Sensory, Motor, and Integrative Systems Chapter 16 Sensory, Motor, and Integrative Systems

General Sensations In this chapter we explore the levels and components of pathways that convey sensory nerve impulses from the body to the brain, and the general sensations (somatic and visceral) that result. We will also examine the activation of motor pathways and movements In chapter 17 we will look at the special senses of sight, hearing, taste, and smell

General Sensations As sensory impulses reach the CNS, they become part of a large pool of sensory input (though not every one will elicit a response) Each piece of incoming information is combined with other arriving and previously stored information in a process called integration

General Sensations Integration occurs at many places along pathways in the spinal cord, brain stem, cerebellum, basal nuclei, and cerebral cortex. Sensations result in and evoke a conscious perception or subconscious awareness that changes have occurred in the external or internal environment. The motor responses are also modified at several of these levels.

General Sensations Examples of complex integrative functions of the brain include wakefulness and sleep, and learning and memory

Sensory Modalities Each unique type of sensation is called a sensory modality, and a given sensory neuron carries information for only one modality, be it somatic, visceral, or “special” Somatic senses include tactile sensations (touch, pressure, vibration, itch, and tickle), thermal sensations (warm and cold), pain sensations, and proprioception (awareness of limb and joint position in space) Visceral senses provide information about conditions within internal organs

Sensory Modalities The process of sensation begins in a sensory receptor, which can be either a specialized cell or the dendrites of a sensory neuron A particular kind of stimulus (a change in the environment) activates certain sensory receptors, while other sensory receptors respond only weakly or not at all – a characteristic known as selectivity

Sensory Modalities For a sensation to arise, four events typically occur: Stimulation of the sensory receptor - an appropriate stimulus must occur within the receptor’s receptive field Transduction of the stimulus - a sensory receptor converts energy in a stimulus into a graded potential. Recall that graded potentials (but not APs) vary in amplitude depending on the strength of the stimulus that causes them, and are not propagated Sensory receptors produce two different kinds of graded potentials—generator potentials and receptor potentials—in response to a stimulus. When stimulated, the dendrites of free nerve endings, encapsulated nerve endings, and the receptive part of olfactory receptors produce a generator potential (Figure 16.1a, b). When a generator potential is large enough to reach threshold, it triggers one or more nerve impulses in the axon of a first-order sensory neuron. The resulting nerve impulse propagates along the axon into the CNS. Thus, generator potentials generate action potentials. By contrast, sensory receptors that are separate cells produce graded potentials termed receptor potentials.

Sensory Modalities The four events that bring about a sensation, cont’d… Generation of nerve impulses – occurs when the sum of graded potentials reach threshold in first-order neurons (the first neuron in a specific tract – in this case from the PNS into the CNS) Integration of sensory input – occurs when a particular region of the CNS integrates a number (and even a variety) of sensory nerve impulses and results in a conscious sensations or perceptions

Sensory Modalities

Sensory Receptors Sensory receptors can be grouped into several classes based on structural and functional characteristics: Microscopic structure – free nerve endings vs encapsulated endings, for example Location…of the receptors and the origin of the stimuli that activate them The type of stimulus detected (nociceptors for pain, mechanoreceptors for pressure, etc.) Interoceptors (visceroceptors) are located in blood vessels, visceral organs, muscles, and the nervous system and monitor conditions in the internal environment. The nerve impulses produced by interoceptors usually are not consciously perceived; occasionally, however, activation of interoceptors by strong stimuli may be felt as pain or pressure.

Sensory Receptors Sensory receptors can be grouped into several classes based on structural and functional characteristics: Microscopic structure – free nerve endings vs encapsulated endings, for example Location…of the receptors and the origin of the stimuli that activate them – exteroceptors near the external surface vs interoceptors (visceroceptors), for example The type of stimulus detected (nociceptors for pain, mechanoreceptors for pressure, etc.) Interoceptors (visceroceptors) are located in blood vessels, visceral organs, muscles, and the nervous system and monitor conditions in the internal environment. The nerve impulses produced by interoceptors usually are not consciously perceived; occasionally, however, activation of interoceptors by strong stimuli may be felt as pain or pressure.

Sensory Receptors Receptors named according to their location include: Exteroceptors, which are located at or near the external surface of the body and respond to external stimuli Interoceptors (visceroceptors), which are located in blood vessels, organs, and muscles and produce impulses which usually are not consciously perceived Proprioceptors, which are located in muscles, tendons, joints, and the inner ear. They provide information about body position and movement of joints Exteroceptors provide information about the external environment. Interoceptors monitor the internal environment. Occasionally activation of interoceptors by strong stimuli may be felt as pain or pressure.

Sensory Receptors Receptors can also denote the type of stimulus that excites them

Sensory Receptors Receptors named according to mode of activation are: Mechanoreceptors, which are sensitive to deformation Thermoreceptors, which detect changes in temperature Nociceptors, which respond to painful stimuli Photoreceptors, which are activated by photons of light Chemoreceptors, which detect chemicals in the mouth (taste), nose (smell) and body fluids Osmoreceptors, which detect the osmotic pressure of body fluids Mechanoreceptors provide sensations of touch, pressure, vibration, proprioception, and hearing and equilibrium. They also monitor the stretching of blood vessels and internal organs.

