NeuroPhysiology_B PDF Notes

 Sensory Receptors Sensory physiology, sensory receptors and receptor potentials-2 Prof.Dr. Ramazan Bal  Next / Today’s topic’s learning objectives...

 Sensory Receptors Sensory physiology, sensory receptors and receptor potentials-2 Prof.Dr. Ramazan Bal  Next / Today’s topic’s learning objectives  At the end of this lecture you will be able: – To list types of receptor – To decribe receptor potential – To classify types of fiber – To classify somatic sensation – To describe functions of types receptors – To explaine adaptation of receptors Prof.Dr. Ramazan Bal  Somatic sensations Senses classified as: (1). Special senses: – Vision, – Hearing, – Smell, – Taste, – Equilibrium. (2) Somatic senses: Physiological classification of somatic senses: – (1) Mechanoreceptive somatic senses, which detect tactile & position sensations – Tactile – Touch – Pressure – Vibration – Itchcing – Position (Proprioseption) – Equlibrium – (2) Pain sense,nociception  Other classifications of Somatic sensations: (1). Exteroreceptive sensations are those from the surface of the body. (2). Proprioceptive sensations are to do with the physical state of the body, – Position sensations, – Tendon &muscle sensations, – Pressure sensations from the bottom of the feet, (3). Visceral sensations are those from the viscera of the body; (4). Deep sensations come from deep tissues (FE: fasciae, muscles &bone) – a). deep pressure, – b). pain & – c). vibration  Which receptors Cellular receptors Sensory receptors 1. On the surface of cellular membrane or 2. Speciliazed receptor cells in the cytoplasma Prof.Dr. Ramazan Bal  Reseptors  Either specilized cells,  Or Free nerve ending Specilized cells, Free nerve ending  A single sensory axon and all of its peripheral branches constitute sensory unit Prof.Dr. Ramazan Bal  Types of sensory receptors  Sensory receptors, inform organism about the changes occuring in the internal and external environment  Each receptor is highly sensitive to one type of stimulus  Modality of sensation: labeled line principle  Each of principal types of sensation (FE: pain, touch) is called a modality of sensation  How does different nerve fibers transmit different modalities of sensation? – Since nerve fibers transmit only impulses.  Each nerve tract terminates at a specific point in the CNS  For example, if a pain fiber is stimulated, the person perceives pain – The stimulus can be electricity, overheating of the fiber, crushing of the fiber,  Modality of sensation: labeled line principle  A phantom limb is the sensation that an amputated part is still attached to the body  ~70% of people experience phantom sensations (especially pain) in their amputated limb – Missing limb often feel as if it is in a distorted and painful position Prof.Dr. Ramazan Bal  Modality of sensation: labeled line principle  Mechanism: Caused by irritation &inflamed in severed nerve endings (called neuromas) – These nerve endings send signals to brain, which is interpreted by the brain as pain  Each locality sends sensory impulses to the brain via a specific sensory pathway. – This is called the law of projection  Other possible cause new projection of fibers from neighboring primary somatic sensory areas into the primary somatic sensory area that amputated limb is projected to Prof.Dr. Ramazan Bal  How do different receptor recognize different sensory stimuli?  Each receptor is sensitive to a certain stimuli that has special form of energy – Receptors can also be stimulated by nonspecific stimuli of extremely large intensity / amplitude – FE: when too big pressure is applied to the receptor, person can perceive it as light (seing stars when you have a blow onto your eyes) Prof.Dr. Ramazan Bal  Transduction of sensory stimuli  Receptor potentials: Different receptors can be excited by: – (1) Mechanical deformation of the receptor stretches receptor membrane which opens ion channels – (2) Application of a chemical to the membrane also opens ion channels – (3) Change of temperature of the membrane alters the permeability of th membrane – (4) by the effects of electromagnetic radiation, such as light on a retinal visual receptor Prof.Dr. Ramazan Bal  Transduction  Receptors transform specific energy forms in physico-chemical nature into electrical signals. This is called transduction 1. Transduction phase:  When a receptor is stimulated by appropriate stimuli, ion channels in the membrane of receptors open and then change of local potential occur.  