2. Stimulus
In general, a stimulus is something that provokes or
causes an action or response.
A stimulus is an external force which when applied to
an excitable cell is capable of producing a change in
the internal environment of the cell.
3.
4. Definition
A stimulus is a detectable change in the internal
or external environment of an organism.
The ability of an organism or organ to detect
external stimuli, so that an appropriate reaction
can be made, is called sensitivity.
5. Sensory receptors can receive information from outside the
body, as in touch receptors found in the skin or light
receptors in the eye, as well as from inside the body ,
in chemo receptors and mechanoreceptors.
When a stimulus is detected by a sensory receptor, it can
elicit a reflex via stimulus transduction.
6.
7.
8. INTERNAL & EXTERNAL
STIMULUS
An INTERNAL STIMULUS is often the first
component of a homeostatic control system.
EXTERNAL STIMULI are capable of producing
systemic responses throughout the body, as in
the fight-or-flight response.
9. INTERNAL STIMULUS EXAMPLE
FEVER
Chemicals released by white blood cells raise the
body temperature by 2-3 degrees causing the
temperature to elevate.
This helps to kill bacteria and inhibit viruses.
Internal Stimuli – Bacterial Infection
Response - Fever
10. EXTERNAL STIMULUS Example
“Fight or Flight Response”
A person has just been confronted with a grizzly bear on a
narrow mountain trail.
External Stimuli – Grizzly Bear
Response – Fight or Flight Response Heart rate
increases, blood flow is shunted to the major muscle
groups, digestive activity slows, blood pressure rises.
11.
12.
13. CLASSIFICATION ON the basis of Strength of
stimulus:
In order for a stimulus to be detected with high probability,
its level of strength must exceed the absolute threshold;
if a signal does reach threshold, the information is
transmitted to the CNS, where it is integrated and a
decision on how to react is made.
Although stimuli commonly cause the body to respond, it is
the CNS that finally determines whether a signal causes a
reaction or not.
14. Threshold stimulus
The minimum strength required for stimuli to initiate the
response for example in a muscle i-e contraction is
called Threshold stimulus in muscle contraction.
The stimuli other than nerve threshold stimulus
are mechanical stimuli like pressure, electrical stimuli like
shock, chemical stimuli.
15.
16. Sub-threshold /Sub-liminal stimulus
Sub-threshold : refers to a stimulus that is too
small/weak in magnitude to produce an action potential in
excitable cells.
In general, a sub-threshold stimulus leads to the
depolarization of the membrane, but the magnitude of the
depolarization is not large enough to reach the threshold
voltage
17.
18. Supra-threshold
Supra-threshold : refers to a stimulus that is large
enough in magnitude to produce an action potential in
excitable cells. ... Therefore, supra-threshold stimuli elicit
action potentials.
A stimulus having its strength more than a threshold
stimulus. It will produce the same response as after
application of the threshold stimulus
20. In this graph to the right, we are shown a neuron and it
shows how different intensities of stimuli affect the voltage
changes.
On the bottom of the graph we see stimuli: starting with a
weak stimulus and slowly going to a stronger one.
On the top we see our mv and when we give a really weak
stimulus we have this small little response labeled sub
threshold potential.
21. That’s not an action potential, it’s called a “local response” or
“local depolarization” or “sub threshold potential.” It
depolarizes a little bit but it’s not an action potential because an
AP is a complete reversal of the polarity of the cell and this is
not a reversal yet. If we keep giving it a stronger and stronger
stimulus, strong enough to pass the threshold potential, then it
generates an action potential. This stimulus is called the
threshold stimulus. The threshold stimulus is the weakest
stimulus that could generate an action potential in a
neuron.
22.
23.
24. RESPONSE
Qualitative or quantitative change in the internal
enviornment of an excitable cell secondary to application of
a stimulus
Depending upon the strength of stimulus it can be:
Local response/ Graded pot
Propagated Action potential
26. 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.
27. Different types/modalities of Stimuli
Nervous stimuli (Sympathetic & Parasympathetic)
Mechanical Stimuli
Chemical Stimuli
Electrical Stimuli
Thermal Stimuli
Hormonal Stimuli
28. Sensory transduction
Because the only way afferent
neurons can transmit information to the CNS about
stimulus 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.
29. 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.
30. 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.
31. 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?
32. 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
33. ADEQUATE STIMULUS
Each sensory receptor has an adequate stimulus, a particular
form of energy to which it is most responsive.
For example, thermo receptors 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.
34. ADEQUATE STIMULUS
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.
35. 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.
36. The afferent neuron with its peripheral receptor that first
detects the stimulus is known as a first-order sensory
neuron. It synapses on a second-order sensory neuron,
either in the spinal cord or the medulla, depending on
which sensory pathway is involved.
37. This neuron then synapses on a third-order sensory
neuron in the thalamus, and so on.
With each step, the input is processed further.
38.
39. 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
40.
41. 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: Tonic receptors
Phasic receptors
42. 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.
43. Phasic receptors
rapidly adapting receptors that respond when they first
are 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.
44. A good example is of the touch receptors. When we wear a ring
in the morning, because these receptors adapt rapidly, you are
not continually conscious of wearing your watch, rings, and
clothing.
the tactile receptors are stimulated. However, because the
stimulation is continuous, after a little while the receptors
show adaptation. Now we are not continuously aware of the
ring while we go about doing different things, otherwise, we
would not be able to focus. When we take off the ring, then the
receptor is switched office. it is stimulated again to make us
aware of the fact that the ring is being taken off.
45. MULLER’S DOCTRINE OF SPECIFIC NERVE
ENERGIES
Muller’s doctrine states that the action potential
produced by all receptors is the same in all nerve
fibers provided the diameter of the nerve fiber is
the same.
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.
46. LAW OF PROJECTIONE
Stimulation of a cold receptor over the knee by cold or
electrical stimulation of the fiber originating from this
receptor before and after its entry into the brain or spinal
cord will all produce a sensation of cold over the knee.
Stimulation of nerve fiber anywhere along its course
produces the specific sensation in the area of the body
from where it originated.
47. THRESHOLD
All receptors need a minimum strength of stimulus to
start showing activity; this strength is called the
threshold.
48. SENSORY UNIT
A sensory unit is just like a motor unit. The sensory
unit is a single primary afferent nerve including all its
peripheral branches.
49. RECEPTIVE FIELD.
THE AREA OF THE BODY WHOSE SENSORY
NERVE SUPPLY COMES FROM a single
SENSORY UNIT IS CALLED A RECEPTIVE FIELD.
