Somatosensory System Flashcards
Describe the dorsal column (Medial Lemniscal System):
Tactile sensation and conscious proprioception are mediated by the dorsal column (dorsal or posterior funiculus) – medial lemniscus system
The dorsal (posterior) funiculus/ dorsal column contains two long ascending tracts
Central axons of these neurons travel in dorsal roots and enter the posterior funiculus of the spinal cord
Axons from sensory neurones below T6 form the medial part of the dorsal funiculus (fasciculus gracilis)
Axons from sensory neurones above T6 form the lateral part (fasciculus cuneatus)
Axons ascend in the spinal cord to 2nd order neurone in the medulla
Describe the Fasciculus Gracilis of the dorsal column:
Located medially and contains ascending fibres from sacral, lumbar and lower thoracic segments T6-T12
Involved in mediating conscious proprioception including kinaesthesia and tactile sensations including discriminative touch
1st order neurones:
- dorsal root ganglia peripheral processes
- innervate the Pacinian and Meissner’s corpuscles of the skin and proprioceptors
- ascend ipsilaterally in spinal cord
2nd order neurones:
- located in the ipsilateral nucleus gracilis in the medulla
- travel as the internal accurate fibres
- decussate at medial lemniscus
3rd order neurones:
- synapse with 2nd order in the contralateral ventral posterolateral nucleus of the thalamus
- terminate in medial aspect of sensorimotor cortex
Describe the Fasciculus Cuneatus of the dorsal column:
Joins spinal cord at level T6 and contains long ascending fibres from the upper body
Enter dorsal funicuclus medial to dorsal horn & ascend dorsal horn in a lateral position
1st, 2nd and 3rd order neurones the same except third order terminate in lateral aspect of sensorimotor cortex
Describe the trigeminal touch pathway:
Trigeminal nerves supply somatic sensation from the face and top of the head to the somatosensory cortex
Gives rise to ophthalmic, maxillary and mandibular divisions
Axons from sensory receptors synapse with 2nd order neurons in the ipsilateral trigeminal nucleus
Decussate and project to VPN of thalamus
Sensory information relayed to somatosensory cortex
Describe the effects of lesions to the dorsal column:
Damage to the dorsal column tracts results in ipsilateral symptoms and below level of spinal cord lesion
These symptoms include loss of tactile sense (vibration, deep touch and two point discrimination) and kinaesthetic sense (position and movement)
Patients with such lesions are unable to identify the position of their limbs when their eyes are closed and do not know if one of their joints is in flexion or extension
Asteregnosis - when the lesion affects the cervical region, can’t identify shape, size or texture of the object in hand ipsilateral to the lesion
Tabes Dorsalis - syphilis, central processes of DRG degenerate resulting in loss of vibration sensation, two point discrimination and conscious proprioception
Describe the spinothalamic tract:
Pain and temperature sensations ascend in the spinothalamic tract that occupies the ventrolateral quadrant of the spinal cord on the contralateral side to where the afferent axons enter
Due to the complexity of pathways modulating pain and its associated components which all ascend in
the ventrolateral quadrant, they are collectively known as the anterolateral system of ascending tracts
Consists of direct and indirect pathways
Describe the direct pathway of the spinothalamic tract:
Neospinothalamic tract - mediates the 1st pain which is sharp and well localised and
relayed rapidly to the cortex
1st order neurones located in the dorsal root ganglia and their central axons enter the spinal cord reaching the dorsal horn
Give off ascending and descending branches which run up or down for one or two spinal segments forming the posterolateral tract of Lissauer
Axons of the 2nd order neurones cross obliquely via the ventral white commissure to enter the contralateral white matter where they ascend in the lateral funiculus
The ascending axons synapse on 3rd order neurones in the ventral posterolateral nucleus of the thalamus
Somatotopically arranged:
- fibres from lower body ascend dorsolaterally
- upper extremities & neck ascend ventromedially
- project to sensorimotor cortex
Describe the indirect pathway of the spinothalamic tract - paleospinothalamic:
