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Chapter 50
Stimuli are either : 1- external
2- internal
We cell receptors that receive stimuli from external environment exterior receptors
Interior receptors found in our bodies and detect any stimuli from the internal environment
like CO₂ percentage in our body, blood pressure and pH.
When sensory input is delivered to the CNS
We have four functions that are done from the time of receiving the stimulus to the time of
delivering the stimulus to the CNS; these functions are repeated independently on the
strength of the stimulus.
The four functions are:
1234-
Sensory reception “detection of the stimulus”
Sensory transduction
Transmission
Perception
The stimuli are different in their energy as they may be: light, heat, pressure and chemicals …
and we deal with the nervous system electrically “due to the movement of ions”, so the
stimulus should let ions move through the membrane to form an action potential.
When stimulus is exposed to the receptor membrane we may have K⁺ outflow or Ca⁺² inflow
or Na⁺ inflow or Cl⁻ inflow depending on the ion channels that are located in the receptor
membrane.
So the stimulus wants to change the membrane potential of the stimulus (changing receptor
potential)
More strength of the stimulus
More ion channels will be opened
Receptor potential will increase
Changing the energy of the stimuli to an energy which is considerable to the nervous system
is called sensory transduction.
Summation and integration are controlled by two types of modification:
1- Amplification
2- Sensory adaptation
Amplification: strengthening of a sensory signal during transduction.
Amplification occurs by two methods:
1- Accessory structures of a complex sense organ
For example : the amplification of the stimuli in the eye ; the action potential conducted for
the eye to the human brain has about 100,000 times as much energy as the few photons that
triggered it.
“They are very sensitive receptor cells”
Even one photon can be felt by your eye due to the amplification , and another example is
the amplification of sounds as you can hear a very low voice; the pressure associated with
sound waves is enhanced by a factor of more that 20 before reaching receptors in the
innermost part of the ear.
2- Through signal transduction pathway
In this method that is often required in sensory receptor cells, second messenger is involved.
We have many steps in this method, and each step will activate 10 steps so in the latest step
we would have lots of enzymes that will catalyze lots of ion channels.
Instead of opening one ion channel by one stimulus, signal transduction pathway allows the
opening of a big number of ion channels; this method is slower but has a bigger effect.
So the amplification of the stimulus
Either by: 1- accessory structures that are in sense organs
Or by: 2- signal transduction pathway (the stimulus works on the receptor cell indirectly-not
direct on ion channels.)
How do we know the strength of the stimulus?
Through the frequency of the action potential.
More strong stimulus
more frequency of the action potential
After amplification : Transmission which means the moving of action potential through the
nervous system to the CNS after transduction of the energy of the stimulus into a receptor
potential.
If the sensory receptors cell themselves are specialized neurons, the action potential will be
directly produced and since they have axons they will extend to the CNS.
If the sensory neuron is a separate epithelial cell (non-neural sensory receptor), the stimulus
will lead to release of neurotransmitters which are in the synaptic vesicles which will move
and fuse with the membrane leading to the move of the neurotransmitters to the receptor
protein of the other cell “afferent neuron”.
More strength of the stimulus
Increase of receptor potential
More neurotransmitters released
Each stimulus reach the brain as an action potential, it could have a high or a low frequency
depending on the strength of the stimulus.
The brain differentiates between the stimuli classifying it to light or sound or chemicals or
heat …etc by perception.
The brain does this due to the connections that link sensory receptors to the brain.
Action potentials from sensory receptors travel along neurons that are dedicated to a
particular stimulus; theses dedicated neurons synapse with particular neurons in the brain or
spinal cord.
Sensory reception
Sensory input (information about the stimuli)
(Through sensory neurons- afferent neurons)
Sensory transduction
changing of the energy of the stimulus to a considerable energy by
the nervous system
Transmission
Movement of the action potential to the CNS
Perception
giving the suitable response to the stimulus
“The function the brain”
(a)
Gentle pressure affected the receptor cell
Low frequency of the action potential
more pressure affected the receptor cell
high frequency of the action potential
(b)
The gentle pressure has affected three receptors as each one of them is connected to a
neuron.
More pressure leaded to the activation of more receptors.
The stimulus may affect one receptor cell or many receptor cells.
When an action potential is generated and delivered to the brain, it is recognized as a strong
or a weak stimulus by the number of axons that transmit the action potential.
