Features Of Sensory Receptors
FEATURES OF SENSORY RECEPTORS
Features include,
- Generator potential (Receptor potential).
- Adaptation.
- Coding of intensity of stimulus (Intensity discrimination).
- Sensory unit & receptor field.
- Law of projection (Sensory projection).
- Muller’s law of specific nerve energies.
- Proprioceptive sensations.
1. GENERATOR POTENTIAL:
- Also referred as “Receptor potential”.
- Steps:
- On adequate stimulus application —> Receptor membrane depolarizes —> Producing “Generator potential”.
- Is a local potential.,
Features:
- Graded (not follows all or none law).
- Magnitude is dependent on the intensity of stimulus.
- Not self-propagated.
- Decremental.
- If magnitude of generator potential exceeds threshold (firing) level → Action potential is generated.
2. ADAPTATION:
- When steady stimulus is applied to a receptor → Frequency of action potential generated declines gradually.
- This gradual decline in receptor response referred as “Adaptation”.
Division of sensory receptors based on adaptation:
- Based on development of speed of adaptation, sensory receptors are divided into –
2a. Tonic receptors:
- Receptors either adapt slowly or do not adapt.
Subtypes:
- Adapt slowly (slow adaptation): Proprioceptors, Merkel’s disc & Ruffini’s end organ.
- Non-adapting receptors: Nociceptors (pain receptors), vestibular receptors & muscle spindle.
2b. Phasic receptors:
- Receptors showing fast adaptation.
- E.g. Meissner & Pacinian corpuscles.
3. CODING OF INTENSITY OF STIMULUS /INTENSITY DISCRIMINATION:
- Intensity coding is done in 2 ways:
- Frequency coding
- Population coding
- (both coexist).
3a. Frequency coding:
- At level of an individual receptor frequency of action potential generated is proportional to magnitude of generator potential
- In turn, magnitude of generator potential is dependent on intensity of stimulus.
Hence,
- If intensity of stimulus increased → Increases amplitude of generator potential.
- In turn, Increases firing rate of sensory neuron (frequency of action potentials generated).
3b) Population coding:
- As intensity of stimulus increases → Number of receptors stimulated increases
- I.e. sensory recruitment.
- Note: Magnitude of sensation felt is proportional to log of stimulus intensity.Re
4. SENSORY UNIT AND RECEPTOR FIELD:
4a. Sensory unit:
- Dendritic ends of a single neuron are connected to several superficial sensory receptors.
- Such neuron, with all its peripheral terminals & their attached receptors, referred as “Sensory unit”.
4b. Receptor field:
- Receptors detecting sensations from larger surface area is referred as “Receptor area”
Mechanism:
- Superficial receptor detect touch in smaller area of skin.
- Primary sensory neuron, due to its dendritic connection with several superficial receptors, detects touch from a relatively large area of skin called “Receptor field”.
Summary:
- Sensory unit (a sensory neuron with its connected superficial receptors) detects touch from an area.
- Receptor field detects sensations greater than from area detected by a single receptor.
4c. Localizing to receptor field:
- On stimulation of single receptor field at multiple spots → Only single sensory unit stimulated (corresponding to receptor area).
- Multiple stimuli will be perceived as solitary stimulus by brain
- Hence, localizing to receptor field.
4d. Two point discrimination:
- Is “Minimum distance between two points that can be perceived as a separate point stimulation.
- Measure of tactile acuity or sensory acuity.
Mechanism:
- For two stimuli to be perceived distinctly
- Impingement on two distinct receptor fields is must.
- In turn, stimulating two different sensory units.
- Hence, discrimination.
Factors influencing:
- Surface area of receptor field:
- Smaller the receptor field → More acute will be power of two-point discrimination.
- I.e., Greater sensory (tactile) acuity.
Magnitude:
- Magnitude of two-point discrimination thresholds varies from place to place on body
- Smallest where touch receptors are most abundant.
- Stimulation points on the back must be separated by at least 65 mm before being distinguished as separate.
- Conversely, on fingertips, two stimuli are recognized with a minimum separation distance of 2 mm.
4e. Lateral inhibition:
- Phenomenon in which “Information from sensory neurons with receptors at peripheral edge of stimulus is being inhibited compared to information from sensory neurons at center of stimulus”.
Importance:
- Enhances contrast between center & periphery of stimulated area
- Increases brain’s ability in localizing sensory input.
- Lateral inhibition is responsible for localization of stimulus site & two-point discrimination.
4f. Receptor field overlapping:
- Receptor fields overlap considerably.
- Greatest overlap seen with pain receptors.
- Relatively less receptos field overlaps with tactile receptors.
Variations in sensation loss:
- On cutting a spinal nerve, variations occur with sensation loss.
- In Tactile sensation:
- Area of tactile loss is greater than pain
- Because lost tactile receptor area has less overlapping fibers from adjacent receptor areas.
- Resulting in loss of tactile sensation in more area.
In Pain:
- In case of lost pain receptor area, adjacent receptor areas overlap & cover more of lost surface area.
W.r.t temperature:
- Receptor area of thermoreceptors does not show any overlap.
- Hence, area of temperature loss will be maximum.
5. LAW OF PROJECTION/SENSORY PROJECTION:
- Regardless of stimulation point for a particular sensory pathway (along its course to cortex), conscious sensations are produced.
- Typical example is “Phantom limb”.
Mechanism:
- Even after limb amputation, complains of pain & proprioceptive sensation in absent (amputated) part occurs.
- Because of nociceptive afferents stimulation at amputation level during amputation.
