Sensory Pathways: An Overview

Objectives

• The main goal is to understand the anatomical pathways that transmit sensory information
• Understand the key anatomical regions associated with these pathways
• Recognize primary and cortical (secondary) sensations
• Be able to explain the anatomical basis of sensory dysfunction and neurological examination

Role of Sensory Pathways

• The brain receives sensory inputs regarding forces/stimuli acting on the body's exterior through sensory pathways
• Sensory pathways detect changes via the sensory system, which then processes information in the CNS
• The body responds via the motor system
• Humans can identify shapes and textures of objects, monitor forces, and detect potentially harmful circumstances because of the sensory pathways.

Sensory Modalities - Classification

• Somatic sensations include mechanical sensations like touch, pressure, vibration, and proprioception
• Somatic sensations includes pain and temperature sensations
• Special sensations include vision, hearing, balance, taste, and smell

Sensory Pathways

• There are two main systems associated with conscious somatic sensations: the Dorsal Column-Medial Lemniscus (DCML) pathway and the Antero-Lateral System (ALS)
• The DCML transmits sensory information for conscious proprioception and discriminative touch (fine touch/pressure/vibration), and also acts as backup for crude touch
• The ALS includes multiple pathways and transmits pain, temperature, and crude touch
• Spino-cerebellar tracts deal with unconscious proprioception

Conscious Proprioception

• Conscious Proprioception, or kinesthesia, is the sense that lets us perceive the location, movement/action, force, and heaviness of body parts
• It combines with other senses to locate external objects relative to the body and contributes to body image
• Unconscious Proprioception is closely tied to the control and coordination of movements.

Touch

• Crude (simple) touch is a tactile sensation felt when lightly touching or stroking something
• Discriminative touch sensations include fine touch, pressure and vibration, which assists accurate localisation.
• The ability to detect detailed information about objects is discriminative touch sensation
• Differentiating between simultaneously touched points is referred to as 2 point discrimination test.
• Stereoagnosis is the the ability to recognise objects with touch
• Graphaesthesia is the ability to identify symbols/patterns drawn on the skin.
• Texture utilizes accurate localisation to identify textures and shapes

Good News!

• The 2 main pathways consist of 3 neurons
  • First neuron starts from the sensory receptor whose cell body is outside the CNS
  • Second neuron decussates close to its origin, with its cell body in the spinal cord or medulla
  • Third neuron starts at the thalamus and ends in the primary sensory cortex

DCML Pathway - Receptors

• Muscle spindles and Golgi tendon organs detect stretching of muscle and tension during contraction.
• These send information to the brain about the degree of muscle contraction and movement
• Joint kinesthetic receptors detect the stretch/tension in ligaments and at joint capsule
• Sensory receptors for DCML include:
  • Crude touch: free nerve ending
  • Fine touch, texture: Merkel receptor
  • Flutter stroking: Meissner corpuscle
  • Vibration/pressure: Pacinian corpuscle
  • Stretch: Ruffini ending

DCML Pathway

• 1st order neurons ascend within the gracile or cuneate fasciculi to reach the medulla and synapse with 2nd order neurons in the gracile or cuneate nucleus
• 2nd order neurons decussate in the medulla before ascending as the medial lemniscus termininating in the the contralateral thalamus
• 3rd order neurons then synapse in the Ventral Postero-Lateral Nucleus (VPL) of the thalamus
• 3rd order neurons then travels via the posterior limb of the internal capsule to somatosensory corex

Somatosensory Pathways from the Face

• All somatic sensations are transmitted through the 3 main branches of the trigeminal nerve to the brainstem.
• Axons of the trigeminal nerve synapse in 3 nuclei in the brainstem:
  • Mesencephalic nucleus for unconscious proprioception
  • Main sensory nucleus for all forms of touch and conscious proprioception
  • Spinal nucleus for pain and temperature
• From those nuclei, the signals go through the ventral trigeminothalamic tract (VTT) to the sensory cortices.

Ventral Trigeminothalamic tract

• (For proprioception, vibration, discriminative touch from the head)
  • 1st order neurons start from the receptors in the head and have their cell bodies located in the trigeminal ganglion. Their central processes enter the Pons.
  • In the Pons, these neurons synapse with the cell bodies of the 2nd order neurons located in the Main Sensory Nucleus of the Trigeminal tract.
  • The 2nd order neurons decussate in the pons and enter the VTT, then ascend in the VTT and synapse with the 3rd order neurons in the Ventral Postero-Medial nucleus (VPM) of the thalamus.
  • The 3rd order neurons enter the posterior limb of the internal capsule, pass through the corona radiata, and reach the head representing region of the Primary somatosensory cortex.

Testing DCML Pathway

• Proprioception tested by moving the patient's big toe (lower limb) or fingers for upper limbs holding lightly perpendicular to movement axis.
• Patients should have their eyes closed during the examination
• Vibration sense is tested through the use of a 128hz tuning fork placed on bony prominences.
• Discriminatory ability is tested using a forceps to determine a patients ability to determine that they are being touched in two places at once.

Romberg Sign

• Cerebellum plays the central role in truncal stability
• Sensory ataxia is a lack of co-ordination of limb and trunk function that aggravates with loss of visual inputs
• Romberg sign involves three sensory inputs which maintain truncal stability.
• At least two must be intact to maintain balance
• Propioception or vestibular function impairment will cause the patient to be unstable with eye closure
• Romberg's test only tests sensory atxia and is not a test for cerbellar function

Gait in Dorsal Column Disease

• Sensory Ataxic Gait (Steppage Gait)- a lack of reception makes knowing when the foot hits the ground challenging.

Lesions involving DCML

• Lesions affecting the sensory cortex can cause complete contralateral sensory loss.
• A lesion affecting the internal capsule above the lower medullar can effect the entire contralateral body.
• Midbrain lesions can cause the loss of DCML signs on the entire contralateral body
• Cervical spine Lesion in left spinal cord can result to ipsilateral spinal cord loss- affect upper and lower limb
• Thoracic spine loss of DCML - ipsilateral lower limb

Dorsal Column Diseases

• Signs of sensory ataxia is unsteadiness that is worse during night and is associated with vibration and proprioception deficits
• Includes a positive Romberg test and can be diagnosed with an examination on ipsilateral loss of vibration and proprioceptors
• *Causes Include:
• Deterioration through many sclerosis
• Compression and calcification of ligaments
• Artery blockage