Somatosensory system Flashcards
What are 3 functions of somatosensory system?
- Proprioception
- Exteroception
- Interoception
What is proprioception?
Receptors are in the skeletal muscle, joint capsule and the skin
Enables us to have conscious awareness of posture and movement of our own body, particularly 4 limbs + head
What happens when there is no sensory feedback from proprioceptors?
The movement are often clumsy, poorly coordinated and inadequately adapted to complex tasks, particularly if visual guidance is absent
What is exteroception?
Sense of direct interaction with external world as it impacts on body
Receptors are located in the skin (mechanoreceptors, thermoreceptors + nociceptors)
Touch + pain mediated by different pathways
What are examples of exteroception?
- Touch [contact, pressure, stroking motion, vibration]
2. Temperature + pain [nociception]
What is Interoception?
Sense of function of major organ systems of body and its internal state
For transmission, what are all somatic senses mediated by?
one class of sensory neurons - dorsal root ganglion neurons
What do axons of primary afferent fibres have?
2 branches, one projecting to periphery and one projecting to CNS
What does axons of each dorsal root ganglion cells serve as?
single transmission line with one polarity between the receptor terminal and CNS
How are DRG guided?
Through development to a specific location in body via various trophic factors
What do peripheral nerve also include?
Motor axon innervating nearby muscles, blood vessels, glands or viscera
What do individual neurons in DRG respond selectively to?
Specific type of stimuli because of morphological and molecular specialisation of peripheral terminals
What are receptors?
Transducers converting stimulus energy into electrochemical energy; receptor potential and nerve action potentials
What do mechanoreceptors sense?
Physical deformation of tissue in which they reside
- light touch, vibration, pressure
- Muscle stretch, contraction, joints angle
What do nociceptors and thermoceptors detect + initiate?
Sensation of painful stimuli + abnormal temperature
What is touch mediated by?
4 types of mechanorecptors in human hand
What are terminals of myelinated sensory nerve innervating hand surrounded by?
Specialised structures that detect contact on skin
What do receptors differ in?
- Morphology
- Innervation pattern
- Location in the skin
- Receptive field size
- Physiological response to touch
What do superificial layers contain?
Small receptor cells: Meissner corpuscle + Merkel cells
What do deep layers of skin + subcutaneous tissue contain?
Large receptors: Pacinian corpuscle + rufinni endings
What does receptive field of mechanoreceptor reflect?
Location + distribution of terminals in skin
What do spike train show?
Responses of each type of nerve when its receptor is activated by constant pressure against skin
What do RA type fibre that innervate Meisnner + pacinian corpuscle adapt to?
Constant stimulation
What do SA type nerve that innervate Merkel cell + Rufinni endings adapt ?
ongoing stimulation
What do rapidly-adapting nerve fibre send information related to?
Changing stimuli
Receptive field in human hand
Smallest at fingertips
What do receptive field of receptors in superificial layers of skin encompass?
Spot-like patches of skin
Where do receptors in deep layers extend across?
Wide region of skin
Where are responses strongest in skin?
Directly over receptor (dark spot)
What is contour map?
Pressure sensitivity throughout the receptive field
What do 2-point threshold measure?
Minimum distance at which 2 stimuli are resolved as distinct
Where are the greatest discriminative capacity afforded in?
Fingertips, lips + tongues, which have the smallest receptive field
Where is tactile acuity in human hand highest in?
Fingertip
What constitutes a dermatome?
The skin + deeper tissues innervated by afferent fibres of a single spinal nerve
What do the 4 mechanoreceptors detect?
- Grip forces (SA1, merkel)
- Hand posture ( SA2, Rufinni)
- Force rate (RA1, Meissner)
- Vibrations at hand contact, lift-off, table contact + release of grasp (RA2, pacinian)
What does a single dorsal root ganglion contain?
around 100,000 neurons
where does incoming afferent information travel
Dorsal root
What do dorsal horn contain?
Orderly arrangement of sensory relay neuron that receive input from periphery
What do ventral horn contain?
Group of motor neurons and interneurons that regulate motor neuronal firing patterns
What does white matter contain?
rostral-caudal ascending and descending tract of myelinated axons
What does ascending pathway carry?
Sensory information to brain
What does descending pathway carry ?
motor command + modulatory signal from brain to muscles
What does the sensory division carry?
Information from muscles + skin into spinal cord + terminate in dorsal aspect of cord
What do different classes of axon within dorsal root convey?
Pain, temperature, touch and visceral sensory information
What do motor division (ventral root) do?
Emerge from ventral aspect of cord and comprises the axon of motor neuron that innervate muscle
What do ventral root from certain levels of SC include?
sympathetic and parasympathetic axon
What do motor neuron of spinal cord comprise
final common pathway through which all higher brain levels controlling motor activity must act
Define referred pain
Disorder of internal organ is sometime perceived as cutaneous pain
What is most common clinical example of referred pain?
Anginal pain
What are 2 main pathways transmitting and processing somatosensory input?
- epicritic sensation - high resolution, light touch = DCMLP
- protophatic sensation - poorly localised, pain + temp
What are part of spinothalamic pathway?
protopathic, pain and temperature
What is the role of dorsolateral tract?
Convery somatosensory information from lower half of body to cerebellum
What does Dorsal-column medial lemniscus pathway mediate?
Light touch, pressure and proprioception
How do many primary afferent travel ?
In a highly-ordered, somatotopic manner in dorsal column
What do dorsal column consist of?
Cuneate = upper limb
Gracile = lower limb
tracts
Where do DCML pathway terminate in?
Cuneate + Gracile nuclei
Axons of second order neuron decussate + project through brainstem as medial lemniscus to thalamus ( ventro-posterior nucleus)
Where do thalamus somatopically project to?
primary somatosensory cortex (S1)
What does the spinothalamic pathway mediate?
Crude touch, pain and temperature
Where does the spinothalamic pathway terminate in?
Thalamus
(Ventroposterior lateral nucleus)
Third order thalamic neuron project to somatosensory cortex
Why are there no clear centre for pain?
widely distributed over many cortical/sub-cortical sites
What is brown-sequard syndrome?
Dissociated sensory loss following a spinal cord hemisection at 10th thoracic level on left side + motor weakness on same side as lesion
who mapped the sensory homunculus?
Penfield (1930’s)
Electrical stimulation of brain
Patient being operated for epilepsy
Where is clear somatotopic map found on?
Face on most lateral surface and lower limb on medial surface
How is receptor density reflected ?
Disproportionately large representation of distal extremities (lips, digits + toes)