- Eg., In mid-thigh amputation, if nerve carrying pain stimulation of great toe is stimulated (at thigh level)
- Pain will be felt in great toe.
- Implying, sensation evoked by a sensory unit will always be projected to (appear to originate from) area innervated by its peripheral endings (receptors).
6. MULLER’S LAW OF SPECIFIC NERVE ENERGIES:
- Law of projection & Muller’s law of specific energies are almost with differences like,
- Law of projection is related to receptor location.
- E.g.toe sensory pain unit will cause pain in toe even after lower limb amputation.
- Law of specific energies is related to receptor’s specific sensation.
- E.g. Touch sensory unit will evoke touch sensation even if stimulated by temperature in its pathway.
7. PROPRIOCEPTIVE SENSATIONS:
- Sensations concerned with physical state of body.
- I.e. Sense of position for tendon & muscle and Sense of equilibrium.
Types of proprioceptive sensations:
- Static & Kinesthesia.
- Sense of static position:
- Ability to determine where a particular limb exactly is in space.
- Kinesthesia:
- Sense of degree of movement.
Receptors & nerve involved:
- Receptors for sense of position – Chiefly located in joint capsules & ligaments around joint.
- Most important receptors – Ruffini’s end organs
- Few Golgi receptors & Pacinian corpuscles.
- Sensations are carried by thick myelinated nerve fibers
- I.e. Group I (Aα) – All proprioception
- Group II (Aβ) for kinesthesia to Dorsal columns of spinal cord.
- Ultimately, conscious proprioceptive sensations reach somatic sensory cortex.
Exam Question
FEATURES OF SENSORY RECEPTORS
Features include,
- Generator potential/Receptor potential.
- Adaptation.
- Coding of intensity of stimulus (Intensity discrimination).
- Sensory unit & receptor field.
- Law of projection (Sensory projection).
- Muller’s law of specific nerve energies.
- Proprioceptive sensations.
1. ADAPTATION:
- When steady stimulus is applied to a receptor → Frequency of action potential generated declines gradually.
- This gradual decline in receptor response referred as “Adaptation”.
Based on development of speed of adaptation, sensory receptors are divided into –
1a. Tonic receptors:
Subtypes:
- Adapt slowly (slow adaptation): Proprioceptors, Merkel’s disc & Ruffini’s end organ.
- Non-adapting receptors: Nociceptors (pain receptors), vestibular receptors & muscle spindle.
1b. Phasic receptors:
- Receptors showing fast adaptation.
- E.g. Meissner & Pacinian corpuscles.
2. CODING OF INTENSITY OF STIMULUS /INTENSITY DISCRIMINATION:
2a. Frequency coding:
- At level of an individual receptor frequency of action potential generated is proportional to magnitude of generator potential
- In turn, magnitude of generator potential is dependent on intensity of stimulus.
- Hence,
- If intensity of stimulus increased → Increases amplitude of generator potential.
- In turn, Increases firing rate of sensory neuron (frequency of action potentials generated).
2b) Population coding:
- As intensity of stimulus increases → Number of receptors stimulated increases
- I.e. sensory recruitment.
- Note: Magnitude of sensation felt is proportional to log of stimulus intensity.
Referred as “Weber-Fechner law”.
3. SENSORY UNIT AND RECEPTOR FIELD:
3a. Sensory unit:
- Dendritic ends of a single neuron are connected to several superficial sensory receptors.
- Such neuron, with all its peripheral terminals & their attached receptors, referred as “Sensory unit”.
3b. Receptor field:
- Receptors detecting sensations from larger surface area is referred as “Receptor area”
Summary:
- Sensory unit (a sensory neuron with its connected superficial receptors) detects touch from an area.
- Receptor field detects sensations greater than from area detected by a single receptor.
- 3c. Two point discrimination:
- Is “Minimum distance between two points that can be perceived as a separate point stimulation.
- Measure of tactile acuity or sensory acuity.
Factors influencing:
3d. Lateral inhibition:
- Responsible for localization of stimulus site & two-point discrimination.
3e. Receptor field overlapping:
- Greatest overlap seen with pain receptors.
- Relatively less receptos field overlaps with tactile receptors.
Variations in sensation loss:
- On cutting a spinal nerve, variations occur with sensation loss.
In Tactile sensation:
- Area of tactile loss is greater than pain.
- Because lost tactile receptor area has less overlapping fibers from adjacent receptor areas.
- Resulting in loss of tactile sensation in more area.
W.r.t temperature:
- Receptor area of thermoreceptors does not show any overlap.
- Hence, area of temperature loss will be maximum.
4. LAW OF PROJECTION/SENSORY PROJECTION:
- Typical example is “Phantom limb”.
5. PROPRIOCEPTIVE SENSATIONS:
- I.e. Sense of position for tendon & muscle and Sense of equilibrium.
- Types of proprioceptive sensations:
- Static & Kinesthesia.
- Kinesthesia – Sense of degree of movement.
- Receptors & nerve involved:
- Receptors for sense of position – Chiefly located in joint capsules & ligaments around joint.
- Most important receptors – Ruffini’s end organs
- Sensations are carried by thick myelinated nerve fibers
- I.e. Group I (Aα) – All proprioception
- Group II (Aβ) for kinesthesia to Dorsal columns of spinal cord.
- Ultimately, conscious proprioceptive sensations reach somatic sensory cortex.
Surface area of receptor field:
- Smaller the receptor field → More acute will be power of two-point discrimination.
I.e., Greater sensory (tactile) acuity.
- Stimulation points on the back must be separated by at least 65 mm before being distinguished as separate
- Conversely, on fingertips, two stimuli are recognized with a minimum separation distance of 2 mm.
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