Sensory Receptors A characteristic feature of most sensory receptors is adaptation, in which the generator potential or receptor potential decreases in amplitude during a sustained or constant stimulus Because there is an accommodation response at the receptor level, the frequency of nerve impulses traveling to the cerebral cortex decreases and the perception of the sensation fades even though the stimulus persists receptors vary in how quickly they adapt (rapidly adapting and slowly adapting receptors) Receptors associated with pressure, touch, and smell are rapidly adapting.

Mechanoreception Many of the mechanoreceptors and nociceptors previously described are located in the skin

Mechanoreception This graphic illustrates some representative examples of general somatic mechanoreceptors and the first-order neurons to which they belong. Receptors for special senses are not shown.

Nociception All of our sensory modalities are important, but pain serves a protective function and is indispensable for survival Nociceptors are chemoreceptive free nerve endings activated by tissue damage from intense thermal, mechanical, or chemical stimuli - they’re found in every tissue of the body except the brain

Nociception There are two types of pain: fast and slow The perception of fast pain (acute, well localized) occurs rapidly because the nerve impulses propagate along medium-diameter, myelinated A fibers By contrast, slow pain begins after a stimulus is applied and gradually increases in intensity over a period of several seconds or minutes. Impulses for slow pain conduct along small-diameter, unmyelinated C fibers. This type of pain may be excruciating and often has a burning, aching, or throbbing quality An example of slow pain is the pain associated with a toothache.

Nociception Pain that arises from stimulation of receptors in the skin is called superficial somatic pain; stimulation of receptors in skeletal muscles, joints, tendons, and fascia causes deep somatic pain Visceral pain results from stimulation of nociceptors in visceral organs In many instances of visceral pain, the pain is felt in or just deep to the skin that overlies the stimulated organ, or in a surface area far from the stimulated organ. This phenomenon is called referred pain

Nociception Common patterns of referred visceral pain are shown in this graphic

Proprioception Muscle spindles are the proprioceptors in skeletal muscles that monitor changes in the muscle length and participate in stretch reflexes By adjusting how vigorously a muscle spindle responds to stretching of a skeletal muscle, the brain sets an overall level of muscle tone (the small degree of contraction that is present while the muscle is at rest)

Proprioception Each muscle spindle consists of several slowly adapting sensory nerve endings that wrap around 3 to 10 specialized muscle fibers. A connective tissue capsule encloses the sensory nerve endings and anchors the spindle to the endomysium and perimysium Muscle spindles are plentiful in muscles that control fine movements and much more sparse in those that control course or forceful movements Free nerve endings and Ruffini corpuscles in the capsules of joints respond to pressure. Pacinian corpuscles respond to acceleration and deceleration of joints during movement.

Somatic Sensory Pathways No matter the type of receptor on the receiving end (where the generator potential is set up), first-order somatosensory neurons are unipolar in structure This means that their cell body is located in the dorsal root ganglia (DRG) just outside the CNS Their other end terminates nearby in the posterior gray horns of the cord, usually at the level where they enter Mechanoreceptors provide sensations of touch, pressure, vibration, proprioception, and hearing and equilibrium. They also monitor the stretching of blood vessels and internal organs.

Somatic Sensory Pathways Second-order neurons conduct ascending impulses from the brain stem where their axons decussate (cross over to the opposite side) before ascending to the thalamus Thus, all somatic sensory information from one side of the body reaches the thalamus on the opposite side

Somatic Sensory Pathways Third-order neurons conduct impulses from the thalamus to the primary somatosensory area of the cortex on the same side.

Somatic Sensory Pathways Somatic sensory neurons (and their axons that convey somatic sensations) are not distributed evenly in the body The peripheral areas with the highest density are represented in the brain with the largest amount of gray matter in the sensory homunculus. The most sensitive areas in the body are therefore the tip of the tongue, lips, and fingertips

Somatic Sensory Pathways There are two major spinocerebellar tracts in the spinal cord that carry proprioceptive impulses to the cerebellum Although they are not consciously perceived, sensory impulses sent to the cerebellum along these two pathways are critical for posture, balance, and coordination of skilled movements

Somatic Motor Pathways Motor activity begins in the primary motor areas of the precentral gyrus and other cerebral integrative centers Any motor neuron that is not directly responsible for stimulating target muscles is called an upper motor neuron (UMN) UMNs connect the brain to the appropriate level in the spinal cord The basal nuclei and cerebellum influence movement through their effects on upper motor neurons.

Somatic Motor Pathways From there, all excitatory and inhibitory signals that control movement converge on second-order motor neurons known as lower motor neurons (LMNs) that descend to innervate skeletal muscle Since only LMNs provide output from the CNS to skeletal muscle fibers they are also called the final common pathway

Somatic Motor Pathways Axons of LMNs extend through cranial nerves to the skeletal muscles of the face and head, and through spinal nerves to innervate skeletal muscles of the limbs and trunk Two of the major LMN tracts are the lateral and anterior corticospinal tracts Only LMNs provide output from the CNS to skeletal muscle fibers. For this reason, they are also called the final common pathway.

Sensory and Motor Pathways (Interactions Animation) Somatic Sensory Pathways You must be connected to the internet to run this animation

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