cranial nerves & blood supply (4?)

Cranial Nerves

  • Olfactory nerve I: Sensory; responsible for the sense of smell.

  • Optic nerve II: Sensory; transmits visual information from the retina to the brain.

  • Oculomotor nerve III: Motor; controls most eye movements (up, down, medial), pupil constriction, and eyelid elevation.

  • Trochlear nerve IV: Motor; innervates the superior oblique muscle, controlling downward and inward eye movements.

  • Trigeminal nerve V: Mixed; responsible for sensation in the face and motor control of mastication (chewing) muscles.

  • Abducens nerve VI: Motor; innervates the lateral rectus muscle, controlling lateral eye movement.

  • Facial nerve VII: Mixed; controls muscles of facial expression, provides taste sensation from the anterior 2/3 of the tongue, and parasympathetic innervation to salivary and lacrimal glands.

  • Vestibulocochlear nerve VIII: Sensory; responsible for hearing (cochlear part) and balance (vestibular part).

  • Glossopharyngeal nerve IX: Mixed; provides taste sensation from the posterior 1/3 of the tongue, sensation from the pharynx, and parasympathetic innervation to the parotid gland.

  • Vagus nerve X: Mixed; a major parasympathetic nerve innervating most visceral organs, also controls laryngeal and pharyngeal muscles, and taste from the epiglottis.

  • Accessory nerve XI: Motor; innervates the sternocleidomastoid and trapezius muscles.

  • Hypoglossal nerve XII: Motor; controls movements of the tongue.

Optic Nerve Anatomy

  • Left/Right optic nerves: Transmit visual information from the retina of each eye.

  • Visual fields divided: Each eye's visual field is divided into temporal (lateral) and nasal (medial) halves.

  • Optic chiasm and tracts: At the optic chiasm, the nasal fibers from each optic nerve decussate (cross over) to the contralateral side. The optic tracts then contain fibers from the ipsilateral temporal retina and the contralateral nasal retina, carrying information representing the contralateral visual field.

  • Visual cortex locations (left/right): Visual information travels via the optic tracts to the lateral geniculate nucleus of the thalamus and then projects to the primary visual cortex in the occipital lobes. The left visual cortex processes input from the right visual field, and vice-versa.

Trigeminal Nerve Components

  • Divided into three branches:

    • Ophthalmic (V1): Primarily sensory, covering the forehead, upper eyelid, cornea, and dorsum of the nose.

    • Maxillary (V2): Primarily sensory, covering the lower eyelid, upper lip, maxilla, nasal cavity, and palate.

    • Mandibular (V3): Mixed; provides sensory innervation to the lower lip, mandible, chin, and outer ear, and motor innervation to the muscles of mastication (masseter, temporalis, medial and lateral pterygoids).

  • Functions:

    • Somatic motor: Innervates the muscles essential for chewing.

    • Somatic sensory: Provides general sensation for the face, scalp, sinuses, and teeth.

    • Sensory to most of face/head

      Motor root is to muscles of mastication (masseter, pterygoid, temporalis muscles)

      Trigeminal nerve

      -3 branches come off trigeminal nerve (V1,2,3) \

      -Somatosensory info of face

      Facial nerve

      - Close proximity to trigeminal

      - Sensory to most of face

Facial Nerve Functions

  • Muscles of facial expression: Innervates all muscles responsible for moving the face (e.g., smiling, frowning, blinking).

  • Taste from anterior 2/3 of the tongue: Transmitted via the chorda tympani, a branch of the facial nerve.

  • Salivary and lacrimal glands control: Provides parasympathetic innervation to the submandibular, sublingual (salivary), and lacrimal (tear) glands.

  • Associated conditions:

    • Bell's Palsy: An idiopathic, acute facial paralysis caused by inflammation or damage to the facial nerve, typically unilateral and temporary, affecting all facial expression muscles on one side.

    • Ramsay Hunt Syndrome: Caused by reactivation of the varicella-zoster virus (shingles) within the geniculate ganglion of the facial nerve. It presents with facial paralysis, a painful vesicular rash in the ear canal or mouth, and often accompanied by hearing loss or vertigo, typically more severe and persistent than Bell's Palsy.

Key Functions of Cranial Nerves

  • Somatic motor and sensory: Direct control over skeletal muscles in the head and neck (e.g., eye, face, tongue, SCM, trapezius) and general sensation from the face and head.

  • Parasympathetic functions: Control of glandular secretions (e.g., salivary, lacrimal) and regulation of visceral organ function (e.g., heart rate, digestion via the Vagus nerve).

  • Special Sensory innervation: Transmitting specialized senses such as taste, smell, vision, hearing, and balance.

Spinal Cord Overview

  • Organized into segments:

    • Cervical (C1-C8): Innervates the neck, shoulders, arms, and hands.

    • Thoracic (T1-T{12}): Innervates the chest, upper back, and abdomen.

    • Lumbar (L1-L5): Innervates the lower back and legs.

    • Sacral (S1-S5): Innervates the pelvis, legs, and feet.

  • Contains gray matter (H-shaped):

    • Dorsal (posterior) horns: Primarily contain sensory neurons that receive afferent (incoming) signals from the body.

    • Ventral (anterior) horns: Contain motor neuron cell bodies that send efferent (outgoing) signals to skeletal muscles.

    • Lateral horns: Present in thoracic and upper lumbar segments, containing preganglionic sympathetic neuron cell bodies.

