Chapter 21 Somatic Sensory and Motor Control

Transcription

1 Chapter 21 Somatic Sensory and Motor Control Overview of Sensations A sensation is the conscious or subconscious awareness of changes in the external or internal conditions of the body. For a sensation to arise, four events typically occur: 1. stimulus- capable of activating specific sensory neurons must occur 2. sensory receptor or sense organ- responds to the stimulus and transduce (convert) it into a nerve impulse 3. nerve impulses are conducted to the brain 4. a region of the brain must receive and integrate the nerve impulses, producing sensation bullets Overview of Sensations Conscious sensations or perceptions are integrated in the cerebral cortex Distinct quality that makes one sensation (e.g., touch) different from another sensation (e.g., pain) is its sensory modality. A characteristic of many sensory receptors is adaptation A decrease in sensation during a prolonged stimulus Receptors vary in how quickly they adapt. 1

2 Overview of Sensations Sensations can be grouped into two classes: 1. General senses, which include both somatic senses (which include touch, pressure, vibration, warm, cold, pain, and proprioceptive sensations) and visceral senses 2. Special senses, which include smell, taste, vision, hearing, and equilibrium (balance). Sensory Receptor Nerve Ending There are three basic types of sensory nerve endings: 1. Free Nerve Ending- bare dendrites 2. Encapsulated Nerve Endings- connective tissue capsule around dendrites 3. Separate Cells- synapse with sensory neurons Sensory Receptor Sensory receptors can be grouped by the type of energy they respond to and are categorized as follows: Photoreceptors are responsive to light. Mechanoreceptors are sensitive to mechanical stimuli. Thermoreceptors are sensitive to heat and cold. Osmoreceptors detect changes in the concentration of solutes in the body fluids and the resultant changes in osmotic activity. Nociceptors or pain receptors detect pain, usually as a result of damage to tissues Chemoreceptors detect specific chemicals. 2

3 Sensory Receptors Sensory receptors can be classified according to location and origin of the stimuli: Exteroceptors- located at or near the surface of the body Proprioceptors- located in muscles, tendons, joints, and the inner ear Interoceptors or visceroceptors- located in blood vessels, visceral organs, muscles, and the nervous system Somatic Sensations Somatic sensations arise from stimulation of sensory receptors embedded in: the skin or subcutaneous layer mucous membranes of the mouth, vagina, and anus muscles, tendons, and joints; and in the inner ear. Some parts of the body are densely populated with receptors (e.g., tip of tongue, lips, fingertips) and other parts of the body have few receptors (e.g., back of neck). Cutaneous sensations- somatic sensations that arise from stimulation of the skin surface Types of Sensations Four types of sensations Tactile Thermal Pain Proprioception 3

4 Tactile Sensations Touch- arises from stimulation of tactile receptors located in the skin and subcutaneous layer Receptors for both crude touch and fine touch include: Corpuscles of touch or Meissner corpuscles for fine touch (encapsulated) Hair root plexuses that detect hair movement (free nerve endings) Type I cutaneous mechanoreceptors or Merkel disks for fine touch (free nerve endings) Type II cutaneous mechanoreceptors or Ruffini corpuscles that detect stretching that occurs during digit or limb movements (encapsulated) Tactile Sensations There are two types of slowly adapting touch receptors. Merkel discs, function in fine touch. These mechanoreceptors are plentiful in the fingertips, hands, lips, and external genitalia. Ruffini corpuscles, located deep in the dermis, and in ligaments and tendons. Structure and location of sensory receptors in the skin 4

5 Pressure and Vibration Pressure- detected by tactile receptors in numerous tissues that are widely distributed in the body Sustained sensation felt over a larger area than touch. Occurs with deformation of deeper tissues. Vibration- from rapidly repetitive sensory signals from tactile receptors. Rapidly adapting touch receptors that contribute to sensations of pressure and vibration are: Corpuscles of touch, or Meissner corpuscles Lamellated, or pacinian corpuscles Itch and Tickle The itch sensation results from stimulation of free nerve endings by certain chemicals, such as bradykinin, often because of a local inflammatory response. Free nerve endings and lamellated corpuscles are thought to mediate the tickle sensation. Thermal Sensations Thermoreceptors are free nerve endings. Two distinct thermal sensations: coldness and warm are detected by different receptors. Cold receptors- located in the stratum basale of the epidermis. Temperatures between 10 and 40 o C ( o F) activate cold receptors. Warm receptors- located in the dermis and are activated by temperatures between 32 and 48 o C ( o F). 5

6 Pain Sensations Pain is indispensable for survival. It serves a protective function by signaling the presence of noxious tissue-damaging conditions. Nociceptors- receptors for pain, are free nerve endings found in every tissue of the body except the brain. Intense thermal, mechanical, or chemical stimuli can activate nociceptors. Types of Pain There are two types of pain: fast and slow. Fast pain occurs very rapidly, usually within 0.1 second after a stimulus. Perceived as localized, acute, sharp, or pricking pain. Fast pain is not felt in deeper tissues of the body. Slow pain begins a second or more after a stimulus. It then gradually increases in intensity over a period of several seconds or minutes. Perceived as diffuse, chronic, burning, aching, or throbbing pain. From skin, organ, or deep tissue origins. Superficial somatic pain- arises from stimulation of receptors in the skin. Deep somatic pain- stimulation of receptors in skeletal muscles, joints, tendons, and fascia. Visceral pain- stimulation of nociceptors in visceral organs. Types of Pain Referred pain is a phenomenon in which visceral pain is felt in the skin overlying the stimulated organ or in a surface area far from a stimulated organ 6

