The Spinal Cord

The Spinal Cord

Functional Anatomy

  • The spinal cord is a cylinder of nervous tissue enclosed in the vertebral canal.
  • Functions:
    • Conduction
    • Neural integration
    • Locomotion
    • Reflexes

Functions: Conduction

  • Contains bundles of axons that conduct information up and down the cord.
  • Moves sensory signals from the body to the brain.
  • Moves motor signals down to the muscles and glands below the head.

Functions: Neural Integration

  • Groups of spinal neurons receive input from a variety of sources, integrate the information, and execute appropriate output.
  • Example:
    • The spinal cord integrates stretch sensation from a full bladder.
    • Issues messages to urinary sphincters to allow for or delay urination.
    • With cerebral input about the appropriate time and place to urinate.

Functions: Locomotion

  • Although the decision to move muscles, such as walking, is made by the brain.
  • Repetitive muscle contractions are coordinated by groups of spinal neurons, called central pattern generators.

Functions: Reflexes

  • Plays a vital role in posture, motor coordination, and protective responses to pain or injury.

Spinal Cord Gray and White Matter

  • Butterfly-shaped gray matter in the center, surrounded by white matter.
  • Left and right halves are connected by gray commissure.

Spinal Cord Gray Matter

  • Posterior wings of the butterfly are called the dorsal horns.
    • Are the receiving points for incoming sensory info.
    • Usually synapse with interneurons in the horn.
  • Anterior wings of the butterfly are called the ventral horns.
    • Location of neurosomas of motor neurons.
    • Axons of motor neurons lead out to skeletal muscles.

Spinal Cord White Matter

  • Surrounds the butterfly.
  • It consists of axons traveling up and down the cord.
  • Serves as a communication pathway.
  • Axons originate from neurons in the brain and from neurons in the gray matter.
  • Bundles of axons are arranged in three groups (columns):
    • Posterior
    • Lateral
    • Anterior
  • Composed of tracts:
    • Ascending
    • Descending

Spinal Cord White Matter - Ascending and Descending Tracts

  • Ascending tracts:
    • Carry sensory information to higher levels of the spinal cord or brain.
  • Descending tracts:
    • Carry motor signals from the brain down the cord to meet motor neurons that control muscles and glands.
  • Decussation:
    • Tracts crossover from the left side of the body to the right and vice versa.
    • Thus, the left side of the brain senses and controls the right side of the body, and the right side of the brain senses and controls the left side of the body.
    • Why strokes on one side of the body cause sensory or motor effects on the opposite side of the body.

Spinal Meninges – Protective Membranes

  • The brain and spinal cord are covered by three membranes, in between nervous tissue and bone.
  • Function: prevent nervous tissue from abrasion and trauma.
  • Dura mater:
    • Outermost membrane – against bone.
    • Tough collagenous membrane.
  • Arachnoid mater:
    • Middle layer.
    • Looks like a spider web.
  • Pia mater:
    • Innermost layer.
    • Thinness directly touches the brain and spinal cord.

Spinal Nerves and Ganglia

  • The PNS is composed of two kinds of structures:
    • Nerves and ganglia
    • Think of nerves as fibrous rope; each fiber is an axon in the nerve.
    • Think of ganglia as like a knot in the rope.

Spinal Nerves

  • Nerves: bundles of neuron axons and blood vessels wrapped in connective tissue.
  • Connective tissue wrapping each axon: endoneurium.
  • Axon bundles: fascicles.
  • Tissue wrapping around fascicles: perineurium.
  • Tissue wrapping around several fascicles: epineurium.
  • Epineurium protects the nerve from stretching and injury.
  • Nerves have high metabolic demands.
  • Wrapping nerves in CT ensures blood vessels can reach all axons.

Ganglia

  • Swelling containing the cell bodies of their peripheral neurons.
  • Ganglia can act as information processing centers of the PNS.
  • Neurons can form synaptic connections with each other.
  • The spinal cord gives off 31 pairs of spinal nerves.
  • Provide a means for the cord and brain to communicate with the rest of the body.
  • Emerge from the spinal cord as nerve rootlets from dorsal and ventral horns.

Ganglia and Nerve Roots

  • Posterior rootlets merge and form a single dorsal root, which expands into a swelling called a posterior root ganglion.
    • The dorsal root is composed of sensory (afferent) neuron axons.
    • Carries signal from sensory nerve endings to the spinal cord.
  • Anterior rootlets merge and form a single ventral root.
    • No ganglion.
    • Ventral root is composed of motor (efferent) neuron axons.
    • Carries signals leaving the spinal cord to muscles and glands.

Reflexes

  • Reflex: a quick, involuntary reaction of a gland or muscle to a stimuli.
  • 4 important properties:
    • Quick
      • Simple neural pathways with relatively few interneurons.
    • Involuntary
      • Occur without consciousness or awareness – impossible to suppress.
    • Stereotyped
      • Occur the same way every time.
    • Stimulation
      • Not spontaneous actions – response to sensory.

Reflexes - Visceral vs Somatic

  • Can involve cardiac muscle, smooth muscle, or glands.
  • EX:
    • Increased heart rate in fear.
    • Constriction of the esophagus when swallowing.
    • Secretion of tears when eyes are irritated.
    • These are examples of visceral reflexes.
  • Reflexes involving skeletal muscle are somatic reflexes.
  • EX:
    • Knee-jerk reflex.
    • Pulling hand away from a hot stove.

Somatic Reflexes: Reflex Arc Pathway

  • A receptor:
    • Sensory nerve ending or simple sense organ in the skin muscle or a tendon.
  • An afferent (sensory) neuron axon:
    • Axon of a unipolar neuron that carries signal from the receptor to the spinal cord or brainstem.
  • An integrating center:
    • Synaptic contact between neurons in gray matter of spinal cord or brain stem.
    • Sometimes an interneuron is involved, or efferent and afferent neurons may directly synapse.
  • An efferent (motor) neuron axon:
    • Carries motor signal to a skeletal muscle.
  • An effector:
    • In this case, is skeletal muscle – could be an organ or cell that carries out the final response.

Somatic Reflexes: Stretch Reflex

  • “Knee jerk” reflex.
  • Doctor taps on the patellar tendon with a rubber mallet.
  • Stretch receptors, called muscle spindles, detect stimuli.
  • The signal travels through an afferent neuron, which synapses with a motor neuron in the spinal cord.
    • Called a monosynaptic reflex arc.
  • The motor neuron carries the signal to the quad to contract.

Somatic Reflexes: Flexor (Withdrawal) Reflex

  • Protective reflex – requires coordination of multiple muscles and is more than a quick jerk.
  • Utilizes a polysynaptic reflex.
    • Afferent and efferent neurons do not synapse directly with each other – synapse with an interneuron in between.
  • Can involve both stimulating and inhibiting muscles.
  • Example: withdrawing from pain in foot.
    • The hamstring is excited.
    • The quad is inhibited.