  • White matter: Surrounds the gray matter, composed of myelinated axons organized into ascending (sensory) and descending (motor) tracts.

    • Ascending tracts: Carry sensory information from the body to the brain (e.g., spinothalamic tracts for pain/temperature, dorsal columns for fine touch/proprioception).

    • Descending tracts: Carry motor commands from the brain to the spinal cord (e.g., corticospinal tracts for voluntary movement).'

  • Spinal roots: ventral root (motor output coming out) dorsal root (sensory info coming in synapses in dorsal horn) àconverge as dorsal root ganglion (where cell bodies for sensory neurons are) to form spinal nerve

    -Motor neurons are in CNS (cell bodies and dendrites) and PN (axons)

    -Sensory neurons axons in CNS and cell bodies in PNS

Spinal Nerve Injuries

  • Common issues:

    • Prolapsed (herniated) disc: Occurs when the soft nucleus pulposus bulges or ruptures through the outer annulus fibrosus, compressing nearby spinal nerves or the spinal cord itself, leading to pain, numbness, and/or weakness.

    • Osteoarthritis (spinal stenosis): Degeneration of spinal joints and discs can lead to narrowing of the spinal canal (central stenosis) or intervertebral foramina (foraminal stenosis), causing compression of the spinal cord or exiting spinal nerves.

  • Nerve injuries lead to sensory/motor deficits:

    • Sensory deficits: Manifest as numbness, tingling (paresthesia), burning pain, or diminished sensation in a dermatomal distribution (an area of skin supplied by a single spinal nerve).

      • Peripheral nerve territories represent more than a single dermatome

        Injury to peripheral nerve à sensory region of that nerve is affected but not other peripheral nerve areas from same spinal root

    • Motor deficits: Include weakness, muscle atrophy, fasciculations (muscle twitching), and altered reflexes, typically in a myotomal pattern (a group of muscles supplied by a single spinal nerve).

Blood Supply to the Brain

  • Internal carotid arteries: Arise from the common carotid arteries and supply the bulk of circulation to the brain, branching into the anterior cerebral arteries (ACA) and middle cerebral arteries (MCA).

  • Vertebral arteries: Secondary source of blood to brain. Arise from the subclavian arteries, ascend through the cervical vertebrae, and join to form the basilar artery. The basilar artery supplies circulation to the brainstem, cerebellum, and posterior cerebral arteries (PCA).

  • Form Circle of Willis: An arterial anastomosis located at the base of the brain that connects the internal carotid and vertebrobasilar systems. This crucial network provides collateral circulation, ensuring continuous blood flow to the brain even if one major artery is occluded or stenosed.

    • Anterior communicating arteries and posterior communicating arteries form the circle of willis

       will not make up for blood loss of carotid arteries because they are small

  • Connects major cerebral arteries: The Circle of Willis links the ACAs, MCAs, and PCAs via anterior and posterior communicating arteries.

  • Cerebral arteries:

  -anterior serves front part of brain

  -Middle comes off middle and serves middle portion of brain

  -Posterior comes off back and serves back part of brain

- If you have blood loss to one stricture, you might still be able to function because of backups (other sources provide blood to brain)

Motor and Sensory Functional Testing

  • ASIA scale (American Spinal Injury Association Impairment Scale): A standardized neurological assessment tool used to classify the severity and extent of spinal cord injury. It categorizes injury into five levels (A-E) based on sensory and motor function segments below the neurological level of injury.

  • Key muscles and sensory points for evaluating function:

    • Motor testing: Involves assessing the strength of specific muscles (myotomes) on a 0-5 scale (0=no contraction, 5=normal strength) at designated levels (e.g., C5 elbow flexors, L4 ankle dorsiflexors).

    • Sensory testing: Involves evaluating light touch and pain/pinprick sensation at specific dermatomal points (e.g., C6 thumb, S1 lateral heel) on a 0-2 scale (0=absent, 1=impaired, 2=normal).

  • Normal vs. impaired motor/sensory function: The scores determine the neurological level of injury and help classify the completeness of the spinal cord lesion.

Brain Blood Supply Anatomy

  • Anterior cerebral arteries (ACA): Supply the medial surfaces of the frontal and parietal lobes. Occlusion can lead to contralateral leg weakness and sensory loss, as well as behavioral changes.

  • Middle cerebral arteries (MCA): Supply the lateral surfaces of the frontal, parietal, and temporal lobes, including the primary motor and somatosensory cortices for the upper limbs and face, and important language areas (Broca's and Wernicke's areas). MCA strokes are the most common type and can result in contralateral hemiplegia/hemianesthesia (arm and face more affected than leg) and aphasia if the dominant hemisphere is involved.

  • Posterior cerebral arteries (PCA): Supply the occipital lobes (primary visual cortex), inferior temporal lobes, and parts of the thalamus. Occlusion can cause visual field deficits (e.g., contralateral homonymous hemianopsia) and memory impairments.

  • Homunculus representation for motor and sensory areas: A somatotopic map found in the cerebral cortex, depicting a distorted human body where each body part's size on the map is proportional to the amount of cortex dedicated to its motor control or sensory input.

    • Motor Homunculus: Located in the precentral gyrus (primary motor cortex), it illustrates which cortical areas control movements of specific body parts.

    • Sensory Homunculus: Located in the postcentral gyrus (primary somatosensory cortex), it shows which cortical areas receive sensory input from specific body parts. Areas like the hands, lips, and tongue have disproportionately large representations due to their highly precise motor control and rich sensory innervation.