Week 11 Introduction to Touch Sensation PDF

Week 11 Introduction to Touch Sensation Touch Overview: ○ Touch is one of the five canonical senses, traditionally understood as a unified sense. ○ However, it encompasses a wide range of sensations, including temperature, texture, shape, weight, and spatial location of objects. Integration with O...

Week 11 Introduction to Touch Sensation Touch Overview: ○ Touch is one of the five canonical senses, traditionally understood as a unified sense. ○ However, it encompasses a wide range of sensations, including temperature, texture, shape, weight, and spatial location of objects. Integration with Other Senses: ○ Tactile cues combine with vestibular and proprioceptive senses to provide a complete understanding of objects and their interactions. ○ Example: Searching for an object in a bag using haptic exploration to identify features like texture or shape without visual cues. Haptic Exploration Definition: Using hands to gather object features through touch. Information Gathered Through Touch: ○ Texture: Rubbing fingers over the surface. ○ Weight: Feeling pressure in the palm. ○ Hardness: Applying pressure and observing the response. ○ Shape: Closing the hand around the object or tracing its contours with fingers. ○ Temperature: Holding the object to sense heat or cold. Sensory Cells and Skin Structure Skin as a Sensory Organ: ○ Largest sensory organ, divided into: Epidermis: Outer layer; mostly dead cells, providing protection. Dermis: Inner layer; contains sensory receptors, vasculature, glands, and hair follicles. Receptor Subtypes in the Skin: ○ Merkel's Discs: Superficial receptors for fine pressure. ○ Meissner's Corpuscles: Detect light touch and texture. ○ Pacinian Corpuscles: Respond to deep pressure and vibration. ○ Ruffini Endings: Sensitive to skin stretch. ○ Hair Follicle Receptors: Detect deflection of hair. ○ Free Nerve Endings: Responsible for temperature and pain sensations. Touch Sensitivity and Spatial Resolution Variability Across the Body: ○ Distribution of receptors differs by area, leading to variation in touch sensitivity. ○ Example: Fingertips are more sensitive than the palm due to receptor density. Two-Point Threshold Test: ○ Method: Calipers press the skin, and participants indicate whether they feel one or two points of pressure. ○ Findings: Fingertips have the smallest threshold (highest resolution). Threshold increases as you move toward less sensitive areas, like the palm. Correlation with Receptor Types: ○ High-resolution areas have dense distributions of Merkel's discs and Meissner's corpuscles (small receptive fields). ○ Low-resolution areas rely on Pacinian corpuscles and Ruffini endings, which have larger receptive fields. Temperature Sensitivity Free Nerve Endings: ○ Located in the upper dermis; sensitive to temperature changes. ○ Critical for encoding ambient temperature and aiding thermoregulation. ○ Helps the body maintain homeostasis by prompting behavioral adjustments (e.g., seeking warmer or cooler environments). Temperature Sensation Temperature-Sensitive Free Nerve Endings: ○ Allow assessment of object temperature through direct skin contact. Example: Parents testing the temperature of infant formula on their hand. Types of Temperature-Sensitive Fibers: ○ Cold Fibers: Respond to temperatures below body temperature. ○ Warm Fibers: Respond to temperatures above body temperature. Mechanism of Temperature Perception: ○ At resting body temperature, both fiber types maintain a baseline activity. ○ Temperature changes cause: Increased output from cold fibers with lower temperatures. Increased output from warm fibers with higher temperatures. ○ Adaptation: Sensitivity decreases with prolonged exposure (e.g., adjusting to a cold pool or hot tub). Pain Sensation (Nociception) Nociceptors: Specialized free nerve endings that detect harmful stimuli and generate pain perception. ○ Subtypes have different mechanisms, leading to changes in pain sensation over time. Purpose of Pain: ○ Functions as an important signal to avoid further damage. ○ Guides behaviors to protect the body and minimize harm. Pathways of Pain Signals 1. Reflex Pathway (Rapid Response): ○ Bypasses the brain for immediate withdrawal from harmful stimuli. ○ Steps: Pain receptors detect a noxious stimulus (e.g., touching a hot object). Signal travels via afferent neurons to the spinal cord. Interneurons in the spinal cord relay the signal to efferent motor neurons. Motor neurons contract muscles to pull the body part away from the source of pain. ○ Occurs within a fraction of a second. 