Lecture: 1 Sensory Receptors and Somatic Sensation 1 Dr. Eyad M. Hussein Ph.D of Neurology Consultant in Neurology Department, Nasser Hospital, Assistant Professor, Faculty of Medicine, Islamic University Faculty of Dentistry, University of Palestine

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The various sensations in the body include: 1. Somatic sensation: from the skin and deep tissues (e.g. muscles, joints and bones), conducted to CNS through the somatic nerves. 2. Special senses: reaching the CNS through specialized cranial nerves: a. Sense of the vision. b. Sense of the hearing and equilibrium. c. Sense of the smell. d. Sense of the taste. 3. Visceral sensation: including all sensations from the internal viscera and reaching the CNS through the autonomic nerves. 4. Organic sensations: e.g. hunger, thirst and sexual sensations. The Sensory System 3

Definition: The sensory receptors are specialized structures located at the peripheral ends of sensory neurons. Functions of the sensory receptors 1.Detectors: they detect energy (stimulus) changes in both the external and internal environments. 2. Transducers: transform the stimulus into action potential (nerve impulses). 3.Generator: generation of nerve impulses in the sensory nerve. 4.They inform the CNS about different sensations or changes occurring inside and outside the body. The Sensory Receptors 4

1.Specificity (differential sensitivity). 2.Excitability (the receptor potential). 3.Discharge of impulses. 4.Adaptation. The CNS becomes almost useless without the sensory receptors 5 Properties of the Sensory Receptors

 Each type of receptors responds to a specific form of energy (stimulus) called Adequate Stimulus and produces a particular sensation.  Some receptors can respond to other stimuli called Inadequate Stimuli: e.g. the adequate stimulus of the retinal receptors is light but can also stimulated by mechanical pressure Specificity (Differential Sensitivity)

 This is the property of converting energy into action potential.  In the resting state (without stimuli) the sensory nerve endings are in the polarized state.  If it is stimulated, the terminal nerve fiber is partially depolarized due to increase Na + influx secondary to Na + channel activation Excitability (Receptor Potential)

 This part of partial depolarization of the sensory nerve ending is called the receptor potential.  The receptor potential is passively conducted to the first node of Ranvier causing its deep depolarization and reaching the CNS by peripheral nerve. 8

 The Weber-Fechner law states that “the frequency of discharge (stimulus strength) from receptors is directly proportional to the logarithm of intensity of the applied stimuli”.  The power law: R = KS A. R- is the magnitude of sensation felt (frequency of discharge). S- is the intensity of the stimulus. K and A are constant (which vary with each type of sensation) Discharge of Impulses

 This is decline in the frequency of discharge of action potentials from receptors that occurs on maintained stimulation by stimuli of a constant strength Adaptation

1.Accommodation of the terminal nerve fiber to the stimulus, due to inactivation (closure) of Na + channels as a result of continues current flow. 2.Remodeling (Readjustment) in the structure of the receptor itself after its initial stimulation. 11 Mechanism of Adaptation

A. Free nerve endings. B. Fast-adapting type:  Meissner’s corpuscles.  Pacinian corpucles.  Hair follicle endings. C. Slow-adapting type:  Merkel’s disks.  Ruffini’s end organs. Cutaneous Receptors “Somatosensory Receptors in Skin” 12

Proprioceptors are located in muscles, tendons, joint ligaments and in joint capsules. In the skeletal (striated) muscle: two types of encapsulated proprioceptors: -Muscle spindles. -Golgi tendon organs. In the joints: encapsulated endings similar to those in skin. Proprioceptive Receptors 13

Muscle spindles: are found in all striated muscles. Function: monitor muscle length (stretch). Golgi tendon organs: are found in the tendons of striated muscles near the muscle-tendon junction. Function: monitors muscle contraction. Joint receptors: are found within the connective tissue, capsule and ligaments of joints: the Ruffini’s and Pacinian corpuscles and the Golgi tendon organs. Proprioceptive Receptors 14

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1.According to site of the receptor. 2.According to site of stimulus. 3.According to specificity (physiological classification). 4.According to adaptation. Classification of the Receptors 17

1.Somatic: - Cutaneous Senses: for pain, temperature and touch. - Deep Sense: from muscles, tendons and joints. 2.Special Senses: e.g. visual, hearing, smell and taste. 3.Visceral Sense: from the internal organs. A.According to Site of Receptor 18

1.Exteroceptors: these include the following: The cutaneous receptors. The teleceptors (distance receptors) or distance receptors of vision, hearing and smell. 2.Interceptors: these include the following: The proprioceptors: for deep sensations. The viceroceptors: e.g. baroreceptors, chemoreceptors, viscerla stretch receptors for visceral pain. The hypothalamic receptors: glucoreceptors, osmoreceptors and hypothalamic thermoreceptors. B. According to Site of Stimulus 19

