SENSORY NERVOUS SYSTEM Dr. Ayisha Qureshi - PowerPoint Presentation

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SENSORY NERVOUS SYSTEM

Dr. Ayisha Qureshi

Assistant Professor

MBBS,

Mphil

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Stimulus & Modalities

A stimulus is a change detectable by the body.Stimuli exist in a variety of energy forms, or modalities, such as heat, light, sound, pressure, and chemical changes.Sometimes we perceive sensory signals when they reach a level of consciousness, but other times they are processed completely at the subconscious level. All the information regarding all these senses is send to the CNS via AFFERENT NEURONS.

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The conversion of stimulus energy into a graded potential is called Sensory transduction and is done by sensory receptors.

Because the only way

afferent

neurons can transmit information to the CNS about stimuli

is via

action potential

propagation,

these forms of energy must be converted into

electrical signals

.

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SENSORY Receptors

Receptors are sensory afferent nerve endings that terminate in periphery as either part of a neuron or in the form of specialized capsulated structures. They act as biological transducers and convert various forms of energy acting on them into action potentials.

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Classification of Receptors

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Receptors

are present in the skin, the mucous membranes, fascia and deeper

parts of the body.

They are responsible for 4 different sensations:

Touch-pressure

Cold

Warmth and

Pain

The receptors are:

Encapsulated

receptors:

consist of multilayered capsules of connective tissue which surround a core of cells in which axons end after losing their myelin sheath.

-

Meissner’s

corpuscle:

sensitive to light touch

&

are rapidly adapting. Are present just below the epidermis in the palmer surface of fingers, lips, margins of the eyelids.

-

Pacinian

Corpuscle:

respond to vibration

&

deep pressure

&

is rapidly adapting. Present in deeper tissues and also in pleura, peritoneum, external genitalia and walls of many viscera. Also present in

periostium

, ligaments and joint capsules.

-

Krause’s end

bulbs:

occur in conjunctivae, papillae of lips and tongue.

Expanded

tips on sensory nerve endings:

-

Merkel’s discs:

which detects light, sustained touch and texture,

and

is slowly adapting

. Present in hairless skin e.g. fingertips.

-

Riffini’s

end organs:

in deeper layer of skin and deeper tissues, e.g.

periostium

, ligaments and joint capsules. They respond

to deep, sustained pressure and stretch of the skin, such as during a massage, and are slowly

adapting.

3.

Naked

or free nerve

endings:

are the most widely distributed receptors in the body and can be excited by touch, cold, warmth and pain.

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SENSORY RECEPTORS

Pacinian Corpuscle

Free nerve ending’s

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General properties of receptors

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The following are the properties of the Sensory Receptors:

Receptor Potential.

Specificity of stimulus & the Adequate stimulus.

Effect of strength of stimulus.

Adaptation (also Desensitization).

Muller’s doctrine of specific nerve energies

Law of projection.

Threshold.

Sensory unit

Receptive field.

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The changes in sensory receptor membrane potential is a graded potential called the receptor potential.

1. RECEPTOR POTENTIAL

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SENSORY TRANSDUCTION

Transduction is the conversion of stimulus energy into information that can be processed by the nervous system, which is an action potential. Stimulus↓ Receptor (SENSORY TRANSDUCTION)↓Graded Potential (RECEPTOR POTENTIAL)↓Afferent Neuron ↓Action Potential

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How is a physical or a chemical stimulus converted into achange in membrane potential?

Stimulus

(chemical/ mechanical/ thermal)

↓

Receptor which is either:

Specialized ending of the afferent neuron, OR

A separate receptor cell associated with a peripheral nerve ending.

↓

Membrane permeability altered

(usually by opening of ligand-gated or stimulus sensitive

cation

channels)

↓

A graded potential is generated. It is called RECEPTOR POTENTIAL.

↓

There is summation, and if the stimulus is strong, it leads to a greater permeability change in the receptor which leads to a large Receptor potential.

↓

If the Receptor Potential is large enough

↓

An Action Potential is generated

(by opening of the voltage-gated Na channels in the afferent neuron next to the receptor)

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THE INITIATION OF THE ACTION POTENTIAL

The initiation site of action potentials in an afferent neuron differs from the site in an efferent neuron or interneuron.In the other two types of neurons (interneuron & the efferent neuron), action potentials are initiated at the axon hillock located at the start of the axon next to the cell body. However, in the afferent neuron, action potentials are initiated at the peripheral end of fiber next to the receptor, a long distance from the cell body.

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2. SPECIFICITY OF STIMULUS & ADEQUATE STIMULUS

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If all stimuli are converted to action potentials in sensory neuronsand all action potentials are identical, how can the central nervoussystem tell the difference between heat and pressure, orbetween a pinprick to the toe and one to the hand?

