The Body Senses and Movement Study Guide

Primary Somatosensory Cortex - Features a somatotopic body map representing different regions of the body. - The cortical area dedicated to a specific body part is proportional to the sensitivity of that body part, rather than its physical size. - The receptive fields are complex, containing excitatory centers and inhibitory surrounds to enhance contrast and precision. - It contains feature detectors specialized for identifying specific qualities: orientation, direction of movement, shape, and texture.
Secondary Somatosensory Cortex - Responsible for integrating sensory information from both sides of the body. - Involved in determining the meaning and reward associated with sensations. - Maintains connections to the hippocampus, likely facilitating memory for tactile experiences.
Posterior Parietal Cortex - Acts as an association area for multiple senses, including body senses, vision, and audition. - Governs orientation in space, the relative location of limbs, and the spatial location of objects. - Specialized for motor-sensory tasks such as posture, reaching, and grasping.

The Vestibular system consists of multiple components to monitor position and movement: - Semicircular canals: There are $3$ of these, responsible for detecting head movement. - Utricle and Saccule: These components monitor head position relative to the force of gravity.
Neural Projections: - The system projects information to the Cerebellum and the Brain stem. - It also projects to the Parieto-insular-vestibular cortex; stimulation in this area is associated with dizziness causing nausea.

The somatosenses are broadly categorized into three systems: - Skin senses: Detect conditions specifically at the body surface. - Proprioception: Monitors the position and movement of the body and limbs. - Interoceptive system: Responsible for sensations originating from internal organs.
Dermatomes and Information Flow - The body is divided into segments known as dermatomes. - Sensory information crosses over from one side of the body to the opposite brain area. - Flow of sensory information: 1. Information enters the spinal cord through spinal nerves (or enters the brain directly via cranial nerves). 2. Signals travel to the Thalamus. 3. Signals are processed in the Somatosensory cortex located in the parietal lobes.

Primary sensations detected include touch, warmth, cold, and pain, and potentially itch.
Superficial Receptors: - Free nerve endings: Responsible for detecting warmth, cold, and pain. - Meissner’s corpuscles: Detect fine touch. - Merkel’s discs: Responsible for perceiving texture and fine detail.
Deep Receptors: - Encapsulated receptors: These detect touch, pressure, and stretch. - Pacinian corpuscles: Involved in detecting deep pressure and vibration. - Ruffini endings: Responsible for detecting stretch and assisting in the perception of grasped objects.

Molecular Mechanism: - Free nerve endings detect temperature and pain through the $TRP$ (transient receptor potential) family of protein ion channels.
Chemical Detection via $TRPA1$ Receptor: - Triggered by chemicals such as tear gas and tobacco smoke. - Triggered by specific foods like garlic and wasabi.
Thermal Detection: - $TRPV1$ : The heat pain receptor. Capsaicin (found in chili peppers) is an agonist. Interestingly, capsaicin can be used to alleviate joint pain by fatiguing these receptors. - $TRPM8$ : The coolness receptor, which is activated by chemicals like menthol.
Types of Pain Fibers: - A-delta fibers: Large, myelinated fibers that transmit pain signals quickly. They are responsible for immediate, localized, sharp pain. - C fibers: Small, unmyelinated fibers that transmit signals slowly. They are responsible for slower, diffuse, dull, and aching pain.
Pain Pathway: - The spinal cord releases glutamate and Substance $P$ . - Information proceeds to the Thalamus, then to the Somatosensory cortex, and finally to various areas responsible for the emotional aspects of pain.

Gate Theory: - Pain triggers the release of endorphins from the periaqueductal gray ( $PAG$ ). - This release inhibits the secretion of Substance $P$ . - Consequently, the pain "gate" is closed within the spinal cord, blocking the signal from traveling further.
Endorphins: - Function as both a neurotransmitter and a hormone. - They act specifically at opiate receptors. - Their effects can be entirely reversed by the administration of naloxone, which acts as an antagonist. - Triggers for Endorphin Release: - Inescapable shock. - Physical stress. - Acupuncture. - Vaginal stimulation (specifically linked to the process of childbirth). - Placebo administration.

Nonsteroid anti-inflammatory drug ( $NSAID$ ) - Examples: Naproxen, ibuprofen, aspirin. - Method of Action: Decreases swelling in tissues and decreases inflammation. - Side Effects: Increased risk of bleeding, stomach irritation, allergic reaction.
$COX-2$ inhibitor (selective $NSAID$ ) - Example: Celecoxib. - Method of Action: Decreased inflammation. - Side Effects: Slight risk of heart attack.
Acetaminophen - Example: Tylenol. - Method of Action: Blocks pain signals in the Central Nervous System ( $CNS$ ). - Side Effects: Liver problems.
Opiates (narcotics) - Examples: Morphine, codeine, hydromorphone, oxycodone, fentanyl. - Method of Action: Stimulates opiate receptors and blocks pain messages in the $CNS$ . - Side Effects: Depressed breathing, sleepiness, addiction, tolerance, euphoria.

Chronic Pain: - Defined as pain that persists after healing has already occurred. - A reduced pain threshold can increase an individual's susceptibility to chronic pain. - Chronic pain states can lead to a measurable loss of gray matter.
Phantom Pain: - Affects between $80\%$ to $90\%$ of amputees. - Occurs because neurons from adjacent body areas invade the cortical area that previously received input from the missing limb. - Treatments include: - Use of a functional prosthesis. - The mirror box technique, which utilizes mirror neurons to trick the brain into thinking the limb is moving or relaxed.