Visual Systems and Photoreceptors

Lecture 4: Special Senses

• This lecture covers the special senses, including the visual, chemical (taste and smell), auditory, and vestibular systems. It draws on Sherwood Chapter 5 for detailed information.

Objectives

• The lecture will cover the visual system, including the eye itself and its accessory structures.
• It will look at chemical senses (taste and smell).
• The auditory system will be explored.
• Finally, the vestibular system will be discussed.

Visual System: The Eye

• 70% of all sensory receptors are located in the eye.
• The eye is largely sheltered by fat and bony orbit.
• Accessory structures include:
  • Eyebrows: shade the eye and prevent perspiration.
  • Eyelids: protect the eye, and have glands that secrete oil.
  • Eye lashes: initiate reflexive blinking.
  • Eye muscles: control eye movement.

Visual System: Extraocular Muscles

• Lateral Rectus Muscle: abducts the eye.
• Medial Rectus Muscle: adducts the eye.
• Superior Rectus Muscle: moves the eye upward.
• Inferior Rectus Muscle: moves the eye downward.
• Superior Oblique Muscle: moves the eye downward.
• Inferior Oblique Muscle: moves the eye upward.

Visual System: Structures of the Eye

• Cornea: transparent, protective covering of the iris and pupil.
• Sclera: protective, white, connective tissue covering.
• Conjunctiva: epithelial layer connecting the sclera to the eyelids.
• Lacrimal glands: secrete aqueous tear film which lubricates the eye.

Visual System: Structures of the Eye

• Iris: the colored part of the eyeball, regulates the amount of light entering the eye.
• Pupil: central opening of the iris. -Pupil constrict in bright light and close vision -Pupils dilate in dim light and during emotional states
• Lens: clear, flexible tissue responsible for focusing.
• Retina: light-sensing tissue at the back of the eye.
  • A delicate, two-layered membrane with photoreceptors

Visual System: Retinal Layers

• Pigmented Layer: absorbs stray light rays.
• Neural Layer: processes visual data.

Visual System: Cells of the Retina

• Photoreceptors:
  • Rods: numerous (~120 million), used for dim light and peripheral vision, do not detect colour, see grayscale.
  • Cones: less numerous (~6 million), detect colour in bright light.
• Ganglion Cells: receive visual information from photoreceptors and send signals to the optic nerve, ON and OFF responses associated with light/dark.
• Bipolar Cells: receive signals from photoreceptors and transmit signals to ganglion cells.
• Horizontal Cells: modulate signaling between photoreceptors and bipolar cells.
• Amacrine Cells: modulate signaling between ganglion and bipolar cells.

Visual System: Cells of the Retina (more detailed)

• Outer Segment: detects light, contains photopigments.
• Inner Segment: metabolic area of the receptor.
• Synaptic Terminal: transmits signals to bipolar cells.

Visual System: Light Sensitive Photopigment

• Photopigments are present in the rods and cones.
• Rhodopsin (rods): absorbs all wavelengths of light.
• Photopsin (cones): three types (red, green, blue) for colour vision.

Visual System: Dark Vision (Rods)

• 11-cis-retinal in the dark.
• All-trans-retinal in the light.
• Light stimulus causes retinal to change shape.
• That starts the conversion of energy (photon) into a nerve impulse
• Conversion of 11-cis-retinal to all-trans-retinal in response to light initiates the process of transduction.
• The conversion of 11-cis-retinal to all-trans-retinal in response to light initiates the process of transduction.

Visual System: Colour Vision (Cones)

• Three types of photoreceptors:
• Photopsin I (red)
• Photopsin II (green)
• Photopsin III (blue)
• Colour vision is the result of combinations of these cone stimulations
• Colour blindness is related to lack of one or more cone types.

Chemical Senses (Taste and Smell):

• Their chemoreceptors respond to chemicals dissolved in solutions.
  • Taste – dissolved in Saliva
  • Smell – dissolved in nasal fluids
  • 5 basic tastes: Sweet, Sour, Salty, Bitter and Umami
• Taste and smell are closely related; smell contributes substantially to taste experience.

Taste Receptors (Gustation)

• Located along edges of small, depressed areas called fissures (on the tongue)
• Stimulated when substance is dissolved in solution
• Taste receptors are often G-protein coupled receptors.
• Taste is sent to the brain via CN VII (facial nerve) and CN IX.