All stimuli once received by the receptor are converted into action potentials and all of them are carried by the afferent neurons. This

means that

the CNS

must distinguish four properties of a

stimulus to be able to specify a stimulus:

(

1) its

nature, or modality

and

(

2) its

location

(3) Intensity

(4) Duration

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Adequate Stimulus

Each

sensory receptor has an

adequate stimulus

, a

particular form

of energy to which it is most responsive. For example,

thermoreceptors

are more sensitive to temperature changes than to pressure, and mechanoreceptors respond preferentially to stimuli that deform the cell

membrane, receptors

in the eye are sensitive to light, receptors in the ear to sound waves, and warmth receptors in the skin to heat energy. Because of this differential sensitivity of receptors, we cannot “see” with our ears or “hear” with our eyes.

Some receptors can respond weakly to stimuli other than their adequate stimulus, but even when activated by a different stimulus, a receptor still gives rise to the sensation usually detected by that receptor type. They respond to most other forms of energy if the intensity is high enough. Photoreceptors of the eye respond most readily to light, for instance, but a blow to the eye may cause us to “see stars”, an example of mechanical energy of sufficient force to stimulate the photoreceptors.

Sensory receptors can be incredibly sensitive to its preferred stimulus.

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Modality/ Nature of the stimulus

The 1:1 association of a receptor with a sensation is called labeled line coding. Stimulation of a cold receptor is always perceived as cold, whether the actual stimulus was cold or an artificial depolarization of the receptor. The blow to the eye that causes us to see a flash of light is another example of labeled line coding. A blow to the eye is seen as “white light” as the photoreceptors were stimulated.

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The table summarizes

how the CNS is informed of the type (what?), location (where?), and intensity (how much?)of a stimulus.

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Location of the stimulus

The location of a stimulus is also coded according to which receptive fields are activated. The sensory region supplied by a single sensory neuron is called a receptive field. For example, touch receptors in the hand project to a specific area of the cerebral cortex. Experimental stimulation of that area of the cortex during brain surgery is interpreted as a touch to the hand, even though there is no contact. Also, lateral inhibition of the less activated regions leads to release of inhibitory NT that inhibits the region around the stimulated area. The contrast leads to a better localization of the stimulated area. (Tactile localization)

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Receptive fields and convergence

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3. Effect of Strength & Duration of Stimulus:

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For individual sensory neurons, intensity

discrimination begins

at the receptor. If a stimulus is below threshold, the

primary sensory

neuron does not respond. Once stimulus

intensity exceeds

threshold, the primary sensory neuron begins

to produce

action potentials.

As

stimulus intensity increases, the

receptor potential

amplitude (strength) increases in proportion,

and the

frequency

of action potentials in the primary sensory

neuron increases

, up to a maximum

rate.

Similarly

, the duration of a stimulus is coded by the

duration of

action potentials in the sensory neuron. In general,

a longer

stimulus generates a

longer series

of action potentials

in the

primary sensory

neuron

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It is the decrease in response of receptors on being continuously stimulated.

4. ADAPTATION

also called Desensitization.

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When a stimulus persists continuously, some receptors adapt, or cease to respond. Thus, the receptor “adapts” to the stimulus by no longer responding to it to the same degree. Receptors fall into one of two classes, depending on how they adapt to continuous stimulation: Tonic receptorsPhasic receptors

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Types of receptors based on their adaptation

TONIC RECEPTORS

Tonic Receptors are slowly adapting receptors that respond rapidly when first activated, then slow down and maintain their response. Pressure sensitive baroreceptors, irritant receptors, and some tactile receptors and proprioceptors fall into this category. In general, the stimuli that activate tonic receptors are parameters that must be monitored continuously by the body. It is important that these receptors do not adapt to a stimulus and continue to generate action potentials to relay this information to the CNS.

PHASIC RECEPTORS

Phasic receptors are

rapidly adapting

receptors that respond when they first receive a stimulus but stop responding if the strength of the stimulus remains constant.

Many of the tactile receptors in the skin belong to this class.

Some phasic

receptors, most

notably the

Pacinian corpuscle,

respond with

a slight depolarization called the

off

response

when

the stimulus is

removed. They are important in situations where it is important to signal a change in stimulus intensity rather than the status quo information.

When

you put something

on, you

soon become accustomed to it, because

of these

receptors’ rapid adaptation. When you

take the

item off , you are aware of its removal

because of

the

“off”

response.

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the nature of perception of a stimulus by the cns is defined by the pathway over which the sensory information is carried. Hence, the origin of the sensation is not important.

5. MULLER’S DOCTRINE OF SPECIFIC NERVE ENERGIES &

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Stimulation of nerve fiber anywhere along its course produces the specific sensation in the area of the body from where it originated.

6. LAW

OF PROJECTION

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All receptors need a minimum strength of stimulus to start showing activity; this strength is called the threshold.

6. THRESHOLD

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The sensory unit is a single primary afferent nerve including all its peripheral branches.