2. Ascending Pathway (Conscious Pain Perception): ○ Pain signals travel to the brain, allowing: Further actions to address the pain. Formation of memories to avoid future harm. 3. Descending Pathway (Pain Modulation): ○ Brain regulates pain perception by releasing endorphins, which: Inhibit pain-sensitive neurons. Reduce the intensity of pain signals sent to the brain. ○ Allows coping in non-threatening situations (e.g., dental visits, workouts). Congenital Insensitivity to Pain (CIP) Genetic Basis: ○ Caused by mutations in the SCN9A gene, affecting sodium channels essential for nociceptor function. ○ Individuals with CIP cannot perceive pain due to non-functional channels. Consequences of CIP: ○ Lack of pain awareness leads to significant harm, such as: Unnoticed injuries (e.g., biting their tongue, walking on broken bones). ○ Pain's protective role is absent, increasing the risk of severe injury or infection. Part 2 Overview of Sensory Integration and Movement Planning The somatosensory system and motor system form a closed loop: ○ Sensory signals travel to the brain via afferent (ascending) pathways. ○ Brain interprets sensory input in the somatosensory cortex. ○ Commands are generated in the motor cortex and travel down efferent (descending) pathways to activate muscles. Role of the Somatosensory Cortex: ○ Interprets tactile and proprioceptive inputs. ○ Integrates sensory cues with spatial awareness to plan interactions with the environment. Focus on Ascending Pathways Purpose: Carry sensory information from the skin and musculoskeletal system to the brain. Pathway type depends on the nature of the sensory signal. Tactile and Proprioceptive Information Follows the dorsal column-medial lemniscal pathway: ○ Processes touch (e.g., texture, pressure) and proprioception (body position in space). ○ Allows fine discrimination of sensory details. 1. Dorsal Root Ganglia: ○ Cell bodies of sensory nerves reside here. ○ Sensory input enters the spinal cord via the dorsal root. 2. Spinal Cord to Medulla: ○ Sensory axons project along the dorsal (back) side of the spinal column. ○ Terminate in the medulla (part of the brainstem). 3. Crossing Over (Decussation): ○ Neurons in the medulla cross to the contralateral side (opposite side of the brain). ○ Ensures signals from one side of the body are processed by the opposite side of the brain. 4. Medial Lemniscus: ○ A bundle of fibers carrying sensory signals from the medulla to the thalamus. 5. Thalamus: ○ Ventral posterior nucleus processes sensory input and relays it to the cortex. 6. Somatosensory Cortex: ○ Located in the parietal lobe. ○ Final processing site for tactile and proprioceptive cues. ○ Integrates signals with spatial awareness for motor planning. Neuroanatomy of Sensory Pathways 1. Dorsal Column-Medial Lemniscal Pathway (Tactile and Proprioceptive Signals): ○ Name Origin: Neurons travel through the dorsal spinal column and the medial lemniscus. ○ Pathway Details: Origin: Sensory neurons in dorsal root ganglia. Spinal Cord: Travel along the dorsal column, terminating in the medulla. Decussation: Cross to the contralateral side in the medulla. Thalamus: Project to the ventral posterior nucleus. Somatosensory Cortex (S1): Signals are processed in areas 1, 2, 3a, and 3b. 2. Spinothalamic Pathway (Pain and Temperature Signals): ○ Name Origin: Neurons synapse in the spine and then the thalamus. ○ Pathway Details: Origin: Sensory neurons in dorsal root ganglia. Spinal Cord: Terminate in the dorsal horn. Decussation: Cross at the spinal cord level. Medulla to Thalamus: Travel via the spinothalamic tract. Somatosensory Cortex (S1) & Insular Cortex: Processed for pain and temperature perception. 3. Hemisection of the Spinal Cord (Brown-Séquard Syndrome): ○ Effect: Loss of tactile/proprioceptive sensation on the same side of the damage. Loss of pain/temperature sensation on the opposite side. ○ Cause: Differential crossing of the dorsal column-medial lemniscal and spinothalamic pathways. Organization of the Somatosensory Cortex (S1) 1. Subregions: ○ Areas 1 & 3b: Input from skin receptors. ○ Areas 2 & 3a: Input from bone and muscle receptors. 2. Location: ○ S1 lies posterior to the central sulcus, stretching from the top of the brain to the temporal lobe. ○ Secondary Somatosensory Cortex (S2): Found at the lower end of S1; integrates sensations from both body sides. 3. Somatosensory Homunculus: ○ Orderly Mapping: Neighboring body parts are represented by neighboring cortical regions. ○ Size Representation: Proportional to sensory importance, not body size (e.g., hands and face dominate). 4. Neuroplasticity: ○ Reorganization: Cortex regions can expand or shrink with experience or injury. Example: Amputation of a finger leads to adjacent cortical areas taking over. ○ Skill-Related Changes: Enhanced representations in professional athletes or musicians. Proprioception and Reflexes 1. Proprioception: ○ Definition: Awareness of body position in space, vital for balance and movement planning. ○ Sensory Inputs: 1. Joint Receptors: Detect joint angles via pressure on bones. 2. Tendon Receptors: Measure stretch in connective tissues. 3. Muscle Spindles: Gauge muscle thickness to infer flexion or extension. ○ Function: Inputs combine to provide accurate spatial awareness. 2. Reflex Arcs: ○ Definition: Rapid, automatic responses to maintain control without involving the brain. ○ Example: Patellar reflex: 1. Stimulus: Tap on patellar tendon. 2. Response: Stretch receptors signal spinal interneurons, which activate motor neurons. 3. Outcome: Quadriceps contract; leg kicks forward. 3. Proprioception and Alcohol: ○ Impairment of proprioception explains difficulty in completing tasks like the touch-your-nose test when intoxicated.