Human Anatomy & Physiology I: The Brain and Spinal Cord Overview (copy)

HUMAN ANATOMY & PHYSIOLOGY I

Chapter 13 & 14: The Brain; Selected Presentation with Extra Picture Guide

SPINAL CORD

  • Spinal nerves: 31 pairs attach to the spinal cord by paired roots.

    • Cervical and lumbar enlargements: Sites where nerves serving the upper and lower limbs emerge.
    • Cauda equina: Collection of nerve roots at the inferior end of the vertebral canal.

  • Conus medullaris: Terminal portion of the spinal cord (Level? ).

  • Filum terminale: Fibrous extension of the pia mater; anchors the spinal cord to the coccyx.

  • Denticulate ligaments: Delicate shelves of pia mater; attach the spinal cord to the vertebrae.

GRAY MATTER AND SPINAL ROOTS

  • Gray matter consists of:

    • Soma (cell bodies).
    • Unmyelinated processes.
    • Neuroglia (glial cells).

  • Gray commissure: Connects masses of gray matter; encloses the central canal.

  • Posterior (dorsal) horns: Contain interneurons.

  • Anterior (ventral) horns: Contain interneurons and somatic motor neurons.

  • Lateral horns: Contain sympathetic nerve fibers.

GRAY MATTER: ORGANIZATION

  • Dorsal half: Contains sensory roots and ganglia.
  • Ventral half: Contains motor roots.
  • Dorsal and ventral roots fuse laterally to form spinal nerves.
  • Four zones evident within the gray matter:
    • Somatic sensory (SS)
    • Visceral sensory (VS)
    • Visceral motor (VM)
    • Somatic motor (SM)

ANATOMY OF A NERVE

WHITE MATTER IN THE SPINAL CORD

  • Fiber directions: Fibers run in three directions – ascending, descending, and transversely.
  • Divisions: Divided into three columns (funiculi):
    • Posterior funiculus
    • Lateral funiculus
    • Anterior funiculus.
  • Each funiculus contains several fiber tracts.
    • Fiber tract names reveal their origin and destination.
    • Fiber tracts are composed of axons with similar functions.

WHITE MATTER: PATHWAY GENERALIZATIONS

  • Pathways decussate: Most pathways cross over to the opposite side.
  • Neuron composition: Most consist of two or three neurons.
  • Somatotopy: Exhibit precise spatial relationships.
  • Pathways are paired: One on each side of the spinal cord or brain.

ASCENDING AND DESCENDING TRACTS

  • Lateral reticulospinal tract
  • Medial reticulospinal tract
  • Lateral vestibulospinal tract
  • Medial vestibulospinal tract
  • Anterior spinocerebellar tract
  • Posterior spinocerebellar tract
  • Posterior column: Gracile fasciculus, Cuneate fasciculus.
  • Descending tracts: Include anterior corticospinal tract, lateral corticospinal tract, and anterolateral system (which contains spinothalamic and spinoreticular tracts).
  • Generalizations about pathways: Need to know names.

SPINOTHALAMIC ASCENDING TRACT

  • Decussation: Pathway decussates in the spinal cord.
  • Neuron structure: Usually a three-neuron pathway.
  • Functions: Involved in touch, tickle, itch, temperature, pain, and pressure sensations.

SPINOCEREBELLAR ASCENDING TRACT

  • Decussation: Pathway does not decussate (occasionally double decussates).
  • Neuron structure: Usually a two-neuron pathway.
  • Functions: Involved in proprioception sensing.

CUNEATE FASCICULUS ASCENDING TRACT

  • Decussation: Pathway decussates in the medulla.
  • Neuron structure: Usually a three-neuron pathway.
  • Functions: Involved in sensations of limbs and trunk position and movement, deep touch, visceral pain, and vibration above T6.
    • Note: For areas T6 and below, the gracile fasciculus is used.

SENSORY AREAS OF THE BRAIN

  • Primary motor area:
  • Premotor cortex:
  • Frontal eye field:
  • Working memory areas:
  • Central sulcus:
  • Primary somatosensory cortex:
  • Somatosensory association area for spatial tasks:
  • Executive area for task management: Broca's area (outlined by dashes).
  • Working memory for object-recall tasks:
  • Solving complex, multitask problems: Prefrontal cortex
  • Gustatory cortex: Taste (in insula)
  • Wernicke's area: (outlined by dashes).
  • Visual areas: Primary and association visual cortex.
  • Auditory areas: Primary and auditory association cortex.

PRIMARY SOMATOSENSORY CORTEX HOMUNCULUS

  • Concept: A cartoon representation of the sensory homunculus, arranged adjacent to the cortical region in which the processing occurs.

