CH 8 Part 1- Nervous System Physiology and Pathology

Physiology and Functional Divisions of the Nervous System

  • The nervous system controls body coordination and activities through three primary mechanisms:

    • Detecting stimuli (sensory input).

    • Processing information (integration).

    • Initiating responses (motor output).

  • This system utilizes electrical and chemical signaling to maintain homeostasis, enable movement, allow communication, and support cognition and consciousness.

  • Major Functions:

    • Sensory Input: Sensory receptors detect changes (stimuli) inside or outside the body. Information travels via afferent (sensory) neurons to the Central Nervous System (CNS). Examples include touch, pain, temperature, vision, hearing, balance, blood pressure, and blood chemistry.

    • Integration: The CNS processes and interprets sensory information. This occurs in the brain and spinal cord and involves conscious perception, decision-making, memory, learning, and reflex coordination.

    • Motor Output: Commands sent from the CNS through efferent (motor) neurons to effectors (muscles and glands). This results in muscle contraction or glandular secretion.

  • Structural Divisions:

    • Central Nervous System (CNS): Includes the brain and spinal cord. It is the site of information processing and integration.

    • Peripheral Nervous System (PNS): Includes cranial nerves, spinal nerves, and ganglia. It connects the CNS to the rest of the body.

  • Functional Divisions of the PNS:

    • Somatic Nervous System: Controls voluntary movement and conscious sensation. Targets skeletal muscles. Responses are typically fast and precise.

    • Autonomic Nervous System (ANS): Controls involuntary body functions (automatic). Targets cardiac muscle, smooth muscle, and glands to maintain homeostasis.

      • Sympathetic Nervous System: "Fight or flight" response. Prepares the body for stress and activity (e.g., adrenaline release).

      • Parasympathetic Nervous System: "Rest and digest" response. Promotes energy conservation and maintenance.

Cellular Components and Neurotransmission

  • The Neuron: The basic functional unit of the nervous system, specialized for rapid electrical communication.

    • Structure of a Neuron:

      • Dendrites: Receive incoming signals.

      • Cell Body (Soma): Integrates the received signals.

      • Axon: Conducts electrical impulses away from the cell body toward other cells.

      • Synaptic Terminals: Transmit signals to the next neuron or target cell.

    • Myelin Sheath: A fatty insulation covering the axon. It increases the speed and efficiency of impulse conduction and provides protection/support.

  • Communication Mechanisms:

    • Action Potential: Rapid electrical impulses traveling along the nerve.

    • Synaptic Transmission: Chemical signaling occurring at the synapse (the gap between neurons or between a neuron and another cell).

    • Neurotransmitters: Chemical messengers that transmit signals across the synapse.

Anatomy of the Brain

  • The brain is divided into four major anatomical regions:

    • Cerebrum: The largest part of the brain. It is responsible for conscious activities and divided into right and left hemispheres, connected by the corpus callosum.

      • Surface Features: Gyri (ridges), Sulci (shallow grooves), and Fissures (deep grooves).

      • Frontal Lobe: Voluntary motor control, decision-making, personality, and speech production.

      • Parietal Lobe: Sensory perception (touch, pain, temperature), spatial awareness, and sensory integration.

      • Temporal Lobe: Hearing, memory, and language comprehension.

      • Hemispheric Lateralization: The left hemisphere is associated with language, logic, and analytical skills. The right hemisphere is associated with spatial ability, creativity, and emotional processing.

    • Diencephalon: Located centrally, connecting the cerebrum to the brainstem.

      • Thalamus: Directs sensory impulses to the cortex; involves motor signal integration and consciousness.

      • Hypothalamus: Maintains homeostasis. Regulates body temperature, hunger, thirst, hormonal activity, and the ANS. It is the site of pleasure/reward centers.

      • Epithalamus: Includes the pineal gland, which secretes melatonin to regulate circadian rhythms (sleep cycles).

    • Brainstem: Connects the brain to the spinal cord and controls vital life functions.

      • Midbrain (Mesencephalon): Visual/auditory reflexes and eye movement.

      • Pons: A bridge between the cerebrum and cerebellum; regulates breathing rhythm and sleep/arousal.

      • Medulla Oblongata: Regulates vital autonomic functions such as heart rate, blood pressure, and breathing. Controls reflexes like coughing, sneezing, swallowing, and vomiting.

