Lecture Notes: Spinal Cord Anatomy and Physiology

Overview of the Spinal Cord

• The spinal cord is the main communication channel between the brain and spinal nerves.
• It is a long, thin tube of nervous tissue.
• Enclosed in three membranes known as the meninges.
• Protected within the bones of the vertebral column.

Structure and Function

• Spinal Nerves:
  • 31 pairs arise from the spinal cord.
  • Emerge from the vertebrae.
• Extent:
  • Extends from the brainstem to the upper lumbar vertebrae.
  • Lower lumbar and sacral regions form the cauda equina with nerve roots descending within the spinal canal.
• Cross Section Anatomy:
  • Central gray matter (butterfly-shaped): Contains cell bodies and dendrites.
  • Surrounding white matter: Composed of axon bundles for conducting information.

Nervous Tissue

• Gray Matter:
  • Contains cell bodies and dendrites of neurons.
  • Site for neuron synapse and information transmission.
• White Matter:
  • Made of axon bundles, conducts information up and down the cord.
  • Organized into spinal tracts with specific functions.

Spinal Tracts

• High-speed cables for information transfer between spinal cord and brain.
• Types of Tracts:
  • Ascending tracts carry sensory information to the brain.
  • Descending tracts carry motor instructions from the brain.
• Tract Pathways:
  • Some tracts cross to opposite side (contralateral transmission).
  • Others stay on the same side (ipsilateral transmission).

Spinal Nerves

• Mixed nerves with sensory and motor fibers.
• Fiber Pathways:
  • Sensory fibers enter via the dorsal root.
  • Motor fibers exit via the ventral root.

Sensory Pathways

• Typically involve 3 neurons:
  • First-order neurons detect stimuli, transmit signals to the spinal cord.
  • Second-order neurons ascend specific tracts to the brainstem or thalamus.
  • Third-order neurons relay information to the sensory cortex.

Motor Pathways

• Typically involve 2 neurons:
  • Upper motor neurons: Start in motor cortex or brainstem.
  • Lower motor neurons: Continue from brainstem or spinal cord to effector organs.
  • Conduct along specific descending tracts.

Somatic Reflexes

• Fast, involuntary responses of skeletal muscles.
• Do not require brain input but can inform the brain afterward.
• Reflex Arc Components:
  • Somatic receptor, sensory neuron, interneuron, motor neuron, effector muscle.
• Complex Reflexes:
  • Involve multiple pathways and central coordination from the brain.
  • Example: Stepping on something sharp requires contralateral pathways for balance.