Sensory Receptors, Neuronal Circuits for Processing Information

neuronal pool

group of neurons that processes signals in a unique way (typically due to its unique organization). A functional group of interconnected neurons - the foundation for the differences in function found in various brain areas. Within neural structures/brain regions there are different types of neuronal pools, subgroupings called nuclei that perform specific subfunctions or process a specific type of information.

neuronal pool examples:

cerebellum performs a different function than the medulla, and the nuclei in the thalamus each perform different functions.

neuronal pools: input neurons

each input neuron branches to form thousands of terminals which synapse with various neurons in the pool.

True or false: each input neuron can only synapse once with an output neuron.

false. each input neuron can synapse multiple times with the same output neuron, or synapse with multiple different output neurons.

neuronal pools: stimulatory field

the neuronal area stimulated by each incoming nerve fiber.

neuronal pools: excitatory/suprathreshold stimulus.

signal from an input neuron is sufficient to elicit an action potential in an output neuron.

neuronal pools: subthreshold stimulus

signal from an input neuron increases membrane potential in an output neuron, but is not sufficient to elicit an action potential. (the output neuron is facilitated).

neuronal pools: facilitated neuron

the neuron is activated/enhanced, now readily excitable for an incoming signal

neuronal pools: discharge zone

all neurons in this zone receive sufficient input to trigger action potentials (usually at the center of the neuronal pool because those neurons receive more terminals from input fibers)

neuronal pools: facilitated zone

neurons in this zone receive excitatory input, but not sufficient to elicit action potentials. (usually at the peripheries of the neuronal pool)

neuronal pools: inhibitory zone

neurons in this zone receive inhibitory input, impeding the formation of action potentials. (The whole pool is an inhibitory zone, but the degree of inhibition is greater at the center of the neuronal pool because there are more terminals receiving more inhibitory neurotransmitters.)

List the 2 main functions of neuronal pools:

relaying signals and prolonging signals

List the 3 mechanisms of relaying signals

divergence, convergence, and reciprocal inhibition circuit

List the 3 mechanisms of prolonging signals:

synaptic after-discharges, reverberatory (oscillatory) circuit, and continuous signal output

Divergence

when a small number of weak signals entering the neuronal pool lead to excitation of many neurons leaving the pool.

What are the 2 types of divergence?

Amplifying divergence and divergence into multiple tracts

Amplifying divergence

An input signal spreads to an increasing number of neurons as it passes through successive neuros in its path

Divergence into multiple tracts

information entering the pool is transmitted in two directions (through 2 tracts) when it leaves the pool

convergence

signals from multiple inputs unite to excite a single neuron

what are the 2 types of convergence?

convergence from a single source and convergence from multiple sources

Convergence from a single source

multiple terminals from a single incoming fiber tract terminate on the same neuron

convergence from multiple sources

summation of information from multiple neural pathways to control activation of a single neuron

Reciprocal inhibition circuit

incoming signal to neuronal pool causes an excitatory signal in one direction and a simultaneous inhibitory signal in another direction

where are reciprocal inhibition circuits important?

important for antagonistic pairs of muscles - muscle groups working in opposition to each other, like for flexion/extension. Also important in the brain to prevent overactivity of certain circuits.

Afterdischarge

neural firing that lasts longer than the incoming signal in order to increase the duration of an incoming signal. discharges can last a few milliseconds to many minutes after the incoming signal is over.

Synaptic afterdischarge

prolonged firing of an output neuron due to prolonged elevation of membrane potential due to long-acting neurotransmitters

Reverberatory (oscillatory) circuit

positive feedback within the neuronal circuit feeds back to re-excite the input of the same circuit. (circuit can discharge repetitively for a long time, important for rhythmical output signals like rhythmic breathing and walking)

What can cause the cessation of a reverberatory circuit?

inhibitory input or fatigue of the synaptic junctions in the circuit. (Fatigue could mean that you run our of neurotransmitters, vesicles, ATP, etc.)

facilitated reverberatory circuits (vs. inhibited and normal):

facilitated circuits have higher output pulse rate for longer period of time - they might recruit more parallel fibers or increase action potential frequency to do this. Their signals are higher, last longer, and they fatigue slower.

list the 2 mechanisms of continuous signal output:

continuous intrinsic neuronal discharge and continuous reverberatory signals.

continuous intrinsic neuronal discharge

some neurons spontaneously discharge because their membrane potential at rest is above threshold

continuous reverberatory signals

continual output occurs in a reverberatory circuit where fatigue does not fully stop reverberation

True or false: because continuous signal outputs are constantly occurring, their rate remains constant.

false. even though they are continuously happening, the rate of firing can be changed with excitatory or inhibitory signals. (example: your heart is always beating but your heart rate changes because the autonomic nervous system upregulates or downregulates the rate.)

what are epileptic seizures?

uncontrolled bursts of electrical activity that spread through the brain due to reverberating signals, state of continuous re-excitation.

what are the 2 mechanisms that stabilize neuronal circuits?

inhibitory circuits and synaptic fatigue

Inhibitory circuits

prevent the excessive spread of signals.