Sense non discriminative touch, pain and temperature
Neurones located in dorsal horn and intermediate gray matter
Axons ascend bilaterally in the ventrolateral quadrant of the spinal cord
Several synapses with reticular formation of brainstem
Project to the midline and intralaminar thalamic nuclei
These nuclei project to the cerebral cortex, especially the limbic regions such as the cingulate gyrus
Describe the indirect pathway of the spinothalamic tract - spinoreticular:
Sense pain, neurones located in dorsal horn and intermediate gray matter
1st order neurones from receptor to dorsal horn
2nd order neurones synapse in medullary and pontine reticular formations
Projections from the spinal cord to the brain are both crossed and uncrossed and these ascending fibres are believed to transmit sensory
information to the reticular formation
Activates the cerebral cortex through secondary and tertiary projections via the midline and intralaminar thalamic nuclei
The thalamocortical projections are highly diffuse and influence wide areas of the cerebral cortex
Describe the indirect pathway of the spinothalamic tract - Spinomesencephalic tract
Projects to midbrain structures involved in pain modulation and emotional-affective responses to pain
located in the anterolateral dorsal horn and intermediate grey matter
Originates Dorsal horn (Lamina I, V)
Decussates at anterior white commisure
Destination is periaqueductal gray and midbrain
Function:
- Modulates pain perception by engaging descending inhibitory pathways (PAG → nucleus raphe magnus → spinal cord dorsal horn).
- Integrates pain with emotional and autonomic responses.
- Helps orient the body to noxious stimuli, coordinating reflexive movements
Describe the sensory receptors of touch modalities, vibration and pressure sensations
Hair follicles:
- embedded in the skin and innervated by nerve endings
- hair is bent causing mechanical deformation + activating nerve ending
Pacinian Corpuscle:
- located in dermis of both hairy and glabarous skin
- consist concentric lamellae of flattened cells with the spaces filled with fluid
- where a myelinated nerve enters the corpuscle the myelin sheath disappears, and a bare nerve terminal occupies the centre of the corpuscle
- receptors are low threshold, rapidly adapting and sensitive to high vibration
Meisner’s Corpuscle:
- stacks of horizontally flattened epithelial cells in a
connective tissue sheath
- located beneath epidermis in ridges of glabrous skin
- low threshold rapidly adapting mechanoreceptors that are sensitive to touch and vibration
Merkel’s discs:
- located in epidermis of glabrous skin
- consist of a nerve terminal and a flattened non- neural epithelial cell
- low threshold, slowly adapting mechanoreceptor responding to pressure
Ruffini’s Corpuscle:
- epidermis of both skin types
- consist of encapsulated bundles of collagen fibrils that are connected with similar fibrils of the dermis
- low threshold, slowly adapting mechanoreceptors responding to stretching of the skin
Describe conscious proprioception:
In conscious proprioception the receptors located in the joint capsules provide sensory information to the cerebral cortex
Then the cerebral cortex uses this information to generate conscious awareness of kinesthesia (joint position, direction and joint velocity)
These receptors which are located in ligaments and joint capsules consist of free nerve endings and encapsulated receptors which are low threshold mechanoreceptors
Slowly adapting receptors - provide information about the ability of an individual to judge the position of a joint without seeing it and without movement
Rapidly adapting receptors - provide information about the ability of an individual to perceive the movement of a joint and the judge the direction and velocity of its movement
Describe non-conscious proprioception:
Non-conscious proprioception refers to the automatic, unconscious awareness of body position and movement, crucial for motor coordination, balance, and posture
does not reach the cerebral cortex for perception — instead, it supports reflexive and coordinated movement via the cerebellum
Enteroceptive:
- from viscera
- results in homeostasis
Proprioceptive:
- from within the body
- result is coordination and refinement of movement
