Phys Ch 46
Types of Sensory Receptors and Stimuli They Detect
 Basic types of sensory receptors: mechanoreceptors (detect mechanical compression or stretching of receptor
or tissues adjacent to receptor), thermoreceptors (detect changes in temperature; either detect hot or cold),
nociceptors (pain; detect damage occurring via physical or chemical means), electromagnetic receptors (detect
light on retina of eye), and chemoreceptors (detect taste in mouth, smell in nose, and content of bodily fluids)
 Modality – each principal type of sensation (pain, touch, sight, sound, etc.)
 Each nerve tract terminates at specific point in CNS, and type of sensation felt when nerve fiber stimulated
determined by point in nervous system to which fiber leads (if pain fiber stimulated, it feels painful, regardless
of what actually started the stimulation)
o Specificity of nerve fibers for transmitting only one modality of sensation – labeled line principle
Transduction of Sensory Stimuli into Nerve Impulses
 All sensory receptors – stimulus exciting receptor has immediate effect to change membrane electrical potential
of receptor (receptor potential)
 Different receptors can be excited by mechanical deformation of receptor (stretches receptor membrane and
opens ion channels), application of chemical to membrane (opens ion channels), change of temp of membrane
(alters permeability), or effects of electromagnetic radiation (i.e., light; directly or indirectly changes receptor
membrane characteristics, allowing ions to flow through membrane channels)
o Basic cause of membrane potential is change in membrane permeability, which allows ions to diffuse
more or less readily through membrane, changing transmembrane potential
 The more the receptor potential rises above threshold level, the greater the AP frequency
 Receptor potential induces local circuit of current flow that spreads along nerve fiber; at first node of Ranvier,
local current flow depolarizes fiber membrane, which sets off typical APs that are transmitted along nerve fiber
toward CNS
 Very intense stimulation of receptor causes progressively less and less additional increase in number of APs;
allows receptor to be sensitive to very weak sensory experience and yet not reach max firing rate until sensory
experience extreme
 All sensory receptors adapt either partially or completely to any constant stimulus after period of time; receptor
responds at progressively slower rate until rate of APs decreases to very few or none at all
o All mechanoreceptors adapt completely given enough time
 Non-mechanoreceptors adapt slower, and some never adapt completely
 Accommodation – even if central core fiber continues to receive stimulus, tip of nerve fiber gradually becomes
accommodated to stimulus; results from progressive inactivation of Na+ channels
 Slowly adapting receptors continue to transmit impulses to brain as long as stimulus present; keep brain
constantly apprised of status of body and relation to surroundings
o Impulses from muscle spindles and Golgi tendon apparatuses allow nervous system to know status of
muscle contraction and load on muscle tendon at all times
o Receptors of macula in vestibular apparatus, pain receptors, baroreceptors of arterial tree, and
chemoreceptors of carotid and aortic bodies all slowly adapting receptors
o Also called tonic receptors
 Receptors that adapt rapidly don’t transmit continuous signal, but react strongly while a change is taking place
o Also called rate receptors, movement receptors, or phasic receptors
o Allows brain to know rate of change and make corrections ahead of time to compensate
o Loss of this predictive function makes tasks like running impossible
Nerve Fibers that Transmit Different Types of Signals and Their Physiologic Classification
 The larger the diameter of nerve fiber, the great the conducting velocity
 Type A fibers – typical large and medium-sized myelinated fibers of spinal nerves
 Type C fibers – small unmyelinated nerve fibers that conduct impulses at low velocities; constitute more than
half of sensory fibers in peripheral nerves, as well as all postganglionic autonomic fibers
 Group Ia – fibers from annulospiral endings of muscle spindles (type Aα in general classification)
 Group Ib – fibers from Golgi tendon organs (also type Aα)

Group II – fibers from most discrete cutaneous tactile receptors and from flower-spray endings of muscle
spindles (type Aβ and Aγ)
 Group III – fibers carrying temp, crude touch, and pricking pain sensations (type Aδ)
 Group IV – unmyelinated fibers carrying pain, itch, temperature, and crude touch sensations (type C)
Transmission of Signals of Different Intensity in Nerve Tracts – Spatial and Temporal Summation
 Spatial summation – increasing signal strength transmitted by using progressively greater numbers of fibers
o Receptor field – entire cluster of fibers covering the area that transmits through one fiber
