Somatic Sensory System I. Background A. Differences between somatic senses and other senses 1. Receptors are distributed throughout the body as opposed to being concentrated at small, specialized locations 2. Responds to many kinds of stimuli (usually mechanical) 3. At least four senses (not one) a. Temperature b. Body position c. Touch d. Pain 4. Place, pressure, sharpness, texture, and duration can be accurately gauges B. Types of somatic sensation receptors 1. Mechanoreceptors--sensitive to physical distortion 2. Nociceptors--respond to damaging stimuli 3. Thermoreceptors--sensitive to changes in temperature 4. Proprioceptors--monitor body position 5. Chemoreceptors--respond to certain chemicals C. Classification 1. Free nerve endings a. Nociceptors b. Thermoreceptors 2. Encapsulated a. Most cutaneous receptors D. Mechanism of function 1. Stimuli applied to skin deform or change receptor a. Alters the ionic permeability of the receptor creating generator potentials i. Trigger action potentials II. Mechanical Senses A. Mechanical energy 1. Easily differentiated a. Stimulus frequency b. Stimulus pressure c. Receptive field
B. Types of receptors 1. Mechanoreceptors a. Pacinian i. Sensitive to vibration (250-350 Hz) ii. Involved in the fine discrimination of texture or other moving stimuli that cause vibrations b. Meissner's corpuscle i. Most common receptor in glabrous skin (smooth, hairless) ii. Sensitive to vibration (low frequency, 30-50 Hz) c. Ruffini's ending-not well understood d. Mercel's disks i. Light pressure and tactile discrimination e. Hair follicle receptor 2. Nociceptors a. Free, unmyelinated nerve endings b. Signal that body tissue is being damaged c. In most tissues, not brain d. Types of damage detected i. Mechanical--strong pressure (sharp objects) ii. Thermal (different from temperature)--active when tissues begin to be destroyed iii. Chemical--environmental agents or those from tissues itself--ph, histamine, etc. 3. Thermoreceptors a. Brain temperature is tightly regulated i. Close to 37C ii. Brain function changes above and below that temperature b. Specialized receptors in our skin that can perceive changes in temperature as small as 0.01C. c. Two types: i. Warm--begin firing at 30C up to 45C (above causes damage and pain) ii. Cold--below 35C to 10C.
Note: Like other sensory receptors, temperature receptors adapt. They respond to sudden changes in temperature. Experiment--three beakers of water: one cold, one hot, one lukewarm. One finger from one hand into hot; one finger from the other hand into cold. After some time period, immerse both simultaneously into the lukewarm. The finger from the hot senses the water to be cold and the finger from the cold senses the same water to be hot. Why? Adaptation--the hot and cold receptors adapted (stopped firing). When immersed in lukewarm, only the un-adapted receptors were available. You need both to sense lukewarm, etc. 4. Proprioceptors a. Body position i. Where the body is ii. Direction of movement iii. Speed of movement b. Receptors in the skeletal muscles (more in movement lecture) c. Two different mechanosensitive proprioceptors: i. Muscle spindles-consist of specialized intrafusal muscle fibers distributed among ordinary (extrafusal) muscle fibers; detect changes in muscle length ii. Golgi tendon organs-distributed among collagen fibers in tendons and detects changes in muscle tension III. Organization of Somatic Sensory Information A. Structure of spinal cord (see Neuroanatomy Lecture) B. Spinal segments 1. 30 spinal segments consisting of paired dorsal and ventral roots 2. Spinal segments are divided into 4 groups: cervical, thoracic, lumbar, sacral 3. Each segment is named after the vertebra from which the nerves a. Cervical: C1 - C8 b. Thoracic: T1 - T12 c. Lumbar: L1 - L5 d. Sacral: S1 - S5
C. Dermatomes 1. Segmental organization of the spinal nerves and sensory innervation of skin are related 2. Area of skin innervated by the dorsal roots of a single spinal segment is a dermatome 3. Characteristics a. Overlap between the dermatomes b. Cervical dermatomes i. Above the sternum c. Thoracic dermatomes i. Top of sternum to waist d. Lumbar dermatomes i. Front of legs and stomach e. Sacral dermatomes i. Back of legs and genitals
IV. Somatic Sensory Pathways A. Two basic systems 1. Pain and temperature 2. Touch and Proprioception B. Pathways 1. Dorsal column-medial lemniscal pathway a. Touch and proprioception 2. Spinothalamic pathway a. Pain and temperature
C. DCML Pathway 1. In the DCML pathway information ascends through the dorsal column on the ipsilateral side of the spinal cord 2. Synapses in the medulla 3. Crosses over and ascends via the medial lemniscus to the thalamus (VP) 4. Synapses in VP thalamus 5. Projects to the cortex D. ST Pathway 1. Information crosses to the contralateral side in the spinal cord 2. Ascends via the spinothalamic tract 3. Synapses in the thalamus (VP) 4. Projects to the cortex. E. Information carried in each pathway remains separate 1. Segregated all the way to the cortex 2. Thalamus a. Ventral posterior (VP) nucleus receives the information and projects to the somatosensory cortex V. Somatosensory Cortex A. Anatomy 1. Parietal lobe a. Post-central gyrus i. Most complex processing occurs in the cortex B. Somatotopy 1. Mapping of the body's surface sensations onto a brain structure 2. Features of the map: a. Not continuous b. Not scaled to the human body c. Relative size of the cortex devoted to each body part is correlated with the density of sensory input (i.e., lips versus the skin on your calf). d. Size is related to the importance of the sensory input (i.e., finger tip versus elbow)
C. Posterior parietal lobe 1. Primary somatosensory cortex receives simple segregated streams of sensory information 2. Integration takes place in the posterior parietal cortex VI. Pain and Its Control A. Nociception 1. Sensory process that provides signals that trigger pain B. Characteristics 1. Pain is influenced cognitively 2. Hyperalgesia a. Tissue already damaged is much more sensitive to pain i. Nociceptors are sensitized by various substances released by damaged tissue (protaglandins, histamines, etc.) C. Regulation of pain 1. Pain can be modified by non-painful sensory input (i.e., rub the skin around a bruise) a. Gate Theory of Pain-circuit in spinal cord dorsal root 2. Several brain regions can act to suppress pain a. PAG (periacqueductal gray matter) project to the raphe (serotonin) that sends axons to the spinal cord (5-HT is inhibitory, block synaptic activity) 3. Brain chemicals a. Endorphins i. Share many opioid properties and bind to opioid receptors in the brain ii. Opioid receptors are throughout the body, but especially in the brain and particularly in brain areas that process and modulate nociceptive information (PA, raphe, and spinal cord)