Vision and Hearing: A Biological Overview
Vision
Pupil Dilation
At night, the pupil dilates. During the day, the pupil contracts.
Insects can see ultraviolet light. Snakes can see infrared light.
Vision Impairment
Night blindness: Difficulty seeing at night due to a Vitamin A (retinal) deficiency.
Eye Structure and Function
Cornea: Allows light to enter the eye.
Anterior chamber (aqueous humor): Nourishes the cells of the cornea.
Pupil: The opening through which light enters and exits.
Iris: Contains two muscles that adjust the pupil’s size.
Lens: Focuses the light beam. Cataracts occur when the lens becomes opaque.
Vitreous humor: Maintains the eye’s shape and contains phagocytic cells for cleaning.
The fovea, a tiny dot at the back of the retina, is where the light beam is focused.
Myopia (nearsightedness): The eye is too long, causing the light beam to focus before reaching the fovea, resulting in difficulty seeing far away. Corrected with concave lenses.
Hyperopia (farsightedness): The eye is too short, causing the light beam to focus behind the fovea, resulting in difficulty seeing up close. Corrected with convex lenses.
Photoreceptor Cells
Cones: Short, activated by bright light, responsible for color vision, daytime vision, and high spatial and temporal resolution. There are three types of cones.
Rods: More abundant and longer, highly sensitive to light, activated by even a single photon, responsible for night vision, and have reduced spatial and temporal resolution. Rods perceive in black and white (monochromatic).
Rods contain the pigment rhodopsin.
Cones contain three pigments: red, green, and blue.
Types of Vision
Scotopic vision: Rod-mediated, highly sensitive.
Photopic vision: Cone-mediated.
The fovea contains more cones, while the periphery of the retina contains more rods.
The blind spot is an area on the retina with no photoreceptors, resulting in no vision in that area.
Phototransduction
Phototransduction: The process by which photoreceptors react to light.
Transduction: The conversion of a light signal into a chemical signal. The entire process is called phototransduction.
In darkness: Sodium (Na) channels are open, leading to depolarization. Sodium enters the cell, neurotransmitters are not released, and calcium channels remain closed.
In light: Sodium channels close, leading to hyperpolarization. Potassium (K) exits the cell, neurotransmitters are released, and calcium channels open.
Receptive Fields
Receptive fields: Areas of the retina containing photoreceptors that activate and stimulate bipolar neurons. The membrane potential of these neurons changes in response to light.
Image sharpness increases with the number of active receptors (cones and rods).
Receptive fields are smaller in the fovea (where cones are concentrated) and larger in areas with more rods.
Ganglion Cells
Magnocellular cells (10%): Perceive motion, receive information from rods, and have large receptive fields.
Parvocellular cells (90%): Perceive color and shape, receive information from cones, and have small receptive fields.
Lateral Geniculate Nucleus (LGN)
The LGN contains six layers of neurons that receive information from different parts of the retina.
Layers 1, 4, and 6 receive information from the left eye.
Layers 2, 3, and 5 receive information from the right eye.
Layers 1 and 2 receive information from magnocellular cells.
Layers 3, 4, 5, and 6 receive information from parvocellular cells.
Damage to the optic tract can result in vision loss in the outer part of one eye and the inner part of the other eye.
Damage to the optic nerve of one eye results in vision loss in that eye.
Damage to the optic chiasm or LGN can result in complete blindness.
Area V4 of the visual cortex is associated with shape and color perception. Damage to this area can result in synesthesia, a condition where stimulation of one sense triggers another sense (e.g., seeing colors when hearing numbers).
Light Activation Cascade
Light activates rhodopsin, which activates the G protein transducin. Transducin activates phosphodiesterase, which hydrolyzes cyclic GMP (cGMP), causing sodium channels to close and the membrane to hyperpolarize. Potassium channels open, potassium exits the cell, and glutamate release decreases. This activates bipolar neurons, which send information to magnocellular and parvocellular ganglion cells. These cells project their axons to form the optic nerve, which carries information to the optic chiasm, optic tract, and LGN. The LGN then sends information to the primary visual cortex.
