SENSORY NERVOUS SYSTEM
Dr. Ayisha Qureshi
Assistant Professor
MBBS, Mphil
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.
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.
THE CONVERSION OF STIMULUS ENERGY
INTO A GRADED POTENTIAL IS CALLED
SENSORY TRANSDUCTION AND IS DONE BY
SENSORY RECEPTORS.
SENSORY NERVOUS
SYSTEM
SOMATIC
SENSES
Mechanoreceptive
Tactile(Touch,
tickle , Pressure,
Vibration)
Position
SPECIAL
SENSES
Thermoreceptive
Hot
Pain
Cold
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.
SENSORY RECEPTORS
Classification of Receptors
RECEPTORS
INTEROCEPTORS
EXTEROCEPTORS
Receptors which give
response to stimuli arising
from WITHIN the body.
Receptors which give
response to stimuli arising
from OUTSIDE the body.
Stretch Receptors
(Heart)
Baroreceptors
(pressure changes in b.v)
VISCEROCEPTORS
Chemoreceptors
(chemical changes in blood)
Osmoreceptors
INTEROCEPTORS
(Osmotic Pressure changes
Urinary Tract & Brain )
Muscle Spindle
Golgi Tendon Organs
PROPRIOCEPTORS
Pacinian Corpuscle
Free Nerve endings
CUTANEOUS
RECEPTORS
EXTERCEPTORS
TOUCH
MEISSNER’S
CORPUSCLE &
MERKEL’S DISC
PRESSURE
PACINIAN
CORPUSCLE
COLD
THERMORECEPTOR
WARMTH
THERMORECEPTOR
PAIN
FREE NERVE
ENDING’S
CHEMORECEPTORS
SPECIAL
SENSES
(Taste & Smell)
TELERECEPTORS
(Vision & Hearing)
Receptors are present in the skin, the mucous membranes, fascia and deeper parts of
the body. They are responsible for 4 different sensations:
1. Touch-pressure
2. Cold
3. Warmth and
4. Pain
• The receptors are:
1. 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.
2. 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.
SENSORY RECEPTORS
Pacinian Corpuscle
Free nerve ending’s
GENERAL PROPERTIES OF RECEPTORS
The following are the properties of the Sensory
Receptors:
1. Receptor Potential.
2. Specificity of stimulus & the Adequate stimulus.
3. Effect of strength of stimulus.
4. Adaptation (also Desensitization).
5. Muller’s doctrine of specific nerve energies
6. Law of projection.
7. Threshold.
8. Sensory unit
9. Receptive field.
1. RECEPTOR POTENTIAL
THE CHANGES IN SENSORY RECEPTOR MEMBRANE
POTENTIAL IS A GRADED POTENTIAL CALLED THE RECEPTOR
POTENTIAL.
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
How is a physical or a chemical stimulus converted into a
change in membrane potential?
Stimulus
(chemical/ mechanical/ thermal)
↓
Receptor which is either:
1. Specialized ending of the afferent neuron, OR
2. 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)
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.
2. SPECIFICITY OF STIMULUS & ADEQUATE
STIMULUS
If all stimuli are converted to action potentials in sensory neurons
and all action potentials are identical, how can the central nervous
system tell the difference between heat and pressure, or
between 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
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.
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.
The table
summarizes how
the CNS is
informed of the
type (what?),
location (where?),
and intensity (how
much?)
of a stimulus.
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)
Receptive fields and convergence
3. Effect of Strength & Duration of Stimulus:
• 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
4. ADAPTATION also called Desensitization.
IT IS THE DECREASE IN RESPONSE OF RECEPTORS ON BEING
CONTINUOUSLY STIMULATED.
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:
1. Tonic receptors
2. Phasic receptors
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.
5. MULLER’S DOCTRINE OF SPECIFIC NERVE
ENERGIES &
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.
6. LAW OF PROJECTION
STIMULATION OF NERVE FIBER ANYWHERE ALONG ITS COURSE
PRODUCES THE SPECIFIC SENSATION IN THE AREA OF THE BODY FROM
WHERE IT ORIGINATED.
6. THRESHOLD
ALL RECEPTORS NEED A MINIMUM STRENGTH OF
STIMULUS TO START SHOWING ACTIVITY; THIS STRENGTH IS
CALLED THE THRESHOLD.
7. SENSORY UNIT
THE SENSORY UNIT IS A SINGLE PRIMARY AFFERENT NERVE
INCLUDING ALL ITS PERIPHERAL BRANCHES.
8. RECEPTIVE FIELD.
THE AREA OF THE BODY WHOSE SENSORY NERVE SUPPLY
COMES FROM A SINGLE SENSORY UNIT IS CALLED A
RECEPTIVE FIELD.
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.
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.
THE SENSE OF TOUCH:
TACTILE SENSE
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
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.
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.
THE DORSAL COLUMN MEDIAL
LEMNISCUS SYSTEM
•
•
•
•
•
•
-
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).
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)
Lesion of the DCML at the level of T8
on the left side causes what kind of
impairment?
Lesion of the DCML at the level of C3
on the left side produces what kind
of impairment?
Lesion of the right medial lemniscus
produces what impairment?
Lesion of the right somatosensory
cortex or the right internal capsule
produces what impairment?
Effects produced by lesions of the posterior white
columns leads to:
The lesion produces the following effects on the
same side:
1. Loss of tactile localization and two-point
discrimination.
2. Loss of vibratory sense.
3. Astereognosis (loss of appreciation of difference
in weight and inability to identify objects placed
in hand by feeling them)
4. Position and movement sense is lost leading to
impairment in the performance of voluntary
motor functions.