Neuroscience Chapter on Sensation

Advanced Human Physiology: Sensory Systems (Somatic Senses)

• This material covers sensory systems and somatic senses, specifically the afferent division of the nervous system.
• Sensory information travels from receptors to the central nervous system (CNS).
• The lecture, from Glorimar Aponte-Kline M.D. at Utah Tech University, focuses on the afferent division of the nervous system.
• This division emphasizes sensory information perceived consciously or subconsciously.
• The process involves stimulus, receptor, ascending signal/pathway, and integration.

Sensation

• Sensation is the conscious or subconscious awareness of changes in internal or external environments.
• Perception is the awareness and interpretation of these changes mediated by the cerebral cortex.
• The process of sensation involves four steps: stimulation of sensory receptors, transduction of stimulus, generation of action potentials, and integration of sensory input.
• Sensory receptors are specific structures associated with sensory neurons.
• Transduction converts stimulus energy (chemical, mechanical, thermal, photo) into a graded receptor potential.
• If the graded potential reaches threshold, action potentials are generated.
• Integration of sensory input in the CNS can modify, continue, or terminate the response.

Sensory Transduction

• Sensory transduction converts energy into graded potentials.
• Stimulus energy is converted into information processed by the CNS.
• This conversion involves opening or closing of channels in the receptor membrane producing graded potentials ("generator potential" or "receptor potential").
• "Adequate stimulus" refers to the specific form of energy a receptor responds to most effectively.
• Threshold is the minimum stimulus energy needed to activate a receptor.
• Receptor potentials are graded potentials that may or may not reach action potential threshold, depending on if they reach the necessary levels for an AP.

Different Types of Sensory Receptors

• Receptors are categorized into five major types: mechanoreceptors, thermoreceptors, photoreceptors, chemoreceptors, and nociceptors.
• Mechanoreceptors respond to mechanical stimuli like pressure, stretching, or bending.
• Thermoreceptors detect changes in temperature.
• Photoreceptors respond to light.
• Chemoreceptors detect chemicals in the mouth (taste), nose (smell), and body fluids.
• Nociceptors respond to painful stimuli.

Receptive Fields

• A receptive field is the area that, when stimulated, results in a response in that sensory neuron.
• The size of the receptive field influences the precision of localization of a stimulus.
• Receptive fields can overlap, enhancing localization ability.
• Neighboring sensory neurons may have overlapping receptive fields, and the relative location of stimulus to the sensory neuron influences a given response.
• Smaller receptive fields lead to better two-point discrimination (ability to perceive two points of touch as separate)

Sensory Coding

• Sensory systems encode four stimulus attributes: modality, location, intensity, and duration.
• Modality refers to the type of sensation (determined by stimulus itself).
• Location is determined by specific neuronal pathways and receptive fields.
• Intensity is determined by frequency of action potentials.
• Duration is determined by the duration of action potentials, and whether the receptor adapts (phasic) or continues to fire (tonic).

Sensory Pathways

• Sensory pathways are groups of parallel neuron chains carrying sensory information from receptors to the cerebral cortex.
• These pathways include first-order neurons that originate at the sensory receptors; second-order neurons that synapse with first-order neurons in the spinal cord or brainstem; third-order neurons that project to the thalamus; and fourth-order neurons in sensory cortex.
• Pathways decussate in some cases, meaning the pathways cross over at some point.
• Important pathways include: the Dorsal Column pathway and the Anterolateral pathway.

Tactile Sensations

• Tactile sensations include touch, pressure, vibration, itch, and tickle.
• Touch is due to stimulation of tactile receptors in the skin, with pressure being from deeper deformation of skin and subcutaneous layers.
• Vibration arises from rapid repetitive signals from receptors.
• Itch involves free nerve endings stimulated by certain chemicals (e.g., histamine).
• Tickle is a sensation originating from someone else touching you.

Thermal Sensations

• Thermal sensations use free nerve endings that act as thermoreceptors, activated by specific temperature ranges, (e.g., cold or warm).
• These receptors involve TRPV3 and TRPM8 channels.

Pain Sensations

• Pain receptors (nociceptors) are important for protecting the body from tissue damage and act as specialized receptors that respond to extreme stimuli.
• Nocireceptors are categorized by the stimuli they respond to; mechanical, thermal, or polymodal (respond to multiple stimuli).
• Pain transduction involves the opening of cation channels in response to noxious stimuli.
• One ion channel that is commonly discussed for pain transduction is TRPV1, triggered by capsaicin (in chili peppers).

Fast and Slow Pain

• Fast pain is sharp, localized pain perceived quickly.
• Fast pain results from myelinated A-delta fibers; small, myelinated neurons.
• Slow pain is dull, aching, and diffuse, and results from unmyelinated C fibers; slow conduction, unmyelinated neurons.

Pain Pathways

• Nociceptors activate both spinal reflexes (unconscious protective responses) and ascending pathways (to the cerebral cortex, for conscious pain perception).
• The ascending pathways involve first, second, and third-order neurons, relaying the signal through the spinal cord and thalamus before reaching the brain (somatosensory cortex).
• Substance P is a neurotransmitter involved in pain signal transmission.

Stimulus Intensity

• Stimulus intensity is encoded by the frequency of action potentials.
• A stronger stimulus produces a higher frequency of action potentials.

Stimulus Duration: Receptor Adaptation

• Adaptation describes the change in responsiveness to a constant stimulus.
• Tonic receptors adapt slowly or not at all.
• Phasic receptors adapt rapidly.

Somatic Sensory Pathways

• Somatic sensory pathways send information from somatic sensory receptors (skin, muscles, joints) to the primary somatosensory cortex in the brain.
• These pathways generally contain a series of first-, second-, and third-order neurons.
• Important pathways include dorsal column and anterolateral pathways.

Referred Pain

• Referred pain is pain felt in a location other than its actual source.
• Visceral and somatic sensory neurons converge on the same ascending tract causing the confusion.

Pain Modulation

• Pain can be modulated by both physical and chemical methods.
• Drugs such as aspirin and ibuprofen decrease inflammation and suppress pain.
• Physical methods like TENS use electrical impulses to stimulate and deflect pain signals.
• Endogenous opioids are hormones and chemicals that block pain signals.

Proprioceptive Sensations

• Three types of receptors (muscle spindle, Golgi tendon, and joint kinesthetic) transmit information about the position and movement of muscles and joints.