    • Cerebellum: Located posteriorly and inferior to the occipital lobes. Coordinates voluntary movement, balance, posture, and motor learning. Damage results in loss of coordination (ataxia) rather than paralysis.

Specialized Structures and Protection

  • Pituitary Gland: A pea-sized "master gland" located at the base of the brain in the sella turcica of the sphenoid bone.

    • It is connected to the hypothalamus by the infundibulum (pituitary stalk).

    • Control: The hypothalamus regulates the pituitary, which in turn regulates other endocrine glands (thyroid, adrenals, gonads).

    • Clinical Note: Accessed surgically through the nose (transsphenoidal) to avoid cutting through brain tissue.

  • Meninges: Three connective tissue layers surrounding the brain and spinal cord:

    • Dura Mater: Tough outer layer.

    • Arachnoid Mater: Web-like middle layer.

    • Pia Mater: Thin inner layer adherent to the brain tissue.

    • Spaces:

      • Epidural Space: Between the skull and dura.

      • Subdural Space: Between the dura and arachnoid.

      • Subarachnoid Space: Between the arachnoid and pia; contains CSF.

  • Cerebrospinal Fluid (CSF): Cushions and protects the CNS, delivers nutrients, and removes waste.

    • Produced by the choroid plexus within the ventricles.

    • Ventricular System: Includes two lateral ventricles, a third ventricle, and a fourth ventricle.

  • Bloody Supply: Supplied by internal carotid arteries and vertebral arteries, joined at the Circle of Willis for collateral circulation.

The Spinal Cord

  • A long cylindrical structure extending from the brainstem through the vertebral canal.

  • Starts at the foramen magnum and ends at levels L1−L2L1-L2 in adults (L3L3 in infants).

  • Conus Medullaris: The tapered distal end of the spinal cord.

  • Cauda Equina: "Horse's tail" of nerve roots extending from the conus medullaris.

Infections of the Central Nervous System

  • Meningitis: Acute inflammation of the meninges (pia and arachnoid).

    • Classic Triad: Fever, stiff neck (nuchal rigidity), and altered mental status.

    • Etiology:

      • Bacterial: Most severe and life-threatening; contagious via respiratory droplets; requires immediate IV antibiotics and corticosteroids.

      • Viral (Aseptic): Most common type; usually less severe and self-limiting.

      • Fungal: Rare; affects immunocompromised patients; requires long-term antifungal treatment.

      • Parasitic and Non-Infectious: Rare; can result from parasites, autoimmune diseases, medications, and cancer

    • Diagnosis: The Lumbar Puncture (LP) is the gold standard. In adults, the needle is inserted between L3−L4L3-L4 or L4−L5L4-L5 to avoid the spinal cord.

  • Encephalitis: Inflammation of the brain parenchyma (tissue).

    • Hallmark: Altered mental status (confusion, personality changes).

    • Etiology: Most commonly viral; Herpes Simplex Virus Type 1 (HSV-1) is particularly fatal/fulminant.

    • Diagnosis: Lumbar Puncture. MRI (most sensitive, showing temporal lobe involvement in HSV) and PCR testing.

  • Brain Abscess: Localized collection of pus within the brain.

    • Causes: Contiguous spread from sinusitis (most common) or otitis media.

    • Clinical Manifestation: Headache (most common)

    • Treatment: High dose IV antibiotics, 6-6 weeks

    • Imaging: MRI with contrast shows "ring-enhancing lesions."

    • Contraindication: Lumbar puncture is avoided due to the risk of brain herniation from mass effect.

  • Subdural Empyema: Collection of pus between the dura and arachnoid.

    • It is a medical emergency due to rapid spread in the barrier-free subdural space.

    • Requires urgent surgical drainage (craniotomy or burr hole).

    • Imaging: “Crescent shaped” fluid collection. MRI with contrast is the gold standard

  • Osteomyelitis of the Skull: Infection of the cranial bones, often from contiguous spread

    • Etiology: Staphylococcus aureus (most common)

    • Imaging: MRI is the gold standard

Primary and Secondary CNS Tumors

  • Definition: Abnormal growths that arise from the brain, spinal cord, meninges, cranial nerves, or supporting tissues

  • General Symptoms: Headaches (often worse in the morning due to natural rises in intracranial pressure), seizures, nausea/vomiting, and focal neurologic deficits.

