Central Nervous System Dr. Hussein Farouk Sakr

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, andthe terminals in the CNS. Also shown is the receptive field of thisneuron.

The Sensory receptors• Specialized microscopic structures

present at the peripheral termination of the afferent neurons.

• Function: they are1. 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.

Sensory receptors transform an external signal into a membrane potential

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

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

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.

• 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

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.

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

• 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

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.

Ionic bases and mechanism of the receptor potentialWhen 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.

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.

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.

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

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.

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.

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.

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.

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).