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Sensory Pathways and the Somatic Nervous System Fundamentals of Anatomy & Physiology Eleventh Edition Chapter 15 Sensory Pathways and the Somatic Nervous System Lecture Presentation by Lori Garrett, Parkland College © 2018 Pearson Education, Inc. Learning Outcomes 15-1 Specify the components of the afferent and efferent divisions of the nervous system, and explain what is meant by the somatic nervous system. 15-2 Explain why receptors respond to specific stimuli, and how the organization of a receptor affects its sensitivity. 15-3 Identify the receptors for the general senses, and describe how they function. 15-4 Identify the major sensory pathways, and explain how it is possible to distinguish among sensations that originate in different areas of the body. 15-5 Describe the components, processes, and functions of the somatic motor pathways, and the levels of information processing involved in motor control. 2 © 2018 Pearson Education, Inc. Sensory Pathways and Somatic Nervous System ▪ Focus for this chapter – Sensory pathways • General senses – Motor pathways • Somatic nervous system (SNS) – Controls contractions of skeletal muscles 3 © 2018 Pearson Education, Inc. 15-1 Sensory and Motor Pathways ▪ Sensory pathways – Series of neurons that relays sensory information from receptors to CNS ▪ Sensory receptors – Specialized cells or cell processes that monitor specific conditions • In the body or external environment – When stimulated, a receptor generates action potentials that are sent along sensory pathways 4 © 2018 Pearson Education, Inc. 15-1 Sensory and Motor Pathways ▪ Afferent division of nervous system – Somatic and visceral sensory pathways ▪ Efferent division of nervous system – Somatic motor portion • Carries out somatic motor commands that control peripheral effectors • Commands travel from motor centers in brain along somatic motor pathways 5 © 2018 Pearson Education, Inc. Figure 15–1 An Overview of Events Occurring Along the Sensory and Motor Pathways. Sensory Pathway Depolarization of Sensory Action Potential CNS Arriving Propagation stimulus Receptor Generation Processing A stimulus produces a If the stimulus depolarizes Axons of sensory neurons Information processing graded change in the the receptor cell to carry information about occurs at every relay membrane potential of threshold, action the type of stimulus synapse. Sensory informa- a receptor cell. potentials develop in the (touch, pressure, tion may be distributed to initial segment. temperature) as action multiple nuclei and centers potentials to the CNS. in the spinal cord and brain. Immediate Involuntary Response Motor Pathway (involuntary) Processing centers in the spinal cord or brainstem may direct an immediate reflex response even before sensations reach the cerebral cortex. Voluntary Response Perception Motor Pathway The voluntary response, which is not (voluntary) Only about 1 percent of immediate, can moderate, enhance, arriving sensations are or supplement the relatively simple relayed to the primary involuntary reflexive response. somatosensory cortex. 6 15-2 Sensory Receptors ▪ Sensory receptors – Processes of specialized sensory neurons – Or cells monitored by sensory neurons ▪ Sensation – Arriving information ▪ Perception – Conscious awareness of a sensation 7 © 2018 Pearson Education, Inc. 15-2 Sensory Receptors ▪ General senses – Describe our sensitivity to • Temperature • Pain • Touch • Pressure • Vibration • Proprioception (body position) 8 © 2018 Pearson Education, Inc. 15-2 Sensory Receptors ▪ Special senses – Olfaction (smell) – Gustation (taste) – Vision (sight) – Equilibrium (balance) – Hearing ▪ Special sensory receptors – Provide sensations of special senses – Located in sense organs such as the eye or ear – Protected by surrounding tissues 9 © 2018 Pearson Education, Inc. 15-2 Sensory Receptors ▪ Detection of stimuli – Receptor specificity • Each receptor has a characteristic sensitivity – Receptive field • Area monitored by a single receptor cell • The larger the receptive field, the more difficult it is to localize a stimulus – Transduction • Conversion of an arriving stimulus into an action potential by a sensory receptor 10 © 2018 Pearson Education, Inc. Figure 15–2 Receptors and Receptive Fields. Receptive Receptive field 1 field 2 Epidermis Free nerve endings 11 15-2 Sensory Receptors ▪ Taste, hearing, equilibrium, and vision – Provided by specialized receptor cells – Communicate with sensory neurons across chemical synapses 12 © 2018 Pearson Education, Inc. 15-2 Sensory Receptors ▪ Interpretation of sensory information – Stimulus reaches cortical neurons via labeled line – Each labeled line carries information about one modality, or type of stimulus (e.g., touch or light) – Frequency and pattern of action potentials contain