spinal cord

Sensory Receptors and Pathways

  • Types of Sensory Receptors

    • Tactile Receptors:
    • Responsible for touch sensation.
    • Information travels via unipolar cells located in the ganglia.
    • Stops at the somatosensory nucleus.
    • Visceral Receptors:
    • Responsible for sensing internal organs.
    • Information stops at the visceral sensory nucleus.

  • Neuronal Communication:

    • Interneuron Functionality:
    • Communicates with sensory neurons.
    • Processes information before passing it on.
      • Motor Information Transmission:
    • Somatosensory pathway sends signals to the somatomotor nucleus affecting muscle movement.
    • Visceral sensory information directs activity to the visceral motor nucleus targeting cardiac and smooth muscles.

Ascending and Descending Tracts

  • Overview of Tracts:

    • Ascending Tracts: Carry sensory information up the spinal cord.
    • Descending Tracts: Carry motor information down from the brain.

  • Contralateral and Ipsilateral Information Processing:

    • Sensory information is processed from the contralateral side of the body (right brain controls left side and vice versa).
    • Decussation: The term for the crossing over of fibers at the midline.
      • Note: Do not confuse "decussate" with "desiccate" (to dry out).

Neuron Pathway Overview

  • Critical Terminology:

    • Dendrites of unipolar neurons receive information and enter the spinal cord.
    • Understand location of neurons:
    • Where does the first neuron's axon start and stop?
    • Where is the second neuron's cell body and where does it decussate?
    • How many neurons in the pathway—typically three or four for sensory pathways.

  • Funiculus and Fasciculus:

    • Funiculi: Columns of spinal cord (posterior, lateral, anterior).
    • Fasciculi: Sections of columns; gracilis fasciculus (medial) and cuneate fasciculus (lateral).

Neuron Naming and Pathways to the Brain

  • Degree of Neurons:

    • First Order Neuron: Unipolar neuron sending sensory info to spinal cord.
    • Second Order Neuron: Often connects to thalamus.
    • Third Order Neuron: From thalamus to cerebral cortex.

  • Pathways to Remember:

    • Posterior Column Medial Lemniscus Pathway: Carries proprioception, fine touch, and pain.
    • Neuron 1: Dendrites located in the skin, axon ends in the spinal cord.
    • Neuron 2: Cell body in the medulla oblongata; decussates at medulla and travels to the thalamus.
    • Neuron 3: From the thalamus to the sensory homunculus in cortical areas for perception.
    • Medial Lemniscus Name Origin: Refers to a ribbon-like structure formed by axons from this pathway.

The Sensory Homunculus

  • Representation in the Brain:
    • Sensory Homunculus: Visual depiction of sensory representation in the cortex.
    • Different body parts are represented, with larger areas dedicated to more sensitive parts.
    • Factors Influencing Sensitivity: Areas with more nerve endings (fingers, lips) occupy larger areas on the homunculus; they are more sensitive.

Detailed Pathway Information

  • Gracilis and Cuneate Fasciculi:

    • Gracilis: Carries signals from lower body.
    • Stops at the gracilis nucleus in the medulla.
    • Cuneate: Carries signals from upper body.
    • Stops at the cuneate nucleus in the medulla.

  • Spinothalamic Tract:

    • Involved in carrying pain and temperature sensation.
    • First order neuron ends in the posterior horn.
    • Second order neuron decussates in the spinal cord and goes to the thalamus.
    • Third allows perception to the cerebral cortex.

  • Spinoreticular Tract:

    • Carries pain information to the reticular formation instead of the thalamus.
    • Third order neuron then goes to the thalamus, and fourth is for reaching cortex for perception.

  • Spinocerebellar Tracts:

    • Carries proprioceptive information to the cerebellum for balance and coordination.
    • Uses only two neurons:
      • The first order stays ipsilateral (same side).
      • The second order may decussate and then return to the ipsilateral side in the cerebellum.

Descending Motor Pathways

  • Corticospinal Pathway:

    • Mandatory for voluntary motor control.
    • Neuron 1 goes from the cortex to the spinal cord (decussation occurs at the medulla).
    • Neuron 2 travels to the skeletal muscle from the anterior gray horn of the spinal cord.

  • Reticulospinal Tract:

    • Functionality: Involved in modulating pain (descending analgesic pathways).
    • Carries information from the reticular formation down the spinal cord.

  • Tectospinal Tract:

    • Starts in the tectum and coordinates head movements in response to visual and auditory stimuli.

  • Vestibulospinal Tract:

    • Associated with balance control, influences extensor muscle response to maintain stability.

Summary of Key Terms and Functions

  • Key Functions of Major Components:
    • Reticular formation: Integrating sensory inputs related to pain.
    • Thalamus: Major relay station for sensory input (except olfaction, which bypasses it).
    • Cortex: Area for processing sensory and motor input.