Pns

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Unformatted text preview: • • PNS Consists of nerve fibers that carry information b/t the CNS and other parts of the body. • Afferent division- sends information form internal and external environment to CNS. • Visceral afferent- incoming pathway for information from internal viscera (organs in body cavities). • Sensory afferents- somatic (body sense) sensation. Sensation arising form the body surface and proprioception. • • • • • • • • • • • • Special senses- vision, hearing, vestibular, taste, smell. Receptors- structures at peripheral endings of afferent neurons. Convert forms of stimulus energy into electrical signals (AP). Process is called transduction. Detect stimuli- change detectable by the body only if AP produced. Minimum threshold of energy required. Photoreceptors- responsive to visible wavelength of light. Mechanoreceptors- sensitive to mechanical energy. Thermoreceptors- sensitive to heat and cold. Receotors-may be specialized endings of an afferent neuron. Separate cell synaptically associated with peripheral endings of a neuron. Transduction- stimulus alters receptor’s permeability which leads to graded receptor potential. • • Usually causes nonselective opening of a small cation channels. This change in membrane permeability leads to the influx of Na+ ions producing receptors (generators) potential. • The magnitude of the receptor potential represents the intensity of the stimulus (this is a graded potential). • A receptor potential of sufficient magnitude can produce a depolarization of Em, leading to an action potential (if > threshold). • • This action potential is propagated along an afferent fiber to the CNS. Receptor membrane potentials return towards resting levels by the process of adaptation. • • • • • • • • • • • May adapt slowly or rapidly to sustained stimulation. 2 types of receptors Tonic receptors Do not adapt at all or adapt slowly. Muscle stretch receptors, joint proprioceptors. Phasic receptors Rapidly adapting receptors Tactile receptors in skin. Receptive Field (Acuity) Small receptor field and large recepto...
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