PT 22 Sensory receptors and Neuronal circuits

Mechanoreceptors

respond to mechanical deformation of receptor or surrounding area

Thermoreceptors

respond to changes in temperature

Nociceptors

respond to noxious stimuli; result in perception of pain

Chemoreceptor

respond to chemical substances and are responsible for taste, smell, oxygen levels in arterial blood, CO2 concentration, and osmolality (concentration gradient) of body fluids

Blood CO2

receptors in or on surface of medulla, aortic and carotid body

Blood glucose, amino acids, fatty acids

receptors in the hypothalamus

Photic(electromagnetic) receptors

respond to light within the visible spectrum

Exteroceptors

sensitive to stimuli arising from outside the body

Interoceptors

receive stimuli from internal viscera

Proprioceptors

sense of position; monitor degree of stretch

Deep sensations

from the fascia, muscles, bone; includes deep pressure, pain and vibration

Pacinian Corpuscles

Vibration or rapid changes in mechanical state of tissues; pressure

Beneath skin & deep in fascia; stimulated by rapid compression

Fast-adapting; needs a lot of stimulus

Hair end organ

Detects movement of objects on surface of body (initial contact)

Around each hair

Fast-adapting

Ruffini Endings

Primarily mediates pressure; heavy, prolonged touch, skin stretch, and kinesthetic sense

Found in joint capsule; pressure must reach dermis area

Slow-adapting

Meissner's Corpuscles

Light touch (spinothalamic tract)

Great sensitivity; elongated & encapsulated nerve ending of large myelinated nerve fiber

Fast-adapting

Merkel's discs

Proprioception or position sense

Epidermis, basal dermis

Expanded tip tactile receptor; important role in deep touch, texture, pressure, and position

Found in hair parts of skin

Slow-adapting; sends steady signals that determine continuous touch

Krause Corpuscles

Bulbous

Conjunctive of eye, mucous membrane of lips and tongue, epineurium of nerve trunks, penis, and clitoris

Thermoreceptors (cold)

Slow-adapting

Muscle Spindles

Changes in length of muscle

Within muscle; slow-adapting

Muscle spindles

Changes in length of muscle

Within muscle; slow-adapting

Golgi tendon organ

Changes in muscle tension

Receptor in OIs of muscles; sensory component of Golgi tendon reflex

Slow-adapting

Free nerve endings

Found everywhere

Fast-adapting for pain & pain-like sensations (itch, tickle, touch)

Slow-adapting for pressure & temperature

Type A

Has the biggest diameter and conducts impulses up to 120 m/s

Myelinated

Has 4 subtype

A-Alpha

- efferent to extrafusal fiber (skeletomotor nerve)

A-Beta

afferent; subserves touch & pressure

A-Gamma

efferent to intrafusal fiber (fusimotor nerve)

A-Delta

fast pain

Type B

Big but not as big as Type A

Myelinated

Includes all preganglionic autonomic, parasympathetic fibers

Type C

Smallest and goes up to 2 m/s

Unmyelinated

Includes dorsal root ganglion pain (slow pain & temperature) & postganglionic sympathetic fibers

Type I

Same fiber diameter as A-alpha

Has 2 subtypes:

Type Ia

afferent to muscle spindle; annulospiral endings

type Ib

GTO

Type II

Same fiber diameter as A-beta

Afferent to muscle spindle; flower spray endings

Type III

Same fiber as A-delta

Fast pain & cold receptors

type IV

Same fiber as C

Slow pain & temperature receptors

Adequate Stimulus

requires least amount of energy to activate receptor

Differential Sensitivities

each type of receptor is highly sensitive to one type of stimulus for which it is designed and yet is almost nonresponsive to other types of sensory stimuli

Labeled Line Principle

specificity of nerve fibers for transmitting only one modality of sensation

Law of Specific Nerve Endings

- Adequate stimulus

- Labeled line principle

- Differential Sensitivities

Intensity

Frequency of action potential; number of neurons stimulated

How the sensation is perceived is determined by the characteristics of the receptor and the central connections of the axon connected to the receptor

Modality of sensation

type of sensation

Mechanisms of receptor potentials

Mechanical deformation

Application of chemicals

Change in temp

Electromagnetic radiation

Mechanical deformation

stretches the membrane and opens ion channels

Application of chemicals

also opens ion channels

Change in temperature

Alters the permeability of the membrane

Electromagnetic radiation

(i.e., light on a retinal visual receptor) that changes the membrane characteristics

Maximum amplitude of receptor potential is?