7

. SENSORY UNIT

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THE AREA OF THE BODY WHOSE SENSORY NERVE SUPPLY COMES FROM a single SENSORY UNIT IS CALLED A RECEPTIVE FIELD.

8. RECEPTIVE FIELD.

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Summary

1. Each receptor is most sensitive to a particular type

of stimulus

.

2. A stimulus above threshold initiates action potentials in

a sensory

neuron that projects to the CNS.

3. Stimulus intensity and duration are coded in the

pattern of

action potentials reaching the CNS.

4. Stimulus location and modality are coded according

to which

receptors are

activated.

5. Each sensory pathway projects to a

specific

region of

the cerebral

cortex dedicated to a particular receptive

field

.

The brain can then tell the origin of each incoming signal.

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SENSORY CLASSIFICATION OF THE NERVE FIBERS

Type A fibers are the typical large and medium-sized myelinated fibers of spinal nerves. Type C fibers are the small unmyelinated nerve fibers that conduct impulses at low velocities. The C fibers constitute more than one half of the sensory fibers in most peripheral nerves, as well as all the postganglionic autonomic fibers. Note that a few large myelinated fibers can transmit impulses at velocities as great as 120 m/sec, a distance in 1 second that is longer than a football field.Conversely, the smallest fibers transmit impulses as slowly as 0.5 m/sec, requiring about 2 seconds to go from the big toe to the spinal cord.

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THE SENSE OF TOUCH: TACTILE SENSE

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Sense of touch is also called the Tactile sense. Sense of pressure is not separate from the sense of touch as it is only sustained touch.

Receptors:

Free nerve endings.

Pacinian corpuscles.

Meissner’s

corpuscles.

Ruffini’s

end organs

Merkel’s discs.

Hair end organs.

Location:

All cutaneous receptors (skin)

Dermal tissue

Within the mouth (tip of tongue esp.)

Tendons

Periostium

Nerve fibers carrying the tactile sensations:

A-beta nerve fibers

C nerve fibers

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Tactile Localization

This is the capacity to localize the area where a touch stimulus is applied. The lips and the fingers have the best developed tactile localization and also possess a low touch threshold.

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TWO-POINT DISCRIMINATION

This is the capacity to distinguish two tactile stimuli when an area of skin is stimulated by two stimuli simultaneously at a certain distance from each other.

A high tactile discrimination is said to present when one can distinguish between the two points.

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THE DORSAL COLUMN MEDIAL LEMNISCUS SYSTEM

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`

DCML is a crossed system. It originates from

mechano

-receptors located in the body wall and projects to the contralateral cerebral hemisphere via a 3-neuron projection system.

Also called:

- Dorsal white column system

- The

Lemniscal

system

It is

constituted

of 2 tracts called:

- Fasciculus

Gracilis

- Fasciculus

Cuneatus

The

dorsal column-medial

lemniscal

system is composed of large,

myelinated

nerve fibers that transmit signals to the brain at velocities of 30 to 110

m/sec.

The

dorsal column-medial

lemniscal

system, as its name implies,

mainly

in the

dorsal columns

of the cord.

FUNCTIONS:

It carries the following sensations:

Fine tactile sensations

Tactile localization

Tactile discrimination

Sensation of vibration

Conscious kinesthetic sense (sensation or awareness of various muscular activities in different parts of the body).

Stereognosis

(It is the ability to recognize the known objects by touch with closed eyes).

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Touch receptor/proprioceptor- Fasciculus Gracilis fibers from sacral, lumbar & lower thoracic ganglion- Fasciculus Cuneatus contains fibers from Upper thoracic and cervical ganglion ↓First order neuron ↓Posterior root ganglion (Cell bodies) ↓Spinal cord (post. Column. Same side)↓Medulla Oblongata (the cuneate & gracile nuclei)↓Second order neurons ↓Internal Arcuate Fibers arise tocross over to form the SENSORY DECUSSATION ↓Ascends as the MEDIAL LEMNISCUS through the pons & midbrain on the contralateral side ↓THALAMUS (Ventral Posterolateral nucleus of thalamus)↓Third order neurons ↓Cerebral cortex(Primary Somatosensory Cortex)

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Lesion of the DCML at the level of T8 on the left side causes what kind of impairment?

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Lesion of the DCML at the level of C3 on the left side produces what kind of impairment?

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Lesion of the right medial

lemniscus

produces what impairment?

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Lesion of the right somatosensory cortex or the right internal capsule produces what impairment?

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Effects produced by lesions of the posterior white columns leads to:

The lesion produces the following effects on the same side:

Loss of tactile localization and two-point discrimination.

Loss of vibratory sense.

Astereognosis

(loss of appreciation of difference in weight and inability to identify objects placed in hand by feeling them)

Position and movement sense is lost leading to impairment in the performance of voluntary motor functions.