Spinal Cord and Sensory Pathways

Overview of the Spinal Cord

• The spinal cord is structured into two major regions: gray matter and white matter.

  • Gray Matter: Divided into three horns:

  • Anterior Horn: Contains motor neurons.

  • Posterior Horn: Contains sensory neurons.

  • Lateral Horn: Present in certain regions, involved in autonomic functions.

  • White Matter: Divided into three columns:

  • Anterior Column

  • Posterior Column

  • Lateral Column

  • These columns contain ascending (sensory) and descending (motor) tracts.

Sensory Pathways

• A typical sensory pathway consists of three neurons:

  • First Order Neuron: Located in the Dorsal Root Ganglion (DRG), responsible for collecting sensory information from the skin or other sensory organs.

  • Second Order Neuron: Can be located either in the posterior horn of the spinal cord or brainstem; it decussates (crosses over) to the opposite side upon ascension.

  • Third Order Neuron: Located in the thalamus, carrying information to the cortex for perception.

Key Terms:

• Decussation: The crossing of axons to the opposite side, crucial for sensory pathways.

• Conscious Sensation: Sensory pathways leading to the cortex allow for conscious awareness.

• Unconscious Sensation: Atypical pathways that do not reach the cortex.

Examples of Sensory Pathways

1. Medial Lemniscal/Dorsal Column Pathway:

   • Carries sensory information about proprioception and fine touch.

   • First order neurons ascend ipsilaterally, second order neurons decussate at brainstem.

   • Terminate at thalamus and then project to cortex.

2. Spinothalamic Pathway:

   • Carries information about pain, temperature, and crude touch.

   • First order neurons enter spinal cord and synapse in the posterior horn.

   • Second order neurons decussate immediately and ascend to the thalamus.

Motor Pathways

• Motor pathways consist of two neurons:

  • Upper Motor Neuron: Located in the cortex or brainstem, responsible for voluntary movement.

  • Lower Motor Neuron: Located either in the anterior horn of spinal cord or in brainstem, innervates skeletal muscles.

Comparison of Motor Pathways:

1. Pyramidal Pathways:

   • Upper motor neurons in the cortex.

   • Lower motor neurons located in the spinal cord or brainstem (decussation occurs either at the medulla or spinal cord).

   • Control voluntary muscle contraction.

2. Extrapyramidal Pathways:

   • Upper motor neurons primarily in the brainstem.

   • Lower motor neurons in the spinal cord.

   • Involve involuntary muscle control and muscle tone adjustment.

Brainstem Structure

• Comprised of three parts:

  1. Medulla Oblongata

  2. Pons

  3. Midbrain

• The brainstem acts as a conduit for all sensory and motor pathways between the cerebrum and spinal cord.

Reticular Formation:

• A network of interconnected nuclei within the brainstem that regulates alertness and awareness.

• Controls vital activities such as breathing and heart rate, and visceral reflexes (e.g., coughing, sneezing).

Summary of Functions:

• Pyramidal System:

  • Voluntary muscle contraction.

• Extrapyramidal System:

  • Controls muscle tone, involuntary muscle actions, posture, and balance.

Important Notes:

• All sensory pathways, except for smell, pass through the thalamus.

• Understanding the decussation of pathways is crucial for predicting which side of the body may be affected by damage to specific brain regions or pathways.