Somatic (sensory) system

Sensory modalities

• Pain e.g. pinprick sensation
• Touch e.g. cotton wool
• Joint position sense (proprioception)
• Deep pain e.g. muscle squeeze
• Vibration e.g. tuning fork test
• Temperature

Nerve pathways

• Third order neuron - Sensory cortex
• Second order neuron - Thalamus
• First order neuron - Dorsal horn spinal cord

Two main pathways

• Spinothalamic pathway - fibres cross in spinal cord (pain, temperature)
• Dorsal columns (Cuneate / Gracilis fasciculus) - fibres cross in brainstem (touch, vibration sense, joint position)
• Fibres all exit the dorsal nerve root from the spinal cord

Clinical relevance

• Lesions in one half of the spinal cord result in loss of all sensory modalities on one side of the body.
• Lesions in one half of the brainstem lead to 'crossed signs' with pain/temp affected on one side of the body and touch, vibration, JPS affected on the other.
• Generalised and gradual loss of peripheral nerve function results in a receding 'glove and stocking' pattern

Glossary of abnormal sensory disturbances

• Dysaesthesia - an unpleasant, abnormal sense of touch, and it may be, or not be, considered as a kind of pain. (sensations, whether spontaneous or evoked, such as burning, wetness, itching, electric shock, and pins and needles.)
• Paresthesia - a sensation of tingling, pricking, or numbness of a person's skin with no apparent long-term physical effect. Often described as the feeling of "pins and needles" or of a limb "falling asleep".
• Allodynia a pain due to a stimulus which does not normally provoke pain and can be either thermal or mechanical.
• Hyperalgesia an increased sensitivity to pain
• Hyperesthesia (or hyperaesthesia) an abnormal increase in sensitivity to stimuli of the sense. When a non-noxious stimulus causes the sensation of pain (see Allodynia)
• Hypoalgesia or hypalgesia - a decreased sensitivity to painful stimuli.