Sensory Systems

Main Categories of Sense Receptors: I. Five main categories based on the types of stimuli:
1. Exteroceptors - respond to external stimuli (e.g., pressure, temperature).
2. Interoceptors - respond to internal stimuli (e.g., blood pressure, blood glucose).
3. Proprioceptors - provide information about position and stretch in muscles and joints.
4. Mechanoreceptors - detect distortion of membrane (pressure, stretch, motion, sound).
5. Chemoreceptors - detect particular chemicals both internally and externally.

Sensation: Activation of sensory receptor cells at the level of the stimulus.
Transduction: Conversion of stimulus energy into changes in sensory receptor membrane potential.
Transmission: Leads to action potentials sent to the CNS for information integration.
Perception: Central processing of sensory stimuli into a meaningful pattern.

Sensory Neurons: Carry sensory information from receptors to the CNS.
Motor Neurons: Transmit responses from the CNS to effectors (muscles/glands).

Classification by Function:
- Nociceptors: Pain receptors that detect harmful stimuli (e.g., excess heat, pressure).
- Thermoreceptors: Respond to heat or cold and help regulate body temperature by signaling surface and core temperatures.
- Photoreceptors: Detect electromagnetic energy such as light (found in the eyes).
- Chemoreceptors: Detect chemicals (e.g., taste and smell).
- Mechanoreceptors: Respond to mechanical distortion (e.g., sound, pressure).
Classification by stimulus location:
- General sense - receptors located throughout body.
- Special sense - dedicated organs (e.g., eyes, nose).

Free Nerve Endings: Located in the dermis and other areas for pain/temperature.
Pacinian Corpuscles: A specialized mechanoreceptor in deep dermis for deep pressure and high-frequency vibrations.
Muscle Spindles: Proprioceptors that detect muscle stretch and inform CNS about muscle length.
Golgi Tendon Organs: Proprioceptors located in tendons that provide information about muscle tension.

Olfactory Receptors: Neurons within olfactory epithelium sensitive to odorant molecules.
Depolarization occurs when odorant molecules dissolve in the nasal mucous, stimulating olfactory hairs.
Action Potential leads through the ethmoid bone to synapse with olfactory bulb neurons, then to the cerebral cortex without a thalamic synapse.

Tongue: Contains papillae with taste buds sensitive to chemical stimuli from food/drink.
Gustatory Receptor Cells: Respond to dissolved chemicals, activating taste hairs, which leads to depolarization and action potential.
Taste Types: Sour, bitter, sweet, salty, umami.
Neural Pathway: Neurons of cranial nerves VII (facial), IX (glossopharyngeal), and X (vagus) relay taste sensations to the thalamus, reaching the primary gustatory cortex of the cerebrum.

Fibrous Tunic:
- Cornea: Main refractive structure.
- Sclera: Support, protection, movement.
Vascular Tunic:
- Choroid: Supplies blood, absorbs light.
- Ciliary Body: Produces aqueous humour, alters lens shape.
- Iris: Regulates light entry.
Retina: Includes pigmented and neural layers; contains photoreceptors (rods and cones).

Refraction occurs primarily at the cornea (75%) and also at the lens (25%).
Accommodation is the lens's ability to change shape for focusing on near versus distant objects.
Convergence of eyes and Constriction of pupils are also critical in forming a clear image.

Rods: Function in dim light; peripheral vision receptors; low acuity and no color vision.
Cones: Operate in bright light; provide high-acuity and color vision; three types (red, green, blue).
Distribution: Rod density is highest in periphery, while cone density peaks at the fovea.

Signal travels from photoreceptors to bipolar neurons, then to ganglion cells, then through optic nerve to optic chiasma, eventually reaching the visual cortex.
Object orientation is processed; images from left field of view project to the right side of the brain and vice versa for the right field of view.

The visual pathway consists of interconnected neural elements, from photoreceptors to the cortex, and involves significant processing and coordination to perceive visual stimuli accurately.