Sensory Systems: Somatosensory and Special Senses

• Sensory systems are crucial for survival
• Sensory systems include somatosensory and special senses
• The Department of Neurology at the Faculty of Medicine, Universitas Indonesia studies these systems

Terminology

• Stimulus: A change in the environment detected by receptors
• Sensory receptors: Specialized cells/structures that collect raw information from the environment
• Sensation: The process of sensing the environment through senses
• Senses: Biological systems that translate information outside the nervous system into neural activity
• Perception: The way the brain interprets information
• Signals are converted via collection, transduction, processing and then action

Sensory Systems

• Sensory systems include somatosensation and special senses
• Somatosensation includes general senses
• Somatosensation is the direct sensation of interaction with the world
• The direct sensation of interaction with the world includes touch
• The specific sensory for direct sensation of interaction with the world includes pressure, contact, vibration, and directional motion of skin
• Interoception is for the detection and appreciation of the internal functioning of the body and the physical state of major visceral organ systems
• Interoception receptors include chemoreceptors, nociceptors, and mechanoreceptors
• Proprioception is the detection and sensing of position and movement in the environment
• The classic sensory end organs for proprioception are muscle tissue, tendon, joint capsule, and ligament
• Cutaneous exteroception is a recent discovery in proprioception

Sensory Processing Levels

• Receptor level involves sensory reception and transmission to the central nervous system (CNS)
• Circuit level involves processing in ascending pathways
• Perceptual level involves processing in cortical sensory centers
• Sensory processing pathway includes the spinal cord, medulla, pons, reticular formation, cerebellum, thalamus, somatosensory cortex, and motor cortex

Physiology: Sensory Pathways

• Sensory pathway reaches the cerebral cortex for conscious perception
• Action potential occurs in the sensory axon
• An upper motor neuron from the cortex executes a motor command
• The upper motor neuron contacts a lower motor neuron in the spinal cord
• The lower motor neuron causes contraction of the target skeletal muscle

Sensory Coding

• Sensory coding steps: transduction, adaptation, amplification, conduction, transmission and perception
• Transduction converts sensation to nerve impulses
• Adaptation diminishes sensitivity to unchanging stimulus

Types of Sensory Receptors

• Tonic receptors are slow acting with no adaptation and continue firing impulses as long as the stimulus is there, e.g., proprioceptors
• Phasic receptors are quick acting, adapt rapidly, and stop firing when stimuli are constant, e.g., smell

Neural Pathways

• Integrates process being actively constructed by the brain and dependent on the memories of past experiences
• Signals travel between third-order neuron, primary somatosensory cortex, thalamus, second-order neuron, first-order neuron, dorsal column, and spinal cord
• Pathways include Dorsal column-medial lemniscal pathway and Spinothalamic tract
• Modality/stimulus, location, intensity, and timing all affect how a stimulus is perceived

Types of Sensory Receptors

• Somatic sensory stimuli reach the conscious level of perception and include general and special senses
  • Pain receptors - Tissue damage (skin)
  • Thermoreceptors – Temp. (Skin)
  • Chemoreceptors – Taste & smell
  • Baroreceptors – Sound, balance
  • Proprioreceptors - Muscle stretch
• Visceral Sensory
  • Pain receptors – Tissue damage (viscera)
  • Thermoreceptors - Temperature
  • Chemoreceptors – Chemical in blood, osmoreceptors
  • Baroreceptors - Blood pressure

Modality: Sensory Stimuli

• Includes gustatory, olfactory, vestibular, auditory, visual, and somatosensory
• Gustatory receptors are chemoreceptors that detect chemical tastants, resulting in the perception of taste via taste buds
• Olfactory receptors are chemoreceptors that detect chemical odorants, resulting in the perception of smell via olfactory receptor neurons
• Vestibular receptors are mechanoreceptors that detect angular and linear accelerations, resulting in the perception of balance via hair cells
• Auditory receptors are mechanoreceptors that detect hydromechanical stimuli, resulting in hearing via hair cells
• Visual receptors are photoreceptors that detect light, resulting in vision via rods & cones
• Somatosensory receptors can be mechano-, thermo-, and chemoreceptors that detect mechanical, thermal, and chemical stimuli
• Touch is perceived via Tactile, proprioceptive, nociceptive & thermal

