Cranial Nerves Module 1

Cranial Nerves

• There are 12 pairs of cranial nerves that arise from the brain stem and mostly innervate the head.
• The 12 cranial nerves and their important functions are:
  • Olfactory nerve: sensation of smell
  • Optic nerve: sensation of vision
  • Oculomotor nerve: movements of the eye and eyelid, parasympathetic control of pupil size
  • Trochlear nerve: movements of the eye
  • Trigeminal nerve: sensation of touch to the face, movement of muscles of mastication (chewing)
  • Abducens nerve: movements of the eye
  • Facial nerve: movement of muscles of facial expression, sensation of taste in anterior two-thirds of the tongue
  • Auditory-Vestibular nerve: sensation of hearing and balance
  • Glossopharyngeal nerve: movement of muscles in the throat, parasympathetic control of the salivary glands, sensation of taste in posterior one-third of the tongue, detection of blood pressure changes in the aorta
  • Vagus nerve: parasympathetic control of the heart, lungs, and abdominal organs, sensation of pain associated with viscera, movement of muscles in the throat
  • Spinal Accessory nerve: movement of muscles in the throat and neck
  • Hypoglossal nerve: movement of the tongue

Cranial Nerve V: Trigeminal

• The trigeminal nerve supplies sensations to the face and other structures of the head.
• It has 3 divisions: ophthalmic, maxillary, and mandibular.
• The mandibular division is motor for the muscles of mastication, including the masseter and temporals.
• Trigeminal neuralgia is a painful condition characterized by extreme, sporadic, sudden shock-like facial pain in the areas of the face where the branches of the nerve are distributed.

Cranial Nerve VII: Facial

• The facial nerve is efferent to the muscles of facial expression, lacrimal (tear) glands, and two salivary glands.
• Bell's palsy is a form of flaccid paralysis of CN VII, characterized by sudden onset, usually unilateral, and may be due to edema, viral infections, trauma, or pressure from a tumor.
• Facial synkinesis is a condition where voluntary muscle movement causes simultaneous involuntary contraction of other facial muscles or glands.

Cranial Nerve VIII: The Vestibulocochlear Nerve

• The vestibulocochlear nerve consists of two separate nerves: the vestibular nerve and the cochlear nerve.
• The vestibular nerve is responsible for maintaining balance, while the cochlear nerve is responsible for hearing.
• The inner ear semicircular canals are filled with fluid and contain calcium carbonate crystals (otoliths) embedded in a membrane outside the semicircular canals, which are involved in the detection of various motions of the head and body.

Cranial Nerve X: Vagus

• The vagus nerve supplies motor parasympathetic fibers to all organs (except the adrenal glands) from the neck down to the second segment of the transverse colon.
• It also controls a few skeletal muscles, including the muscles of the larynx.
• The vagus nerve is responsible for various tasks such as heart rate, gastrointestinal peristalsis, sweating, and muscle movements in the mouth, including speech.

Cranial Nerve XI: Accessory

• This nerve is motor to sternocleidomastoid (SCM) and trapezius.
• Patients with acquired torticollis (wry neck) present with the head twisted to the affected side and the chin pointed toward the opposite shoulder.

The Meninges

• The meninges protect the CNS from the overlying bone and are composed of:
  • Epidural space: adipose and connective tissue filled space between the wall of the vertebral canal and the dura mater
  • Dura mater: outermost layer, a tough inelastic surrounding
  • Subdural space: interstitial fluid-filled space between the dura mater and arachnoid membrane
  • Arachnoid membrane: middle layer with a spiderweb consistency
  • Subarachnoid space: CSF-filled space between arachnoid membrane and pia mater
  • Pia mater: innermost layer that adheres closely to the surface of the brain

Meningitis

• Meningitis is an inflammation of the meninges, usually caused by viral and bacterial infections.
• Symptoms include neck stiffness, vomiting, fatigue, and confusion.
• Other causes of meningitis include CSF leaks that can cause fluid to leak into the ears, nose, and spinal canal.

The Ventricular System

• The ventricular system is a fluid-filled cavern and canal system inside the brain.
• CSF is produced by special tissue called choroid plexus in the ventricles of the cerebral hemispheres.

Immune System Functions

• T-cells work through cell-to-cell contact or by secreting messenger compounds to communicate with immune system cells.
• T-cells can:
• Attract cells
• Prevent cells from leaving the area
• Amplify the effectiveness of cells
• Increase or decrease cell reactions
• Activate and regulate B-cells
• Signal natural killer cells
• Increase local blood flow to facilitate immune cell movement
• Determine if humoral or cellular immunity is needed

T-Cell Functions

• T-cells are involved in:
• Differentiating between self vs. non-self recognition and reaction
• Activating most cells and IgE's in allergen responses
• Rejecting foreign tissue grafts
• Controlling intracellular (viral) infections
• Killing tumor cells

Types of T-Cells

• T-Helper cells (CD-4 cells):
• Up-regulate immune responses
• Release T-messenger compounds to activate and regulate other cell types
• Activate B and T cells correctly depending on the type of immune challenge
• Regulatory T-cells (Tregs):
• Down-regulate immune responses
• Suppress/modify immune responses
• Decrease immune cell production
• Control the mechanism to ensure the response matches the situation and healthy self-cells are not killed
• T-cytotoxic cells (CD-8 cells):
• Destroy identified/presented antigens
• Kill virus-infected cells by various means (lytic enzymes, cytotoxins, perforins, interferon)
• Involved in attacking cellular problems that antibodies cannot influence

Self vs. Non-Self Recognition

• T-cells recognize and respond to an antigen by:
• Presenting the antigen through another cell or an antibody
• Coding the antigen with a Major Histocompatibility Complex (MHC)
• MHC determines tissue and blood compatibility
• In humans, MHC is often called Human Leukocyte Antigens (HLA)

Acquiring Immunity

• Immunity can be acquired through:
• Active natural immunity: develops through direct exposure to an antigen
• Active artificial immunity: develops through purposeful introduction to an antigen (e.g., vaccines)
• Passive immunity: passed from a mother to fetus, protecting the infant during the first few months of life
• Passive artificial immunity: injecting antibodies from one system to another to help fight a current infection

Auto-Immunity

• Auto-immunity occurs when the immune system:
• Mounts a response against the body's own tissues
• Fails to differentiate between self- and non-self antigens
• Examples of auto-immune diseases include:
• Rheumatoid arthritis
• Ankylosing spondylitis
• Multiple sclerosis
• Diabetes
• Lupus (SLE)

Type I Hypersensitivity

• Type I responses vary in severity:
• Mild, but annoying (e.g., seasonal allergies)
• Severely debilitating (e.g., asthma)
• Life-threatening (e.g., anaphylaxis)
• Type I responses can vary based on the site of involvement:
• Local (atopic) reactions
• Systemic reactions (anaphylaxis)

Local/Atopic Reactions

• Examples of local/atopic reactions include:
• Allergic rhinitis
• Asthma
• Urticaria (hives)
• Angioedema (Quincke's edema)
• Gastrointestinal food allergies

Type IV Hypersensitivity

• Type IV hypersensitivity is mediated by:
• Specifically sensitized T-lymphocytes
• Manifests as sub-acute or chronic inflammation with infiltration of the tissue by lymphocytes and macrophages
• Examples of Type IV hypersensitivity include:
• Contact dermatitis
• Graft rejection