100mV

Receptor Potential

In the receptor

Graded

Doesn't follow all or none law

Can be summated

Unpropagated

No refractory period

Action Potential

In the sensory nerve fiber

Not graded

Obeys all or none law

Not summated

Propagated

Absolute and relative refractory periods

Generator Potential

Occur in specialized nerve endings

Stimulus causes local current flow

Local current flow opens channels of the afferent AP generating neuron

If threshold is reached, an action potential can be generated

receptor potential (generator potential)

Occur in separate receptor cells

Stimulus causes graded potential by chemical messenger

Can trigger NT release and generate postsynaptic potentials

Do not generate APs within same neuron; can cause release of NTM on second neuron to generate AP

Tonic Receptors

produce constant rate of firing as long as stimulus is applied; keep brain constantly apprised of body status

Slow; include receptors of the macula in vestibular apparatus, pain receptors, baroreceptors, & chemoreceptors

Phasic Receptors

burst of activity but quickly reduce firing rate (adapt) if stimulus maintained; react strongly while a change is taking place

Also called rate receptors and movement receptors

Sensory adaptation; cease to pay attention to constant stimuli

Rate & strength of response is related to rate & intensity of stimulus

Predicts future position or condition of body; important for balance & movement

Rapidly-adapting

Receptive fields

Area of skin whose stimulation results in changes in the firing rate of the neuron.

Area size of each receptor field is INVERSELY PROPORTIONAL with the density (amount) of receptors in the region

Back and legs have few sensory endings; receptive field is large

Fingertips have a large number of cutaneous receptors; receptive field is small

Similarly, 2-point discrimination distance is greater in the back vs the fingertips due to difference in receptive fields

Neuronal Pools

Groups of neurons with special characteristics of organization

different Neuronal Pools

Converging

Diverging

Reverberating Circuits

parallel after discharge circuit

Stimulatory Field

area stimulated by each incoming nerve fiber

Afterdischarge

prolonged output discharge

Divergence

two major types; involves weak signals entering neuronal pool to excite far greater numbers of nerve fibers leaving the pool

Amplifying

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

Divergence into multiple tracts

signal is transmitted in two directions from the pool

Convergence

signals from multiple inputs uniting to excite a single neuron

Convergence from single source

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

Convergence from multiple sources

summation of information from different sources resulting in summated effect

Parallel after discharge circuit

incoming neurons stimulate several neurons in parallel arrays

Continuous Intrinsic Discharge

neurons can continue to fire once the level of excitatory membrane potential increases to a certain degree, this level may be enough to cause continuous emission of impulses even without excitation

Ex. cerebellum; interneurons in the SC

Continuous Reverberatory signals

some reverberating circuits do not fatigue enough to stop the reverberation providing continuous source of impulse

Excitatory input will increase output signal; inhibitory input signal decreases the output

Stabilization of Neuronal Discharge

Each part of the brain is connected to each other directly or indirectly

Excitation can create a series of continuous cycles or re-excitation; the brain could be inundated by uncontrolled signals

synaptic fatigue

short term and acute adjustment of sensitivity

Neuronal Inhibitory Circuits

Gross inhibition

Feedback inhibition

Gross inhibition

neuronal pools exerting gross inhibition (i.e. basal nuclei)

Feedback Inhibition

terminal pathways inhibition to source or interneurons

downregulation

long-term stabilization through modification of the receptor availability (internalization or externalization)

regulation of receptor proteins when there is overactivity

Upregulation

- long-term stabilization through modification of the receptor availability (internalization or externalization)

the regulation of receptor proteins when there is underactivity

Lateral Inhibition

Sharpening of sensation; when a blunt object touches the skin, sensory neurons in the center areas are stimulated more than the neighboring fields

Stimulation will gradually diminish, but lateral inhibition will sharpen perception of area being stimulated; will be perceived as a single touch with well defined borders

Occurs within CNS

Reciprocal Inhibition circuit

excitatory signal in one direction & inhibitory signal in another direction simultaneously (i.e. prevent muscles from opposing movement)