PRIMARY MOTOR CORTEX HOMUNCULUS

  • Represents body areas concerning motor control, including:
    • Wrist
    • Hand
    • Fingers
    • Thumb
    • Neck
    • Brow
    • Eye
    • Face
    • Lips
    • Jaw
    • Tongue
    • Swallowing

DESCENDING TRACT ORGANIZATION

LATERAL CORTICOSPINAL DESCENDING TRACT

  • Decussation: Pathway decussates in the medulla.
  • Neuron structure: Usually a two-neuron pathway (sometimes includes an interneuron).
  • Functions: Involved in fine motor control of limbs.

ANTERIOR CORTICOSPINAL DESCENDING TRACT

  • Decussation: Pathway decussates in the spinal cord.
  • Neuron structure: Usually a two-neuron pathway (sometimes includes an interneuron).
  • Functions: Involved in fine motor control of limbs.

SPINAL CORD TRAUMA: PARALYSIS

  • Paralysis: Loss of motor function.

  • Flaccid paralysis:

    • Results from severe damage to the ventral root or anterior horn cells.
    • Lower motor neurons are damaged; impulses do not reach muscles.
    • No voluntary or involuntary control of muscles.

  • Spastic paralysis:

    • Caused by damage to only upper motor neurons of the primary motor cortex.
    • Spinal neurons remain intact, muscles are stimulated irregularly; no voluntary control.

SPINAL CORD TRAUMA: TRANSECTION

  • Description: Cross-sectioning of the spinal cord at any level results in total motor and sensory loss in regions inferior to the cut.
  • Paraplegia: Transection between T1 and L1 (loss of function in the lower body).
  • Quadriplegia: Transection in the cervical region (loss of function in all four limbs).

SPINAL NERVES

  • General information: There are thirty-one pairs of mixed spinal nerves that arise from the spinal cord, supplying 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 (Co1)

EMERGENCE OF SPINAL NERVES

  • C1 nerve: Emerges between the skull and atlas.
  • C2 nerve: Emerges between the atlas and axis, with a prominent and palpable DRG (dorsal root ganglion).
  • C1 - C7: Emerge superior to their correspondingly numbered vertebrae (e.g., C5 above C5).
  • C8 nerve: Emerges inferior to vertebra C7.
  • C8 - Co1: Emerge inferior to their correspondingly numbered vertebrae (e.g., T1 below T1).

SPINAL NERVES: BRANCHES

  • Distal to vertebral foramen, the nerve divides into the following:
    • Anterior ramus: Innervates the anterior and lateral skin and muscles of the trunk.
    • Posterior ramus: Innervates muscles and joints in that region of the spine and skin of the back.
    • Meningeal branch: Reenters the vertebral canal and innervates the meninges, vertebrae, and spinal ligaments.

CUTANEOUS INNERVATION AND DERMATOMES

  • Dermatome: A specific area of the skin that receives sensory input from a pair of spinal nerves.
  • Dermatome map: Diagram showing cutaneous regions innervated by each spinal nerve; overlaps edges by as much as 50%.
  • Testing spinal nerve damage: Assess by testing dermatomes with pinpricks to note areas lacking sensation.

SHINGLES

  • Chickenpox: A common disease during early childhood caused by varicella-zoster virus; produces an itchy rash that resolves without complications. The virus remains in the posterior or dorsal root ganglia (DRG) for life.
  • Shingles (herpes zoster): Localized disease caused by the virus traveling down sensory nerves when the immune system is compromised.
    • Symptoms include a painful trail of skin discoloration and fluid-filled vesicles along the nerve's path, typically affecting one side of the body (usually the chest and waist).
    • Symptoms may include postherpetic neuralgia (pain and itching).
    • Adult vaccines are available for healthy individuals over 60 years of age.

NERVE PLEXUSES

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

CERVICAL PLEXUS

  • Formation: Formed by ventral rami of C1-C5.
  • Branches: Mostly cutaneous nerves of the neck, ear, back of the head, shoulders.
  • Important nerve: Phrenic nerve (C3-C5); it is sensory and the sole motor supply to the diaphragm.

BRACHIAL PLEXUS

  • Formation: Formed by C5-C8 and T1 (C4 and T2 may also contribute).
  • Innervation: Gives rise to nerves that innervate the upper limb.
  • Anatomy: Passes over the first rib; the presence of a cervical rib is correlated with Thoracic Outlet Syndrome (TOS).

