CNS: Spinal Cord Characteristics

The CNS: The Spinal Cord - Characteristics

  • Encased in the vertebral column: This refers to the protective bony structure of the spine that encases the spinal cord.

  • Extends to the 2nd lumbar vertebra: The spinal cord typically terminates around the second lumbar vertebra (L2) in adults.

  • 31 pairs of spinal nerves exiting the spinal cord: Each pair of spinal nerves corresponds to a segment of the spinal cord. There are 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal pairs.

  • Intervertebral foramina: These are the openings between the vertebrae where the spinal nerves exit the spinal column.

A Longitudinal Perspective on the Spinal Cord

  • Decreasing diameter of the spinal cord: The spinal cord's diameter varies along its length except for two major enlargements:

    1. Cervical intumescence (enlargement): This region is associated with nerves that control the upper limbs and allows the nerves servicing the upper limbs to enter and exit the spinal cord.

    2. Lumbar enlargement (intumescence): This bulging region corresponds to nerve roots innervating the lower limbs, enabling the nerves associated with the lower limbs to leave the spinal cord.

    3. Conus medullaris: Located just inferior to the lumbar enlargement, the conus medullaris marks the tapered end of the spinal cord, resembling a cone.

    4. Cauda equina: A structure similar to a horse's tail, composed of a collection of nerve roots (lumbar and sacral) that extend beyond the end of the spinal cord.

A Transverse Perspective - Cross-section of the Spinal Cord

  • Outer White Matter: Organized into three columns:

    1. Dorsal Column: Located at the back of the spinal cord.

    2. Lateral Column: Positioned in the middle.

    3. Ventral Column: Found at the front or belly side.

  • Fasciculi: Also known as nerve tracts, these consist of groups of similar axons that carry action potentials (AP's) to and from the brain.

    • A single fasciculus: A specific grouping of similar axons that transmit the same type of information.

  • Central Gray Matter: Organized into horns:

    1. Dorsal (posterior) horn: Where axons from sensory neurons synapse with interneurons (or association neurons).

    2. Ventral (anterior) horn: Contains the cell bodies of motor neurons that convey signals to effectors (muscles and glands).

    3. Lateral horn: Contains the cell bodies of autonomic neurons, primarily involved in involuntary responses.

The Roots of the Spinal Cord

  • Dorsal Root: Enters the spinal cord at the posterior horn, carrying afferent signals from sensory receptors.

  • Dorsal Root Ganglia: Composed of bipolar neurons, these ganglia contain the cell bodies of afferent neurons.

  • Ventral Root: Exits the spinal cord at the ventral horn, transmitting efferent action potentials from the spinal cord to effectors (muscles and glands).

  • Spinal Nerves: Formed by the union of dorsal and ventral roots on each side of the spinal cord.

Membranes Around the Spinal Cord and the Brain

  • Brain Meninges: Composed of three layers that protect the brain beneath the skull:

    1. Dura Mater: The outermost membrane that lies directly underneath the skull, extending into the brain's fissures.

    2. Arachnoid Mater: A web-like membrane situated beneath the dura mater, includes a subdural space that contains small amounts of fluid for moisture.

    3. Pia Mater: The innermost layer, which closely binds to the surface of the brain. It contains the subarachnoid space filled with cerebrospinal fluid (CSF).

  • Flow of CSF: Cerebrospinal fluid is produced in the choroid plexus of each ventricle, creating pressure and allowing flow through the arachnoid layer facilitated by ciliated ependymal cells.

  • Differences in Meninges Between the Spinal Cord and Brain:

    1. Dura Mater Attachment: In the brain, the dura mater is attached to the skull periosteum; however, in the spinal cord, it is not attached directly to the vertebral bones.

    2. Epidural Space: Present around the spinal cord, located between the vertebral column and the dura mater.

Ascending Pathways Carrying Sensory (Afferent) Information

Two Ways Sensory & Motor Action Potentials Move Through the Spinal Cord

  1. The Reflex Arc (Example: pricked by a tack):

    • Pathway:
      i. Pain receptors in the skin (PNS) are stimulated.
      ii. An afferent message is sent to the spinal cord (CNS).
      iii. The spinal cord sends an outgoing message to the arm's flexor muscle.
      iv. The hand quickly pulls away.

    • Primary Purpose: Provides a quick response to pain, bypassing the brain.

    • Four Stages of Events:
      i. A sensory receptor is activated.
      ii. An afferent neuron transmits information to the spinal cord.
      iii. An association neuron (within the spinal cord) connects the incoming signal.
      iv. An efferent neuron returns the message to the effector organ.

  2. Voluntary Muscle Action:

    • The Anterior Spinothalamic Tract - Ascending Pathway:
      i. Signals originate in sensory receptors of the skin.
      ii. Afferent AP's are produced in primary sensory pathways.
      iii. The primary neuron axon synapses with an association neuron.
      iv. Axons cross over in the spinal cord. AP travels up the anterior spinothalamic tract to the thalamus, where it synapses with secondary neurons, which send signals to the somatosensory cortex for interpretation.

    • Sensory Function of the Cerebrum - for locating specific sensory inputs:
      i. Primary Somatic Sensory Area (PSSA) or Cortex: Receives signals from every body part and localizes sensation sources.
      ii. Somatosensory Association Area (SSAA): Functions to:
      a) Discriminate various stimuli such as touch, poke, slap, or punch.
      b) Maintain sensory memory.

Corticospinal Tract - Descending Pathways Carrying Motor (Efferent) Information

  • Function: Carries signals from the CNS to the muscles. The decision to move originates in the brain.

  • Pathway:

    1. Signals originate in the Primary Motor Cortex:
      a. Preparatory work occurs in the Premotor Cortex.
      b. Upper Motor Neurons (UMNs) located in the motor cortex send action potentials down through the midbrain, pons, and medulla.

    2. Decussation occurs: Axons cross over at the Medullary Pyramid.

      • Approximately 80% of axons decussate (cross over) at this point.

      • About 20% decussate within the spinal cord.

    3. The UMN synapses with an association neuron.

    4. The association neuron synapses with the Lower Motor Neuron (LMN).

    5. The LMN carries the signals to the neuromuscular junction of the muscle.

Signs of Upper Motor Neuron (UMN) and Lower Motor Neuron (LMN) Lesions

  • Signs (Yes/No Indicators):

    • Weakness:

      • UMN Lesions: Yes

      • LMN Lesions: Yes

    • Atrophy:

      • UMN Lesions: No*

      • LMN Lesions: Yes

      • (*Mild atrophy may develop due to disuse)

    • Fasciculations:

      • UMN Lesions: No

      • LMN Lesions: Yes

    • Increased Reflexes:

      • UMN Lesions: Yes

      • LMN Lesions: Decreased

    • Decreased Tone:

      • UMN Lesions: Decreased

      • LMN Lesions: Increased

Meningitis

  • Definition: Inflammation of the meninges, which can be caused by bacterial or viral infections.

  • Pathophysiology: Accumulation of pus in the subarachnoid space can block the flow of cerebrospinal fluid (CSF).

  • Consequences: Can lead to serious complications such as paralysis, coma, or death due to infection.