Textbook notes-Chapter 10-Sensory Physiology

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University of Toronto Scarborough
Biological Sciences
Ingrid L.Stefanovic

MAMMILIAN PHYSIOLOGY Chapter 10 Sensory Physiology Special Senses: Include vision, hearing, smell, taste, equilibrium Somatic Senses: include touch, temperature, pressure, pH, painitch, proprioception (the awareness of body movement and position in space mediated by sensory receptors in muscles and joints) General Properties of Sensory Stimulation All sensory pathways begin with a physical stimulus that acts on a receptor. The receptor acts as a transducer and converts the stimulus to an intracellular signal. If it is above the threshold, it is transferred to the CNS where it is integrated either consciously or unconsciously Sensory receptors vary in complexity. The most basic, simplest receptor is the neuron with naked (free) nerve endings (i.e. somatosensory receptors). In the complex ones, the nerve endings are encased in connective tissue The most complex type of sensory receptor is that of the special senses. All of them except for smell use non-neuronal receptor cells that synapse onto secondary neurons (smell goes straight to the brain through one of the cranial nerves) When activated, they release neurotransmitters that initiate action potentials that send information to the brain Non-neuronal accessory structures are very important to help receptors. I.e. the cornea of the eyes help focus light for photoreceptors; the hairs on ones arm help sense movement of air close to skin. These attributes enhance information gathering Receptors are divided into 4 major groups: 1. Chemoreceptors: respond to chemical ligands that bond to receptors. I.e. taste, smell 2. Mechanoreceptors: Respond to various types of mechanical energy. I.e. pressure, vibrations, acceleration, sound (i.e. hearing) 3. Thermoreceptors: Respond to temperature 4. Photoreceptors: Respond to light www.notesolution.comTransduction The first step of converting physical stimuli into an electrical one is transduction (conversion of stimulus energy into information that can be processed by the nervous system) Adequate Stimulus: Each type of receptor has a particular form of energy it is most responsive to (Thermoreceptors J heat; Mechanoreceptors J pressure) Although receptors may have adequate stimuli, they are also responsive to different forms of stimuli, given it has enough intensity. (I.e. photoreceptors are sensitive to light but if you punch someone in the eye, they will see stars (mechanoreceptors) The threshold J the amount of stimulus required to activate a receptor J is much lower for their specific adequate stimuli Usually the physical stimulus causes the opening or closing of ion channels in the receptor membrane. Opening causes an influx of Na or other cations causing depolarization whereas closing it or an influx of K causes hyperpolarisation The change in sensory receptor membrane potential is called graded potential or receptor potential. In some cells, this causes an action potential to the nervous system. In other cells, it causes the release of neurotransmitters which alters electrical activity in associated neurons Receptive Fields Somatic sensory receptors and visual neurons are activated if stimuli fall within a certain physical area J called a receptive field Each receptive field has a primary neuron which relays electrical info to the secondary neuron located in the CNS It is possible for receptive fields (and therefore their primary neurons) overlap and converge into the same secondary neuron. This causes their individual receptive fields to merge into a single, large secondary receptive field The size of the secondary receptive field determines sensitivity: the large the secondary receptive field (more convergence), the less sensitive I.e. for skin cells, two-point discrimination is used to determine sensitivity on arms and legs. 2 pins placed 20 mm away from each other are viewed as the same prick (high convergence, large secondary receptive field). However in other areas, 2 pins placed 2mm away from each other can be interpreted as 2 different pricks (smaller secondary receptive field, little or no convergence) www.notesolution.com
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