4 ch 15 Spinal Cord Sensory and Motor Tracts Study Guide

Introduction to Spinal Cord Tracts

  • Most sensory and motor signals traveling between the brain and the body move through bundles of axons in the spinal cord known as spinal tracts.
  • Ascending Tracts (Sensory Tracts): These pathways receive sensory information from the Peripheral Nervous System (PNS) and deliver it to the brain.
  • Descending Tracts (Motor Tracts): These pathways send motor output from the brain to effectors in the PNS.
  • Neuron Relays: Each spinal tract consists of multiple neurons that relay signals from one neuron to the next until the destination target is reached.

Organization and Naming Conventions

  • Functional Grouping: Spinal tracts with similar functions are often grouped together geographically in the spinal cord.
    • Example: Tracts carrying information about painful sensations are located in the spinothalamic tracts within the lateral columns of the spinal cord.
  • Naming Structure: Spinal tracts are typically named based on the origin of the signal and its final destination.
    • Prefix: Indicates where the signal comes from (e.g., "spino-" for the spinal cord).
    • Suffix: Indicates where the signal is being sent (e.g., "-thalamic" for the thalamus).
    • Example: The spinothalamic tracts carry information coming into the spinal cord to the thalamus in the brain.

Sensory (Ascending) Tracts: Neuronal Hierarchy

Ascending tracts carry somatic sensory signals (touch, pain, temperature, etc.) and typically involve a chain of either 22 or 33 neurons.

  • First-Order Neurons:
    • These are the sensory neurons that deliver sensations directly to the Central Nervous System (CNS).
    • Structure: They are all pseudounipolar neurons.
    • Cell Body Location: Located in the dorsal root ganglia or a cranial nerve ganglion.
    • Process: They receive somatic sensory information and send it through the dorsal roots into the spinal cord.
  • Second-Order Neurons:
    • The axon of the first-order neuron synapses on a second-order neuron.
    • Cell Body Location: Located in either the spinal cord or the brainstem.
    • Destination: Depending on the specific tract, the axon sends information to either the thalamus or the cerebellum.
  • Third-Order Neurons:
    • These neurons are present only in tracts that send information to the cerebrum; tracts terminating in the cerebellum lack third-order neurons.
    • Cell Body Location: Located in the thalamus.
    • Process: They receive signals from second-order neurons and carry the information to the primary somatosensory cortex for processing.

Decussation and Contralateral Processing

  • Decussation: This refers to the crossing over of an axon from one side of the body to the other (e.g., left to right or right to left).
  • Mechanism: In every ascending tract, the axon of either the first-order or second-order neuron will undergo decussation.
  • Result: Sensory information from the left side of the body is processed by the right side of the brain, and vice-versa.
    • Example: Touching an object with your right hand sends a sensory signal that is processed in the left hemisphere of the brain.

The Somatosensory Cortex and Sensory Homunculus

  • Primary Somatosensory Cortex: Information reaching the thalamus is forwarded to neurons in this area of the cerebrum.
  • Somatic Mapping: Every neuron in the primary somatosensory cortex corresponds to a specific sensory receptor in a specific part of the body.
    • Example: A light touch on the tip of the pinky finger always stimulates the same specific group of neurons in the somatosensory cortex.
  • Sensory Homunculus: A map of the somatosensory cortex that depicts which locations correspond to different body parts.
    • Size Representation: The size of each body part on the homunculus is proportional to the amount of cortex devoted to that part, representing the density of sensory receptors.

Major Ascending (Sensory) Tracts

There are 33 major ascending tracts in the spinal cord:

  • Posterior Columns:
    • Location: White matter in the posterior of the spinal cord.
    • Sensory Input: Body position, fine touch, pressure, and vibration.
    • Destination: Somatosensory cortex.
  • Spinothalamic Tracts:
    • Location: Anterior and lateral white columns.
    • Sensory Input: Crude touch, pressure, pain, and temperature.
    • Destination: Thalamus, then the somatosensory cortex.
  • Spinocerebellar Tracts:
    • Location: Lateral white columns.
    • Sensory Input: Body position.
    • Destination: Cerebellum.
    • Special Characteristics: These tracts do not have third-order neurons and do not decussate.

Motor (Descending) Tracts: Neuronal Hierarchy

Motor tracts are part of the Somatic Nervous System (SNS) and carry signals from the brain to skeletal muscles. These pathways typically involve two neurons.

  • Upper Motor Neurons:
    • Cell Body Location: Cerebrum, specifically in the primary motor cortex.
    • Process: They carry somatic motor information from the brain down to the brainstem or spinal cord.
    • Decussation: The axons of upper motor neurons cross over, meaning the left brain controls the right side of the body.
  • Lower Motor Neurons:
    • Cell Body Location: Brainstem or spinal cord.
    • Process: They receive information from upper motor neurons and send it to skeletal muscles through the ventral roots into the PNS.

The Motor Homunculus and Corticospinal Tracts

  • Motor Homunculus: Similar to the sensory homunculus, this maps the primary motor cortex. Different parts of the cortex control different body parts. Note that the sizes of body parts on the motor homunculus may differ from those on the sensory homunculus.
  • Corticospinal Tracts: These are the major descending tracts that carry signals from the primary motor cortex for voluntary somatic motor control.
    • Location: Anterior and lateral columns of the spinal cord.
    • Corticobulbar Tract: This is a specialized corticospinal tract that does not extend into the spinal cord; instead, it sends motor signals through the cranial nerves.

Subconscious Motor Pathways

  • Function: Control subconscious and reflexive movements.
  • Upper Motor Neurons: Located throughout various parts of the brain (distinct from the primary motor cortex).
  • Lower Motor Neurons: These pathways share the same lower motor neurons as the voluntary corticospinal tracts.
  • Interaction: Subconscious pathways can stimulate or inhibit lower motor neurons, allowing for the subconscious alteration of voluntary muscle signals.
  • Practical Example (Balance): The body constantly alters muscle tone via these pathways to maintain balance and prevent falling over.