This is called receptor potential  Amplitude &duration of receptor potential depends on amplitude &duration of stimuli 2. Genesis of action potential phase :  Transduction & genesis of action potential phasis can occur in different cells Prof.Dr. Ramazan Bal  Transduction of sensory stimuli  Maximum receptor potential amplitude: ~100mV  Receptor potentials are graded potentials, depending on stimulus intensity  Receptor potentials are not action potential and therefore they are not are all-or- none  If the receptor potential rises above the threshold for eliciting action potentia in the nerve fiber attached to the receptor, then action potential occur Prof.Dr. Ramazan Bal  Transduction of sensory stimuli Receptor potential of the pacinian corpuscle  Corpuscle has a central nerve fiber & surrounded by multiple concentric capsule layers  Compression on the outside of the corpuscle will deform the central fiber – Na+ channels open, which creates depolarization (receptor potential)  If the threshold is reached, action potentials are generated and transmitted along the nerve fiber toward the CNS – The frequency of repetitive action potentials increases in proportion to the increase in receptor potential Prof.Dr. Ramazan Bal  Adaptation of receptors  If a continuous sensory stimulus is applied, receptor responds with a progressively slower rate – Rapidly adapting (phasic): Meissner corpuscle end-organs, Pacinian corpuscle end-organs, Hair end-organ – Slowly adapting (never adapt completely) (tonic): Baroreceptors, chemoreceptors, Muscle spindle, golgi tendon organ, Receptors of macula Merkel cell end-organ: Ruffini corpuscle Joint capsule Pain receptors,  Adaptation of receptors Mechanism  The mechanism of receptor adaptation is different for each type of receptor, – For example, in the eye, rods &cones adapt by changing level of light-sensitive chemicals  Slowly adapting receptors detect continuous stimulus strength: tonic receptors. – Slowly adapting receptors transmit impulses to the CNS as long as the stimulus is present  Rapidly adapting receptors detect changes in stimulus strength: phasic receptors  In, the pacinian corpuscle, sudden pressure excites this receptor for a few ms, and then its excitation is over even though the pressure continues. Prof.Dr. Ramazan Bal Encoding stimulus intensity How the CNS perceive stimulus intensity? 1. by changing the frequency of impulses induced by receptor  Amplitıde of the receptor potential increases, as stimulus intensity increases  Which, in turn, increases requency of action potential in the fibers and / or 2. Number of receptor (also fiber) stimulated increases Prof.Dr. Ramazan Bal  Classification of fibers Prof.Dr. Ramazan Bal  Next / Today’s topic’s learning objectives  At the end of this lecture you will be able: – To describe what conduction velocity depends on – To classify types of fibers Prof.Dr. Ramazan Bal  Classification of nerve fibers  Diameter of nerves ranges from 0.5 to 20 m – Conduction velocity is between 0,5 and 120 m/sec  A stumilus applied to the finger tip has a distance of ~1 m to spinal cord – Signal is conducted at 25 m/sec with A fiber in 0.04 sec. – Signal is conducted at 0.5 m/sec with C fiber in more than 2 sec.  If the signal is conducted by C fibers, the finger would be burnt before the signal reach to the CNS  General classification of nerve fibers Type A fibers  Large &medium-sized myelinated fibers of spinal nerves Type B fibers:  Myelinated preganglionic autonomic fibers Type C fibers  Small unmyelinated nerve fibers. – Some sensory fibers &postganglionic autonomic fibers A B C  Classification of nerve fibers by sensory physiologists Diameter Speed (m/sec) 12-20 m 70-120 4-12 m 30-70 2-8 m 25-70 A 10-50 1-5 m 3-30 crude touch &pricking