information • About strength, duration, and variation of stimulus – Your perception of the nature of a stimulus • Depends on the path it takes in CNS 13 © 2018 Pearson Education, Inc. 15-2 Sensory Receptors ▪ Adaptation – Reduction of receptor sensitivity in the presence of a constant stimulus – Peripheral adaptation in PNS – Central adaptation in CNS – Nervous system quickly adapts to painless, constant stimuli 14 © 2018 Pearson Education, Inc. 15-2 Sensory Receptors ▪ Tonic receptors – Always active – Slow-adapting receptors • Show little peripheral adaptation – Pain receptors • Remind you of an injury long after damage has taken place 15 © 2018 Pearson Education, Inc. Figure 15–3a Tonic and Phasic Sensory Receptors. Increased Normal Normal Stimulus Frequency of action potentials Time a Tonic receptors are always active. Action potentials are generated at a frequency that reflects the background level of stimulation. When the stimulus increases or decreases, the rate of action potential generation changes accordingly. 16 15-2 Sensory Receptors ▪ Phasic receptors – Normally inactive – Provide information about intensity and rate of change of a stimulus – Fast-adapting receptors • Respond strongly at first but then activity decreases 17 © 2018 Pearson Education, Inc. Figure 15–3b Tonic and Phasic Sensory Receptors. Increased Stimulus Normal Normal Frequency of action potentials Time b Phasic receptors are normally inactive. Action potentials are generated only for a short time in response to a change in the conditions they are monitoring. 18 15-3 General Sensory Receptors ▪ Classifying sensory receptors – Exteroceptors • Provide information about external environment – Proprioceptors • Report positions of skeletal muscles and joints – Interoceptors • Monitor visceral organs and functions 19 © 2018 Pearson Education, Inc. 15-3 General Sensory Receptors ▪ General sensory receptors – Divided into four types by nature of stimulus • Nociceptors (pain) • Thermoreceptors (temperature) • Mechanoreceptors (physical distortion) • Chemoreceptors (chemical concentration) 20 © 2018 Pearson Education, Inc. 15-3 General Sensory Receptors ▪ Nociceptors (pain receptors) – Free nerve endings with large receptive fields – Are common • In superficial portions of skin • In joint capsules and within periostea of bones • Around walls of blood vessels – May be sensitive to • Temperature extremes • Mechanical damage • Dissolved chemicals (as released by injured cells) 21 © 2018 Pearson Education, Inc. 15-3 General Sensory Receptors ▪ Nociceptors – Myelinated Type A fibers • Carry sensations of fast pain (prickling pain) such as that caused by injection or deep cut • Sensations reach CNS quickly and often trigger somatic reflexes • Relayed to primary somatosensory cortex and thus receive conscious attention 22 © 2018 Pearson Education, Inc. 15-3 General Sensory Receptors ▪ Nociceptors – Unmyelinated Type C fibers • Carry sensations of slow pain (burning and aching pain) • Sensations cause generalized activation of reticular formation and thalamus • You become aware of the pain but only have a general idea of the area affected 23 © 2018 Pearson Education, Inc. 15-3 General Sensory Receptors ▪ Thermoreceptors (temperature receptors) – Free nerve endings located in • Dermis • Skeletal muscles • Liver • Hypothalamus – Sensations are conducted along same pathways that carry pain sensations • Sent to reticular formation, thalamus, and (to a lesser extent) the primary somatosensory cortex 24 © 2018 Pearson Education, Inc. 15-3 General Sensory Receptors ▪ Mechanoreceptors – Sensitive to physical stimuli that distort their plasma membranes – Membranes contain mechanically gated ion channels that open or close in response to • Stretching • Compression • Twisting • Other distortions of the membrane 25 © 2018 Pearson Education, Inc. 15-3 General Sensory Receptors ▪ Three classes of mechanoreceptors – Tactile receptors provide sensations of • Touch (shape or texture) • Pressure (degree of mechanical distortion) • Vibration (pulsing pressure) – Baroreceptors • Detect pressure changes in blood vessels and in digestive, respiratory, and urinary tracts – Proprioceptors • Monitor positions of joints and skeletal muscles 26 © 2018 Pearson Education, Inc. 15-3 General Sensory Receptors ▪ Tactile receptors – Fine touch and pressure receptors • Extremely sensitive • Narrow receptive fields • Provide detailed information about source of stimulation – Crude touch and pressure receptors • Large receptive fields • Provide poor localization • Give little information about stimulus 27 © 2018 Pearson Education, Inc. 15-3 General Sensory Receptors ▪ Six types of tactile receptors in skin 1. Free nerve endings • Sensitive to touch and pressure • Situated between epidermal cells • Tonic receptors
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