Chapter 10: Sensory Physiology

Adequate Stimulus

Concept of sensory receptors having a stimulus that it is most sensitive to (eyes → Light, Olfactory Cells → Odorants) but can still be activated by other stimuli.

Stimulus Threshold

Minimum stimulus required to activate a receptor.

Receptor Potential

A graded potential that enacts changes in sensory receptor membrane potential.

Receptive Field

The physical space that a sensory receptor will activate from a stimulus within. Frequently overlap with neighboring fields and can be referred to as primary or secondary depending on if they immediately receive the stimulus or are the first neurons to receive a synaptic signal to sum the stimuli.

Primary Sensory Neuron

The presynaptic neuron that activates from a sensory stimulus and sends the signal to the secondary sensory neuron (CNS). Known as first-order neurons.

Secondary Sensory Neuron

Neuron that receives a synapse signal from a primary sensory neuron, part of the CNS. Known as second-order neurons.

Convergence

When multiple first-order neurons send an input signal to a smaller amount of second-order neurons to better sum the stimulus. Can apply to receptive fields as well.

Two-Point Discrimination Test

Tests the area of a secondary receptive field by stimulating the receptive fields of neurons that signal to it. Example: Placing 2 needles X amount of mm away from each other on someone’s skin will feel like they are being poked in the same place.

Perceptual Threshold

The level of stimulus intensity necessary for you to be aware of a particular sensation.

Habituation

Decreased perception of a stimulus. Accomplished through inhibitory modulation which diminishes the stimulus above threshold until it is below.

Olfactory Sense

The only sense that doesn’t get processed/modified by the thalamus before going to the brain for integrating. May be the basis for why scents are so strongly linked to memories.

The CNS must distinguish four properties of a stimulus:

1) Modality

2) Location

3) Intensity

4) Duration

Sensory Modality

The modality of a stimulus is indicated by which sensory neurons are activated and by where the pathways of the activated neurons terminate in the brain.

Labeled-Line Coding

Association the brain makes with certain sensory receptors and their usual modality (nature). Means that a stimulus that activates a certain sensory receptor will result in the same perception regardless of what type of stimulus it was. Also relates to location of receptors.

Lateral Inhibition

When a receptive field is activated closest to the stimulus it may inhibit those near it to contrast the stimulus to one location (Example: Fields being activated radially away from a pin prick will be inhibited against the field closest to the pin).

Population Coding

The way multiple receptors function together to send the CNS more information than would be possible from a single receptor.

How is the intensity of a stimulus calculated?

1) Number of receptors activated (population coding).

2) Frequency Coding: Frequency of action potentials coming from activated receptors.

How is the duration of a stimulus calculated?

By the duration of action potentials in the sensory neuron. If stimuli persist, then some receptors adapt and cease to activate.

Types of Receptor Adaptation

Tonic Receptors: Fire rapidly at first then become slower and maintain action potential frequency.

Phasic Receptors: Fire very rapidly at first then cease to respond completely as stimulus persists. Good for measuring changes in a parameter.

Tertiary Sensory Neurons

3rd order neurons that receive their signal from secondary neurons after crossing the midline of the body, sending their signal to the somatosensory region of the cerebral cortex.

Somatosensory Cortex

Region of the brain that recognizes where ascending sensory tracts originate from.

Pacinian Corpuscles

Large phasic receptors that sense vibration. Respond best to high-frequency vibrations, whose energy is transferred through the connective tissue capsule to the nerve ending, where the energy opens mechanically gated ion channels.

Free Nerve Ending

Receptors that operate on the simplest system of receiving and parsing stimuli. They terminate immediately in the subcutaneous layer of the skin (Example: Skin temperature receptors). Operate within 20-40 degrees F, higher or lower values will initiate pain receptors as well.

Nociceptors

Free nerve ending receptors that respond to noxious stimuli, found in the skin, joints, muscles, bones and various internal organs, but not in the central nervous system. Can respond as either fast pain (A-Delta Fiber) or slow pain/itch (C fiber) depending on the stimulus.

Different Types of Pain

Fast Pain: Sharp and localized, is rapidly transmitted to the CNS by the small, myelinated Ad fibers (Examples: Cold, mechanical stimuli)

Slow Pain: Duller and more diffuse, is carried on small, unmyelinated C fibers (Examples: heat, cold, mechanical stimuli)

Inflammatory Pain

Increased sensitivity to pain at sites of tissue damage.

Substance P

Peptide released by sensory neurons at sites of tissue damage.

2 Pathways of Nociception:

(1) Reflexive protective responses that are integrated at the level of the spinal cord (Example: withdrawal reflex).

(2) Ascending pathways to the cerebral cortex that become conscious sensation (pain or itch).

Referred Pain

Concept of pain in internal organs (visceral pain) being felt in other parts of the body instead (Example: Ischemia in the heart tissue being felt on the left shoulder/arm). Occurs because visceral and somatic sensory pain inputs converge on a single ascending tract.

Gate Control Theory of Pain

Ab fibers carrying sensory information about mechanical stimuli help block pain transmission. The Ab fibers synapse on the inhibitory interneurons and enhance the interneuron’s inhibitory activity. If simultaneous stimuli reach the inhibitory neuron from the Ab and C fibers, the integrated response is partial inhibition of the ascending pain pathway so that pain perceived by the brain is lessened.

Example: Rubbing a sore muscle to alleviate pain.

Olfactory Bulb

The extension of the forebrain that receives input from the primary olfactory neurons.

How is sound transmitted through the ear?

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Function of the Cochlea

Depending on the frequency of the incoming sound wave, the displacement of the wave in the basilar membrane determines how the sound is perceived. Higher frequency waves displace closer to the oval window and lower frequency waves displace further down the membrane near the flexible distal end.

Anatomy of the Eye

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Cornea

A transparent disk of tissue that is a continuation of the sclera. Exterior light enters this as the first step of the visual pathway.

Steps of the Visual Pathway

1) External light hits the cornea.

2) This light is modified by the size of the pupil and the shape of the lens. The pupil forms depth of field.

3)The lens and cornea together bend incoming light rays so they focus on the retina, the light-sensitive lining of the eye that contains the photoreceptors.

4) Phototransduction occurs in the rods and cones of the retina, light converts into electrical signals.

5) The electrical signals are processed through bipolar cells before converging on retinal ganglion cells (RGCs).

6) RGC axons form the retinal nerve fiber layer and exit the globe at the optic disc, where photoreceptors are absent, creating the physiologic blind spot.

7) The signals travel through the optic nerve through the optic chiasm, where neve fibers cross. Allows for consensual reflex.

8) The optic nerves travel through the Lateral geniculate nucleus of the thalamus ,a gray matter relay that preserves eye-specific and functional segregation.

9) The vision neurons terminate in the visual cortex of the occipital lobe.

https://www.ncbi.nlm.nih.gov/books/NBK553189/

Types and Functions of Photoreceptors:

Rods: Function well in low light and are used in night vision, when objects are seen in black and white rather than in color.

Cones: Responsible for high-acuity vision and color vision during the daytime, when light levels are higher.

Fovea

Along with the macula, forms the center of the field of vision. Absent of neurons and blood vessels, it receives light directly. Cones are abundant here.