Receptors kinds:
The stimuli represent different kinds of energy.
Depending on the type of the energy of the stimulus, the receptors are classified to:
1-
Mechanoreceptors: The stimulus is anything mechanical.
Like the touch as it makes deformation of the membrane of cells existed in the skin.
Pressure (as in fig 50.4), sound, motion and stretch are other examples of mechanical stimuli.
One of the known mechanoreceptors is the hair cells.
Like in the ear, in the cochlea there is a fluid and there is a membrane where there are cells
which are separate receptor cells and on each (hair) cell there are hairs (cilia). When there is a
sound (stimulus) the hair cells will vibrate, when they vibrate in one direction there will be
depolarization and when vibrating in another direction there will be hyperpolarization. And
because the hair cells are non-neural cells they will make chemical synapses with the
dendrites of other sensory neurons.
Another example is the knee-jerk reflex; the long big quadriceps muscle contains of very
short fibers. Groups of 2-12 of these fibers formed into a spindle shape and surrounded by a
connective tissue, paralleled to other, muscle fibers, they are scattered.
We call these fibers stretch receptors.
Dendrites of the sensory neurons spiral around the middle of certain (stretch) skeletal muscle
fibers.
Stretch of the muscles
Stretch of the spindle fibers
Depolarization
Action potential
Via sensory neurons, it moves to the spinal cord
In the spinal cord, they analyze the input to send the suitable reflex
Contracting and jerking the lower leg forward
Another example is the mechanoreceptors in the skin.
We have nerves that include big number of neurons, the dendrites of the neurons are in the
epidermis, and some dendrites are naked meaning that they are exposed while others are
encapsulated by layers of connective tissue.
So: when we have a gentle touch in the skin
Receptors are close to the surface; so as to respond to any gentle touch.
If the touch is deep
The receptors are in the dermis
There are receptors for different stimuli like gentle touch and pain (receptors in theses
examples are close to the surface).
There are dendrites of sensory neurons existed in the base of hair cells, and this is very
important for animals that depend on whiskers at night to feel what is in front of them; as
they feel anything by their hair cells, signals are delivered to the brain to give them
perception.
So, our skin has a very big number of receptors.
Pressure and touch are examples of mechanical stimuli.
2-
Chemoreceptors
: The stimuli is chemical
They are classified as:
A- General chemoreceptors: they detect concentration of the total solutes.
For example: osmoreceptors in the hypothalamus that detect the total concentration of
chemicals. (Solute concentration if more or less than 300 mOsm)
B- We have specific chemoreceptors: they detect the concentration of single solute.
-
Like CO₂ receptors in the wall of aorta; their function is to detect CO₂
concentration.
-
We have other examples like glucose, O₂ and amino acids receptors.
3- Electromagnetic receptors
Like visible light, electricity and magnetism
We differentiate between the colors by receptors of the visible light (electromagnetic
radiation that can reach the air) that are called photoreceptors.
Electromagnetic receptors are found in some animals specially fishes
They make electrical current if any other animal comes near to them; so by these receptors
they can feel any near movement of other animals.
Another example is that the migratory birds have in their brain magnetite which contains iron
and this leads them to know the right directions in their migration.
4-
Thermoreceptors : they detect heat (hot or cold)
For example when you eat a spicy food, there is a natural product called capsaicin that
activates receptors( that are part of Ca⁺² channels) leading to the opening of these channels,
as a result Ca⁺² will enter to the receptors leading to an action potential.
The same receptors are activated when exposing to a heat ≥ 42°C, like when you drink or eat
something hot.
That’s why you feel the same when you eat spicy food or drink something hot.
When there is a cold temperature (<28°C) you will feel the same when you eat a menthol;
cold food and temperature have the same receptors.
Thermoreceptors are classified into groups each group detects a particular temperature
range.(like TRP-type receptor specific for high temperature is sensitive to capsaicin ).
5-
Pain receptors (nociceptors): the stimulus is anything that causes pain
(damage to the tissue)
(Nocere from the Latin means to hurt
These receptors cause withdrawal from danger to save yourself; when feeling the pain you
can avoid doing what causes pain and to get a suitable cruel.
Any damage of the tissue
Producing prostaglandins (from the phospholipids of the damaged membrane)
More sensitive receptors
Lowering the threshold to have an action potential
Brain
Pain
Aspirin and ibuprofen
Stopping the synthesis the prostaglandins