The document discusses the central nervous system and sensory system. It describes the main components of the sensory unit including sensory axons, peripheral branches, and receptors. The main types of sensory receptors are described based on the stimuli they detect such as mechanoreceptors, chemoreceptors, photoreceptors, and thermoreceptors. The properties of sensory receptors including specificity, excitability through receptor potentials, adaptation, rate of discharge, and neural coding of sensory information are explained.

1. Central Nervous System
Dr. Hussein Farouk Sakr

2. The Sensory system and the sensory receptors
• The sensory system is the division of
the nervous system concerned with
the perception, conduction and
integration of sensory inputs from the
external or the internal environment.
• The sensory unit: it is formed of:
1. Single sensory axon.
2. Its peripheral branches.
3. The connected receptors.
• The Receptive field of the sensory unit
is the area from which the stimulus
causes action potential generation. A sensory unit including the location of sensory receptors, the
processes reaching peripherally and centrally from the cell body, and
the terminals in the CNS. Also shown is the receptive field of this
neuron.

3. The Sensory receptors
• Specialized microscopic
structures present at the
peripheral termination of the
afferent neurons.
• Function: they are
1. Detectors: they detect the
changes in the external or
internal environment.
2. Transducers: they transform
any form of energy into a
receptor potential.
The cutaneous sensory receptors. Each of these structures is associated with a
sensory (afferent) neuron. Free nerve endings are naked, dendritic branches that
serve a variety of cutaneous sensations, including that of heat. Some cutaneous
receptors are dendritic branches encapsulated within associated structures.
Examples of this type include the Pacinian (lamellated) corpuscles, which provide
a sense of deep pressure, and the Meissner’s corpuscles, which provide
cutaneous information related to changes in texture.

4. Sensory receptors transform an external signal into
a membrane potential

5. Classification: according to nature of the stimulus
Mechanoreceptors Chemoreceptors Pain
receptors
Photoreceptors Thermoreceptors
- Stretch receptors.
- Baroreceptors
- Tension receptors.
- Vibration
receptors.
- Touch receptors.
- Joint receptors.
- Auditory and
vestibular
receptors
Internal:
- Glucoreceptors.
- Osmoreceptors.
- Chemoreceptors for
the blood gases.
External:
- Taste &
smell
receptors
Responds to
injurious stimuli
that causes
tissue damage.
Rods and cones of
the retina
stimulated by
electromagnetic
light rays
Warm receptors:
stimulated between 30-
43 oC
Cold receptors:
stimulated between 15-
35 oC

6. Properties of the Receptors:
1. Specificity (the Adequate stimulus)
2. Excitability (the Receptor potential – the generator
potential)
3. Adaption
4. Rate of discharge
5. Neural coding of sensory information

7. 1- Specificity: (the Adequate stimulus)
Muller's Law of specific nervous energy:
• Each type of receptors is highly sensitive to only one type of stimuli called
the adequate stimulus and when it is stimulated it yields a specific
sensation.
• The receptor has low threshold to its adequate stimulus.
• For example:
1. Photoreceptors are stimulated by light giving vision sensation.
2. Touch receptors are highly sensitive to light mechanical stimulus touch
sensation.
3. Cold Receptors are stimulated by the decrease in the environmental
temperature.

8. • However sometimes the receptors can be stimulated by a stimulus
other than the adequate stimulus, if it is of high energy.
• Meaning that, the receptors have high threshold to any stimulus
other than the adequate stimulus.
• For example, application of a pressure to the eye can produce visual
sensation of flashes of light in the affected eye

9. 2- Excitability (the Receptor potential – the generator potential):
• The ability of the receptors to respond
to its adequate stimulus.
• Each receptors respond to its
adequate stimulus by producing
changes in the membrane potential
called the receptor potential or
generator potential.
• The receptor potential is a state of
depolarization in the unmyelinated
receptor region.
• All the receptors are stimulated by
depolarization except the
photoreceptors are stimulated by
hyperpolarization
The receptor (generator) potential. Sensory stimuli result in the
production of local graded potential changes known as receptor, or
generator, potentials (numbers 1–4). If the receptor potential
reaches a threshold value of depolarization, it generates action
potentials (number 5) in the sensory neuron.

10. Each type of receptors has a specific mechanism
for the generation of the receptor potential
• Mechanoreceptors: application of the
mechanical stimulus produces deformity
in the receptors  stretch of the
membrane  open Na+ channels  Na+
influx  depolarization.
• Chemoreceptor:(olfactory) the chemical
substance alter the membrane
permeability of the receptor so increase
the membrane permeability to Na+  Na+
influx  depolarization
• Thermoreceptors: with an increase in the
environmental temperature  increased
metabolic activity of the receptors 
accumulation of metabolites  Na+ influx
 depolarization
Mechanism of stimulation of Mechanoreceptors
Mechanism of stimulation of olfactory receptors

11. • Photoreceptors which are highly
sensitive to light  exposure to
light  hydrolysis of the
photosensitive pigment 
closure of Na channels 
hyperpolarization.
Mechanism of stimulation of photoreceptors

12. Characteristics of the receptor potential:
1- Graded Amplitude:
• The receptor potential is directly proportionate with intensity of the stimulus.
2- Passive Localized spread:
• The receptor potential is conducted electrtonically with decrement until reaching to the impulse
generator area which is the first node of Ranvier.
3- Summation:
• The receptor potential can be summated either spatially with other potential at the same time
and at different sites, or temporally with other potentials following it at the same site.
4- Does Not obey all or non law.
5- Has no refractory period.
6- Can not be blocked by local anesthesia.

