Somatic Sensory System Visceral Sensory System November 14, 2018 sensory physiology

Introduction Our awareness of bodily sensation Sensory experience Depend on processing of Sensory information from different sensory receptors Touch, pain, temperature, auditory receptors Sensory experience Has several parameters November 14, 2018 sensory physiology

Introduction Sensory parameters Quality or modality (kind of sensation) Warmth, cold, sweet,bitter Intensity Location Timing November 14, 2018 sensory physiology

Modality Different types of sensation that we are aware off include Smell, touch, hearing, and taste Pain, temperature,position sense, visceral sensation Information transmission to CNS By action potential November 14, 2018 sensory physiology

Modality Action potential from How is it possible for nerves Touch receptor not different from that of pain receptor How is it possible for nerves To transmit information about Different sensation November 14, 2018 sensory physiology

Determination of Modality Specific pathways (Mullers doctrine) Modality of sensation depends What end organ or nerve fiber is stimulated Each nerve tract Terminate at specific point in CNS Type of sensation felt Determined by activation of specific area to which the fibers lead November 14, 2018 sensory physiology

Determination of Modality There are specific pathways (labeled lines) For different (modalities) kind of sensation Stimulation of pain fibers by what ever mechanisms Electrical, heat, crushing Will elicit pain Stimulation of retina by what ever mechanism Visual sensation November 14, 2018 sensory physiology

Intensity Discrimination Weak stimulus activate Receptors with low threshold only Strong stimulus activate both Low threshold & high threshold receptors Hence the number of AP  Frequency of AP  November 14, 2018 sensory physiology

Intensity Discrimination The stronger the stimulus The greater the frequency of AP The greater the number of sensory units activated This is interpreted by the brain As  in intensity November 14, 2018 sensory physiology

Determination of Location Determined (encoded) by The location of sensory projection to cerebral cortex No matter where particular sensory pathways is stimulated Brain Skin A B November 14, 2018 sensory physiology

Determination of Location Conscious sensation Referred to the location of the receptor Gives the sensation of Phantom limbs in people with amputation Brain Skin A B November 14, 2018 sensory physiology

Determination of Location Each sensory neuron Receive information From particular sensory area Called receptive field Sensory nerve Sensory receptors Receptive field November 14, 2018 sensory physiology

Determination of Location However Extensive overlap of the receptive fields Application of stimulus at Point “A” stimulates the blue nerve only Point “C” stimulates the red nerve only Point “B” stimulate both Sensory nerves A C B November 14, 2018 sensory physiology

Determination of Location Transmit AP Stimulus localization Can be made more precise by Lateral inhibition Inhibited Inhibited Lateral inhibition Lateral inhibition November 14, 2018 sensory physiology

Nerve Fiber Classification Type Group Diameter (m) Speed (m/s) Function A I 12 – 20 72 – 120 Muscle spindles, motor to skeletal muscles A II 6 – 12 36 – 72 Touch, pressure, muscle spindles A III 1 – 6 6 – 36 Pain, temperature A 5 15 – 30 Motor to muscle spindles C IV < 1m 0.5 – 2 Pain B Pre-ganglionic Autonomic nerves November 14, 2018 sensory physiology

Sensory Pathways Peripheral Tactile sensation Touch, pressure, proprioception, vibration Required for accurate localization on skin Transmitted through Rapid conducting fibers Group II (type A) 30 – 72 m/sec November 14, 2018 sensory physiology

Sensory Pathways Less critical type of sensation Crude touch Transmitted in Slow conducting fibers Group IV (type C) 1 m/sec November 14, 2018 sensory physiology

Sensory Pathways Muscle spindle & tendon organs Transmit by group I (type A) 72 – 120 m/sec Sensation from cold and warm receptors Transmitted by group III (type A) 5 – 30 m/sec November 14, 2018 sensory physiology

Sensory Pathways Sensation from pain receptors Transmitted through group III (type A) and Group IV (type C) November 14, 2018 sensory physiology

In the Spinal Cord Sensory afferents enter the spinal cord Dorsal root fibers Segregate according to function Large fibers (A, A) From special mechano-receptor Skin, muscles, joints, viscera Mediate fine touch, pressure, proprioception Enter the dorsal column Posterior/dorsal column A, A Lateral spinothalamic tract A & gp IV Anterior spinothalamic tract November 14, 2018 sensory physiology

