4.2 Vision and auditory systems

Vision and hearing are crucial sensory systems that allow us to perceive and interact with our environment. The visual system processes light through specialized cells in the retina, transmitting information to the brain via the optic nerve.

The auditory system converts sound waves into electrical signals in the cochlea. These signals travel through the auditory nerve to the brain, where they're processed to create our perception of sound. Both systems work together to help us navigate the world.

Visual System

Photoreceptors and Retina

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• Photoreceptors are specialized cells in the retina that detect light and convert it into electrical signals
  • Rods are responsible for low-light and peripheral vision, enabling us to see in dim conditions and detect motion
  • Cones are responsible for color vision and high-acuity central vision, allowing us to distinguish different hues and see fine details
• The retina is a light-sensitive layer at the back of the eye that contains photoreceptors and other neural cells
  • Photoreceptors synapse with bipolar cells, which then synapse with ganglion cells to transmit visual information to the brain
  • The fovea is a small region in the center of the retina with a high density of cones, providing the sharpest vision (macula)

Optic Nerve and Visual Cortex

• The optic nerve is a bundle of nerve fibers that carries visual information from the retina to the brain
  • Ganglion cell axons converge at the optic disc to form the optic nerve
  • The optic chiasm is where the optic nerves from both eyes partially cross, allowing for binocular vision and depth perception
• The visual cortex is the primary area of the brain responsible for processing visual information
  • Located in the occipital lobe, it receives input from the lateral geniculate nucleus of the thalamus
  • Different regions of the visual cortex are specialized for processing specific aspects of visual information (color, motion, form)
  • Higher-order visual areas integrate information to create a coherent visual perception (ventral and dorsal streams)

Auditory System

Cochlea and Hair Cells

• The cochlea is a spiral-shaped structure in the inner ear that converts sound waves into electrical signals
  • Sound waves cause vibrations in the fluid-filled cochlea, which stimulate hair cells along the basilar membrane
  • Hair cells are specialized sensory cells with stereocilia that bend in response to fluid movement, opening ion channels and generating electrical signals
• The basilar membrane is a flexible structure in the cochlea that vibrates at different frequencies along its length
  • High-frequency sounds cause vibrations at the base of the cochlea, while low-frequency sounds cause vibrations at the apex (tonotopic organization)

Auditory Ossicles and Cortex

• The auditory ossicles are three small bones in the middle ear (malleus, incus, and stapes) that transmit sound vibrations from the eardrum to the cochlea
  • The ossicles act as a mechanical amplifier, increasing the efficiency of sound transmission and protecting the inner ear from loud sounds
• The auditory cortex is the primary area of the brain responsible for processing auditory information
  • Located in the temporal lobe, it receives input from the medial geniculate nucleus of the thalamus
  • Different regions of the auditory cortex are specialized for processing specific aspects of sound (pitch, timbre, location)
• Sound localization is the ability to determine the direction and distance of a sound source
  • The brain uses differences in the timing and intensity of sounds reaching each ear to calculate the location of the source (binaural cues)
  • The shape of the outer ear (pinna) also helps to filter sounds and provide additional localization cues (monaural cues)

Balance and Orientation

Vestibular System

• The vestibular system is responsible for maintaining balance, spatial orientation, and coordinating eye movements
  • Located in the inner ear, it consists of the semicircular canals and otolith organs (utricle and saccule)
• The semicircular canals detect rotational movements of the head
  • Three fluid-filled canals arranged at right angles to each other sense rotation in different planes
  • Hair cells in the ampullae of the canals bend in response to fluid movement, generating electrical signals
• The otolith organs detect linear accelerations and head tilt
  • The utricle senses horizontal movements, while the saccule senses vertical movements
  • Hair cells embedded in a gelatinous matrix with calcium carbonate crystals (otoconia) bend in response to gravity and linear acceleration
• Vestibular information is integrated with visual and proprioceptive inputs in the brain to maintain balance and coordinate movements
  • The vestibular nuclei in the brainstem process vestibular signals and project to the cerebellum, spinal cord, and cortex
  • The vestibulo-ocular reflex (VOR) stabilizes gaze during head movements by generating compensatory eye movements in the opposite direction