D’YOUVILLE COLLEGE
BIOLOGY 659 - INTERMEDIATE PHYSIOLOGY I
SENSORY RECEPTORS, SUMMATION
Lecture 11: Sensory systems, Sensory neurons, Neuron pools
1.
Sensory Receptors, Neurons:
• part of afferent pathway of reflex arc: receptor  sensory neuron  CNS
• types of sensory receptor (table 46 – 1):
- mechanoreceptors: tactile, position senses respond to touch, pressure,
vibration, tickle & itch (fig. 46 – 1 & ppt. 1); hearing & equilibrium (special senses);
pressure, stretch in internal organs (proprioceptors)
- thermoreceptors: respond to temperature changes on body surfaces or
deeper tissues (somatic)
- chemoreceptors: respond to chemicals dissolved in mucus, tissue fluid, etc.;
sense of smell, taste (special senses); level of chemicals in body fluids
- photoreceptors: respond to electromagnetic energy (visible spectrum); sense
of vision
- nociceptors: pain receptors respond to various sensory modalities that are
extreme: mechanical, thermal, chemical, tissue damage
• levels of sensory systems (ppt. 2):
- receptor level involves sensory receptor and sensory nerve fiber (afferent
pathway of reflex arc)
- circuit level involves ascending pathways and processing (amplification or
damping out &/or distribution of signal to different destinations); second and third order
neurons participate in delivery of signal to higher brain levels (e.g. cerebral cortex,
cerebellum, thalamus, reticular formation)
- perceptual level involves processing of sensory input at cerebral cortex
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• labeled line principle: localization of stimulus & perception of sensory
modality is not a property of a receptor, but is determined by destination in sensory
area of brain
• sensory adaptation (fig. 46 – 5 & ppt. 3): chronic stimulation causes decline
in sensitivity to stimulus, often to zero sensitivity; speed of adaptation varies widely
from fraction of second to minutes or hours (e.g. pain receptors)
- slowly adapting receptors detect persistent stimuli (tonic receptors);
examples include receptors monitoring musculoskeletal activity, blood pressure, blood
chemistry or static equilibrium; these tonic receptors keep brain apprised of constant
conditions
- rapidly adapting receptors detect rates of stimulation, vibration, movement
across sensory field; they generate a signal at onset of any change in stimulus then
quickly adapt, but generate another signal if the stimulus is removed or changes strength;
examples include receptors monitoring tactile stimuli, rate of change of position, dynamic
equilibrium; these phasic or rate receptors help anticipate changes in body position
over the next few seconds so adjustments can be made to maintain balance (e.g. rounding
a curve while running)
• receptor potential: sensory receptors, regardless of modality (physical
distortion, heat, chemicals), respond to their own modality by transducing one form of
energy (physical, chemical, etc.) to electrical energy (change in membrane potential);
generates receptor potential
• signal strength: stronger stimuli generate stronger receptor potentials, which are
of prolonged duration; if sensory neuron is raised to threshold  action potential
results; if sensory neuron is kept above threshold  volley of action potentials results
(temporal summation) (figs. 46 – 2, 46 - 8 & ppts. 4 & 5)
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- spatial signal strength: stronger stimuli excite larger numbers of receptors in
sensory field, resulting in signal spread within sensory nerve (fig. 46 - 7 & ppts. 6 & 7)
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• classification of nerve fibers (fig. 46 – 6 & ppt. 8): variability in velocity of
transmission, based on axon diameter and state of myelination or non-myelination
- A fibers (myelinated) – fast neurons; fastest = A least fast = A; somatic
sensory & motor nerve fibers
- C fibers (non-myelinated) are slow neurons; less precise sensory fibers
(tickle & itch) and some pain fibers
• alternative classification (certain sensory nerve fibers) (fig. 46 – 6 & ppt. 8):
also based on axon diameter and state of myelination or non-myelination
- IA fibers (myelinated) – correspond to A orfaster A
- II & III fibers (myelinated) – correspond to slower A, A & A
- IV fibers (non-myelinated) – correspond to C fibers
2.
Processing in Neuron Pools:
• patterns of signals in neuron pools: synaptic connections of input neurons
may range from few to hundreds or thousands of synaptic terminals; postsynaptic (output)
cells may have elaborate dendritic branching
- determination of signal pattern that passes within CNS is based upon # of
input synapses firing at a given postsynaptic (output) cell (fig. 46 – 9 & ppt. 9)
- if input produces subthreshold potential (EPSP), output cell doesn’t fire but is
facilitated
- if input produces EPSP at or above threshold, output cell is excited (i.e.
discharges APs)
- organization of neuron pools promotes excitatory output from center of
input & facilitation of output cells peripheral to center of input (fig. 46 – 10 & ppt.
10)
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• some typical patterns (designed to exploit summation effects) (figs. 46 – 11
to 46 – 14 & ppts. 11 to 14):
- divergence of output: may result in amplification or distribution to multiple
destinations
- convergence: designed to maximize temporal &/or spatial summation
- lateral inhibition: input transmits to excitatory and parallel inhibitory output
 sharpens output signal
- after discharge: slowly acting neurotransmitters or synapses with presynaptic
cells may produce prolongation of output signal
- reverberating circuits exploit self-stimulation to produce a continuous output
for prolonged period of time (e.g. sleep - wake cycle, breathing)
- rhythmic outputs result from collaboration of reverberating circuits with
excitatory vs. inhibitory outputs