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Biology 232 Human Anatomy and Physiology Chapter 15 Lecture Outline Sensory and Motor Pathways sensation – conscious or subconscious awareness of internal or external stimuli perception – conscious awareness and interpretation of sensations (occurs in thalamus and cerebral cortex) Basic Sensory Pathway 1) sensory receptor – specialized cell or dendrites that detect stimuli stimulus – change in internal or external environment specificity – most receptors are most sensitive to a particular type of stimulus (modality) receptive field – area in which a stimulus can be detected varies in size for different receptors and body regions 2) transduction – stimuli produce graded potentials receptor potentials – size depends on strength of stimulus 3) generation of an action potential first-order neuron – conducts sensory impulse from PNS to CNS 4) integration of sensory input in CNS spinal cord – spinal reflexes brain stem – cranial reflexes, autonomic reflexes lower brain regions – subconscious motor regulation cerebral cortex – precise perception and complex integration Sensory Adaptation – amplitude of graded potentials produced by some receptors decreases during constant stimulation and perception of stimulus decreases fast-adapting receptors – detect changes in stimulus (eg. touch, smell) slow-adapting receptors – continue signaling as long as stimulus persists (eg. pain, proprioception, visceral receptors) (central adaptation – occurs in sensory pathways in the CNS) SENSORY RECEPTORS Structural Classifications of Sensory Receptors: free nerve endings – bare dendrites (eg. pain, temperature, touch) encapsulated nerve endings – dendrites enclosed in capsule of connective tissue (eg. touch, pressure) specialized cells – cells that produce receptor potentials which trigger release of neurotransmitters at a synapse with the first-order neuron 1 Locational Classifications of Sensory Receptors: exteroceptors – at or near body surface; sensitive to external stimuli consciously perceived (SNS) interoceptors – within deep tissues and visceral organs; sensitive to internal stimuli; not usually consciously perceived (ANS) proprioceptors – in muscles, tendons, joints, inner ear; detect body position and movements (SNS) FUNCTIONAL CLASSIFICATIONS OF SENSORY RECEPTORS – based on the type of stimulus that excites them 1) nociceptors – detect pain sensations free nerve endings in most tissues (fewer in visceral organs) stimulated mainly by chemicals released by tissue damage fast pain (acute or sharp pain) – conduct rapidly by Type A axons well localized sensations, mainly superficial slow pain (aching or throbbing pain) – brief delay, then intensity increases conducted by Type C axons not well localized referred pain – pain felt in another area served by the same spinal cord segment occurs mainly with visceral pain that cannot be localized well due to fewer receptors and less well-established pain pathways slow-adapting receptors – pain continues until tissue damage ends 2) thermoreceptors – detect temperature free nerve endings with in skin, muscles, hypothalamus cold receptors – detect temperatures ranging from 50-105 degrees F warm receptors – detect temperatures ranging from 90-118 degrees F lower or higher temperatures mainly stimulate pain receptors fast-adapting receptors – detect mainly changes in temperature 3) mechanoreceptors – detect distortions of cell membrane (touch, pressure, vibration) adaptation speed varies Meissner (tactile)corpuscles – encapsulated nerve endings in dermal papillae of skin hair root plexuses – free nerve endings around hair follicles detect touches that disturb hairs Merkel discs (tactile discs) – flattened free nerve endings that contact Merkel cells Ruffini corpuscles – encapsulated receptors in deep dermis; detect stretching lamellated (Pacinian) corpuscles – encapsulated receptors; widely distributed in internal and external tissues; detect pressure sensations proprioceptors – mechanoreceptors that monitor location of body parts located in muscles, tendons, joint capsules, (inner ear) slow-adapting kinesthesia – perception of movement of body part in relation to another 2 muscle spindle – nerve endings wrapped around 3-10 intrafusal muscle fibers and enclosed in a capsule; measure muscle length; stimulated by stretching brain regulates sensitivity to maintain muscle tone tendon organs – in tendon/muscle junction; nerve endings entwined with collagen fibers and wrapped in a capsule; stimulated by tension joint capsule receptors – variable nerve endings in joint capsules stimulated by pressure, tension, or movement baroreceptors – mechanoreceptors in blood vessels and organs detect pressure in vessels or organs – stimulated by stretch of walls mainly function in autonomic reflexes 4) chemoreceptors – detect concentrations of specific chemicals mainly function in autonomic reflexes special senses – smell, taste 5) photoreceptors – detect light; function in vision SENSORY PATHWAYS TO CEREBRAL CORTEX 1) first-order neurons – from somatic receptors to CNS (spinal cord or brain stem) cranial nerves – areas of face and mouth spinal nerves – neck, body, posterior head 2) second-order neurons – from brain stem or spinal cord to thalamus decussate (cross over) in medulla or spinal cord before ascending to thalamus all sensory information from one side of the body goes to the cerebral cortex on the opposite side 3) third-order neurons – from thalamus to cerebral cortex primary somatosensory area – in postcentral gyri of parietal lobes sensory homunculus – each region of cortex receives input from specific body area; size of cortical region depends on number of sensory receptors in body part visual cortex, auditory cortex, olfactory cortex, gustatory cortex Not all sensory inputs reach the sensory cortex and are perceived. proprioceptive inputs to cerebellum – subconscious movements visceral (autonomic) sensations central adaptation 3 Somatic Motor Pathways – from cerebral cortex and subconscious motor centers of lower brain to skeletal muscles primary motor area – in precentral gyri of frontal lobes motor homunculus – each region of cortex controls muscles of specific area; size of cortical region depends on number of motor units in area Motor Pathway Neurons: lower motor neurons (LMNs) – cell bodies in brain stem or spinal cord; axons in cranial nerves or spinal nerves; extend to skeletal muscle upper motor neurons (UMNs) – cell bodies in primary motor cortex, motor association areas, or subconscious motor centers of lower brain plan and initiate voluntary movements regulate muscle tone, posture and balance basal ganglia neurons – help initiate and terminate movements; influence muscle tone; suppress unwanted movements cerebellar neurons – monitor motor commands (UMNs) compare with actual movements (proprioceptors, inner ear, eyes) provide feedback to UMNs to correct movements; coordinate movements, help maintain posture and balance direct motor pathways – UMNs from cortex synapse directly with LMNs; 90% decussate in medulla, other 10% in spinal cord control voluntary body movements motor impulses from cortex control skeletal muscles on opposite side of body indirect motor pathways – UMNs from brain form complex pathways with input from motor cortex, basal ganglia, thalamus, cerebellum, reticular formation and nuclei in brain stem control subconscious movements responsible for muscle tone, posture and balance, coordination 4