o Arborizing fibrils overlap from other fibers
o Pinprick in center of receptive field of particular pain fiber has larger degree of stimulation than when it
is in periphery of field because number of free nerve endings in middle of field much greater
 Temporal summation – increasing frequency of nerve impulses to increase strength of signal
Transmission and Processing of Signals in Neuronal Pools
 Entire cerebral cortex could be considered single large neuronal pool; other neuronal pools include different
basal ganglia and specific nuclei in thalamus, cerebellum, mesencephalon, pons, and medulla
 Each neuronal pool has its own organization that causes it to process signals in its own way, allowing total
consortium of pools to achieve multitude of functions of nervous system
 Stimulatory field – neuronal area stimulated by each incoming nerve fiber; large numbers of terminals from each
input fiber lie on nearest axon in field, and progressively fewer terminals lie on neurons farther away
 Discharge zone (excited zone or liminal zone) is area in center of input nerve fiber where all neurons stimulated
by incoming fiber; area outside the discharge zone is facilitated zone (subthreshold or subliminal zone)
 Inhibitory zone – entire field of inhibitory branches; degree of inhibition highest in center of this zone
 Divergence – signals entering neuronal pool excite far greater numbers of nerve fibers leaving pool
o Amplifying divergence characteristic of corticospinal pathway in control of skeletal muscles
o Divergence into multiple tracts – signal transmitted in 2 directions from pool
 Convergence – multiple inputs uniting to excite single neuron; allows summation of info from different sources
o Interneurons of spinal cord receive converging signals from peripheral nerve fibers entering cord,
propriospinal fibers passing from one segment of cord to another, corticospinal fibers from cerebral
cortex, and several other long pathways descending from brain into spinal cord
 Reciprocal inhibition circuit – responsible for controlling all antagonistic pairs of muscles (inhibits antagonists so
they don’t interfere with movement of agonists); also prevents overactivity in many parts of brain
 Afterdischarge – prolonged output discharge after incoming signal over
o When excitatory synapses discharge on surfaces of dendrites or soma of neuron, postsynaptic electrical
potential develops in neuron and lasts, especially when long-acting synaptic transmitter substances
involved; as long as potential lasts, it can continue to excite neuron, causing continuous train of APs
o Reverberatory (oscillatory) circuit – caused by positive feedback in neuronal circuit that feeds back to reexcite input of same circuit; circuit may discharge repetitively for a long time
o Intensity of output signal increases to high value early in reverberation and decreases to critical point, at
which it suddenly ceases entirely; due to fatigue of synaptic junctions in circuit
 Fatigue beyond certain critical level lowers stimulation of next neuron in circuit below threshold
level so circuit feedback suddenly broken
 Duration of total signal controlled by signals from other parts of brain that inhibit or facilitate
circuit
 Neuronal circuits that emit APs continuously even w/o excitatory input signals; can be continuous intrinsic
neuronal discharge or continuous reverberatory signals
o Membrane potential normally high enough to cause them to emit APs continually; seen in many
neurons of cerebellum and most interneurons of spinal cord
o Used by ANS to control vascular tone, gut tone, degree of constriction of iris in eye, and heart rate
 Neuronal circuits can emit rhythmical output signals (rhythmical respiratory signal originates in respiratory
centers of medulla and pons); some like breathing just go, and some like walking require input stimuli into
respective circuits to initiate rhythmical signals
o Almost all rhythmical signals result from reverberating circuits or succession of sequential reverberating
circuits that feed excitatory or inhibitory signals in circular pathway from one neuronal pool to next
Instability and Stability of Neuronal Circuits
 Uncontrolled reverberating signals cause epileptic seizures (brain bombarded by continuous loop)
 Inhibitory feedback circuits that return from termini of pathways to initial excitatory neurons of same pathways
(occur in virtually all sensory nervous pathways and inhibit either input neurons or intermediate neurons in
sensory pathway when termini become overly excited) can prevent excessive spread of signals in brain
 Some neuronal pools exert gross inhibitory control over widespread areas of brain (many basal ganglia exert
inhibitory influences throughout muscle control system)
 Can control stimulation by upregulating receptor proteins when not much stimulation and downregulating
receptor proteins when overstimulated