Smell Receptors (Olfaction)

• Located in the olfactory epithelium (upper part of nasal cavity)
• Chemicals detected must be dissolved in solution
• Must "sniff" to bring odors upward.

Hearing:

• The ear (inner, outer, and middle ear) contains hearing and balance receptors.
• The outer and middle ear are involved in hearing only.
• Receptors for hearing and balance respond to different stimuli independently.

The Ear (overall summary of structures):

• Houses hearing and equilibrium
• The three parts of the ear: external (outer), middle and inner ear; and receptors for hearing and balance.
• Hearing and balance receptors respond to separate stimuli.
• They are activated independently

Anatomy of the Ear

• External ear (outer): pinna and external auditory meatus, lined with ceruminous glands that secrete cerumen
• Middle ear (tympanic membrane): sound waves cause vibration that moves the auditory ossicles
• Inner ear (bony labyrinth): vestibule, semicircular canals and cochlea,
• These structures have hair cells -In the inner ear, hair cells are mechanoreceptors.

Internal Ear

• Comprised of bony chambers in the temporal bone that house parts for hearing and balance -The vestibule and semicircular canals are used for balance
  • The cochlea contains hair cells, mechanoreceptors for hearing

The Cochlea (of the Inner Ear)

• Contains the spiral organ of Corti.
• The spiral organ of Corti contains thousands of hair cells.
• These hair cells are receptors for hearing.

Organ of Corti (detailed look at the cochlea)

• Contains hair cells that are the hearing receptors.
• Sound waves, travelling through the ear, causes vibration in fluids of the cochlea.
• These vibrations stimulate hair cells which then transmit signals through the cochlear nerve and onto the auditory cortex.

Sound Transduction:

• Sound waves vibrate the eardrum.
• These vibrations are transmitted across the middle ear and vibrate the oval window (of inner ear).
• Vibrations cause fluid movement in the cochlea.
• This movement causes bending of hairs on inner hair cells located on the basilar membrane which leads to transduction of vibrations into electrical signals to be sent further along.

Auditory Pathway

• The auditory pathway includes the cochlea, vestibulocochlear nerve, medulla, midbrain and the medial geniculate body of the thalamus and the primary auditory cortex.
• Vibrations are transduced and carried to the auditory cortex in the temporal lobe.

Deafness

• Sensorineural deafness: issues with hair cells, cochlear nerve or neurons.
• Could be caused by prolonged drug exposure, loud noise.
• Conduction deafness: sound vibrations cannot reach inner ear due to buildup of earwax or injury to eardrum.

The Vestibular System

• Responds to head movements.
• Equilibrium receptors in the inner ear.
  • Static equilibrium - Vestibule (responds to head position relative to gravity)
  • Dynamic equilibrium - Semicircular canals (responds to head position/movement relative to body motion)

Anatomy of the Vestibular Apparatus

• Consists of the bony and membranous labyrinths. -Bony labyrinth – filled with perilymph, -Membranous labyrinth - suspended within the perilymph and filled with endolymph.

Static Equilibrium

• Macula: patches of hair cells embedded in otolithic membrane w/ tiny stones (otoliths)
• Otoliths' movement in response to head position pulls on the gel, bending hair cells, sending impulses to the cerebellum.

Dynamic Equilibrium

• Cristae ampullaris: Hair cells in semicircular canals covered by the cupula.
• When the head moves, endolymph lags behind causing the cupula to deflect, stimulating hair cells, sending vestibular nerve impulses to cerebellum.

Balance and Orientation Pathways

• Three input modes: vestibular receptors, visual receptors, and somatic receptors.
• These receptors allow the body to respond reflexively to maintain balance and orientation.

Input and output of the Vestibular Nuclei

• Visual, cutaneous, and proprioceptive inputs are used in coordination

Homeostatic Imbalance: Motion Sickness

• Sensory mismatch between vestibular and visual inputs from movement results in motion sickness

Lecture 4 Pop Quiz Questions

• These are questions for a quiz. The questions are about the different special senses. -Function of middle and inner ear -Process of transmission of impulses to auditory cortex -Difficulties differentiating between conduction and sensorineural deafness -Static and dynamic equilibrium differences -Description of how dynamic equilibrium works -Case study examining vertigo, semicircular canals and how the brain processes conflicting impulses to generate motion sickness

Explore the intricate details of photoreceptors and their role in the visual system through this quiz. Dive into topics such as the effect of light on photoreceptors, the function of glutamate in bipolar cells, and the crossing of visual information. Test your understanding of the visual pathway and its components.