Lecture Notes on Sensory Receptors and Perception

Receptive field: The area innervated by a nerve ending, rather than pinpoint accuracy.
Touch is the easiest sense to understand in terms of receptive fields.
Sensitivity varies across the body. Areas like fingertips have many nerve endings for fine touch.
Fingertips:
- Small receptive fields.
- High density of nerve endings.
- Greater sensitivity.
Arms:
- Larger receptive fields.
- Lower density of nerve endings.
- Less precise localization of touch.
- Example: Knowing an arm is touched, but difficulty pinpointing the exact location.
- Mosquito bites illustrate this, where the itchy area may be larger than the bite itself due to larger receptive fields.

Some areas require more sensitivity for specific functions.
- Hands vs. elbow.
Why not have everywhere be sensitive?
- Sensory overload.
- Too many signals hitting at once.
- Difficulty filtering information.
- Higher energy cost.
  - More nerves require more energy for function and processing.
Small receptive fields:
- Allow for more precision.
- Increase energy cost due to the need for more nerves.
The body balances energy expenditure and sensitivity.
Numerous small receptive field nerve endings are used where needed (e.g., fingertips).
Large receptive fields are used where less precision is needed (e.g., armpits).
- Armpits being ticklish is related to large receptive fields.

Adaptation: Reduction in sensitivity to a continually applied stimulus.
- Examples: Not constantly feeling clothing or jewelry, forgetting you're wearing glasses.
Nerves get tired and ignore continuous stimuli.
Nerves reset themselves to treat the current condition as the new norm.
Pain adaptation:
- Tooth pain may eventually fade, but it doesn't mean it's healed.
- Nerves get used to the pain.
- Nurses manage pain medication with consideration of adaptation.
- Prioritize other care methods before increasing pain meds.

General senses: Somatic and visceral receptors.
- Tactile (touch).
- Proprioceptors (joint position).
- Chemical receptors.
- Temperature receptors.
Somatic vs. Visceral:
- Somatic: Consciously controlled.
  - Example: skeletal muscle
- Visceral: Organs.
Tactile: Touch.
Chemical: Flavors, reactions to proteins and lipids.
Proprioceptors: Body's ability to sense its position in space.
- Not equilibrium (balance).
- Sense limb position (e.g., knowing if fingers are raised or lowered without looking).
Proprioceptors detect body and limb movements, skeletal muscle contraction, stretch, and joint pressure.

Exteroreceptors, interoreceptors (more direct terminology).
Five types of receptors:
- Chemoreceptors.
- Thermoreceptors.
- Photoreceptors.
- Mechanoreceptors.
- Nociceptors.
Thermoreceptors: Respond to changes in temperature, like a thermostat, detecting hot and cold.
Nociceptors: Detect pain. Some individuals lack functional nociceptors.
- They must be cautious to avoid injuries because they won't feel pain.
Mechanoreceptors: Detect distortion of cells.
- Three types:
  - Baroreceptors.
  - Proprioceptors.
  - Tactile receptors.
Baroreceptors: Detect changes in pressure caused by stretch or distension.
- Especially in the heart (blood pressure) and lungs (breathing).
Photoreceptors: Detect changes in color, intensity, and movement of light.
- Located in the retina of the eyes.

Referred pain: Inaccurate localization of sensory signals.
- Pain felt in a location other than the actual source.
Mechanism:
- Cutaneous and visceral sensory neurons conduct signals on the same ascending tracts of the spinal cord.
- The brain falsely localizes the pain stimulus.
- Decussation occurs in the medulla before the foramen magnum.
Example:
- Person having a heart attack may feel pain along their left arm.

Some odors cause visceral reactions (e.g., gagging from rotten food).
Olfactory pathway: Olfactory tract projects directly to the primary olfactory cortex, hypothalamus, amygdala, and other regions.
Gag reflex is a result of the hypothalamus being shared.
The body is designed with the receptor to be in line with the receptors that would make things come back out.

Eyebrows serve two purposes:
- Communication.
- Prevent sweat from dripping into the eyes.
Eye Structure:
- Divided by the lens into two components.
  - Anterior chamber has aqueous humor.
  - Posterior chamber has viterous humor.
Iris:
- Functions as a diaphragm to control the size of the pupil.
Pupil is the hole through which light enters the eye.
The colored part of the eye.
Layers of the eye (from outermost to innermost):
- Sclera.
- Choroid (contains veins and arteries).
- Retina (inner layer with rods and cones).
Optic nerve (cranial nerve II):
- Enters the back of the eye.
- Causes a blind spot due to the absence of retina at that location.
Fovea centralis: The focal point for light entering the eye.
- Area with the highest concentration of rods and cones for clearest vision.

Astigmatism: Unequal focusing due to the eye losing its round shape, becoming more football-shaped.
- Causes skewed images.
Presbyopia: Age-related loss of lens elasticity (senior eyes).
- Difficulty seeing close-up.
- Lenses of their eye are no longer round because their ligaments slacken.
Nurses should refer patients with visual impairments to specialists but explain normal age-related changes.
Eye exercises can strengthen eye muscles and improve eyesight affected by age-related issues.

Rods and cones: Photoreceptors in the eyes.
- Named for their shapes under a microscope.
Rods:
- Sensitive to light.
- See in black and white.
Cones:
- See color and detail.
- Fewer in number than rods.
Fovea centralis has the highest concentration of cones.
Rods function in dim light.
The retina provides vitamin A, which is required for pigmentation of the retinal layer and improves eyesight.

Optic chiasm: Crossing point of optic nerves.
- Located underneath the brain.
- Axons from the medial region of each retina cross to the opposite side of the brain.
- Lateral sides of the nerves for all your rods and cones actually stay on the same side
Two eyes enable stereoscopic vision (depth perception).
- The brain combines two images into one with depth.
- One-eyed vision has limited depth perception.
Nerve Pathways:
- Only the medial side crosses over at the optic chiasm, and the lateral sides stay on the same sides
  *The reason you do that is because when you look at stuff, you actually only see one picture.

Ear also contains:
- malleus bones and the eardrum
- tubes with fluid and hairs inside to detect fluid movement and maintain balance.
Fluid Movement:
- Fluid movement that isn't correct will affect ability of the body to orient itself. Movements and medicines can affect the hairs in your tubes that make you lose your bowels.
Pitch: Measured in hertz (Hz), indicates how high or low a note is.
Decibels (dB): Measure of loudness, on a scale of tens.
- Frequency determines pitch.
- Amplitude determines loudness.