Chapter 15 Notes: Sensory Pathways

Sensory Pathways

• If there is time left, there will be a review session. The goal is to complete chapter 15, which covers sensory pathways – specifically, the somatic sensory pathways that transmit sensory input from the body’s periphery to the central nervous system.

Spinal Cord Location

• The posterior column pathway, the spinocerebellar pathway (yellow), and the spinothalamic pathway (green) are located in the spinal cord.

Spinothalamic Pathway

• Relays crude touch, pressure, pain, and temperature sensations to the brain.
• Information enters through the dorsal root of the spinal nerves into the posterior gray horn, crosses over, and ascends through the medulla to the midbrain, then to the thalamus, and finally synapses in the primary somatosensory area.
• The anterior spinothalamic tracts carry crude touch and pressure sensations.
• Lateral spinothalamic tracts follow a similar path, also crossing over in the posterior gray horn to reach the primary somatosensory area.
• The involvement of the thalamus enables the perception of these sensations.
  • The thalamus acts as a final relay center determining awareness of bodily sensations.
• Abnormalities in the spinothalamic pathway can lead to conditions such as phantom limb syndrome, where painful sensations are perceived in a limb that is not there (either due to amputation or congenital absence).
  • This occurs because developmental pathways are established early, and even without the limb, sensory neurons and interneurons may still be activated.
  • Example: A friend with an amputated foot experienced itching or pain in the missing foot due to intact developmental pathways.
• Referred pain is another abnormality.
  • Example: During a heart attack, pain may be felt down the left arm, even though the heart is the source.
  • Activity in interneurons in the CNS can stimulate the primary somatosensory area, causing visceral pain to be perceived as originating from the body's surface.
  • Heart pain can radiate to the neck and left arm; ureter issues can cause groin pain; liver and gallbladder problems can manifest as shoulder pain.

Posterior Column Pathway

• Located on the posterior side of the spinal cord.
• Information enters the posterior gray horn and travels up the pyramids into the medulla, then goes to the thalamus and finally to the primary somatosensory area.
• First-order neurons enter the spinal cord and ascend the pyramids to meet second-order neurons in the medulla, then travel through the midbrain.
• Third-order neurons synapse in the thalamus, leading to conscious awareness of sensations.
• Carries fine touch, vibration, certain pressure sensations, and proprioception (body position).

Spinocerebellar Pathway

• The thalamus is not involved.
• Information ascends through the pons to the cerebellum.
• Involved in proprioception and maintaining balance/equilibrium.
• Golgi tendon organs, muscle spindles, and joint capsule receptors monitor body position, sending this data to the cerebellum.
• The anterior spinocerebellar tract crosses over before ascending, while the posterior tract remains on the same side.
• Communication from both sides of the body is crucial for maintaining position and balance.

Motor Pathways

Pyramidal Cells

• Located in the primary motor cortex, shaped like pyramids (hence the name).
• Send information down the brain stem and spinal cord.

Cranial Nerves with Somatic Motor Function

• Oculomotor, trochlear, and abducens (eye movement).
• Accessory.
• Hypoglossal (tongue movement).

Reticular Formation

• Spans various brainstem regions.
• Regulates consciousness.
• Initiates involuntary motor responses.
• Processes sensory and motor information.

Tectum of the Midbrain

• The roof of the midbrain houses the colliculi (superior and inferior).
• The superior colliculi receive visual sensations.
• The inferior colliculi receive auditory sensations.

Vestibulo Portion of Cranial Nerve VIII (Vestibulocochlear Nerve)

• Monitors equilibrium.

Somatic Motor Pathways

• Involve at least two neurons: upper and lower motor neurons.
• Upper motor neurons have cell bodies in a CNS processing center.
  • They communicate with and either activate or inhibit lower motor neurons.
• Lower motor neurons have cell bodies in the brainstem or spinal cord nuclei.
  • Axons extend outside the CNS to innervate single motor units for skeletal muscle contraction.
  • Acetylcholine neurotransmitter is secreted to communicate with skeletal muscles and produce contractions.
• Damage to lower motor neurons disrupts communication between the CNS and skeletal muscles, affecting voluntary and involuntary motor functions.
  • Motor neurons also serve visceral organs, so damage can affect both voluntary and involuntary muscle contractions.

Motor Pathways Overview

• Corticospinal pathways (purple).
• Lateral pathways (red).
• Medial pathways (orange).

Corticospinal Pathway

• It Provides skeletal muscle control, originating in the primary motor cortex.
• Pyramidal cell bodies are located here.
• Axons descend through the brainstem and into the spinal cord, synapsing on lower motor neurons.
• Three pairs of descending tracts exist:
  • Upper motor neurons originate in the motor cortex, extend through the midbrain to the pyramids, where some cross over to form the lateral corticospinal tract.
  • After crossing, they synapse on a motor neuron in the anterior gray horn and exit through the ventral root to innervate muscles.
  • Some fibers remain on the same side and cross over at the spinal cord level.
  • The lateral tracts cross over in the medulla region. A few stay on the same side until they reach the spinal cord level.
  • Both types activate lower motor neurons in the anterior gray horn, leading to skeletal muscle control.
• Corticobulbar tracts synapse on corticospinal tracts in cranial nerve nuclei.
  • These tracts are concerned with head and neck innervation.