What are the 2 categories of inhibitory circuits?

inhibitory feedback circuits and widespread inhibition

inhibitory feedback circuits

prevent self-re-excitation

widespread inhibition

some neuronal pools function to inhibit large areas of the brain.

synaptic fatigue

synaptic transmission becomes progressively weaker the more prolonged or more intense the excitation stimulus.

what are the 2 types of mechanisms for controlling synaptic activity?

short-term control and long-term control

short-term control

(fatigue) decrease in sensitivity of neural excitation when the neuronal pathway is overused due to depletion of neurotransmitter or vesicles, and vice versa

long-term control

decreasing the number of receptor proteins at the synapses when there is overactivity and vice versa

Recovery flashcard

sensory receptors

specialized cells that transduce a physical stimulus from the environment (or from within the body) into neural signals

list the types of sensory receptors

mechanoreceptors, thermoreceptors, nociceptors, photoreceptors, and chemoreceptors.

mechanoreceptors

transduce mechanical stimuli (pressure, vibration, stretching, etc.)

thermoreceptors

transduce temperature

nociceptors

pain receptors, detect physical or chemical damage to the body

photoreceptors

electromagnetic receptors, transduce light

chemoreceptors

transduce chemical stimuli (taste, smell, blood oxygenation, etc.)

differential sensitivity

each type of sensory receptor is highly sensitive to one type of stimulus but unresponsive to other types of sensory stimuli (your retinas won't react when someone shouts at you lmao)

Receptor potential

change in membrane potential due to incoming sensory stimulus

list the mechanisms of eliciting receptor potentials:

mechanical deformation (stretches membrane and opens ion channels), chemical applied to membrane (opens ion channels), change temperature of membrane (alters membrane permeability), and light applied to membrane (opens ion channels or changes membrane characteristics leading to a change in potential).

true or false: receptor potentials are graded potentials

true. the amplitude of the receptor potential scales with the strength of the input stimulus, very sensitive at weak stimuli but can still detect changes even at strong stimuli)

what happens when the receptor potential surpasses the threshold?

action potentials occur. action potential frequency scales with the amplitude of the receptor potential.

Receptor adaptation

decrease in response of a receptor over time due to a continuous prolonged stimulus.

mechanisms of adaptation:

generally depends on the type of receptor. However, ALL receptors have accomodation.

Accomodation:

progressive inactivation of sodium channels in the neuron membrane (closing of inactivation gates over time)

what are the 2 types of receptors (in terms of adaptation?)

slowly-adapting receptors and rapidly-adapting receptors

Slowly-adapting receptors

detect continuous stimulus strength (aka tonic receptors). they continue to transmit signals as long as a stimulus is present, keeping the brain up to date on the current state of the body.

rapidly-adapting receptors

detect changes in stimulus strength (aka rate receptors). transmit info about changes in stimuli acting on the body. have a predictive function - the brain uses info about movement speed to predict future position of the body to make anticipatory corrections.

What are the 4 dimensions of sensation?

modality, location, intensity, and duration.

modality

what the stimulus is (type of input)

location

where the stimulus is. depends on receptive fields and innervation density.

receptive fields

area of the skin where a stimulus will excite the nerve fiber. (each neuron branches to several terminals, receptive fields overlap.)

innervation density

the number of neurons supplying a region of the skin per unit area. relates to spatial acuity (spatial sensitivity) of touch.

intensity

how strong the stimulus is. based on spatial and temporal summation.

spatial summation

additive effects based on number of neurons recruited

temporal summation

change in the frequency of action potentials

duration

when the stimulus is, timing of action potentials

labelled line hypothesis

each neuron transmits information only about one modality of stimulus. (separate receptors, separate tracts, separate areas in the brain for processing.)

Neural transmission time (bonus flashcard)

different senses have different transmission times based on transduction time (how long it takes to detect stimuli) and conduction velocity (how fast the signal travels), and for touch, distance from the brain. Is important during intersensory synchrony (when different senses should be perceived simultaneously). There are temporal windows of integration to deal with this - anything within a certain window of integration is perceived as from the same phenomenon.

Recovery flashcard