FINISH lecture 21 - nervous system1: skin sensation Flashcards
what is the somatosensory system?
the neural sense concerned with body sensations
what are the 3 divisions of the somatosensory system?
cutaneous (skin) sensations
visceral - internal organs and deep tissues
proprioception - position of the limbs and body in space
• without it movement is impossible
functions of the somatosensory system
recognition - perception doesn’t always correspond to reality
exploration - control of movement
communication - close, social and intimate
why do all sensory systems require receptors?
to convert stimulus energy
converted into a receptor/generator potential
size depends on stimulus strength
isn’t an AP as its graded
what happens when a receptor potential exceeds threshold?
nerve fibres fire an AP
stimulus strength coded by firing rate or by pattern of firing
this is called sensory transduction
where can cutaneous receptors be found?
in the skin - eg. hair receptors
underneath the skin:
• superficial - just below the epidermis
• deep - in subcutaneous fat
some have capsules around them but some of them are free nerve endings
superficial cutaneous receptors
- Merkels disk
- epidermal-dermal border
- free nerve endings
- Meissners corpuscle
Merkels disk and Meissners corpuscle are capsulated receptors
deep cutaneous receptors
- pacinian corpuscle
* Ruffinis corpuscle
how are cutaneous receptors distinguished?
- appearance
- location - deep or superficial
- size of receptive fields
- physiological properties - rapidly or slow adapting
what fibres are associated with cutaneous receptors?
served either by:
• large (A-beta) myelinated fibres
• small (A-delta) myelinated fibres
C fibres are unmyelinated fibres
what is a receptive field?
area of skin over which a stimulus activates a single nerve fibre
what are anerve fibres?
a bundle of nerve fibres
how do receptive fields differ in cutaneous receptors?
Meissners corpuscle and Merkels disk have feil small receptive fields
pacinian corpuscle has large receptive field
Ruffinis corpuscle prefers movement across the receptive field in a certain way
deep receptors have larger receptive fields
what is receptor adaptation?
when sensory receptors change their sensitivity to a stimulus
you get slow and rapidly adapting receptors
slowly adapting receptors
tonic receptors
when you apply stimulus, you initially get a generator response that is maintained
maintained firing of AP throughout duration of stimulus
rapidly adapting receptors
phasic receptors
AP at onset and offset of stimulus but no other
highlights when the stimulus changes - important for detecting threats and opportunities
why is rapidly adapting receptors more useful?
only processes the start and the end of the stimulus
this reduces computational load
doesn’t use as much energy
temperature receptors
warm - free nerve endings
• C fibres: unmyelinated
cold - free nerve endings
• A-delta fibres
detect temperature in discrete spots - cold sensation only felt at specific locations
noxious receptors
respond to noxious or painful stimuli
- mechanical - crushing, pricking
- thermal - painfully hot or cold
small unmyelinated fibres (A-delta): first pain - initial sharp sensation
unmyelinated C fibres: second pain - nagging pain that follows first pain
what cutaneous receptors are slow adapting?
Ruffini
Merkel
what cutaneous receptors are fast adapting?
Meissner
Pacinian
hair follicles
rapid adaptation of the pacinian corpuscle
depends on its mechanical properties - comes from the capsule around the nerve fibre
capsule present - rapid adaptation of receptor potential
capsule removed - much less adaptation of receptor potential
what 2 pathways transmit somatosensory information in the spinal cord?
dorsal column
spinothalamic pathways
what are the 2 components of the spinal cord?
grey matter - butterfly shape and made of neurones
white matter - consists of nerve fibres covered in myelin
• fattiness gives the white colour