7 Proprioceptive Sensations Proprioceptive sensations and kinesthesia provide an awareness of positions and movements of parts of the body. Muscle spindles located within skeletal muscles Tendon organs located within tendons Joint kinesthetic receptors located within synovial joint capsules Hair cells of the inner ear (provide information for maintaining balance) Proprioceptive Sensations Proprioceptive nerve impulses pass along ascending tracts in the spinal cord to the thalamus to the somatosensory area of the cerebral cortex proprioceptive impulses also pass along spinocerebellar tracts to the cerebellum. Somatic Sensory Pathways Somatic sensory pathways- relay information from somatic receptors to the primary somatosensory areas in the cerebral cortex and to the cerebellum: First-order neurons carry signals from somatic receptors into the brain stem or spinal cord via cranial nerves or spinal nerves Second-order neurons carry signals from the spinal cord and brain stem to the thalamus axons of second-order neurons decussate to the opposite side before ascending to the thalamus Third-order neurons project from the thalamus to the primary somatosensory areas where conscious perception of sensations results 7

8 Somatic Sensory Pathways There are two general pathways by which somatic sensory signals entering the spinal cord ascend to the cerebral cortex: Posterior column-medial lemniscus pathways: transmit nerve impulses for conscious proprioception and most tactile sensations axons Anterolateral (or spinothalamic) pathways: transmit nerve impulses for pain and temperature, as well as, tickle, itch, crude touch, pressure and vibrations Somatic Sensory Pathways Somatic Sensory Pathways Primary somatosensory area in each parietal lobe has been mapped out: Some parts of the body are represented by large areas, other parts of the body are represented by small areas Proprioceptive information reaches the cerebellum via two pathways: posterior spinocerebellar tract anterior spinocerebellar tract 8

9 Somatic Sensory and Somatic Motor Maps in the Cerebral Cortex Somatic Motor Pathways Motor cortex: Primary motor area of the cerebral cortex is the major control region for initiation of voluntary movements Primary somatosensory area also contributes fibers to the descending motor pathways Motor Neurons Lower Motor Neurons (LMN)- Alpha motor neurons run from the brain stem and spinal cord to the skeletal muscle Upper Motor Neurons (UMN)- Connect the cerebral cortex or brains stem to the lower motor neurons via: Direct Motor Pathway- connects cortex to LMN directly Indirect Motor Pathway- connects cortex, basal ganglia, cerebellum, to the brain stem then to the LMN 9

10 Direct Motor Pathways Nerve impulses for voluntary movements from the cerebral cortex to LMN via the direct motor pathways, also known as the pyramidal pathways. Lateral corticospinal: Nerve impulses from motor cortex of cerebrum to skeletal muscles on opposite side of body for precise, voluntary movements of distal parts of free limbs. Anterior corticospinal: Nerve impulses from motor cortex to skeletal muscles on opposite side of body for movements of trunk and proximal parts of free limbs. Corticobulbar: Nerve impulses from motor cortex to skeletal muscles of head and neck to coordinate precise, voluntary movements. Direct Motor Pathways (Fig. 21.6) Indirect (Extrapyramidal) Pathways Rubrospinal: Nerve impulses from the red nucleus to contralateral skeletal muscles that govern precise, voluntary movements of the distal parts of the free upper limbs. Tectospinal: Nerve impulses from the superior colliculus to contralateral skeletal muscles that reflexively move the head, eyes, and trunk in response to visual or auditory stimuli. Vestibulospinal: Nerve impulses from the vestibular nucleus to ipsilateral skeletal muscles of the trunk and proximal parts of the free limbs for maintaining posture and balance in response to head movements. Medial and lateral reticulospinal: Nerve impulses from reticular formation to ipsilateral skeletal muscles of the trunk and proximal parts of the free limbs for maintaining posture and regulating muscle tone in response to ongoing body movements. 10

11 Roles of the Basal Ganglia The functions of the basal ganglia: 1. Play a major role in the initiation and termination of movements 2. Suppress unwanted movements by their inhibitory effects on the thalamus and superior colliculus. 3. Influence muscle tone. 4. Influence many aspects of cortical function, including sensory, limbic, cognitive, and linguistic functions. Role of Cerebellum Roles of the Cerebellum: maintaining proper posture and balance, active in learning and performing rapid, coordinated, highly skilled movements such as hitting a golf ball, speaking, and swimming. Cerebellar function involves four activities. 1. The cerebellum monitors intentions for movement by receiving impulses from the motor cortex and basal ganglia via the pontine nuclei in the pons regarding what movements are planned. 2. The cerebellum monitors actual movement by receiving input from proprioceptors in joints and muscles that reveals what actually is happening. 3. The cerebellum compares the command signals with sensory information 4. If there is a discrepancy between intended and actual movement, the cerebellum sends out corrective feedback to upper motor neurons. 11