THORACIC OUTLET SYNDROME (TOS)

  • Description: Compression of the Brachial Plexus, the subclavian artery, and its first branch (the vertebral artery).
  • Causes: Compression can occur between the scalenes or due to a cervical rib or rib subluxation.
  • Signs & Symptoms: Sharp, burning, aching pain in the neck, pectoral, clavicular, and axillary regions. Symptoms may involve digit 4 and 5 (and potentially digits 1, 2, and 3), along with coldness, weakness, and tingling; transient blindness may occur due to vertebral artery involvement.

BRACHIAL PLEXUS: NERVE INNERVATION

  • Musculocutaneous nerve (C5 - C7): Sends fibers to biceps brachii, brachialis.
  • Axillary nerve (C5 - C6/7*): Innervates deltoid and teres minor.
  • Radial nerve (C5 - T1): Innervates essentially all extensor muscles.
  • Median nerve (C5 - T1): Branches to most flexor muscles of the arm and thenar group.
  • Ulnar nerve (C8 - T1): Supplies flexor carpi ulnaris and hypothenar group.

LUMBAR PLEXUS

  • Formation: Arises from L1-L4; innervates thigh, abdominal wall, and psoas muscle.
  • Major nerves: Obturator and femoral nerves.
    • Obturator nerve (L2 - L4): Supplies most medial adductors of the thigh.
    • Femoral nerve (L2 - L4): Supplies quadriceps, sartorius, and iliacus.

SACRAL PLEXUS

  • Formation: Arises from L4-S4; serves buttock, lower limb, pelvic structures, and perineum.
  • Major nerve: Sciatic nerve, which is the longest and thickest nerve of the body (composed of tibial and common fibular (peroneal) nerves).

SCIATIC NERVE

  • Supplies muscles: Biceps femoris, Semitendinosus, and Semimembranosus, as well as most anterior, lateral, posterior muscles of the leg, and intrinsic foot muscles.
  • Anatomical course: Passes through the greater sciatic notch and ends at the popliteal fossa as it diverges.

COCCYGEAL PLEXUS

  • Formation: Arises from S4-Co1, gives rise to the Anococcygeal nerve which supplies the skin overlying the coccyx and the sacrococcygeal joint.

NERVE PARALYSIS

RADIAL NERVE PARALYSIS

  • Caused by compression from injury, compression, or ischemia (avascular damage).
  • Symptoms: Pain, weakness, numbness, and tingling (paresthesia).
  • Condition can vary from recovery of intact nerve to requiring regeneration.
  • Examples of causes: Crutch paralysis, Saturday night palsy, honeymooner’s palsy.

SCIATIC NERVE PARALYSIS

  • Caused by compression of the sciatic nerve.
  • Symptoms: Sharp pain often traveling from the gluteal region down the back of the thigh and leg, sometimes reaching the ankle.
  • Increased severity of pain if it reaches the knee.
  • Common causes: Disc herniation, osteoarthritis, spinal stenosis, spondylolisthesis, hip subluxations/dislocations, sitting on large wallets, pregnancy, injections.

REFLEX ARC INTRODUCTION

  • Definition: A reflex is a rapid, predictable motor response to a stimulus.
    • Characteristics:
    • Quick [Rapid].
    • Involuntary.
    • Requires sensory stimulation.
    • Stereotyped [Same – essentially predictable].
  • Types of reflexes: Can be inborn (intrinsic) or learned (acquired).
    • Involve only peripheral nerves and the spinal cord.

SOMATIC SEGMENTAL REFLEX ARC

  • Components of a reflex arc:
    1. Somatic receptor: Site of stimulus.
    2. Afferent fiber: Transmits the afferent impulse to the CNS (Posterior horn).
    3. Integration center: Monosynaptic or polysynaptic region within CNS (cord or brainstem).
    4. Efferent fiber: Conducts efferent impulses from integration center to an effector.
    5. Effector: Muscle fiber or gland that responds to efferent impulse.

WITHDRAWAL (FLEXOR) REFLEX

  • Concept: Quick contraction of flexor muscles resulting in the withdrawal of a limb from an injurious stimulus.
  • Mechanism: Requires contraction of flexors and relaxation of extensors in that limb.
  • Pathway: Polysynaptic reflex arc where signals travel over many synapses back to muscle.

FLEXOR AND CROSSED EXTENSOR REFLEXES

  • Flexor reflex: Initiated by a painful stimulus, causing automatic withdrawal of the threatened body part.
  • Crossed extensor reflex: Has two parts:
    • The stimulated side is withdrawn.
    • The contralateral side is extended.

SUPERFICIAL REFLEXES

  • Initiation: By cutaneous stimulation.
  • Example: The plantar reflex is initiated by stimulating the lateral aspect of the sole of the foot.
    • Response: Downward flexion of toes, indirectly testing for proper corticospinal tract functioning.
    • Babinski’s sign: An abnormal plantar reflex indicating corticospinal damage (great toe dorsiflexes, smaller toes fan laterally).