  • Imaging: MRI with contrast is the gold standard for tumors of CNS

  • Primary Tumors:

    • Gliomas: Arise from glial cells (supportive cells). Most common type of primary brain tumors in adults

      • Astrocytoma: Most common glioma; arise from astrocytes (star shaped)

        • grades I−IVI-IV . Grade IVIV is highly malignant.

      • Oligodendroglioma: Arises from oligodendrocytes. Slow-growing; generally better prognosis due to treatment responsiveness.

      • Ependymoma: Arise from ependymal cells lining the ventricles; often causes hydrocephalus.

    • Medulloblastoma: Highly malignant brain tumor in children (peak age 3−83-8). Located in the posterior fossa; can spread via CSF ("drop metastasis").

    • Meningioma: Benign, extra-axial tumor arising from the meninges; common in the elderly.

    • Acoustic Neuroma: Benign tumor on Cranial Nerve VIIIVIII, arising from Schwann cells. Causes unilateral sensorineural hearing loss, tinnitus, and vertigo.

      • Audiologic testing: Asymmetric sensorineural hearing loss

    • Pituitary Adenoma:

      • Functioning (Hormone secreting) adenomas

        • Prolactinoma: Most common functioning type; secretes excess prolactin

          • Causes galactorrhea, amenorrhea, and infertility.

        • Growth Hormone Secreting: excess growth hormone

          • Causes Acromegaly in adults (enlarged hands/feet/face) and Gigantism in children.

        • ACTH Secreting: excess ACTH stimulates the adrenal glands to produce too much cortisol

          • Causes Cushing’s Disease (moon face, buffalo hump, truncal obesity, purple striae).

        • TSH Secreting Adenoma: rare; produces excess thyroid stimulating hormone (TSH)

          • Leads to secondary hyperthyroidism

    • Craniopharyngioma: Benign tumor near the pituitary; often contains calcifications; affects vision and growth in children.

      • Most common in people <20 years old

      • Treated with MR guided stereotactic resection

    • Pineal Tumors: rare tumors that arise in or around the pineal gland

      • Germ cells (most common) → germinoma (most common)

      • Diagnosis: B-hCG (human chorionic gonadotropin) and ATP (alpha fetoprotein)

      • Can cause Parinaud Syndrome (inability to look upward, pupillary light-near dissociation) and obstructive hydrocephalus.

  • Secondary (Metastatic) Tumors: Most common brain tumors overall. Spread via bloodstream, most frequently from lung, breast, or melanoma.

Traumatic Processes of the Brain and Skull

  • Emergency CT has replaced all other radiographic investigations in patients with suspected neurologic dysfunction resulting from head inju

  • Skull Fractures:

    • Linear: Simple break with no displacement; most common.

    • Depressed: Bone fragment pushed inward; higher risk of brain injury.

    • Basilar: Base of skull fracture. Signs: Battle sign (bruising behind ear) and Raccoon eyes (periorbital ecchymosis).

  • Epidural Hematoma: Bleeding between the skull and dura, usually from a torn artery.

    • Classic Sign: "Lucid interval" followed by rapid decline.

    • Imaging: Non-contrast CT of the Head. Biconvex or lens-shaped mass that does not cross suture lines.

  • Subdural Hematoma: Bleeding between the dura and arachnoid, usually from torn bridging veins (venous blood).

    • Imaging: Non-contrast CT of the Head. Crescent-shaped mass that crosses suture lines.

    • Acute: Develops within 7272 hours. Chronic: Common in elderly/alcoholics, develops over weeks.

  • Cerebral Contusion: Bruise of the brain tissue; most common TBI. Often involves frontal and temporal lobes.

    • Imaging: Non-contrast CT of the Head

  • Intracerebral Hemorrhage:

    • AKA Intracerebral Hemorrhage (ICH)

    • Bleeding within the brain parenchyma. Major cause is chronic hypertension.

    • Imaging: Non-contrast CT of the Head

  • Subarachnoid Hemorrhage: Bleeding into the subarachnoid space.

    • Symptoms: "Worst headache of my life" (thunderclap headache) and sentinel headaches (warning leaks).

    • Major Cause: Ruptured berry aneurysm at the Circle of Willis.

    • Imaging: Non-contrast CT of the Head