Sensory Receptors
Receptors
types
externoceptor - i.e. thermoreceptor
internoceptor - i.e. baroreceptor (BP)
proprioceptor - body position
nocieptors = pain
transduction
always some form of energy: chem, mech, therm, EM
generator potential (receptor pot): min stim required to get receptor response
sequence of events: generator potential; AP; synaptic events in neural network; motor activity
stim changes membrane potential: could be depolarizing or hyperpolarizing
nerve ending could be free (pain, therm), encapsulated
- encapsulated could be attached or not (i.e. rec in hair not attached, uses NT)
adaptation
long adapting (tonic) - long stim produces long response; good for transducing static stimuli
rapidly adapting (phasic) - only produce short response; good for transducing changing stimuli
tonic and phasic receptors allow sense of different facets of a stimulus
receptive fields
region where stimulation can cause receptor potential
allows for pinpointing of stim (two-point descrimination)
- related to density of receptors
- also more complex signal processing – fatigue, practice, stress
may explain phantom limb effect
- demonstrates that signal can be blocked out or exaggerated
- phantom limb could also be from stimulation of nerve endings to give false signal
Sensory Coding
determined by:
- types and number of rec activated
- response of receptors
- information processing in signal path (i.e., inhibitory + excitatory)
aspects of signal coding:
- modality
: readily recognized type of sensation
- labeled line – set of neurons dedicated to single modality
- spatial location
: detemined by overlap of receptive fields or combinaion of excitatory and inhibitory
- threshold (adequate) stimulus
: minimum signal required to create AP in afferent neuron
- may need to excite several neurons to get response in CNS
- low threshold = high sensitivity
- intensity
: frequency of APs + number of receptors activated
- frequency vs. intensity usually nonlinear
- frequency
- duration
: usually only for long adapting receptors
divergence: branching of axon to many cells
convergence: innervation of one cell by many nerves
somatotropic maps: cortical or subcortical arrangements of sensory paths that reflect organization of body
recruitment: small motor units are activated before large; due to ‘size principle’
- excitability of motor units inversely proportional to size of unit
- small fast-twitch units used for low muscle effort
- longer stimulus = larger muscle effort = larger muscle units excited