Sensory System PDF

Sensory System Dr Greg Davis Martini Chapter 15 Copyright notice  Learning objectives and reading At the end of this lecture you should be able to: 1. Specify the components of the afferent and efferent divisions of the nervous system and explain what is meant by the so...

Sensory System Dr Greg Davis Martini Chapter 15 Copyright notice  Learning objectives and reading At the end of this lecture you should be able to: 1. Specify the components of the afferent and efferent divisions of the nervous system and explain what is meant by the somatic nervous system 2. Explain why receptors respond to specific stimuli, and how the organisation of a receptor affects its sensitivity 3. Identify the receptors for the general senses and describe how they function Reading guide (textbook): Sections: 15-1, 15-2, 15-3 (leave 15-4, 15-5) Figures: 15-2, 15-3, 15-4, Questions: 1, 2, 3, 6, (14)  Senses General senses Special senses – Temperature – Smell – Pain – Taste – Touch – Vision – Pressure – Hearing – Vibration – Equilibrium – Proprioception – Chemical detection  General senses General sensory receptors are scattered throughout the body Relatively simple in structure Some information sent to CNS reaches the primary sensory cortex (sensation) & some reaches conscious awareness (perception) Receptive field  Receptive field The area monitored by a single receptor cell is its receptive field Small receptive fields enable the localization of the stimulus accurately – e.g. fingertips and tongue: less than 1 mm in diameter Large receptive fields give only a rough idea about the location of a light touch – e.g. in upper arm, receptive fields could be almost 7 cm in diameter  Adaptation Reduction in sensitivity in the presence of a constant stimulus Peripheral adaptation: level of receptor activity changes – Fast-adapting receptors (phasic) – Slow-adapting receptors (tonic) Central adaptation: sensory neuron is active, but the central nervous system adapts – Conscious or unconscious; can also increase sensitivity  Tonic and phasic Slow Rapid  Nociceptors and thermoreceptors Nociceptors are pain receptors – Free nerve endings in the superficial skin, joint capsules, periostea of bones, walls of blood vessels – No adaptation (reminder of injury or inflammation of tissues) Thermoreceptors are temperature receptors – Free nerve endings in dermis, skeletal muscle, liver, hypothalamus – Cold receptors are 3−4 x more numerous than warm receptors – Very active when temperature is changing, but quickly adapt to a stable temperature  Baroreceptors Monitor changes in pressure in an organ, – e.g. blood vessel, or part of respiratory, digestive or urinary tract Adapt rapidly (phasic) Monitor blood pressure in major blood vessels – e.g. the aorta and the carotid artery  Proprioceptors Internal receptors Information processed unconsciously; – no adaptation (tonic) Sense of body position partly arises from these receptors – Sense of equilibrium in inner ear is important too Monitor – position of joints (joint capsules) – tension in tendons and ligaments (Golgi tendon organs) – state of muscular contraction (muscle spindle receptors)  Chemoreceptors Information processed unconsciously Detect small changes in concentrations of specific chemicals – pH (acidity), – CO2 (carbon dioxide), – O2 (oxygen) Important in respiratory centers of the brain Tactile receptors in the skin All are mechnoreceptors  Free nerve endings Sensitive to touch and pressure Small receptive fields Important in corneal surface of the eye Nociceptors and thermoreceptors are also this type  Root hair plexus These receptors are associated with hairs § Monitor distortions and movements across the body surface § Quick to adapt. §  Merkel cells and tactile discs § Fine touch and pressure § Extremely sensitive § Slow to adapt § Very small receptive fields Tactile (Meissner) corpuscles § Fine touch and pressure, low- frequency vibration § Present in eyelids, lips, fingertips, nipples, external genitalia  Lamellated (Pacinian) corpuscles § Sensitive to high-frequency vibration (optimum 250 Hz) § Quick to adapt § Throughout the dermis, fasciae and internal organs § Ruffini corpuscles § Sensitive to pressure and skin distortion § Slow to adapt § Located in deep dermis §  Summary Division between special and general senses Receptive fields – Definition – Practicalities Adaptation Types of receptors – Nociceptors – Thermoreceptors – Baroreceptors – Chemoreceptors – Tactile receptors Free nerve endings Hair root Merkel cells Meisner corpuscles Pacinian corpuscles Ruffini endings