Physiology Ch 46 p559-569
Sensory Receptors, Neuronal Circuits for Processing Information
There are 5 basic types of receptors: mechanoreceptors, thermoreceptors, nociceptors,
electromagnetic receptors, chemoreceptors
Differential Sensitivity of Receptors – each receptor is highly sensitive to one type of stimulus
for which it is designed and nonresponsive to other types of stimuli; rods and cones are sensitive
to light but nonresponsive to changes in temperature
Modality of Sensation – The Labeled Line Principle – the principle type of sensation, such as
pain, touch, sight, sound, and so forth – is called modality of sensation
-each nerve tract terminates at specific point in CNS, and type of sensation is determined by
point in nervous system to which the fiber leads
-if pain fiber is stimulated, person perceives pain regardless of what stimulus excites fiber
-specificity of nerve fibers for transmitting only one modality of sensation is called labeled line
principle
Transduction of Sensory Stimuli into Nerve Impulses – sensory receptors have one feature in
common: effect of any stimulus is to change the electrical potential: receptor potential
Mechanisms of Receptor Potentials – receptors can be excited in different ways:
1. Mechanical deformation of receptor to stretch membrane and open channels
2. Application of chemical to membrane which opens channels
3. Change in temperature of membrane to alter permeability
4. Effects of electromagnetic radiation (light) on retinal visual receptor
Maximum Receptor Potential Amplitude – maximum amplitude of most sensory receptor
potentials is 100mV, but only at very high stimulus levels
Relation of Receptor Potential to Action Potential – when receptor potential rises above
threshold for action potential, then an action potential will occur
Receptor Potential of a Pacinian Corpuscle – corpuscle has a central nerve fiber extending
through its core surrounded by multiple concentric capsule layers; compression anywhere will
on outside will elongate, indent, or deform central fiber
-compression will cause ion channels to open in the membrane and allow Na ions to diffuse
inside to create positivity inside fiber, which is the receptor potential, and causes a local circuit
of current flow that spreads along fiber
-at the first node of Ranvier inside the capsule of pacinian corpuscle, the local current
flow depolarizes fiber membrane here to set off action potential
Relation Between Stimulus Intensity and Receptor Potential – amplitude of potential increases
rapidly at first but progressively decrease at high stimulus strength
-in turn, the frequency of repetitive action potentials from sensory receptors increases
in proportion to the increase in receptor potential
-very intense stimulation of receptor causes less and less additional increase in numbers
of action potentials
-allows sensitivity to weak stimuli until sensory experience is extreme
Adaptation of Receptors – receptors adapt to constant stimulus after a period of time;
receptors responds at a high impulse rate after first being stimulated, but becomes slower until
action potentials decrease to very few or none at all
-pacinian corpuscle adapts very rapidly, hair within 1s, joint/muscle VERY SLOW
-pacinian corpuscles adapt to extinction within 1/100th of a second, and hairs within a second,
showing that some receptors adapt to a greater extent than others
-other mechanoreceptors are non-adapting receptors, such as carotid/baroreceptors,
taking 2 days to completely adapt
-chemoreceptors and pain receptors never adapt
Mechanisms by Which Receptors Adapt – in the eye, rods/cones adapt by changing
concentrations of light-sensitive chemicals
-in mechanoreceptors, the pacinian corpuscle adapts in two ways:
1. the corpuscle is a viscoelastic structure, so when a distorting force is applied to one side,
the force is instantly transmitted by viscous component of the corpuscle directly to the
same side as the central nerve fiber to elicit potential, but within 1/100 of a second, the
fluid redistributes and receptor potential s no longer elicited
2. results from accommodation resulting from inactivation of Na channels in nerve fiber
membrane, which means that Na current flow through channels causes them to
gradually close
Slowly Adapting Receptors Detect Continuous Stimulus Strength – “Tonic” Receptors – slow
adapting receptors continue to transmit impulses as long as stimulus is present to keep brain
constantly aware of body in relation to surroundings
-muscle spindles and golgi tendon organs allow nervous system to know status of muscle
contraction and load on muscle tendon at each instant
-other slow adapting receptors are: receptors of macular in vestibular apparatus , pain
receptors, baroreceptors of arterial tree, chemoreceptors of carotid and aortic bodies
Rapidly Adapting Receptors Detect Change in Stimulus Strength – Rate Receptors, Movement
Receptors, or Phasic Receptors – receptors that adapt rapidly cannot be used to transmit
continuous signal because they are stimulated only when stimulus strength changes, but they
react strongly WHILE a change is actually taking place, calling them rate receptors, movement
receptors, or phasic receptors
-pacinian corpuscle is excited to sudden pressure, then the excitation is over even if
pressure continues. Later, it transmits signal again when pressure is released
Importance of Rate Receptors – if you know the rate at which some change in body is taking
