Physiology of Excitable tissue L 10 Sensation I

Physiology of Excitable tissue L 10 Sensation I Prof. Fakhir Al-Ani fakeralani 2000@yahoo. com

Nervous system

The sensory system: Carry information from the external & internal environment To the CNS to get response (reaction) The response could be Reflex Weak stimulus Sensation Strong stimulus

Reflex It is an involuntary response to a stimulus, usually without orientation. Reflex could be Autonomic Sweating. heart rate, blood Pr. Urination, defecation Reaction of the pupil Somatic Painful reflex Tendon Jerk Blinking

Sensation Strong stimulus: will ascend to the higher centers of the sensory system (Thalamus & Cerebral cortex) So the subject will be oriented about the different stimuli that is referred to as sensation or feeling.

Sensation & Perception Sensation Awareness of sensory stimuli in the higher centers. (Depend on sensory system) Feeling without full appreciation. Perception Meaningful interpretation Conscious understanding of sensory data (Need memory, thinking & higher function)

Sensory Unite Sensory Unit: Sensory neurons + all of its branches + all the receptors attached to it

Sensory Neuron Sensory neuron body at the dorsal root ganglia It is Bipolar nerve cell that sends two axons 1. A long peripheral one that end with receptors. 2. A short central one that synapse with the second order neuron within the CNS. Receptors Cell body Axon Nerve terminal in CNS

Sensory & Motor Neuron

Receptor Classification Different type of classification

A. Physiological classification: 1. Mechanoreceptors = Mechanical 2. Thermo-receptors = Heat 3. Nociceptors = Damaging stimulus 4. Electro-receptors = Electromagnetic waves 5. Chemo-receptors = Chemical

According to types of receptor transduction evolves: 1. Mechanical deformation: Stretching of the membrane will open ion channels (touch, pressure, stretch receptors) 2. Chemical substances: Cause opening of chemical gated ion channels. (Ach, Adrenaline, KCl salt, e. t. c. ) 3. Thermal stimuli: kinetic energy that membrane permeability lead to opening of the ion channels. 4. Electromagnetic waves: Alter the permeability of the membrane either directly or indirectly that end with excitation of the nerve fibers

Classification of receptors B. Structural classification: 1. Naked nerve endings: 2. Corpuscles: 3. Muscle spindle: 4. Vestibular apparatus: 5. Special senses:

1. Naked Nerve Endings

2. Corpuscle

3. Muscle spindle

4. Vestibular Apparatus

5. Special senses

C. Clinical classification: 1. Superficial receptors: 2. Deep receptors: 3. Visceral receptors: 4. Special sensation:

Properties of the receptors: 1. Specificity & adequate stimulus. 2. Responses according to stimulus degree. 3. Adaptation. 4. Modality of sensation. 5. Localization of sensation. 6. Recruitment.

1. Specificity & adequate stimulus Receptor is a transformer: Convert various types of energy form into Electrical. (Mechanical, chemical, thermal, electromegnatic) Each receptor is adopted to respond to one type of energy called adequate stimulus But can respond to another form of energy with higher intensity (higher threshold level). Eye: Electromagnetic energy. Osmoreceptors: Osmolarity. Pain : Damaging energy.

Transduction of sensory stimuli into nerve impulses: The receptor is a transformer: It changes any type of energy into electrical nerve impulse (AP). That means initiation of: 1. Receptor potential (Local response) in the R. 2. Generator potential (Propagated response)

2. Response of receptor according to intensity Weak: Receptor potential (Local changes) Strong: Generator potential (Propagated P) Stronger: - Frequency & amplitude of impulses. - Recruitment (excitation of many receptors). - Noiciceptive sensation (painful stimulation).

3. Adaptation: receptor firing rate with time even if the stimulus intensity is still constant. It is a general property of sensory receptors. All receptors are adaptable but to a variable degree. - Pacinian corpuscles for touch sensation - Hairy receptor for touch sensation. - Proprioceptive receptors. - Baro-receptors - Pain receptors.

Adaptation Rate of firing impulses by the receptor although the intensity of stimulus is still constant.

Other characteristics of adaptation Adaptation Partial Complete. Adaptation Peripheral At receptor level Central Stimuli descend from a central inhibitory fibers.

Types of receptors according to speed of adaptation Phasic receptors Tonic receptors Fast adaptation Slow adaptation like Pacinian corpuscles. like Baroreceptors

Mechanism of adaptation: 1. The receptor structure itself adapts to deformity Due to the viscoelastic properties of the corpuscles (redistribution of the fluid). 2. Accommodation of nerve fiber. Due to partial inactivation of the Na+ channels 3. Descending inhibitory impulses from CNS.

4. Modality of sensation § Modality of sensation depend on all the factors: The receptor. The nerve fiber. The neural pathway to the CNS. The localized area in the cerebral cortex. The specificity of a nerve fiber to transmit only one modality of sensation is called Labelled line principle.

5. Localization of sensation It depend mainly on Doctrine principle For each area in the body there is a specific area in the central nervous system that is stimulated This Depend on additional factors Like Experience Innervations pathway and convergence Phantom Syndrome

6. Recruitment Intense stimulation of a receptor After maximum response Will lead to Activation of adjacent receptors.

Not all of your perception are correct