travel through the olfactory bulb and terminate in the primary olfactory cortex.
• This nerve solely carries afferent impulses for the sense of smell.

Cranial Nerve II: Optic

• This nerve originates from the retina of the eye.
• The optic nerves pass through the optic canals and converge at the optic chiasm, where they cross over each other.
• They continue to the thalamus where they synapse, and then the optic radiation fibers run to the visual cortex.
• This nerve is solely responsible for carrying afferent impulses for vision.

Cranial Nerve III: Oculomotor

• Fibers extend from the ventral midbrain, passing through the superior orbital fissure to the extrinsic eye muscles.
• This nerve controls the raising of the eyelid, directing the eyeball, constricting the iris, and adjusting lens shape.
• Parasympathetic cell bodies are located in the ciliary ganglia.

Cranial Nerve IV: Trochlear

• Fibers emerge from the dorsal midbrain and enter the orbits via the superior orbital fissures, innervating the superior oblique muscle.
• Primarily a motor nerve that directs the eyeball.

Cranial Nerve V: Trigeminal

• Composed of three divisions: ophthalmic (V1), maxillary (V2), and 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).
• This nerve is responsible for conveying sensory impulses from different areas of the face (V1, V2) and provides motor fibers (V3) for mastication.

Cranial Nerve VI: Abducens

• Fibers leave the inferior pons and enter the orbit via the superior orbital fissure.
• This is primarily a motor nerve that innervates the lateral rectus muscle.

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.
• This nerve is a mixed nerve with five major branches.
• Motor functions include facial expression and transmitting autonomic impulses to lacrimal and salivary glands.
• Sensory function includes taste from the anterior two-thirds of the tongue.

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.
• It has two divisions: cochlear (hearing) and vestibular (balance).
• Its sole functions are sensory, responsible for equilibrium and hearing.

Cranial Nerve IX: Glossopharyngeal

• Fibers emerge from the medulla, leave the skull via the jugular foramen, and run to the throat.
• This nerve is a mixed nerve with motor and sensory functions.
• Motor functions include innervating part of the tongue and pharynx, and providing motor fibers to the parotid salivary gland.
• Sensory functions include conducting taste and general sensory impulses from the tongue and pharynx.

Cranial Nerve X: Vagus

• This is the only cranial nerve that extends beyond the head and neck.
• Fibers emerge from the medulla via the jugular foramen.
• This nerve is a mixed nerve.
• Most motor fibers are parasympathetic fibers to the heart, lungs, and visceral organs.
• Its sensory function is in taste.

Cranial Nerve XI: Accessory

• This nerve is 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.
• Primarily a motor nerve, it supplies fibers to the larynx, pharynx, and soft palate.
• It also innervates the trapezius and sternocleidomastoid muscles, which move the head and neck.

Cranial Nerve XII: Hypoglossal

• Fibers arise from the medulla and exit the skull via the hypoglossal canal.
• This nerve innervates the extrinsic and intrinsic muscles of the tongue, important for swallowing and speech.

Spinal Nerves

• Thirty-one pairs of mixed nerves arise from the spinal cord and supply all parts of the body except the head
• They are named according to their point of issue
  • 8 cervical (C1-C8)
  • 12 thoracic (T1-T12)
  • 5 Lumbar (L1-L5)
  • 5 Sacral (S1-S5)
  • 1 Coccygeal (C0)
• Each spinal nerve connects to the spinal cord via two medial roots
• Each root forms a series of rootlets that attach to the spinal cord
• Ventral roots arise from the anterior horn and contain motor (efferent) fibers
• Dorsal roots arise from sensory neurons in the dorsal root ganglion and contain sensory (afferent) fibers

Nerve Plexuses

• All ventral rami except T2-T12 form interlacing nerve networks called plexuses
• Plexuses are found in the cervical, brachial, lumbar, and sacral regions
• Each resulting branch of a plexus contains fibers from several spinal nerves
• Fibers travel to the periphery via several different routes
• Each muscle receives a nerve supply from more than one spinal nerve
• Damage to one spinal segment cannot completely paralyze a muscle

Spinal Nerve Innervation

• The back is innervated by dorsal rami via several branches
• The thorax is innervated by ventral rami T1-T12 as intercostal nerves
• Intercostal nerves supply muscles of the ribs, anterolateral thorax, and abdominal wall

Cervical Plexus

• The cervical plexus is formed by ventral rami of C1-C4
• Most branches are cutaneous nerves of the neck, ear, back of head, and shoulders
• The most important nerve of this plexus is the phrenic nerve
• The phrenic nerve is the major motor and sensory nerve of the diaphragm

Brachial Plexus

• Formed by C5-C8 and T1 (C4 and T2 may also contribute to this plexus)
• It gives rise to the nerves that innervate the upper limb
• There are four major branches of this plexus
  • Roots – five ventral rami (C5-T1)
  • Trunks – upper, middle, and lower, which form divisions
  • Divisions – anterior and posterior serve the front and back of the limb
  • Cords – lateral, medial, and posterior fiber bundles

Brachial Plexus: Nerves

• Axillary – innervates the deltoid and teres minor
• Musculocutaneous – sends fibers to the biceps brachii and brachialis
• Median – branches to most of the flexor muscles of arm
• Ulnar – supplies the flexor carpi ulnaris and part of the flexor digitorum profundus
• Radial – innervates essentially all extensor muscles

Lumbar Plexus

• Arises from L1-L4 and innervates the thigh, abdominal wall, and related muscles
• The major nerves are the femoral and the obturator

Sacral Plexus

• Arises from L4-S4 and serves the buttock, lower limb, pelvic structures, and the related sacral areas
• Major nerve is the sciatic, the longest and thickest nerve of the body
• The sciatic is actually composed of two nerves: the tibial and the common fibular nerves

Dermatomes

• A dermatome is the area of skin innervated by the cutaneous branches of a single spinal nerve
• All spinal nerves except C1 participate in dermatomes

This quiz covers essential concepts about the Peripheral Nervous System (PNS), including its structure, functions, and classification of nerves. It provides an understanding of the neural structures outside the brain and spinal cord, highlighting the components that connect the body to the external environment.