place, you can predict the state of mind or body a few seconds or minutes later
-receptors of semicircular canals in vestibular apparatus of ear detect rate at which head begins
to turn, and a person can predict how he/she will turn in next 2 seconds and adjust motion of
legs AHEAD OF TIME to keep from losing balance
-receptors in joints help detect rates of movement of different body parts to predict where feet
will be when running so that appropriate motor signals can be transmitted to muscles of legs
Nerve Fibers that Transmit Different Types of Signals – some signals transmitted rapidly, some
slowly; some nerves have large diameters, some small; greater the diameter, greater the
conducting velocity
General Classification of Nerve Fibers – fibers are divided into types A and C; type A is divided
into α, β, γ, δ fibers
-Type A fibers are large/medium MYELINATED fibers of spinal nerves
-Type C fibers are small UNMYELINATED fibers conducting at low velocities, constituting 50% of
sensory fibers in peripheral nerves as well as all postganglionic autonomic fibers
Alternative Classification Used by Sensory Physiologists –
1. Group Ia – fibers from annulospiral endings of muscle spinders (α-type A fibers)
2. Group Ib – fibers from golgi tendon apparatus, also type α A fibers
3. Group II – fibers from cutaneous tactile receptors and some muscle spindles (β and γ)
4. Group III – fibers carrying temperature, crude touch, and prickling pain (δ type A)
5. Group IV – unmyelinated fibers carrying pain, itch, temp, crude touch (Type C fibers)
Transmission of Signals of Different Intensity in Nerve Tracts – Spatial and Temporal
Summation – each signal must convey the signal intensity; for example: pain has many different
gradations of intensity, and can be transmitted using increasing numbers of parallel fibers OR
increasing numbers of action potentials along a single fiber; called spatial summation and
temporal summation, respectively
Spatial Summation – increasing signal strength is transmitted by using progressively greater
numbers of fibers; each of these splits into hundreds of free nerve endings for pain sensation
-entire cluster of fibers from one pain fiber covers an area of skin 5cm in diameter (called the
receptive field)
-number of receptors is great in center of the field and diminishes toward periphery, so
pricking near a center of fiber will elicit more pain since there are more receptors
Temporal Summation – increasing frequency of nerve impulses for one fiber can increase signal
strength
Transmission and Processing of Signals in Neuronal Pools – CNS composed of millions of
neuronal pools, containing from a few to many neurons
-entire cerebral cortex can be considered a neural pool; other pools are basal ganglia, thalamus,
cerebellum, mesencephalon, pons, medulla; dorsal gray matter of spinal cord can be neuronal
pool
-each neuronal pool has its own special organization that causes it to process signals in its own
unique way to achieve functionality
Relaying of Signals Through Neuronal Pools – Organization of Neurons for Relaying Signals –
there exist input fibers and output fibers; each fiber divides hundreds of times to provide
thousands of terminal fibrils that spread into large area in pool to synapse with dendrites or
somas of neurons in the pool
-the neuronal area stimulated by each incoming nerve is called its stimulatory field
-most terminals from each input connect to the nearest output neuron, with less
terminals connecting to neurons further away
Threshold and Subthreshold Stimuli – Excitation or Facilitation – discharge of a single
excitatory presynaptic terminal almost never causes an action potential in a postsynaptic
neuron, and that large numbers of input terminals must discharge on same neuron either
simultaneously or in rapid succession to excite the neuron
-if a neuronal input has many terminals to cause an excitatory discharge, stimulus is said to be
excitatory and is called suprathreshold stimulus because it has more than enough for excitation
-nerve fibers from that same neuron branching to further neurons will not be enough to excite
the postsynaptic neuron completely, but will cause it to be more excited, so these will be called
subthreshold stimuli and the neurons are said to be facilitated
-in the center of an input neuron’s field, all the neurons are stimulated by the incoming fiber, so
this is called the discharge zone (excited zone, liminal zone)
-to each side the neurons are facilitated but not excited, so these areas are called the
facilitated zone, or subthreshold zone or subliminal zone
Divergence of Signals Passing Through Neuronal Pools – it is important for weak signals
entering neuronal pool to excite far greater numbers of nerve fibers leaving the pool; this is
called divergence
-an amplifying divergence means that input signal spreads to increasing number of neurons as it
passes through successive orders of neurons
-typical of corticospinal pathway in control of skeletal muscles, with a single cortical
pyramidal cell capable of exciting as many as 10,000 muscle fibers
-second type of divergence is divergence into multiple tracts, where signal is transmitted in two
directions from pool
-information from spinal cord travels up dorsal columns in 2 courses: into the
cerebellum and on through lower regions of brain to thalamus and cerebral cortex
Convergence of Signals – in this path, multiple signals unite to excite a single neuron,
