The Somatosensory System Flashcards
Define Somatosensory function
Ability to interpret bodily sensations o mechanical o thermal o proprioceptive o nociceptive
What does the somatosensory system consist of?
o Sensory receptors in skin, tissues & joints
o nerve cells & tracts in body & spinal cord
o brain centres that process & modulate sensory information
What is referred to as somatosensory modalities?
Includes all conscious senses: o touch o proprioception o temperature o pain
What are the 3 main receptors making up the somatosensory modalites?
o Mechanoceptor
o Thermoceptors
o Nociceptors
Describe the nerve endings for the 3 main receptors making up the somatosensory modalities?
o FREE nerve endings:
- thermoceptors
- nociceptors
o ENCLOSED nerve endings
- mechanoceptors
Nerve Fibre Classification?
FIBRE A:
o Alpha - proprioception (MOTOR)
o Beta - touch, pressure
o Gamma - MOTOR to muscle spindle
o Delta - pain, cold & touvh
FIBRE B:
o POSTganglionic autonomics
FIBRE C:
o Dorsal root - pain, temperature & mechanoception
o Sympathetic - POSTganglionic sympathetic
Main SENSORY neurones?
o A-beta
o A-delta
o C-fibres
Properties of the MAIN SENSORY neurones?
A-delta & C-fibres heavily invovled in PAIN (noxious)
A-beta heavily involved in TOUCH & PRESSURE (non-noxious)
Speed of conduction of the MAIN SENSORY neurones?
A-beta = MOST QUICKLY (most abundant)
A-delta = SMALLER but myelinates
C-fibres = SMALLEST & NO myelination
Define sensory receptor
Transducers
that convert energy from the environment into neuronal APs
Fibres associated with thermoreceptors and their properties?
A-DELTA & C-FIBRES
(have FREE nerve endings)
Work via. TRP ION CHANNELS
Specific TRP channels associated with thermoreceptors and what they’re activated by?
Transient Receptor Potential (Ion Channels)
o TRPV1-4 - HEAT activated
o TRPM8 & TRPA1/V1 - COLD activated
Thermoreceptors that have sense overlap?
TRPV - heat & spice
TRPM8 - cold & menthol
TRPV1 - hear & cold
5 example of Mechanoreceptors?
o Meissner’s Corpuscles
o Merkel Cells
o Pacinian Corpuscle
o Ruffni Endings
o Hair follicle
Meissner’s Corpuscle?
Mechanoreceptor
o Fine, discriminate touch
o LOW frequency vibration
Merkel Cells?
Mechanoreceptor
o Light touch
o Superficial pressure
Pacinian Corpuscles?
Mechanoreceptor
o Detects deep pressure
o High frequency vibration & tickling
Ruffni Endings?
Mechanoreceptors
o Continous pressure
OR
o Touch & stretch
Fibres associated with mechanoreceptors?
A-BETA
Define Stimulus Threshold
Described as the weakest stimulus detectable
The point of intensity at which the person can DETECT the PRESENCE of a STIMULUS 50% OF THE TIME (absolute threshold)
Define Stimulus Intensity
Info. regarding this is conveyed by a variation of:
o Frequencies of APs generated
o Number of seperate receptors activated (recruitment)
What does Stimulus Threshold vary with?
o Anatomical location
o Inter-individual differences
Explain the Neural Adaptation phenomenon
If stimulus of CONSTANT STRENGTH is maintained for a period time, the FREQUENCY of APs DIMINISHES
Tonic receptors?
SLOWLY adapting (e.g. Merkel cells)
o Detect CONTINOUS stimulus strength (continue to transmit impulse to brain as long as stimulus present)
Allows for (to be perceived):
o superficial pressure
o fine touch
Phasic receptors?
RAPIDLY adapting (e.g. pacinian receptors)
o Detect a CHANGE in stimulus strength (transmit impulse @ start & end of stimulus)
It:
o sudden pressure excites receptor
o transmits signal again when pressure released
Define Receptive Field
Region on skin which causes activation of a SINGLE SENSORY NEURONE when activated
Difference between SMALL & LARGE Receptive Fields
Small R.F:
o allows for detection of fine detail over a SMALL area
o precise perception
Large R.F:
o detects changes over a WIDER area
o less precise perception
Can Receptive Fields overlap?
YES
Recruitment of adjacent field can INCREASE STIMULUS INTENSITY = INCREASE APs
Define 2-point Discrimination
MINIMUM DISTANCE at which TWO POINTS are perceived as separate
What does 2-point discrimination depend on?
- Peripheral mechanoreceptors (density of innervation area of receptive fields)
- Spinal posterior column (convergence of signals)
- Cortical function (sensory homunculus)
Where are the sensory cell bodies for the body & face?
Body:
o Dorsal Root Ganglia
Face:
o Trigeminal Ganglia
Define Lateral Inhibition
Activation of one neural unit INHIBITS the activation of other neural units
Prevents the OVERLAP of RECEPTIVE FIELDS - enhances sensory perception (discrimination)
What mediates lateral inhibition?
Inhibitory INTERNEURONES within the dorsal horn
2 main types that the dorsal horn neurone can be divided into?
o Projection neurones - project to brin
o Interneurones - remain in SC
