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Collin County Community College BIOL 2401 : Anatomy/ Physiology PNS Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Peripheral Nervous System (PNS) PNS – all neural structures outside the brain and spinal cord Includes sensory receptors, peripheral nerves, associated ganglia, and motor endings Provides links to and from the external environment Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings 1 PNS in the Nervous System Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 13.1 Sensory Receptors Structures specialized to respond to stimuli Activation of sensory receptors results in depolarizations that trigger impulses to the CNS The realization of these stimuli, sensation and perception, occur in the brain Sensory Receptors can be specialized cells closely associated with peripheral endings of sensory neurons or specialized regions of sensory neurons. Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings 2 Receptor Classification by Stimulus Type Mechanoreceptors – respond to touch, pressure, vibration, stretch, and itch Thermoreceptors – sensitive to changes in temperature Photoreceptors – respond to light energy (e.g., retina) Chemoreceptors – respond to chemicals (e.g., smell, taste, changes in blood chemistry) Nociceptors – sensitive to pain-causing stimuli Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Receptor Class by Location: Exteroceptors Respond to stimuli arising outside the body Found near the body surface Sensitive to touch, pressure, pain, and temperature Include the special sense organs Receptor Class by Location: Interoceptors Respond to stimuli arising within the body Found in internal viscera and blood vessels Sensitive to chemical changes, stretch, and temperature changes Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings 3 Receptor Class by Location: Proprioceptors Respond to degree of stretch of the organs they occupy Found in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles Constantly “advise” the brain of one’s movements Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Receptor Classification by Structural Complexity Receptors are structurally classified as either simple or complex Most receptors are simple and include encapsulated and unencapsulated varieties Complex receptors are special sense organs Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings 4 Simple Receptors: Unencapsulated Free dendritic nerve endings Respond chiefly to temperature and pain Merkel (tactile) discs Hair follicle receptors Simple Receptors: Encapsulated Meissner’s corpuscles (tactile corpuscles) Pacinian corpuscles (lamellated corpuscles) Muscle spindles, Golgi tendon organs, and Ruffini’s corpuscles Joint kinesthetic receptors Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Receptor Density • Receptors vary in terms of abundance relative to each other. • For example, there are far more pain receptors than temperature receptors in the body. • Receptors also vary in terms of the concentration of their distribution over the surface of the body • The fingertips having far more touch receptors than the skin of the back of the hand. The figure shows the distribution of temperature receptors in the skin by area. Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings 5 Simple Receptors: Unencapsulated Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Table 13.1.1 Simple Receptors: Encapsulated Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Table 13.1.2 6 From Sensation to Perception Survival depends upon sensation and perception Sensation is the awareness of changes in the internal and external environment Perception is the conscious interpretation of those stimuli and of the external world from a pattern of different sensory nerve impulses via the sensory receptors. Some perceptions are indeed integrated compound sensations such as for example “wetness” ( touch, pressure and thermal input…. there is no such thing as a “wetreceptor”) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Organization of the Somatosensory System Input comes from exteroceptors, proprioceptors, and interoceptors The three main levels of neural integration in the somatosensory system are: Receptor level – the sensor receptors Circuit level – ascending pathways Perceptual level – neuronal circuits in the cerebral cortex Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings 7 Processing at the Receptor Lever The receptor must have specificity for the stimulus energy ( temperature, touch, pressure, light,…) The receptor’s receptive field must be stimulated Stimulus energy must be converted into a graded potential If the receptive field is in the same neuron that generates the action potential, we call it a generator potential. If the receptive field is in a separate cell, it is called a receptor potential. If summed up to reach threshold, hhis will then release neurotransmitters in order to excite the associated sensory neuron. Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Processing at the Receptor Lever The steps in formation of a generator potential are not known for every receptor, but where it has been studied the start of the generator potential usually results from an increase in the permeability of the membrane of the receptor to all small ions Usually, the ion furthest from its electrochemical equilibrium and in greatest concentration, namely sodium, contributes the greatest current. ( and thus results in EPSP’s) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings 8 Processing at the Receptor Lever Stimulus (1) Action Potential (3) Generator Potential (2) Stimulus (1) N.T. release (3) Receptor Potential (2) Action Potential (3) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Example : Muscle spindle • Muscle spindles are composed of 3-10 intrafusal muscle fibers that lack myofilaments in their central regions, are noncontractile, and serve as receptive surfaces • They inform the body of the muscle tone and length of a muscle. They become activated when stretched and send sensory impulses to the CNS. Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings 9 Example : Muscle spindle • The figure shows the graded responses of the muscle spindles when the muscle is stretched. • The amplitude of the generator potentials increase with increasing stimulus strength. Different amounts of muscle stretch ( as shown by the heights in the lower trace) resulted in the graded series of generator potentials shown in the upper trace. Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Example : Pacinian Corpuscle Pacinian corpuscles are present in the skin, some mucous membranes etc. They are mechanoceptors, responding to pressure, or any kind of mechanical stimulus causing a deformation of the corpuscle. The Pacinian corpuscle has a single afferent nerve fiber. Its end is covered by a sensitive receptor membrane whose sodium channels will open when the membrane is deformed in any way. It is surrounded by several concentric capsules of connective tissue, with a viscous gel between them. Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings 10 Example : Pacinian Corpuscle In the resting state, a crosssection through the corpuscle looks something like this Now, if the skin over the corpuscle is touched, it will be deformed and make a nuisance of itself: But the viscous gel between the capsules will move and allow the nerve ending to resume its normal shape: If the pressure is now released, the corpuscle as a whole will resume its original shape, but the nerve ending will be deformed in the process: The viscous gel will then flow back, and soon we are back at the beginning. Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Example : Pacinian Corpuscle The result is two generator potentials; one when pressure is applied and one when pressure is released. This system is thus very good for picking up vibrations. Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings 11 Adaptation of Sensory Receptors To a certain extent, the duration of the generator potential depends upon the duration of the stimulus. However, some receptors have generator potentials that last only a short time, no matter how long the stimulus is maintained. We refer to a decrease in the amplitude of the generator potential or the frequency of discharge of the sensory fiber in the face of a persisting, constant stimulus as adaptation. Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Adaptation of Sensory Receptors Some receptors are fast adapting, some are slow adapting. Those that adapt fast are called phasic receptors. Thos that adapt slow or not at all are called tonic receptors. Receptors responding to pressure, touch, and smell adapt quickly Receptors responding slowly include Merkel’s discs, Ruffini’s corpuscles, and interoceptors that respond to chemical levels in the blood Pain receptors and proprioceptors do not exhibit adaptation There is purpose to these differences. Tonic (slowly-adapting) receptors are important in situations where constant information about a stimulus is important ( they thus send information about ongoing stimulation) Phasic (rapidly-adapting) receptors send information related to changing stimuli. They stop responding to a maintained stimulus, but when the stimulus is removed, they respond again Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings 12 Adaptation of Sensory Receptors Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Tonic and Phasic Receptors Diagram showing the differences and effects of tonic and phasic receptors Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings 13 Information Coding Any stimulus contains within it certain features that are of interest to the body. Stimuli have • intensities or strengths • locations or sites of application • frequencies of application • rates of application • modalities Modality, broadly speaking, is a class of sensations that are referred to a single type of receptor. Vision, hearing, touch, smell, and taste are all modalities ( energy forms). Sensory receptors may be sensitive to different kind of energies. For example, putting pressure on the eye cause you to see light flashes, although the function of the eye receptors is to detect light. Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Information Coding • Doctrine of Specific Nerve Energies, as formulated by Johannus Müller, says that, although a sense organ may be sensitive to many forms of stimulus energy other than its real stimulus (called the adequate stimulus), the sensation evoked is always like that associated with the adequate stimulus, no matter what kind of energy was applied. • For example : electrical stimulation of the optic nerve, does not result in an electric shock; the sensation evoked is one of seeing light. • The doctrine of specific nerve energies implies that the modality or submodality of a sensation is determined not by the stimulus, but by what specific receptor or nerve fiber is stimulated. The doctrine also implies that the subjective qualities of a modality are determined, not in the receptors themselves, but in the central nervous system. (in this case for the optic nerve, it is determined by the visual cortex). Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings 14 Processing at the Circuit Level Chains of three (3) neurons conduct sensory impulses upward to the brain First-order neurons – soma reside in dorsal root or cranial ganglia, and conduct impulses from the skin to the spinal cord or brain stem Second-order neurons – soma reside in the dorsal horn of the spinal cord or medullary nuclei and transmit impulses to the thalamus or cerebellum Third-order neurons – located in the thalamus and conduct impulses to the somatosensory cortex of the cerebrum Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Processing at the Circuit Level • Neuronal signals from skin and deeper structures are segregated in the spinal cord. • For pain, temperature and the less discriminative aspects of touch, neurons in the dorsal horn have axons that cross in the spinal cord and ascend via the spinothalamic tract • For discriminative touch and for conscious proprioception, the axons of primary sensory neurons ascend ipsilaterally ( do not cross over) in the dorsal funiculus (either gracile or cuneate fasciculus) and end in the gracile or cuneate nucleus. Fibers arising in these nuclei cross in the medulla and ascend in the medial lemniscus, which is near the midline in the medulla and shifts to a lateral location in the midbrain. • The differences between the two main ascending somatosensory pathways are important functionally and clinically. Specific lesions within the spinal cord can thus results in specific loss of sensations in the body. Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings 15 Processing at the Circuit Level Discriminative touch, conscious proprioception Simple touch, temperature, pain Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Processing at the Perceptual Level Both spinothalamic tract and the medial lemniscus terminate in the ventral posterior nucleus of the thalamus. The thalamus projects fibers to: The somatosensory cortex of postcentral gyrus Sensory association areas First one modality is sent, then those considering more than one The result is an internal, conscious image of the stimulus Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings 16 Main Aspects of Sensory Perception Perceptual detection – detecting that a stimulus has occurred and requires summation Magnitude estimation – how much of a stimulus is acting Spatial discrimination – identifying the site or pattern of the stimulus Feature abstraction – used to identify a substance that has specific texture or shape Quality discrimination – the ability to identify submodalities of a sensation (e.g., sweet or sour tastes) Pattern recognition – ability to recognize patterns in stimuli (e.g., melody, familiar face) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings 17