In the Spinal Cord Ascend to medulla Second order neuron Synapse in gracilis & cuneatus nuclei Second order neuron Cross over to join the medial leminiscus End up in ventral basal & ventral posterior nuclei of thalamus Posterior/dorsal column A, A Lateral spinothalamic tract A & gp IV Anterior spinothalamic tract November 14, 2018 sensory physiology

In the Spinal Cord Small fibers A ( gp III) & C (gp IV) Ascend or descend a few segments in the tract of Lissauer Synapse with dorsal horn cells Cross over to ascend Lateral & ventral spinothalamic tract Posterior/dorsal column A, A Lateral spinothalamic tract A & gp IV Anterior spinothalamic tract November 14, 2018 sensory physiology

The Sensory Cortex The ventro-basal nucleus of the thalamus Connected by both ascending and descending axons From the two cortical areas Sensory cortex I & II The periphery of the body Projected onto the cortex Topographic organization November 14, 2018 sensory physiology

The Sensory Cortex Concerned with perception of Concerned with Most stimuli that enter into consciousness Concerned with Recognition of form, texture, size, weight, consistency of objects Stereo-gnosis Discrimination of size & shape November 14, 2018 sensory physiology

The Sensory Cortex Change in position of different parts of the body Accurate localization and recognition of Nature of stimuli applied to body November 14, 2018 sensory physiology

The Sensory Cortex Discrimination between Ability to relate 2 simultaneously applied stimuli 2 point discrimination Ability to relate Sensory experience with Other experiences November 14, 2018 sensory physiology

Somatic Sensation Can be classified into Mechano-receptive somatic sense Mechanical stimulation of tissue Thermo-receptive sense Detect heat or cold Pain sense Activated by factors that damage tissue November 14, 2018 sensory physiology

Mechano-receptive Sensation Sensation produced by Mechanical stimulation of the skin Divided into 3 sub modalities Vibration Touch Pressure November 14, 2018 sensory physiology

Mechano-receptive Sensation Vibration sensation Results from Rapid repetitive sensory signals Encoded by pacinian corpuscles Rapid adapting receptors November 14, 2018 sensory physiology

Mechano-receptive Sensation Deformation of nerve terminals  memb permeability to Na+, K+ Produce depolarizing receptor potential Frequency of discharge by pacinian corpuscle Equals the frequency of vibration November 14, 2018 sensory physiology

Mechano-receptive Sensation Pressure sensation Result from deformation of deeper tissues Encoded by Ruffini’s corpuscle Capsule contain liquid filled collagen structure gp II sensory nerve Ruffini’s corpuscle November 14, 2018 sensory physiology

Mechano-receptive Sensation Any deformation or stretch of the collagen Causes nerve terminals to depolarize Generate action potential Gp II sensory nerve Ruffini’s corpuscle November 14, 2018 sensory physiology

Mechano-receptive Sensation Ruffini’s corpuscle are slow adapting AP continue As long as stimulus is present The magnitude of stimulus Is encoded by frequency of AP Gp II sensory nerve Ruffini’s corpuscle November 14, 2018 sensory physiology

Mechano-receptive Sensation Meisser’s corpuscle Touch sensation Result from stimulation of Tactile receptors In skin or tissues immediately beneath Encoded by both Meisser’s corpuscles Merkel’s disks Merkel’s disks November 14, 2018 sensory physiology

Mechano-receptive Sensation Meisser’s corpuscle Meissner’s corpuscles & merkle’s disks Numerous on Fingers, lips Relative scarce on trunk Merkel’s disks Receptor cell November 14, 2018 sensory physiology

Mechano-receptive Sensation Meisser’s corpuscle Meissner’s corpuscles Rapidly adapting Receive group II sensory nerve Frequency of firing Proportional to rate of stimulation Missner’s encode The velocity of stimulus application Merkel’s disks Receptor cell November 14, 2018 sensory physiology

Mechano-receptive Sensation Meisser’s corpuscle Merkle’s disks Transducer is not on on nerve terminal On epithelial cell that make up the disk Disk make synaptic contact Group II sensory nerve Merkel’s disks Receptor cell November 14, 2018 sensory physiology

Mechano-receptive Sensation Meisser’s corpuscle Disk is 0.25 mm Can only be stimulated If stimulus is applied directly on disk Ideal for recognition Exact point of body being stimulated Merkel’s disks Receptor cell November 14, 2018 sensory physiology