Medial Pathway

• Controls muscle tone and gross motor movements of the trunk and proximal limbs.
• Upper motor neurons originate in several areas:
  • Vestibular Nuclei: Located at the pons-medulla border, receiving input from the vestibulocochlear nerve, monitoring body position and equilibrium.
    • Descending fibers form the vestibulospinal tracts.
  • Superior and Inferior Colliculi: Found in the tectum (roof) of the midbrain.
    • Superior colliculi receive visual sensations, while inferior receive auditory sensations.
    • They are important for balance.
      • Visual Input: Closing the eyes can cause imbalance. Visual stimuli contribute to reflexive changes for equilibrium.
      • Auditory sensations via the vestibulocochlear nerve also play a role.
    • Axons descend in the tectospinal tracts.
  • Reticular Formation: Axons of upper motor neurons descend into medial and lateral reticulospinal tracts.
    • Centered in the reticular formation, integrating sensory and motor information for reflexive postural changes.

Lateral Pathway

• Involved in muscle tone and precise movements of distal limb parts (fine motor control).
• Axons cross over to the opposite side in the brain and descend into the cervical spinal cord only.
• Form the rubrospinal tracts.
• The rubrospinal tract is usually insignificant but can take over motor control if the corticospinal tracts are damaged.

Review Chapter 15

• Perceive a stimulus: Conscious awareness of sensations through the thalamus to the cerebral cortex.

• General vs. Special Senses:

  • General senses: temperature, pain, touch, pressure, vibration, proprioception.
  • Special senses: olfaction, gustation, balance, hearing, vision (located in specialized organs like the eye, ear, nose).

• Receptor Sensitivity:

  • Receptors are very specific to their stimulus.
  • Exception: Pain receptors can be activated by temperature or pressure changes in addition to tissue damage.

• Receptive Field:

  • The area controlled by a single receptor.
  • Smaller fields mean more precise stimulus localization; larger fields make it difficult to pinpoint location.
  • Fingertips have small, precise fields, allowing the blind to use touch as eyes.

• Transduction: Converting an arriving stimulus into an action potential by a sensory receptor, then passed to a sensory neuron & propagated to the CNS.

• Adaptation: Reducing receptor sensitivity when the stimulus is constant.

  • Olfactory receptors adapt quickly.
  • Pain receptors adapt slowly.

• Classify Receptors:

  • Tonic vs. Phasic
    • Tonic: always active, slow adapting (e.g. pain receptors).
    • Phasic: normally inactive, fast adapting.
  • Location
    • Exteroreceptors: in the skin.
    • Enteroreceptors: in internal organs.
    • Proprioceptors: in tendons, muscle spindles, joint capsules.
  • Nature of Stimulus
    • Nociceptors: pain receptors.
    • Thermoreceptors: monitor temperature.
    • Mechanoreceptors: distortion of cell membranes.
    • Chemoreceptors: monitor chemical concentration.
    • Photoreceptors: rods and cones in the retina.

• Nociception:

  • Receptors have large receptive fields and can be sensitive to temperature changes, mechanical damage, and dissolved chemicals from injured cells.

• Other Receptor Locations:

  • Thermoreceptors: skin, skeletal muscles, liver, hypothalamus.
  • Mechanoreceptors: Physical stimuli that distort cell membranes.

• Tactile Receptors: Free nerve endings, root hair plexus, tactile discs, bulbous corpuscles, lamellar corpuscles, tactile corpuscles.

• Baroreceptors: Monitor pressure changes.

  • Example: Carotid arteries, aorta, GI tract, lungs, urinary bladder.

• Proprioceptors: Monitor balance and body position.
  Muscle spindles, Golgi tendon organs, joint capsule receptors.

• Sensory Pathways: Three neurons:

  • First-order: from receptor to CNS.
  • Second-order: interneuron in CNS to thalamus.
  • Third-order: thalamus to primary somatosensory area.

• Somatic Sensory Pathways - Types:

  • Spinothalamic: Crude touch and pressure, pain & temperature sensations.
  • Posterior Column: Fine touch, vibration, pressure, proprioception.
  • Spinocerebellar: Balance and body posture (no thalamus).

• Phantom Limb Pain: Painful sensations are NOT produced where they are perceived to originate. Nerve pathways are developmentally programmed and still somewhat intact even with missing limb.

• Somatic Motor Pathways - Compared to Upper and Lower Motor Neurons:

  • Upper motor neurons: cell bodies in a CNS processing center, axons extend down through the brainstem to the spinal cord, and can create a motor stimulus or less depending on the messages being sent.
  • Lower motor neurons: leave the CNS and go to the periphery.

• Pyramidal Cells: Pyramid-shaped cell bodies of motor neurons in the primary motor cortex.

• Medial Pathway: Muscle tone and gross movements of neck, trunk, and proximal limb muscles

• Lateral Pathway: Muscle tone and precise movements of distal parts of the upper limbs.

• Lateral Pathway Importance: If lateral corticospinal tracts are damaged, the lateral pathway will take over motor function.