Exteroceptors

• Include Merkel disc, Ruffini ending, Meissner corpuscle, and Pacinian corpuscle
• Cutaneous sensory endings are in the Epidermal edge
• Best stimulus includes pressure, edges, point, corner, & curve
• Functional includes Texture & shape perception
• Mean threshold to response is 30μm

Modalities

• Nerve ending, hair follicle are rapid
• Cold receptor, dermis, is phasic and tonic components
• Warm receptor, dermis, is phasic and tonic components

Pain Receptors (Nociceptors)

• Sensitive to pain-causing stimuli like extreme heat/cold, excessive pressure, inflammatory chemicals
• Free nerve ending
• Injured cells release arachidonic acid
• Arachidonic acid is converted into prostaglandins by the interstitial enzyme cyclo-oxygenase
• Prostaglandins activate nociceptors
• Many pain medications function to inhibit cyclo-oxygenase
• Pain levels are modulated by endorphins which inhibit CNS function

Thermoreceptors

• Sensitive to temperature changes and are located in the skin, skeletal muscle, liver, and hypothalamus
• Free nerve endings and phasic receptors
• Cold receptors are superficial
• Heat receptors are deeper
• Temperature out of range of thermoreceptors activates nociceptors

Regulation of Body Temperature

• The hypothalamus regulates body temperature via thermosensory afferents and effector organs
• Body temperature decreases include heat given off by dilated blood vessels, sweating, and skin
• Body temperature increases include heat conserved by constricted blood vessels, no sweating, and skin

Mechanoreceptors

• Detects membrane distortion
• Specific receptors have different sensitivities for each stimulus
• Include Meissner corpuscle, Merkel cell and Pacinian corpuscle

Chemo and Baroreceptors

• Chemo – Detects change in concentration of chemical substance, e.g. pH, CO2
• Baro - Detects pressure changes in blood vessels and visceral organs, e.g. Blood pressure & heart rate regulation

Modality: Proprioceptors

• Muscle spindles: Detect muscle stretch and are composed of intrafusal fibers wrapped by primary afferent endings
• Skeletal muscle fibers: Extrafusal fibers (Skeletal muscle fibers) la afferent axon: primary afferent endings intertwined with collagen fibrils of tendon

Location

• The brain must possess some means of “mapping” or representing external space to central neural structures
• A stimulus that lands within the confines of a receptive field will activate a given sensory receptor
• A stimulus falling outside of that region would result in no response from that receptor
• Spinal afferents innervating dermatomes overlap one another to some degree
• The transition of innervation zones for adjacent dermatomes more blurred rather than sharp
• Complete anesthesia for any given dermatome requires not only the removal of input from that dermatome

Location

• Somatosensory Cortical Map includes leg, trunk, shoulder, arm, palm & wrist
• Include Finger, Feet, Upper and Lower face

Intensity

• Graded receptor potential, Action potential, Transmitters released
• Can be weak, moderate or strong

Timing

• The timing dictates how signals/stimuli are interpreted through sensory receptors.

Sensory Pathways

• Fine touch, proprioception, vibration
• Nociception, temperature, coarse touch
• Sensory pathways synapse in the thalamus
• Pain, temperature, and coarse touch cross the midline in the spinal cord
• Fine touch, vibration, and proprioception pathways cross the midline in the medulla

Major Somatic Sensory Pathways

• Anterolateral pathway: Conscious sensations of Touch & pressure, Pain & temperature
• Posterior Collumn: Sensation of precise touch, vibration, proprioception
• Spincocerebellar Pathway: Subconscious informations of Coordinate movements

Thalamus

• The relay station for most sensory impulses
• Referred to as ""Gateway to cerebral cortex”

Conclusion

• Sensory systems are somatosensory and special senses
• Sensory system is a complex integrative process, necessary to survive changes in environments
• Physiology includes transduction, conduction, transmission, and perception
• Perception includes knowing what type of stimulus and where it came from.