Somatosensory Receptors and Sensory Pathways

Learning Outcomes

• The aim is to identify and describe key somatosensory receptors.
• Describe the structure and function of Dorsal Columns, Spinothalamic tract, and Spinocerebellar tract.
• Understand the connection from the sensory pathways to the cerebral cortex.

Definitions

• Sensory system involves sensory receptors receiving stimuli from external or internal environment through neural pathways to the brain.
• Sensation occurs when sensory information reaches consciousness.
• Perception is the understanding of the sensations meaning.
• Somatic receptors are receptors in the outer body, including skin, skeletal, muscle, tendons, and joints.
• Special senses include the vestibular system, visual system, auditory system, and chemical senses (taste, smell).
• Perceptual processing involves understanding and interpreting information.

Afferent Nerve Endings

• There are various types of stimuli and classifications based on structure, functional type, and type of stimuli.
• Types of afferent nerve endings include mechanoreceptors, thermoreceptors and nociceptors.

Major Sensory Pathways Overview

• First-order neurone is the primary afferent neuron.
• Second-order neurone decussates.
• Third-order neurone projects to the somatosensory cortex.
• Dorsal columns are responsible for proprioception and discriminative touch.
• Spinothalamic is responsible for pain, temperature and non-discriminative touch.

Dorsal Columns

• Responsible for proprioception and discriminative touch.
• Fasciculus gracilis handles information from the lower limb.
• Fasciculus cuneatus handles information from the upper limb.
• The Nucleus gracilis and Nucleus cuneatus both decussate.
• Medial leminicus projects from the thalamus.
• Information processed projects from the thalamus to the sensory cortex.

Spinothalamic Tract

• Deals with pain, temperature and non-discriminative touch.
• Synapses in the dorsal horn and decussates.
• Spinothalamic tract travels contralaterally to the thalamus.
• Fibers pass through the spinal lemniscus to the thalamus.
• The processed information is then sent from the thalamus to be interpreted in the sensory cortex.

Spinocerebellar Tract

• Has a role is unconscious proprioception.
• Important for control of posture and co-ordination of movement.
• Associated receptors are muscle spindles, golgi tendon organs and tactile receptors

Consolidation

• The topics covered include the type and function of sensory receptors.
• Also the different nerve fibre types and their conduction speeds.
• Name and describe the 3 sensory pathways and the type of information related to each.
• Discern what information it conscious, and why.

Case Based Discussion

• Four months ago Naisha sustained a spinal cord injury at level T10 after falling off a horse.
• A recent assessment found a complete loss of voluntary movement and sensation in the right/left lower limbs + sacral area
• The assessment also found abnormal and normal upper limb reflexes.
• This presentation is explained by knowledge of the somatosensory system.