Summary PHYSIOLOGY : Sensory Physiology Introduction

Sensory Physiology Introduction • The survival of any organism depends on having adequate information about the external environment, where food is to be found and where hazards abound. • Also important for maintaining the function of a complex organism is information about the state of numerous internal bodily processes and functions. Introduction • Events in the external and internal environment must first be translated into signals that the nervous systems can process • Sensation process involves sampling selected small amounts of energy from the environment and using it to control the generation of action potentials or nerve impulses Sensory Receptors • Are biological structures that can be as simple as a free nerve ending or as complicated as an eye or the ear. • They respond to a particular modality of environmental stimuli. • usual stimulus for the eye is light & that for the ear is sound • They transduce (change) different forms of sensation to nerve impulses that are conducted to CNS. • factor in the environment that produces an effective response in a sensory receptor is called a stimulus Law of Specific Nerve Energies • Usual and appropriate stimulus for a receptor is called its adequate stimulus • Sensation characteristic of each sensory neuron is that produced by its normal or adequate stimulus. • Adequate stimulus: – least amount of appropriate stimuli required to activate a receptor. • Regardless of how a sensory neuron is stimulated, only one sensory modality will be perceived. – Allows brain to perceive the stimulus accurately under normal conditions. Adequate stimuli • For adequate stimuli, receptor has lowest threshold ,ie, the lowest stimulus intensity that can be reliably detected • Receptor may respond to other stimuli but threshold for inappropriate stimulus much higher ,eg, extreme temps may be percieved as pain, pressing the outer corner of the eye will produce a visual sensation caused by pressure, not light etc Stimulus type • Mechanoreceptors- deformed by force – Touch, pressure (BP), vibration, stretch, itch • Thermoreceptors- changes in temperature • Photoreceptors- light energy • Chemoreceptors- chemicals in solution – Smell, taste, blood chemistry • Nociceptors- pain All receptors can interpret pain if overstimulated! Sensory Receptors Labeled line principle • Each nerve tract terminates at Sp. Pt in CNS • Sp type of sensation felt determined by point in CNS where nerve leads • When pain fiber stimulated, pain felt irrespective of type of stimulus whether heat, electrical or crushing of fiber • Electrical stimulation of touch fiber, touch is perceived • Information arriving by optic nerve always perceived as light not sound Cortical Sensory Areas Transduction-transformation of stimulus to electric