Somatic Sensory System Light touch, vibration, pressure, position of limbs (sense of self), pain, temperature. Leads to the ability to identify shapes and textures of objects. Monitors internal and external forces acting on the body. Detects potentially harmful circumstances.

Overview Receptor neurons located in skin or joints receive stimuli. Information is carried to brain via the spinal cord> brainstem> thalamus> postcentral gyrus of the parietal lobe> other cortical areas. Projections are topographic with respect to body and the amount of cortical space allocated to various body parts is proportional to the density of sensory receptors in that area.

Somatic Sensory System Touch and Pain have different routes to the brain.

General Organization of the Somatic Sensory System PN09012.JPG

Different receptors are classified by different properties Function- Pain, temperature, touch, muscle length. Morphology- free nerve endings or encapsulated. Nociceptors and thermoceptors have free nerve endings, most others encapsulated. Conduction velocities , fast vs. slow Location: skin, muscle, tendon, hair Rate of adaptation: slow vs fast

General Properties stimuli applied to skin change the nerve endings, produce graded receptor potentials that trigger action potentials. quality of stimulus (what it represents and where it is) is determined by the neuron’s targets in the brain. quantity or strength of stimulus signalled by rate of action potentials.

Slowly Adapting and Rapidly Adapting Mechanoreceptors Respond Differently to Stimulation

Low threshold (or high sensitivity) mechanoreceptors Provide information about touch, pressure, vibration, and skin tension. Four major types of encapsulated mechanoreceptors: Meissner’s corpuscle Pacinian corpuscle Merkel’s disk Ruffini’s corpuscle. Called low-threshold because even weak stimulation causes action potentials. Innervated by large myelinated axons.

Receptive fields Receptive field is the region of the skin within which a tactile (touch) stimulus evokes a sensory response in the cell or its axon. Receptive fields are small in the finger tip (1-2 mm), but larger (5-10 mm) in the palms.

The Skin Mechanoreceptors

Differences in mechanosensory discrimination across the body surface Accuracy of of touch varies over the body. Tested with two-point discrimination. Fingers can distinguish things 2mm apart, forearms 40 mm apart. Mechanoreceptors are more numerous in finger tips and have smaller receptive fields. Doesn’t explain everything. Two-point thresholds vary with practice (can be learned), and depend on the stimulus. Phantom limbs- central processing can occur in the absence of peripheral receptors.

Sensitivity of Tactile Discrimination Varies with Location on the Body Surface

The route of sensory information Mechanoreceptors and propioception receptors use the Dorsal-column-medial lemniscus pathway Pain and temperature use spinothalamic (anterolateral pathway).

The Main Mechanosensory Pathways Medial lemniscus cross in the medulla upper and lower body use slightly different pathways.

Trigeminal pathway PN09063.JPG info from head and face

Somatic sensory cortex Located in parietal lobe-post central gyrus Divided into regions, Broadmann’s areas 3b and 1: cutaneous 3a: proprioception 2: tactile and proprioception Each area contains a somatotopic “map” of the body homunculous

Schematic Representation of the Main Mechanosensory Pathways PN09061.JPG

Somatotopic Order in Human Primary Somatosensory Cortex PN09081.JPG

The Somatic Sensory Portions of the Thalamus and Cortical Targets PN09070.JPG Upper body medial Lower body lateral

Human Primary Somatoensory Cortex areas of high receptor density get more cortical space somatotopy

The Humunculus reflects sensory receptor density

receptive fields of somatosensory cortex Area 3b- simple, responses generally from stimulations of a single finger 1 and 2- respond to stimulation of multiple fingers. 1- often a particular direction is important, 2- particular shapes. SI is organized in columns, by receptive field and modality. Stick an electrode vertically, all neurons share same region of body and slowly adapting vs fast adapting mechanosensory neurons

Higher order processing SI sends out projections to other areas of cortex. SII, adjacent to SI. Receives info from SI and sends it to amygdala and hippocampus. Plays roles in fear conditioning and tactile learning and memory.

Summary - organization First order neurons: cell bodies in dorsal root and cranial nerve ganglia Second order: brainstem nuclei Third order: thalamus ….project to SI Fourth order; SI projects to SII Topographical arrangement throughout

Pain Submodality of the sense of touch, warns of injury and things that should be avoided. More subjective that the other senses. The same stimulus can produce different responses in different individuals, or in the same individual in different circumstances.

Pain involves specialized neurons not just extra stimulation of touch receptors. scheme for transcutaneous nerve recording. Nociceptor doesn’t fire until pain is felt. Other thermorecptors fire at all temps and at about the same frequency. Pain threshold is about 45 degrees C

Detecting pain Ion channel receptors open in response to heat as well as capsaicin, called TRP (transient receptor potential) channels. When open allow in Ca++ and Na+ that generate action potentials.

Heat gated ion channels Capsaicin receptors are nonselective cation channels opened by heat, low pH, and capsaicin (the hot in hot peppers). Mice without VR1 (vanilloid receptor – 1 or TRPV1) have impaired sensitivity to pain. Can drink capsaicin as if it were water.

Nociceptors Activation of VR1 channels by pure capsaicin and extracts of various peppers. Nature 1997 Oct 23;389(6653):816-24

Two types of pain: Two types of axons first pain (sharp), Ad fibers second pain (dull, longer lasting) C-fibers

Referred pain Few if any dorsal horn neurons are specialized solely for the transmission of visceral pain. It is conveyed relayed to brain via dorsal horn neurons that also get inputs from skin. Therefore a person may feel pain at a site far from its source.

Descending Systems Modulate Transmission of Ascending Pain Signals Descending pathways from cortex and hypothalamus…

Descending Systems Modulate Transmission of Ascending Pain Signals Through periaqueductal gray rostral medulla reduces spinothalamic tract activation.

opioids Opioid receptors (metabotropic) in descending pain pathway. Ligands- enkephalins, endorphins, and dynorphin. Found in all descending pain areas. Opiate antagonist naloxone blocks morphine-induced analgesia. Opioids decrease the chance that a nociceptor will fire, cause inhibition.

Endogenous opioid peptides hyperpolarize cells to Endogenous opioid peptides hyperpolarize cells to decrease action potential firing

Placebo Effect Sugar pills can reduce perception of pain. The effect can be blocked by naloxone, a competitive antagonist of opioid receptors. The placebo effect is based on a biochemical change in the brain.

Endogenous Opioids PN10T20.JPG