SUMMARY OF WHITE MATTER PATHWAYS

  • Key fiber tracts to remember:
    • Lateral reticulospinal tract
    • Medial reticulospinal tract
    • Lateral vestibulospinal tract
    • Medial vestibulospinal tract
    • Anterior spinocerebellar tract
    • Posterior spinocerebellar tract
    • Posterior column: Gracile fasciculus, Cuneate fasciculus
    • Anterior corticospinal tract
    • Ascending tracts
    • Descending tracts
    • Anterolateral system (including spinothalamic and spinoreticular tracts)
    • Tectospinal tract
    • Lateral corticospinal tract

PROTECTION OF THE BRAIN

  • Mechanisms: The brain is protected by bone, meninges, and cerebrospinal fluid (CSF).
  • Blood-Brain Barrier (BBB): Protects the brain by separating bloodborne substances from neurons, allowing selective passage of nutrients.

MENINGES

  • Description: Three connective tissue membranes lie external to the CNS:
    • Dura mater: Strong, leathery meninx composed of two fibrous connective tissue layers.
    • Arachnoid mater: Middle meninx forming a loose brain covering, separated from dura mater by the subdural space.
    • Pia mater: Deep meninx composed of delicate connective tissue tightly clinging to the brain.

DURA MATER

  • Structure: Composed of two layers that separate to form dural sinuses.
  • Dural septa: Three separate layers extend inward:
    • Falx cerebri: Dips into the longitudinal fissure.
    • Falx cerebelli: Runs along the vermis of the cerebellum.
    • Tentorium cerebelli: Horizontal dural fold extends into the transverse fissure.

ARACHNOID MATER

  • Structure: Forms a loose covering around the brain; filled with CSF and large blood vessels.
  • Arachnoid villi: Protrude superiorly to allow CSF absorption into venous blood.

CEREBROSPINAL FLUID (CSF) – 150 ML

  • Composition: Watery solution similar to blood plasma, with less protein and different ion concentrations.
  • Functions: Provides buoyancy, protects the CNS from trauma, nourishes the brain, and carries chemical signals.
    • Movement driven by: Pressure, ependymal cilia, and rhythmic pulsations from the brain (heartbeat and breath).

CHOROID PLEXUSES

  • Function: Clusters of capillaries that filter tissue fluid, regulate ion concentrations in CSF, and cleanse CSF by removing wastes.

BLOOD-BRAIN BARRIER (BBB)

  • Description: Mechanism maintaining a stable environment for the brain, allowing nutrients to pass freely while blocking harmful substances.
    • Structure: Endothelium of capillaries, thick basal lamina, and bulbous feet of astrocytes.
    • Limitations: Ineffective against substances that can diffuse through plasma membranes (e.g., fat-soluble substances). Present in areas where monitoring chemical composition is necessary (e.g., vomiting center, hypothalamus).
    • Effect of stress: Increases permeability of BBB.

HYDROCEPHALUS IN A NEWBORN & ADULT

  • Newborns: Caused by CSF obstruction, leading to swelling and pressure on brain structures.
  • Adults: Acute CSF obstruction can cause fluid accumulation, compressing blood vessels and nerve tissue.

TRAUMATIC BRAIN INJURY (TBI)

  • Causes: Typically results from coup injuries (localized site) and contrecoup injuries (ricocheting effect).

TBI CONSEQUENCES

  • Concussion: A change in brain function (headache, dizziness, confusion, loss of consciousness) following a blow to the head. Multiple concussions can have cumulative effects (known as MTBI).
  • Cortical contusions: Result in permanent neurological damage.
  • Brainstem contusions: May lead to coma depending on severity of damage to the RAS (reticular activating system).
  • Hemorrhage: Ruptured blood vessels can lead to accumulations in the brain, increasing intracranial pressure and compressing brain tissue. Severe cases may force the brainstem through the foramen magnum.

CEREBRAL EDEMA

  • Description: Swelling of the brain due to excess fluid accumulation.

PLASTICITY AND REPAIR

  • Note: Discusses the brain's ability to adapt and repair itself after injury or disease.

MENINGITIS

  • Definition: Inflammation of the meninges, especially common between 3 months to 2 years of age, with a slightly elevated risk in college students living in crowded situations.
  • Causes: Bacterial and viral infections of the CNS.
  • Consequences: Bacterial meningitis can cause swelling of the brain, enlargement of ventricles, and hemorrhage, with symptoms including high fever, stiff neck, drowsiness, intense headache, and potential progression to coma or death.
  • Diagnosis: Often confirmed by examining CSF through lumbar puncture (spinal tap).