Somatosensation and Pain Hour 1 PDF

Somatosensation Sara Jane Ward Neuroanatomy Kornberg School of Dentistry Spring 2025 Session Objectives Following the required reading and today’s presentation, you should be able to: 1.Define and describe somatosensory receptors and their properties 2.Understand how stimuli, especially touch...

Somatosensation Sara Jane Ward Neuroanatomy Kornberg School of Dentistry Spring 2025 Session Objectives Following the required reading and today’s presentation, you should be able to: 1.Define and describe somatosensory receptors and their properties 2.Understand how stimuli, especially touch stimuli, get transduced into a neural language the brain can understand 3.Identify basic somatosensory receptors found in the skin 4.Identify and localize the ascending pathway from the periphery to the brain that mediates touch Outline Sensory receptors and transduction Types of somatosensory receptors  Mechanoreceptors  Thermoreceptors, nociceptors Somatosensory pathways  Dorsal column system  Anterolateral System / Spinothalamic tract Sensory division CENTRAL NERVOUS SYSTEM (brain and spinal cord) Sensory, or afferent division PERIPHERAL NERVOUS SYSTEM (sensory receptors, sensory nerves, and ganglia) Special Somatic sensory sensory receptors receptor s Visceral RECEPTORS sensory receptors  Sensory Transduction How can the nervous system cell sense and perceive visual, auditory, touch etc information? Signal transduction is the conversion of (an external) Cell body stimulus energy into the language of the nervous Cell body Cell body system (electricity and chemical transmission!) Occurs at the level of the sensory receptor Sensory receptors Receptors are modified epithelial cells OR modified dendrites of sensory neuron  Stimulation of receptor will transduce stimulus from the environment to a neural signal  sends information to a defined region within the brain where the signals are received and interpreted. Modified dendrite type: Mechanoreceptor of the somatosensory system Modified epithelial cell type: Hair cell of the auditory system The Transduction process couples stimulus detection (i.e., mechanical deformation) to the opening or closing of ion channels  Leads to changes membrane voltage  Leads to increased or decreased synaptic vesicle release in specialized receptor cells, or direct generation of membrane potentials in receptor endings of sensory neurons.  This may lead to a threshold change in the membrane potential of the sensory neuron that can trigger an action potential Sensory receptors and transduction Receptor on sensory receptor Don’t confuse a sensory receptor with the kind of receptor that binds a neurotransmitter!!! Those kinds of receptors can be found “on” the sensory receptor Sensory receptor Somatosensory system Somatosensation The somatosenses provide information about what is happening on the surface of our bodies (our “somas”) as well as internally Proper functioning is necessary for us to interact with our environments, to perform simple and complex motor tasks, to feel pleasure, and to feel pain Impairment in somatosensation greatly reduces quality of life and is probably the leading cause we seek medical treatment Somatosensation Different somatosensory receptors respond to several different types of external stimuli Information about different types of stimuli travel to the brain along two different pathways 1. Touch and position pathway Mechanoreceptors 2. Pain and Temperature pathway Temperature, chemical, painful touch, painful chemical, painful temperature Next hour! Mechanoreceptors The sensations of light touch, pressure and vibration are caused by movement of the skin by an external stimulus.  movement of the skin moves the dendrite of the mechanoreceptor  Many mechanoreceptors have layers surrounding this dendrite serve to magnify the movement  Others are just free nerve endings  Deformation opens “stretch receptor” ion channels on the nerve terminal Pacinian Corpuscle Cutaneous mechanoreceptors come in different shapes and sizes at different locations within the skin and possess distinct properties  Mechanoreceptors React to multiple stimulus types Differing Morphology Some are rapidly adapting (RA), others slow adapting (SA) All are low threshold (LT). Varying receptive field sizes Multiple sensations perceived Nature Reviews Neuroscience 12, 139-153 (March 2011) Sensory receptors can code an amazing amount of detailed information when they are activated: 1. Intensity (amplitude or quantity of stimulus) 2. Duration (time between start & end of a stimulus) 3. Location (where stimulus is located or originates) 4. Modality (type or quality of stimulus)  1. Encoding stimulus intensity (amplitude or quantity) Most mechanoreceptors signal somatic sensations using frequency and population coding. 1. Frequency code = amplitude of stimulus  How many action potentials are coming in from that sensory receptor being stimulated? 2. Population Code = quantity of stimulus  How many sensory receptors are being stimulated? 