Regulation and Integration of the Body

Organization of the Spinal Cord Gray Matter

  • The gray matter of the spinal cord is split into two halves:

    • Dorsal Half (Sensory)
    • Ventral Half (Motor)

  • Dorsal and Ventral Roots:

    • Part of the Peripheral Nervous System (PNS), not the spinal cord.

  • Zones of Spinal Gray Matter:

    • Somatic Sensory (SS): Interneurons receiving input from somatic sensory neurons.
    • Visceral Sensory (VS): Interneurons receiving input from visceral sensory neurons.
    • Visceral Motor (VM): Autonomic neurons.
    • Somatic Motor (SM): Somatic motor neurons.

White Matter of the Spinal Cord

  • Comprised of myelinated and nonmyelinated nerve fibers, facilitating communication:

    • Ascending Tracts: Carry sensory impulses to higher centers.
    • Descending Tracts: Relay motor signals to the spinal cord.
    • Transverse Fibers: Connect the left and right sides of the spinal cord.

  • White matter is divided into three columns (funiculi):

    • Dorsal (Posterior) Funiculus
    • Lateral Funiculus
    • Ventral (Anterior) Funiculus

  • Each funiculus contains several fiber tracts named for their origin and destination.

Spinal Cord Trauma and Disorders

  • Spinal Cord Injury: Direct pressure can lead to the loss of function:

    • Dorsal Roots Damage: Causes sensory loss or abnormal sensations (paresthesia).
    • Ventral Roots Damage: Leads to paralysis.

  • Types of Paralysis:

    • Flaccid Paralysis: Lack of nerve impulses to muscles, causing loss of voluntary and involuntary movement, leading to atrophy.
    • Spastic Paralysis: Damage to upper motor neurons; maintains reflex activity but loses voluntary control, leading to potentially permanent muscle shortening.

  • Transection of the Spinal Cord:

    • Paraplegia: Injury between T₁ and L₁ affecting lower limbs.
    • Quadriplegia: Cervical injuries affecting all limbs.
    • Hemiplegia: Paralysis of one side of the body, usually from brain injury.
    • Spinal Shock: Temporary loss of function post-injury.

Poliomyelitis

  • Caused by the poliovirus affecting ventral horn motor neurons.
    • Symptoms include fever, muscle pain, and potentially paralysis and atrophy of affected muscles.
    • Vaccines have drastically reduced its occurrence.
  • Postpolio Syndrome: Survivors may experience muscle weakness and atrophy later in life due to neuron loss.

Amyotrophic Lateral Sclerosis (ALS)

  • A progressive neurodegenerative disease that destroys motor neurons in the ventral horn and pyramidal tracts.
    • Symptoms include loss of muscle control and eventually leads to death usually within five years.
    • Involves environmental and genetic factors.

Neuronal Pathways

  • Key Features of Neuronal Pathways:
    • Decussation: Most pathways cross sides at some point.
    • Relay: Usually involves a chain of 2 or 3 neurons.
    • Somatotopy: Body map reflected among tract fibers.
    • Symmetry: Pathways are paired symmetrically on both sides.

Ascending Pathways to the Brain

  • Ascending pathways consist of three successive neurons: first-order, second-order, and third-order.

    • First-order Neurons: From sensory receptors to spinal cord or brainstem.
    • Second-order Neurons: Synapse in spinal cord or brainstem and transmit impulses to thalamus or cerebellum.
    • Third-order Neurons: Relaying information to the somatosensory cortex.

  • Key Pathways:

    • Dorsal Column-Medial Lemniscal: Carries discriminative touch and proprioception signals.
    • Spinothalamic Tract: Transmits pain and temperature to the thalamus.
    • Spinocerebellar Tracts: Relay information on proprioception to the cerebellum (subconscious).

Descending Pathways and Tracts

  • Divided into two groups: Direct (Pyramidal) Pathways and Indirect Pathways.
    • Direct Pathways: Originate in pyramidal cells, controlling fast and skilled movements.
    • Indirect Pathways: Involve subcortical nuclei and control balance and posture through complex multisynaptic connections.