Proprioception In the conscious state This ability is known as We are constantly aware of Position Movements of different parts of body This ability is known as Proprioception or deep sensibility Stimuli to receptors Derived from the body itself Not from the surrounding November 14, 2018 sensory physiology

Proprioception Receptors lie in The sense of position Muscles, tendon, joints The sense of position Ability to know the position of ones limb The orientation of their part With respect to one another Static position November 14, 2018 sensory physiology

Sense of Position Inform us of From this information Angles at each joint From this information Overall orientation of the limb is derived Sense of position exhibit Little adaptation November 14, 2018 sensory physiology

Sense of Movement When we change the angle of joints We are aware of both the Direction and Velocity of movement This quality of proprioception Called sense of movement Kinaesthesia November 14, 2018 sensory physiology

Thermal Sensation Cutaneous temperature sensibility Sensation of cold Sensation of warmth (heat) Temperature slightly Above body temp perceived as warmth Temp below body temp Perceived as cold Thermoneutral range Cold  Below Above warm 37OC ± 0.5OC Body temp November 14, 2018 sensory physiology

Thermal Sensation There are discrete Cold spots are numerous on Cold sensitive spots Warm sensitive spots Are 4 – 10 times as many as Are warm spots Cold spots are numerous on Lips, finger tips, earlobes November 14, 2018 sensory physiology

Thermal Sensation Warm receptors Begin to respond at 30OC Reach maximum at 45oC Cease to operate at 47OC Warm fibers Discharge rate Cold fibers 40 10 20 30 50 Increase in Temperature OC November 14, 2018 sensory physiology

Thermal Sensation Cold receptors Begin to respond at 15OC Reach maximum at 25oC Cease to operate at 42OC Warm fibers Discharge rate Cold fibers 40 10 20 30 50 Increase in Temperature OC November 14, 2018 sensory physiology

Thermal Sensation If the temp > 45OC This is threshold Cold receptor begin to fire again This is threshold For activating pain receptors Thus if the skin is heated >45OC Sensation of pain accompanied by feeling of coolness Paradoxical cold Warm fibers Discharge rate Cold fibers 40 10 20 30 50 Increase in Temperature OC November 14, 2018 sensory physiology

Pain Sensation Purpose is not to inform the brain But to indicate that About the quality of stimulus But to indicate that The stimulus is damaging Unpleasant Leads to removal of the damaging stimulus November 14, 2018 sensory physiology

Pain Sensation Peripheral mechanism Receptors Nociceptors Respond to noxious stimuli Free nerve endings of small Myelinated A fibers Unmyelinated C fibers November 14, 2018 sensory physiology

Pain Sensation Nociceptors Respond to damaging or potentially damaging Specific for painful stimuli Respond to damaging or potentially damaging Mechanical, chemical, electrical, thermal stimuli November 14, 2018 sensory physiology

Pain Sensation Adequate stimulus for nociceptors Assumed that Not known Assumed that Damaged cells by painful stimulus Release chemicals such as Histamine, H+, K+, bradykinin, 5HT These chemicals then Activate the nociceptors November 14, 2018 sensory physiology

Types of Pain Two types of pain sensation Fast (initial) pain Result from strong noxious stimuli Fast (initial) pain Discrete, well localized Pinprick sensation Results from activation of A fibers November 14, 2018 sensory physiology

Types of Pain Slow or delayed pain Poorly localized Dull ache, burning sensation Results from activation of C fibers November 14, 2018 sensory physiology

Types of Pain Two types of pain sensation use Fast pain Different pathway to reach centers for consciousness Fast pain Conducted by A (gp III) Velocity of conduction is 12 – 30 m/sec AP travel faster Reach the brain before slow pain fibers November 14, 2018 sensory physiology

Types of Pain The A sensory fibers Slow pain Use spinothalamic tract to sensory cortex Topographically represented on cortex Slow pain Conducted by group IV type C fibers Velocity of conduction is 1 - 2 m/sec Give collaterals to Reticular formation Activate fibre tracts that produce the Emotional perception accompanying pain November 14, 2018 sensory physiology

Types of Pain The two types of pain elicit different reflexes Fast pain Evoke withdrawal reflex Sympathetic response  BP  mobilization of body energy supplies November 14, 2018 sensory physiology

Types of Pain Slow pain evoke Nausea Profuse sweating  BP Generalized reduction in skeletal muscle tone November 14, 2018 sensory physiology