Darkness Cascade
In darkness, rhodopsin is inactive, transducin is inactive, and phosphodiesterase is inactive. cGMP levels are high, sodium channels are open, the membrane is depolarized, calcium influx decreases, and glutamate release increases.
Hearing
Sound Properties
Frequency: Measured in Hertz (Hz), determines the pitch of a sound (high or low).
Amplitude: Measured in decibels (dB), determines the loudness of a sound. High amplitude corresponds to loud sounds, while low amplitude corresponds to soft sounds.
Timbre: The mixture of frequencies that gives a sound its unique quality. A simple wave has a single frequency, while a complex wave has multiple frequencies (e.g., the human voice).
Hearing Mechanism
Sound waves enter the outer ear canal, strike the tympanic membrane (eardrum), and cause it to vibrate. This vibration is transmitted to the ossicles (small bones in the middle ear), which then transmit the vibration to the oval window. The vibration is then transmitted through the fluid-filled cochlea to the organ of Corti, where hair cells are located. These hair cells send information to neurons, which transmit the information via the 8th cranial nerve (auditory nerve) to the brain.
Cochlea
The cochlea is filled with fluid and consists of three chambers: the scala vestibuli (contains perilymph, rich in sodium), the scala media (contains endolymph, rich in potassium), and the scala tympani (contains perilymph).
The organ of Corti is located on the basilar membrane of the scala media and contains the auditory sensory cells (hair cells).
Tonotopy refers to the spatial arrangement of hair cells within the cochlea, where different locations respond to different frequencies. Hair cells closer to the oval window perceive high-frequency sounds, while those further away perceive low-frequency sounds.
Hair Cells
Hair cells have stereocilia, which are small hair-like projections that move in response to sound vibrations. The movement of the stereocilia against the tectorial membrane triggers the opening of potassium channels.
When stereocilia move towards the kinocilium (the tallest stereocilium), potassium channels open, leading to depolarization, calcium influx, and increased neurotransmitter release.
When stereocilia move away from the kinocilium, potassium channels close, leading to hyperpolarization and decreased neurotransmitter release.
Vestibular System
The vestibular system, responsible for balance and spatial orientation, includes the semicircular canals, otolith organs (utricle and saccule).
Motor System
Movement Types
Voluntary movement: Controlled by the brain.
Involuntary movement: Controlled by the spinal cord (ventral horn).
Muscle Components
Satellite cells: Involved in muscle repair.
Extrafusal fibers: Responsible for muscle contraction.
Intrafusal fibers: Sensory fibers within the muscle.
Proprioceptors: Provide information about muscle position and movement.
Muscle Disorders
Lambert-Eaton myasthenic syndrome (LEMS): Calcium channels are affected, resulting in slower movements and reduced acetylcholine release.
Myasthenia gravis: Acetylcholine receptors are affected, resulting in reduced muscle contraction due to less depolarization. Both LEMS and myasthenia gravis can be treated with acetylcholine agonists.
Muscle Structure
Thick filaments are composed of myosin.
Thin filaments are composed of actin, troponin (binds calcium), and tropomyosin (covers binding sites on actin).
Muscle Types
There are three types of muscle: smooth, cardiac, and skeletal.
Muscular dystrophy: A group of genetic disorders that cause progressive muscle degeneration and weakness.
Muscle Fibers
Slow fibers (Type I): High endurance, aerobic metabolism.
Fast fibers (Type II): High power and speed, anaerobic metabolism. There are two subtypes: IIa and IIx.
Activation of a myosin gene can convert slow fibers into fast fibers.
The protein calcineurin can increase the number of slow fibers.
Muscle Growth
Hypertrophy: Increase in muscle size.
Hyperplasia: Increase in the number of muscle cells.