Download sensory receptors, neuronal circuits for processing information

SENSORY RECEPTORS,
NEURONAL CIRCUITS FOR
PROCESSING INFORMATION
Physio 1
Types of sensory receptors

Five basic types of sensory receptors
1.
Mechanoreceptors

2.
Thermoreceptors

3.
Detects physical or chemical damage
Electromagnetic receptors

5.
Detects changes in temperature
Nociceptors

4.
Detects mechanical compression or stretching of receptor tissues
Detects light in the retina
Chemoreceptors

Detects taste, smell, O2 levels, osmolality, CO2 concentration, others

How do two types of sensory detect different
stimuli?
 By
differential sensitivities
 Each

type of receptor is sensitive to one type of stimulus
Example:

Rods & cones of the eyes sensitive to light, nonresponsive to
heat, cold

Pain receptors in skin not stimulated by touch but active
when the tactile stimuli is severe enough to damage the
tissues

Modality of sensation—the “labeled line "principle
 Each
nerve tract terminates at a specific point in the
nervous system
 Type
of sensation felt when nerve fiber is stimulated is
determined by the point in the nervous system to
which the fiber leads
 Pain
fibers activated regardless of cause
 Fibers

pain sensation
in the ear terminate in the auditory areas of brain
This specificity of nerve fibers for transmitting only one modality
of sensation is called the labeled line principle
Transduction of sensory stimuli to nerve
impulses

Transduction: conversion of an impulse to an action
potential or change in the transmembrane potential

Mechanisms on how this is achieved
1.
Mechanical deformation of receptor- opens ion channels
2.
Chemical application to membrane- opens ion channels
3.
Change of temperature of the membrane- alters permeability
4.
Effect of electromagnetic radiation- allows ions to flow
Sensory Pathway
Stimulus
Sensory receptor (= transducer)
Afferent sensory neurons
CNS
Integration, perception
Sensory Pathways

Stimulus as physical energy  sensory receptor acts as a
transducer

Stimulus > threshold  action potential to CNS

Integration in CNS  cerebral cortex or acted on
subconsciously
Film somatosensory system

https://www.youtube.com/watch?v=6OpeT6hcwBQ&list=PLNCmf4qh1CfFRfLc
kNpsKoXKEvuw595Qt

Mechanoreceptors Specialized to Receive Tactile Information

Four major types of encapsulated mechanoreceptors are specialized to
provide information to the central nervous system about touch, pressure,
vibration, and cutaneous tension:
1.
Meissner's corpuscles,
2.
Pacinian corpuscles,
3.
Merkel's disks,
4.
Ruffini's corpuscles
These receptors are referred to collectively as low-threshold (or high-sensitivity)
mechanoreceptors because even weak mechanical stimulation of the skin induces
them to produce action potentials. All low-threshold mechanoreceptors are
innervated by relatively large myelinated axons ensuring the rapid central
transmission of tactile information.
The Major Classes of Somatic Sensory Receptors
Receptor type
Location
Function
Rate of adaptation Threshold of
activation
Meissner's
corpuscles
Principally
glabrous skin
Touch, pressure
(dynamic)
Rapid
Low
Pacinian corpuscles Subcutaneous
tissue,
interosseous
membranes,
viscera
Deep pressure,
vibration
(dynamic)
Rapid
Low
Merkel's disks
Touch, pressure
(static)
Slow
Low
Stretching of skin
Slow
Low
All skin, hair
follicles
Ruffini's corpuscles All skin
Example of receptor function—the
Pacinian Corpuscle
Relation between stimulus intensity &
receptor potential

The frequency of repetitive action potentials
transmitted from sensory receptors increases
approximately in proportion to the increase in
receptor potential
 Very
intense stimulation of receptor causes
progressively less and less additional increase in
numbers of action potentials.
Adaptation of receptors

Receptors adapt either partially or
completely to any constant stimulus after
a period of time

When a continuous sensory stimulus is applied,
the receptor responds at a high impulse rate at
first and then at a progressively slower rate
until finally the rate of action potentials
decreases to very few or often none at all.

Adaptation is reduction in sensitivity due to
repeated stimulation
Mechanisms by which receptors adapt
Tonic receptors

Slowly adapting receptors detect continuous stimulus
strength

Slowly adapting receptors continue to transmit impulses to the
brain as long as the stimulus is present

Examples:

Impulses from the muscle spindles and Golgi tendon apparatus allow
the NS to know the status of muscle contraction and load on the
muscle tendon at each instant

Receptors of the macula in the vestibular apparatus

Pain receptors

Baroreceptors of the arterial tree

Chemoreceptors of the carotid and aortic bodies
Phasic receptors (or Rate receptors,
Movement Receptors)

Rapidly adapting receptors detect change in
stimulus strength
 Stimulated
 React
only when the stimulus strength changes
strongly while a change is actually taking place
 Example:

Pacinian corpuscle- sudden pressure applied to the tissue excites
this receptor for a few milliseconds, and then the excitation is
over even though the pressure continues. Later, it transmits a
signal again when the pressure is released
Transmission of signals of different
intensity in nerve tracts

Spatial and Temporal Summation

Spatial summation


Increasing signal strength is
transmitted by using progressively
greater number of fibers
Temporal summation

Increasing signal strength by
increasing the frequency of nerve
impulses in each fiber
Neuronal Pools

The CNS composed of
thousands of neuronal pools

Each input fiber divides
hundreds of times to
synapse with dendrites or
cell bodies of the neurons in
the pool

Amplifying divergence
 Input
signals spreads to an
increasing number of
neurons as it passes
through successive orders
of neurons in its path
Somatic Sensations. General
Organization

Tactile & Position Senses
 Somatic
senses are the nervous mechanisms that
collect sensory information from all over the body.
 In
contradistinction to the special senses (hearing,
vision, smell, taste, & equilibrium)

Classification of somatic senses
(Physiologic types)
1.
Mechanoreceptive somatic sensesstimulated by mechanical displacement
of some tissue of the body
a.
Tactile sensations
b.
Position sensations
2.
Thermoreceptive senses- detect heat and
cold
3.
Pain sense- activated by factors that
damage the tissues

Let us focus on the mechanoreceptive tactile &
position senses
 Tactile
Senses
 Touch
 Pressure
 Vibration
 Tickle
 Position
 Static
 Rate
Senses
position sense
of movement sense
Detection & Transmission of Tactile
Sensations

Although touch, vibration & pressure are frequently
classified as separate sensations, they are detected by the
same types of receptors.

Three principal differences
1.
Touch sensation results from tactile receptors in the skin or in
tissues beneath the skin
2.
Pressure sensation generally results from deformation of deeper
tissues
3.
Vibration sense results from rapidly repetitive sensory signals
Tactile receptors
Sensory pathways for transmitting somatic
signals into the central nervous system

Almost all sensory
information from the
somatic segments of the
body enters the spinal cord
through the dorsal roots of
the spinal nerves

Signals are carried through
1.
The dorsal column-medial
Lemniscal system
2.
Anterolateral system
Somatosensory tracts

https://www.youtube.com/watch?v=vrmKqH8d1RM&list=PLYLMBYHSuMTmOku
UvY86IUdj_Sjb7pKVN&index=2

Carries signals upward
to the medulla of the
brain mainly in the
dorsal columns of the
cord. Then after the
signals synapse and
cross to the opposite
side in the medulla,
they continue upward
through the brain
stem to the thalamus
by way of the medial
lemniscus