1.Mechanoreceptors: they are stimulated by mechanical forms energy: Touch or tactile receptors: present in the skin. The proprioceptors: in the muscles, ligaments and joints. The cochlear and vestibular (equilibrium) receptors: in the internal ear. Pressure receptors in the skin and deep tissue. Baroreceptors: blood pressure (in the carotid sinus and aortic arch). Stretch receptors: e.g. receptors in the wall of the atrium, urinary bladder and the lung alveoli. C. According to Specificity (Physiological Classification) 20

2. Chemoreceptors: detect the changes in the concentration of certain chemicals: The peripheral (in carotid body and aortic arch) and central chemoreceptors (in medulla): to respiration. The taste receptors: the taste buds. The smell receptors: in the olfactory mucous membrane. The glucoreceptors and osmoreceptors: in the hypothalamus. 3. Nociceptors: these are free nerve endings responding to pain sensation. 4. Thermoreceptors: these are stimulated by thermal forms of energy and those present in the skin. 5. Photoreceptors (electromagnetic receptors): present only in the eyes (rods and cones), which are stimulated by the electromagnetic waves of light. 21

1.Rapidly adapting receptors: include mainly the touch receptors. 2.Moderately adapting receptors: include the smell, taste and thermo- receptors. 3.Slowly adapting receptors: include mainly the pain receptors, muscle spindles and the baroreceptors. 22 D. Classification According to Adaptation of Receptors

The somatic sensations can be classified in two ways: A. According to the Site of the Sensation: 1. Superficial (exteroceptive) sensations: the sensations from the skin (pain, touch and temperature). 2. Deep sensations: the sensations from skeletal muscles, tendons, joints, bones and ligaments: a. Proprioceptive sensations: sense of position and the sense of movement (kinesthetic sensation). b. Pressure sense. c. Muscle tension sense. d. Muscle sense. Sometimes the vibration sense is included in this group. 3. Combined or synthetic senses: stereognosis, tactile discrimination and vibration sense. Classification of the Somatic Sensation 23

B. According to the Modality (type) of the Sensation: (More common classification) I. Mechanoreceptive sensations: these include the: 1.Touch sensations. 2.Pressure sensations. 3.Muscle tension sense. 4.Vibration sensations. 5.Tickle and itch sensations. 6.Proprioceptive sensations. II. Thermoreceptive sensations: heat and cold sensations. III. Pain sensation. 24

1. Touch (Tactile) Sensation There are 2 types of touch sensation: a. Crude touch: this is a poorly-localized gross tactile sensation.  Central pathway: ventral spinothalamic tract.  Testing: by stroking the skin lightly with a piece of cotton. b. Fine touch: this include tactile localization and discrimination, steriognosis and the sense of texture of material.  Central pathway: the Gracile and Cuneate tracts. I. The Mechanoreceptive Sensations 25

 Tactile Localization (Topognosis): the ability to localize a touched skin point while the eyes are closed.  Tactile Discrimination (2 point discrimination): the ability to distinguish 2 separated point of touch (minimal distance between them i.e. 5 mm in finger & 5 cm over back).  Steriognosis: the ability to recognize the nature of object (a familiar object) by touch without using vision. 26

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Tested by: asking the patient to differentiate between various weights by place them in his hand without moving the limb or hand and with closed eyes The Pressure Sensation 3. The Muscle Tension Sense Tested by: asking the person to differentiate between various weight placed in his unsupported hand.

 Pathway: Gracile and Cuneate tracts.  Vibration is closely related to proprioception.  Tested by: place the tuning fork on a bony prominence The Vibration Sense

 Tickle sensation: is a pleasurable sensation that result from light tactile stimulation of the skin.  Itch sensation: is annoying sensation that results from skin irritation by either moving tactile stimuli or certain chemical substances.  Pathway: Ventral spinothalamic tract The Tickle and Itch Sensations

 These sensations arise mainly from receptors in deep structures (tendon, ligaments, muscles and joints).  Pathway: Gracile and Cuneate tracts.  The proprioceptive sensation include two types: a.Sense of position (static proprioception): perception of the position of different parts of the body b.Sense of movement (dynamic proprioception): sensation of movement of joints The Proprioceptive Sensations

Joint Sense: "sense of movement". Hold the big toe from each side (or the index finger in U.L.) and dorsiflexion or plantar flexion. Ask the patient about which finger and direction of movement (with closed eyes). Muscle Sense: done by pinching the calf or abdominal muscles. 32

33 II. The Thermoreceptive Sensations  There are two types of thermoreceptors: a.Internal thermoreceptors: located in the hypothalamus. b.External thermoreceptors:  The warmth (Heat) receptors: Respond to temperature from about 30 ºC to 45 ºC.  The cold receptors: Respond to temperature from about 10 ºC to 38 ºC.  The thermosensitive pain receptors: respond to temperature below 10 ºC and above than 45 ºC.  Pathway: lateral spinothalamic tract.

34 III. Pain Sensations

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