Sensation - anatomy, physiology of the superficial and deep sensibility
Definition: Sensation is the ability to perceive and interpret stimuli from the external and internal environment. It requires:
- Intact receptors. Peripheral nerves. Ascending sensory pathways. Thalamus. Sensory cortex (postcentral gyrus)
Classification of Sensibility
1. Superficial Sensibility (Exteroceptive): Arises from skin and mucous membranes
- Modalities: Pain. Temperature. Crude (non-discriminative) touch
2. Deep Sensibility (Proprioceptive): Arises from muscles, tendons, joints, and deep tissues
- Modalities: Joint position sense. Movement sense (kinesthesia). Vibration sense. Deep pressure
A. Superficial Sensibility Pathway
Receptors
- Free nerve endings → pain, temperature
- Mechanoreceptors → crude touch
Spinothalamic Tract (Anterolateral System)
1. First-order neuron: Cell body: dorsal root ganglion. Enters spinal cord via dorsal root
2. Second-order neuron
- Location: posterior horn of spinal cord
- Decussates within 1–2 spinal segments
- Ascends in:
- Lateral spinothalamic tract → pain & temperature
- Anterior spinothalamic tract → crude touch
3. Third-order neuron
- Thalamus: ventral posterolateral (VPL) nucleus
- Projects to: Primary sensory cortex (postcentral gyrus, Brodmann 3,1,2)
Clinical Importance
- Lesion in spinal cord → contralateral loss of pain and temperature below lesion
- Due to early crossing of fibers
B. Deep Sensibility Pathway
Receptors: Muscle spindles. Golgi tendon organs. Joint receptors. Pacinian corpuscles (vibration)
Posterior Column–Medial Lemniscus System
1. First-order neuron
- Cell body: dorsal root ganglion
- Ascends ipsilaterally in posterior columns: Fasciculus gracilis → lower limb. Fasciculus cuneatus → upper limb
2. Second-order neuron
- Nuclei in medulla: Nucleus gracilis. Nucleus cuneatus. Fibers decussate in medulla. Form medial lemniscus
3. Third-order neuron: Thalamus (VPL nucleus). Projects to primary somatosensory cortex
Functions (Deep Sensibility): Vibration sense. Proprioception (position sense). Kinesthesia (movement sense). Fine/discriminative touch
Clinical Importance
- Lesion causes: Loss of vibration sense. Loss of position sense. Sensory ataxia. Positive Romberg sign
C. Cortical Sensory Functions: Require intact superficial + deep sensation
1. Stereognosis: Identification of objects by touch (eyes closed)
2. Graphesthesia: Recognition of written symbols on skin
3. Two-point discrimination: Ability to distinguish two close points
4. Barognosis: Ability to perceive weight differences