13. Ionic bases and mechanism of the receptor potential
When pressure is applied to the Pacinian corpuscles  compress the elastic fibers and the viscous
fluid  deformity of the nerve terminal  increase in the membrane permeability to Na+
depolarization of the nerve terminal  receptor potential generation.
The receptor potential spreads passively creating local current circuits which spread for short
distance.
The strength of the local current is directly propionate with the magnitude of the stimulus.

14. Ionic bases and mechanism of the receptor potential
If the receptor potential is below certain level, it will be insufficient for the generation of action potential at
the first node of Ranvier.
With applying stronger stimuli the receptor potential increase gradually until reaching to the threshold needed
to generate nerve impulse which is transmitted along the sensory nerve.
After the generation of the nerve impulse the node of Ranvier become repolarized, and if the receptor
potential is still higher the threshold  reactivates the node generation of another impulse.
If the receptor potential remain higher the threshold, the first node will continue to produce repetitive
impulses, so it is called the spike generating zone of the receptor or the impulse generator area.

15. 3- Rate of discharge
Weber –Fechner Law:
"The rate of discharge from the receptor
is directly proportionate with the
logarithm of the intensity of the
stimulus".
For example:
Increased stimulus intensity 10 times 
increased rate of discharge 1 time.
Increased stimulus intensity 100 times 
increased rate of discharge 2 times.

16. Detection of the stimulus intensity:
1. The rate of discharge:
Increasing the intensity of the stimulus
leads to an increase in the receptor
potential generation end consequently
increased the rate of discharge. So it is
called the frequency code.
2.The number of stimulated receptors:
Increasing the intensity of the stimulus 
increased the number of the stimulated
receptors.
It is known as recruitment of the receptors. Increased stimulus intensity is associated with increased rate of discharge

17. 4- Adaptation of the receptors:
• The decline in the frequency of discharge of nerve impulses despite of
constant stimulation.
Rapidly Adapting Receptors
(phasic receptors)
Slowly Adapting receptors (Tonic receptors)
Group of receptors which show rapid
decline in the frequency of discharge with
constant stimulation. i.e Pacinian corpuscles.
Group of receptors which continues to
discharge with the constant stimulation, so
showing limited adaptation. i.e
they detect:
1) Onset of the stimulus.
2) Offset of the stimulus.
3) Change of the velocity of the stimulus.
They detect:
1) Onset of the stimulus.
2) The duration of the stimulus.
- Proprioceptors: continue to discharge to
help in equilibrium and production of
voluntary movements.
- Pain receptors: continue to discharge to
avoid injurious agents.
- Baroreceptors: continue to discharge to
regulate arterial blood pressure.
- Chemoreceptor of the carotid body.

18. Mechanism of adaptation:
1- Adaptation due to the visco - elastic nature of the receptor:
• When pressure is applied to the Pacinian corpuscles  compression of the layers of the
elastic fibers and viscous fluid  deformity of the free nerve endings  depolarization of
the receptor.
• The viscous fluid moves and redistribute itself to equalize the pressure around the free nerve
ending, no deformity no discharge,
• Once the pressure is removed the unbalanced distribution of the viscous fluid with
movement of the elastic fibers suddenly in response to the release of pressure  deformity
of the nerve ending  depolarization of the receptor.
• The fluid redistribute itself to the original position  no deformity  stoppage of discharge.
2 - Accommodation of the nerve fiber:
• Adaptation is causes by the inactivation of Na+ channels at the 1st node of Ranvier  no
generation of impulses.

19. 5- Neural coding of sensory information:
• The ability of the brain to identify the locality, the modality and the
intensity of the stimulus.
Sensation modality (modality discrimination)
The ability of the brain to detect the type of stimulus.
Mechanisms:
Peripheral mechanism Specificity of the receptors:
- This occurs at the level of the receptors.
1- Each type of receptors is highly sensitive to only
one type of stimuli called the adequate stimulus and
when it is stimulated it yields a specific sensation.
2- The receptors have low threshold to its adequate
stimulus.
Central mechanism Labeled line principal:
- Each type of sensation has a particular afferent
and a separate pathway to reach to a specific
sensory in the cerebral cortex, such as visual,
auditory , olfactory and somatic sensory area.
- So the sensation is felt when the pathway of any
receptor is activated and the corresponding area in
the cortex is also, activated.
- Specific pathways for different sensory modalities
in the CNS.

20. Sensation Intensity (Intensity
discrimination)
The ability of the brain to detect the
strength of stimulus.
Mechanisms:
- Peripheral mechanisms: the intensity of
the stimulus is detected by the receptors
according to:
1. The rate of discharge.
2. The number of the stimulated
receptors.
-Central Mechanism: Central facilitation –
lateral inhibition:
The stronger sensory inputs are enhanced
whereas the weaker inputs are inhibited at
the same time focusing of the sensory
cortex with the strong stimulus.
A pencil tip pressed against the skin activates receptors under the pencil tip and in the
adjacent tissue. The sensory unit under the tip inhibits additional stimulated units at
the edge of the stimulated area. Lateral inhibition produces a central area of excitation
surrounded by an area where the afferent information is inhibited. The sensation is
localized to a more restricted region than that in which all three units are actually
stimulated.

21. Localization of sensation (Locality discrimination)
The ability of the brain to detect the locality of the
stimulus.
- Each receptor on the body surface is represented
on the cerebral cortex.
- There is a precise anatomical relationship
between the receptors on the skin and the sensory
in the cerebral cortex.
- When an area in the cortical sensory area is
stimulated by impulses discharged from receptors
in the related region of the body, it is not felt in
the cortical area, but is referred to the region of
the body (Law of projection).