Part 1:Sensory Systems,General Principles,Somatic Sensation

• The sensory system is a portion of the nervous system.
• The sensory system is made of: sensory receptors (main focus of this unit), nerve pathways that conduct sensory information from receptors to CNS, and parts of the brain that process sensory information.
• Sensory receptors are the peripheral ends of the afferent neuron.
• Reminder: afferent neurons are neurons that carry information from sensory receptors at their peripheral endings towards the CNS (from textbook).
• There are two types of sensory receptors: one that's a complete afferent neuron with its own receptor membrane, and the other has a receptor cell that is SEPARATE from the afferent neuron that receives the stimulus first (then it send neurotransmitters to the afferent neuron next to it).
• Receptor potentials are graded potentials that are made in response to a stimulus. They can initial action potentials that then travel into the CNS (transductions, process of converting something or a message into another form).
• The harder the pain (such as a pinch), the more stimulus. The more stimulus, the more potential is generated.
• A stimulus is a form of energy, which then activates the receptor membranes to make receptor potentials.
• There are 5 types of sensory receptors: chemoreceptors, mechanoreceptors, photoreceptors, thermoreceptors, and nociceptors.
• Chemoreceptors are sensitive to chemical changes (such as oxygen levels, pH levels, and organic molecules like glucose).
• Mechanoreceptors are sensitive to mechanical (maybe even considered physical) changes (such as pressure (baroreceptors), cell stretch (osmoreceptors), vibration, acceleration, and sound).
• Photoreceptors are sensitive to changes in light (photons of light).
• Thermoreceptors are sensitive to changes in heat/temperature (such as temperature constantly changing, varying degrees of heat).
• Nociceptors are sensitive to stimuli (pain). This is what we usually use as an example when talking about neurons and transmitting signals.
• Adequate stimulus is the modality of stimulus to which a particular sensory receptor is most sensitive (from textbook e15). For example, when you touch a hot surface, the sensory receptor that will pick up that stimulus are thermoreceptors. Photoreceptors will not reaction to a hot surface. However, multiple sensory receptors can be stimulated (e.g., when you touch the hot surface for too long and now it hurts (nociceptors stimulated)).
• Basically, adequate stimulus is the form of energy that a receptor is most responsive to.
• There are four properties to all stimuli: modality, location, intensity, and duration.
• Modality is the specific type of stimulus (touch, temperature, light change, etc).
• The location of stimuli is identified by receptive fields. The larger the receptors in the field, the less it detects (less receptors because there are big ones). The smaller the receptors, the more densely packed the nerves are, so they can feel more and send more signals to the brain.
• Receptive fields are areas of the body that result in activity in that neuron when stimulated (from textbook e15). Basically, the field that a particular receptor is responding to.
• Receptive fields can overlap, so even though a stimulus is felt in one field, the adjacent fields can generate potential too.
• Lateral inhibition increases contrast between active receptive fields and inactive neighbors. It also increases the brain's ability to localize a sensory input. Basically, more action potential is felt where the most stimulus is felt. The neurons that feel the most stimulus release more neurotransmitters, while the neighboring neurons release a lower amount of neurotransmitters. So when action potential travels down a line of neurons towards the brain, only the exact location of the stimulus is known by the brain. Hence, exact localization occurs.
• Intensity of stimulus is coded by the number of receptors activated (population coding) and how often action potentials occur (frequency coding).
• Duration of stimulus coded by the duration of action potentials (the longer the stimuli (e.g., pinch), the more action potentials are fired). Some receptors can adapt or stop responding if the action potential has gone on for too long. The brain would rather prioritize other matters than worry about very long action potentials.
• The frequency of action potentials are proportional to stimulus intensity.
• The duration of a series of action potentials is proportional to stimulus duration.
• There are two types of adapting receptors: rapidly adapting phasic receptors and slowly adapting tonic receptors.
• Somatic receptors are initiated by multiple kinds of sensory receptors, and sensory receptors stimulate sensation in skin, bones, skeletal muscle, tendons, and joints.
• Somatic senses include touch, temperature, pain, itch, and proprioception.
• Special senses include: smell, hearing, vision, taste, and equilibrium.
• Meissner's corpuscle are rapidly adapting mechanoreceptors that are sensitive to touch and pressure.
• Merkel's corpuscle is a slowly adapting mechanoreceptors that is sensitive to touch and pressure (specifically texture and shape sensing).
• Free neuron endings are slowly adapting (they are parts of nociceptors, itch receptors, thermoreceptors, and mechanoreceptors).
• Pacinian corpuscles are rapidly adapting mechanoreceptors that are sensitive to vibration and deep pressure.
• Ruffini corpuscles are slowly adapting mechanoreceptors that are sensitive to skin stretch (e.g., stretching cheeks).