2. Encoding of Stimulus Duration Duration time between start & end of a stimulus Encoded by two types of sensory receptors: slow adapting and rapidly adapting receptors  Adaptation: The process of the diminution of a sensation induced by a stimulus, until it is no longer recognized; ex. touch of clothes during the day Good for touch Good for motion, vibration discrimination and stretch (Meissner, Pacinian) (Merkel, Rufini)  3. Encoding of Stimulus Location Encoded by the receptive fields of the primary sensory afferent endings, and then the route by which they project through the CNS The route the information takes along specific axons projecting into the CNS (spinal cord or brainstem) and to the cerebral cortex is organized into tight patterns that are termed “somatotopic maps”  Somatotopy is the point-for-point correspondence of an area of the body to a specific point on the central nervous system.  These maps serves to provide a representation of the peripheral location of the stimulus.  Receptive Fields The size of the receptive field of a mechanoreceptor varies as a function of peripheral innervation density. The greater the density of receptors, the smaller the receptive fields of individual afferent fibers. Receptive fields vary in size across body, as does density of receptors. Greatest 2-point discrimination and point localization accuracy is in skin areas with smallest receptive fields and afferent nerve highest density, e.g. digits.  4. Encoding of Modality Modality = type or quality of stimulus energy. Most sensory receptors are optimally sensitive to a single stimulus energy = probabilistic receptor specificity. Specificity = labeled line code (tracts)  A common feature of sensory systems are modality-specific pathways  Input from distinct sets of afferent neurons largely stay separate as information travels up the central neuroaxis. Pathologies in touch sensation Paresthesia: usually Anesthesia: total painless and loss of sensation described as tingling or numbness, skin crawling, or itching These can be temporary or chronic when associated with Hypoesthesia: nerve injury or reduced sense of touch or sensation disease Somatosensory pathways The Somatosensory Pathways Somatosensory information from the body travels to the brain along two distinct pathways  Information about touch (from the mechanoreceptors) travels along the dorsal column / Medial lemniscus pathway  Information about pain and temperature (from the free nerve endings) travels along the anterolateral pathway / spinothalamic tract Somatosensory information from the face travels along the trigeminal system  The Somatosensory Pathways Pathway Origin Termination Decussation Sensory Information (crossing) Point Transmitted Dorsal column/ Spinal Somatosensory Brain stem Fine touch, vibration, medial lemniscus cord Cortex (Medulla pressure, and Oblongata) proprioception, Carried by Aβ, wide diameter, myelinated fibers (80 -120 m/s) Anterolateral Spinal Somatosensory Spinal Cord Crude touch, pressure, System / Cord Cortex Pain, temperature, Spinothalamic tract Aδ (20m/s) C fibers (2 m/s) The Somatosensory Pathways Dorsal column pathway Anterolateral/ Spinothalamic pathway  Dorsal Column/Medial Lemniscus  First order sensory neurons (primary afferents) cell bodies are located in the dorsal root ganglia of the spinal nerve, just outside the spinal cord. The axon enters the dorsal spinal cord  The axon ascends through two dorsal columns – the fasciculus gracilis and the fasciculus cuneatis  Sensory information from the upper half of the body is carried in the cuneate tract, and information from the lower body is carried in the gracile tract. Medial lemniscus Dorsal column Dorsal column / Medial lemniscus The first order neurons terminate in the nucleus gracilis and nucleus cuneatus of the lower medulla onto second order neurons These second order neurons are known as the internal arcuate fibers They cross the midline and travel Medial lemniscus as a pathway called the medial Dorsal column lemniscus  Dorsal Column/ Medial lemniscus  Axons of second order neurons ascend to the thalamus and synapse onto third order neurons in the ventral posterior lateral nucleus of the thalamus  These third order neurons project to the primary somatosensory cortex in the parietal lobe Medial lemniscus Dorsal column  Somatosensory cortex Third order neurons synapse onto fourth order neurons located in the somatosensory cortex (S1) S1 has somatosensory homunculus similar to that in the motor cortex The VPL thalamus is also somatotopically organized and these connections track to S1 Trigeminal System Trigeminothalamic Tracts - fine touch, vibration from face; - pain and temp (covered in next lecture) Trigeminal System 3 neuron pathway – Pons to thalamus to SI face region. Neuron #1 Part of the Trigeminal nerve Receives projections from mechanoreceptors in face and oral cavity. Travel to the Main Sensory Nucleus in the mid Pons and synapse onto Neuron #2 Axons of this nucleus project to VPM of thalamus Neuron #3 Axons of this nucleus project of somatosensory cortex for perception of mechanical touch of face