Chapter 16 Sensory, Motor, and Integrative Systems
Chapter 16 Sensory, Motor, and Integrative Systems Sensation Definition: The conscious or subconscious awareness of changes in the external or internal environment. Thus,
sensation may occur without perception. Sensory Modality- Each unique type of sensation e.g. touch, pain, vision, hearing. Modalities can be grouped into two classes: 1. General Senses-somatic, tactile, thermal, pain, proprioceptive. 2. Special Senses-smell, taste, vision, hearing, balance.
The Process of Sensation A sensory receptor responds only to a particular type of stimulus (a property known as selectivity). Four events in sensation: 1. Stimulation of the sensory receptor. 2. Transduction of the stimulus.
3. Generation of nerve impulses. 4. Integration of sensory input. Sensory Receptors Types of sensory receptors 1. Free nerve endings: bare dendrites lacking structural specializations-pain, thermal, tickle, itch, and some touch.
2. Encapsulated nerve endings: dendrites are enclosed in a connective tissue capsule that has a distinctive structure that helps to enhance the sensitivity or selectivity of the receptor. 3. Cellular receptors: hair cells (ear), photoreceptors (eye). Sensory receptors produce two types of graded potentials: 1. Generator potentials 2. Receptor potentials
Sensory Receptor Potentials Generator Potential: Stimulated dendrites of free nerve endings, encapsulated nerve endings, and olfactory receptors produce a generator potential which can reach threshold and generate and action potential. Receptor Potential: Hair cells (ear), taste receptors, and
photoreceptors do not generate action potentials. Their graded potentials trigger NT release and generate PSPs (postsynaptic potentials) in 1st order neurons. Types of Sensory Receptors Adaptation in Sensory Receptors
Adaptation: A generator or receptor potential decreases in amplitude during a maintained constant stimulus. This results in a decrease in the frequency of nerve impulses in the 1st order neuron. The result of this property is that the sensation of a stimulus may fade even in the presence of a persisting stimulus. Rapidly adapting (pressure, touch, and smell) vs Slowly adapting receptors pain, body position).
Somatic Sensations Somatic sensations: arise from stimulation of sensory receptors in the skin, subcutaneous layer, mucous membranes of the mouth, vagina, anus, muscles, tendons, joints, and the inner ear. Cutaneous sensations: Somatic sensations arising in the skin. Four types: tactile, thermal, pain, proprioception.
Tactile: touch, pressure, vibration (encapsulated receptors); itch, tickle (free nerve endings). Touch: crude vs fine. Rapidly adapting receptors: Meissner corpuscles located in dermal papillae of non-hairy skin. Found in fingertips, hands, eyelids, tip of tongue, lips, nipples, penis. Hair root plexuses: rapidly adapting crude touch receptors found in hairy skin. Detect movement on skin surface that disturb hairs. Pressure and Vibration: Pressure is a sustained sensation felt over a larger area than
touch (Pacinian corpuscle), rapidly adapting, widely distributed in the dermis and subcutaneous layers as well as certain viscera (pancreas and urinary bladder). Itch and Tickle: Itch results from stimulation of free nerve endings by chemicals such as bradykinin (inflammatory response). Free nerve endings and lamellated corpuscles arise when someone else touches you-not when you touch yourself. Sensory Receptors in the Skin
Pacinian Corpuscles Distribution of Referred Pain Proprioceptive Sensations
Definition: Sense of the location of head and limbs and how they are moving even if we are not looking at them. Kinesthesia- perception of body movements. Receptors are called proprioceptors. Found embedded in muscles and tendons and sense the amount of contraction, tension on tendons, positions of joints. Slowly adapting. Allow us to estimate the weight of objects quickly.
Muscle Spindles- proprioceptors in skeletal muscle-monitor changes in skeletal muscle length. Several slowly adapting sensory nerve endings wrap around specialized muscle fibers (intrafusal fibers). Sudden or prolonged stretching of these spindles stimulate sensory nerve endings which project to the somatosensory cortex and cerebellum. Gamma motor neurons terminate at both ends of the intrafusal fibers and adjust the tension in the muscle fibers.
Alpha motor neurons innervate extrafusal muscle fibers that surround the intrafusal fibers and are activated by sensory input from the spindles. Tendon Organs- located at the junctions of a tendon and muscle monitor the force of muscle contraction. Tendon reflexes decrease muscle tension by causing muscle relaxation.
Muscle Spindle (length) and Tendon Organ (force) Somatic Sensory Pathways Sensory and Motor Maps in the Cerebral Cortex
Posterior Column: fine touch, conscious proprioception, stereognosis, kinesthesia, vibration. Primary axons project ipsilaterally to the medulla where they synapse with 2nd order neurons decussate travel in the medial lemniscus and synapse in the thalamus. 3rd order fibers project to the somatosensory cortex.
Degeneration of these neurons occurs in late stage Syphilis. Lateral spinothalamic: pain, and thermal sensations. Primary afferents synapse with 2nd order neurons in the dorsal horn which then decussate and enter the lat spinothalamic tract on the contralateral side. These fibers travel in the medial lemniscus and synapse in the
thalamus. 3rd order fibers project to the somatosensory cortex. Anterior spinothalamic: itch, tickle, pressure, and unlocalized touch. Primary afferents synapse with 2nd order neurons in the dorsal horn which then decussate and enter the ant. spinothalamic tract on the contralateral side. These fibers travel in the medial
lemniscus and synapse in the thalamus. 3rd order fibers project to the somatosensory cortex. Somatic Motor Pathways Descending Motor Pathways
Connections of the Cerebellum The Reticular Activating System Chapter 16 END
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