-can be from a single source (multiple terminals from single fiber), important because a single
terminal doesn’t elicit action potential, but multiple terminals can
-can be from multiple input signals (both excitatory and inhibitory): interneurons from spinal
cord receive converging signals from (1) peripheral nerve fibers, (2) propriospinal fibers passing
from one segment of cord to another, (3) corticospinal fibers from cerebral cortex, (4) several
other pathways descending from brain
-convergence shows summation from different sources
Neuronal Circuit with both Excitatory and Inhibitory Output Signals – an incoming signal to
neuronal pool can cause output excitatory signal in one direction and inhibitory signal in a
different direction
-this type of circuit controls antagonistic muscle pairs, called reciprocal inhibition circuit
-input fiber directly excites the excitatory output pathway, but stimulates an intermediate
inhibitory neuron to secrete different neurotransmitter to inhibit second pathway
Prolongation of Signal by Neuronal Pool – “Afterdischarge” – signal entering a pool can cause
prolonged output discharge lasting up to a few minutes after the signal is over. Mechanism:
1. Synaptic Afterdischarge – When excitatory synapses discharge on surfaces of dendrites
or soma of a neuron, a postsynaptic potential develops in neuron that lasts for many
milliseconds; as long as potential lasts, it can excite the neuron to transmit continuous
impulses, causing a single instantaneous input to maintain a sustained signal
2. Reverbatory (Oscillatory) Circuit as a Cause of Signal Prolongation – Reverberatory
circuits are the most important in the nervous system; caused by positive feedback
within the neuronal circuit that re-excites the input of the same circuit
a. Several varieties exist, with more than one neuron, with facilitatory or inhibitory
interneurons, many parallel fibers in an array
Characteristics of Signal Prolongation from a Reverberatory Circuit – input may last 1ms, but
the output can last several minutes; intensity of output signal usually increases to a high value
EARLY in reverberation and then decreases to a critical point at which it ceases entirely
-Cause of cessation of reverberation is FATIGUE of synaptic junctions in the circuit
Continuous Signal Output from Some Neuronal Circuits – some neuronal circuits emit
continuous output signals caused by either (1) continuous intrinsic neuronal discharge or (2)
continuous reverberatory signals
1. Continuous Dischange Caused by Intrinsic Neuronal Excitability – neurons discharge
repetitively if excitatory membrane potential rises above certain threshold
a. Membrane potentials of many neurons are high enough to fire continuously;
occurring in cerebellum, interneurons of spinal cord
b. Rates of impulses can differ with EPSPs or IPSPs
2. Continuous Signals Emmitted from Reverberating Circuits as a Means for Transmitting
Information – a reverberating circuit that does not fatigue enough to stop reverberation
is a source of continuous impulses. EPSPs can increase output, IPSPs decrease output
a. This is a Carrier wave of information transmission, where EPSPs and IPSPs do
not CAUSE the output signal, but they do CONTROL the level of intensity
b. Used by autonomics for vascular tone, gut tone, iris constriction, heart rate
Rhythmic Signal Output – many neuronal circuits emit rhythmical output signals, such as the
rhythmical respiratory signal in medulla and pons
-almost all rhythmic signals are a result of reverberating circuits
-excitatory or inhibitory signals can increase or decrease amplitude of rhythmical signal output
-when carotid is stimulated by O2 deficiency, frequency and amplitude of respiratory rhythmical
output signal increases progressively
Instability and Stability of Neuronal Circuits – normally, signals are sent everywhere in the brain
continuously, which if widespread and uncontrolled, can cause epileptic seizures. Brain
compensates through (1) inhibitory circuits and (2) fatigue of synapses
Inhibitory Circuits as a Mechanism for Stabilizing Nervous System Function – two types of
inhibitory circuits in widespread areas of brain help prevent excessive spread:
1. inhibitory feedback circuits returning from termini of pathways back to initial excitatory
neurons of same pathways; occur in all sensory nervous pathways and inhibit either the
input neruons or intermediate neurons in sensory pathway
2. some neuronal pools exert gross inhibitory control over widespread areas of brain; basal
ganglia exert inhibitory influences throughout muscle control system
Synaptic Fatigue as a Means of Stabilizing Nervous System – synaptic transmission becomes
weaker the more prolonged and intense the period of excitation
Automatic Short-Term Adjustment of Pathway Sensitivity by Fatigue Mechanism – pathways
that are overused in brain usually become fatigued, so sensitivities decrease
-those that are underused become rested and sensitized
-thus, fatigue and its recovery are a means of moderating sensitivities of different neuronal
circuits
Long-term Changes in Synaptic Sensitivity Caused by Automatic Down-regulation or Upregulation of Synaptic Receptors – long-term sensitivities of synapses can be changed
tremendously by up-regulating number of receptor proteins at synaptic sites
-ER/Golgi are constantly synthesizing receptors; when synapses are overused, many receptors
are inactivated and removed from synaptic membrane