Cns 3

Sensory Receptors

Perceptions of the world are created by the brain (CNS) from info derived from the sensory receptors (neurons or epithelial cells) of the PNS.

Receptors transduce (change) different forms of sensation into nerve impulses.

Specialized cell or cell process that monitors specific conditions

Sensory receptor

Sensory Receptors

Structures specialized to respond to stimuli

Activation of sensory receptors results in depolarizations that trigger impulses to the CNS

The realization of these stimuli, sensation and perception, occur in the brain

Peripheral Sensory Receptors

Two main categories of sensory receptors Special nerve endings of sensory neurons

Monitor general sensory information Independent receptor cells – specialized

epithelial cells or small neurons Monitor most types of special sensory information

Sensory Receptors

General senses Pain Temperature Physical distortion Chemical detection

Receptors for general senses scattered throughout the body

Special senses Located in specific sense organs Structurally complex

Senses

Sensory Receptors

Sensory receptors also classified according to: Location Type of stimulus detected Structure

Receptor Class by Location: ExteroceptorsRespond to stimuli arising outside the

bodyFound near the body surfaceSensitive to touch, pressure, pain, and

temperatureInclude the special sense organs

Receptor Class by Location: InteroceptorsRespond to stimuli arising within the bodyFound in internal viscera and blood

vesselsSensitive to chemical changes, stretch,

and temperature changes

Receptor Class by Location: ProprioceptorsRespond to degree of stretch of the

organs they occupyFound in skeletal muscles, tendons, joints,

ligaments, and connective tissue coverings of bones and muscles

Constantly “advise” the brain of one’s movements

Receptor Classification by Stimulus TypeMechanoreceptors – respond to touch,

pressure, vibration, stretch, and itchThermoreceptors – sensitive to changes in

temperaturePhotoreceptors – respond to light energy

(e.g., retina)Chemoreceptors – respond to chemicals (e.g.,

smell, taste, changes in blood chemistry)Nociceptors – sensitive to pain-causing stimuli

Classification by Structure

Widely distributed Nerve endings of sensory neurons monitor:

Touch, pressure, vibration, stretch Pain, temperature, proprioception

Divided into two groups I Simple receptors Free nerve endings Encapsulated nerve endings

II Complex receptors are special sense organs

Simple Receptors: Free Nerve Endings

Abundant in epithelia and underlying connective tissue

Respond to pain and temperatureMonitor affective sensesTwo specialized types of free nerve endings

Merkel discs – lie in the epidermis Slowly adapting receptors for light touch

Hair follicle receptors – wrap around hair follicles Rapidly adapting receptors

Unencapsulated Nerve Endings

Simple Receptors: Encapsulated

Meissner’s corpuscles (tactile corpuscles) Pacinian corpuscles (lamellated

corpuscles)Muscle spindles, Golgi tendon organs, and

Ruffini’s corpusclesJoint kinesthetic receptors


Table 13.1.2


Table 13.1.3


Table 13.1.4

From Sensation to Perception

Survival depends upon sensation and perception

Sensation is the awareness of changes in the internal and external environment

Perception is the conscious interpretation of those stimuli

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Transduction of Sensory signal to Nerve Impulses

Regardless of the type of stimulus, the effect on all receptors is the same: a change in the electrical potential of the receptor: the receptor potential.

Heat, cold, electromagnetic radiation, mechanical deformation, O2 content of the blood – all produce a change in electrical potential in the appropriate receptor type.

Depolarization

Receptor potential or Generator Potentials

In response to stimulus, sensory nerve endings produce a local graded change in membrane potential.

Potential changes are called receptor or generator potential. Analogous to EPSPs.

a

a

Deformation of the nerve ending results in opening of Na+ channels.

Pressure

Pacinian Corpuscle

If depolarization above Threshold = AP

Properties of Receptors

1. Specificity of response or Law of adequate stimulus

The receptors respond maximally only when an appropriate specific stimulus is applied. It responds to lowest threshold of stimulus.


2. Adequate stimulus 3. Muller’s Doctrine of specific nerve energies Sensation characteristic of each sensory neuron is

that produced by its normal or adequate stimulus. Adequate stimulus:

Requires least amount of energy to activate a receptor. Regardless of how a sensory neuron is stimulated,

only one sensory modality will be perceived. Allows brain to perceive the stimulus accurately under

normal conditions.


4. Law of projection Phantom limb: A person whose limb is

amputated often complains of pain in the limb which is not present.

5. Adaptation When a stimulus of constant strength is applied

to the receptor, the frequency of action potential in its sensory nerves decreases over a period of time.

Response

Phasic response: Generator potential increases with increased stimulus, then as stimulus continues, generator potential size diminishes.

Tonic response: Generator potential proportional to intensity of stimulus.

Sensory Adaptation

Tonic receptors: Produce constant

rate of firing as long as stimulus is applied.

Pain. Phasic receptors:

Burst of activity but quickly reduce firing rate (adapt) if stimulus maintained.

Sensory adaptation: Cease to pay

attention to constant stimuli.


6.Localization: This is due to stimulation of specific receptors present in the area. It is achieved by lateral inhibition

Sensory Modality

Lateral inhibition

Properties

7. Law of intensiity discrimation Receptor potential amplitude and action

potential frequency are proportional to stimulus intensity

Weber – Fechner law: The magnitude of sensation is proportional to the log of intensity of the stimulus.

Power function: Sensation felt = KSn

Cns 3

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