Physiology of CNS Sensory System By Dr Abdel

Physiology of CNS Sensory System By Dr. Abdel Aziz M. Hussein Lecturer of Physiology Member of American Society of Physiology

• Sensory receptors are classified functionally according to the following criteria, : 1. their location in the body 2. the nature of tissues in which they are found 3. the nature of stimuli acting on them 4. the morphological structure of the receptor

• A specific stimulus produces a receptor potential by: a. inhibiting Na + influx into receptor b. inhibiting K + efflux from receptor c. enhancing Na + influx into receptor d. enhancing K + efflux from receptor

• Rapidly adapting receptors are involved in : • a. initiation of rapid reflex responses • b. detection of joint movements • c. regulation of heart rate • d. only ‘a’ and ‘b’ are correct

• Receptors detect stimulus intensity by : a. b. c. d. lowering the threshold for stimulating receptors raising the threshold for stimulating receptors decreasing the magnitude of receptor potential raising the magnitude of receptor potential

Fine Touch Def. , • Highly localized touch sensation produced by application of a well localized object to the skin e. g. a tip of a pencil or a head of a pin or teeth of a comb.

Fine Touch Receptors: 1) Meissner's corpuscles (rapidly adapting). 2) Merkel's discs (slowly adapting) • These receptors are numerous in the distal than the proximal parts of the body (more in tips of fingers – tip of the nose – lobule of ear – and nipples of the breast).

Fine Touch Afferents: • A-beta rapidly conducting nerve fibers (30 -70 meters/second Pathway : • Dorsal column – medial lemniscal system (Gracile & Cuneate tracts) • Characterized by a. Fast rate of transmission b. Good spatial arrangement of its fibers c. Accurate information about site and size of fine touch

Fine Touch PVNT Sensory Radiations Gracile and Cuneate Nuclei A beta Receptors Medial Leminiscus Gracile and Cuneate tracts

Fine Touch Types: • It includes : 1. Tactile localization: • It is the ability to localize exactly the point of touch with eyes closed 2. Tactile (2 point) discrimination : • It is the ability to feel 2 points of touch with eyes closed as 2 separate points of touch provided that the distance between the 2 points is greater than minimal or threshold stimulus

Fine Touch Types: Minimal distance: • It is the distance between 2 points of fine touch below it the 2 points are felt as a single point whereas at or above it the 2 points are felt as separate points. • It is very short at tip of tongue (1 mm), finger tips (3 mm) , lips (2 mm) and nipples of the breast. • It is long in the back of trunk (70 mm)

Fine Touch Types: • Tactile localization and discrimination are more accurate on the extremities than on the proximal parts due to: 1. Great number of receptors 2. Great number of afferents 3. Little convergence of afferents centrally 4. Wide area of cortical representation →good analysis and interpretation.

Fine Touch Types: 3. Stereognosis: • It is the ability to recognize a familiar object with eyes closed e. g. recognition of a key, a pen or a coin. 4. Texture of materials: • Ability of the person with his eyes closed to recognize the nature of an object e. g. glasses and wood.

Fine Touch N. B. Gracile and cuneate tracts carry the following sensations: 1. Fine touch 2. Vibration 3. Pressure 4. Proprioceptive sensations

Pressure Sensations

Pressure Sense Def : • It is feeling produced by the application of heavy mechanical stimuli to the skin (which can cause deformation of the different skin layers). Receptors: a. Rapidly adapting receptors (Pacinian corpuscles). b. Slowly adapting receptors (Ruffini multi-branched nerve endings) • They continuously inform the C. N. S about the pressure stimuli • Are present in the deeper layers of the skin

Pressure Sense Significance: • Plays a role in the orientation of body position • Are important for discrimination of weight Pathway: • Gracile and Cuneate tracts (but the fibers end on different neurons in area 3, 1 & 2).

Rapidly adapting mechanoceptive Slowly adapting mechanoceptive Proprioceptive sensations Deep pressure

Vibration Sense

Vibration Sense Touch- Pressure Sense Def : • It is a feeling of rhythmic pressure changes produced by the rapid repetitive stimulation of certain mechanoreceptors. • Receptors: a. Pacinian corpuscles responds to vibrations up to 700 Hz b. Meissner's corpuscles respond to vibration up to 200 Hz

Vibration Sense Touch- Pressure Sense Pathway: • Gracile and Cuneate tracts • Afferents are A beta fibers Test: • Placing the base of vibrating tuning fork on the skin over bony prominence

Proprioceptive Sensations

Proprioceptive Sense Def : • It is the orientation of the position of the different parts of the body in relation to each other as well a movements of joints • Or feeling caused by stimulation of proprioceptors • It arises from deep structures e. g. skeletal ms, ligaments and joints Types: 1. Sense of position 2. Sense of movement • Both senses are called kinesthetic senses

Proprioceptive Sense 1. Sense of position (static sense): • It is conscious orientation of the relative position of the different parts of the body to each other. 2. Sense of movement (dynamic sense): • It means conscious orientation of the changes in the relative position of the different parts of the body to each other as regard, onset, termination, direction and the rate or velocity of this change.

Proprioceptive Sense Receptors: • A) Muscle receptors : are a. Muscle spindle in fleshy part of ms b. Golgi tendon organs in ms tendons • Both inform CNS about; 1) Existing length of the muscles 2) Changes in the muscle length & the rate of this charge “velocity 3) Degree of tension developed in the muscle “force generated in the muscles during contraction”.

Proprioceptive Sense Receptors: • B) Joint receptors : are a. Ruffini endings are slowly adapting receptors • continuously discharge impulses to inform the C. N. S about the position of the joints. b. Pacinian corpuscles are rapidly adapting receptors • Inform the C. N. S about, the onset, the termination and the velocity of the movement

Proprioceptive Sense A) At conscious level Pathway From body: dorsal column – medial leminiscal system “Gracile & Cureate tracts” From face: trigeminal pathway B) At subconscious level From body spinocerebellar tracts a) Dorsal spinocerebellar tract. b) Ventral spinocerebellar tract. From face: Fibers from the main sensory nucleus in pons, relay through the inferior cerebellar peduncle to the cerebellum

Fine Touch PVNT Sensory Radiations Gracile and Cuneate Nuclei A beta Receptors Medial Leminiscus Gracile and Cuneate tracts

cerebellar cortex Vermis of cerebellum Clark’s nucleus

Proprioceptive Sense A) At conscious level B) At subconscious level Site of perception 1 ry sensory area in CC Cerebellar cortex Significance Give rise to the senses Help in the regulation of position & movement. of body equilibrium and feedback regulation of voluntary movements.

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