Visual System in Fish, Amphibians and Reptiles

Sensory Reception and Transduction

• Human sensory capabilities are only average, with limits in sensory reception
• Examples of superior sensory capabilities in other animals:
  • Elephants can hear and produce sounds less than 20 Hz and more than 120,000 Hz
  • Bats can hear and produce ultrasonic sounds
  • Dogs can detect odors and hear ultrasonic sounds that humans cannot
• Human superiority lies in color vision

Energy Transduction

• Definition: the process of converting energy received by a receptor into action potentials
• Example: mechanical energy (touch) converted into electrical energy (action potential)

Touch Receptors

• Displacement of a single hair results in an action potential interpreted as touch
• Dendrite of a sensory neuron wrapped around the base of the hair is stretched, triggering an action potential
• Stretch-sensitive Na+ channels open, allowing an influx of Na+ ions, which can lead to an action potential

Other Modalities

• Hearing and balance receptors also have hairs that, when displaced, activate stretch-sensitive channels
• Visual receptors activated when light particles strike chemicals in the retina
• Olfactory receptors activated when odor molecules bind to specific receptors
• Pain receptors activated when tissue is damaged, releasing chemicals that activate ion channels

Receptive Fields

• Definition: a specific part of the world that a receptor organ or cell responds to
• Example: the part of the world that the eyes see at a given moment
• Receptive fields can change with movement or stimulation

Detection and Localization of Sensation

• Receptors detect the presence and duration of stimulation
• Rapidly adapting receptors respond to presence, slowly adapting receptors respond to duration
• Localization of stimuli through overlapping receptive fields of individual receptors

Sensory Systems and the Brain

• Each sensory system is connected to the cortex through a sequence of three to four intervening neurons
• There is no straight-through point-to-point correspondence between one relay and the next
• Different stages in the relay allow the sensory system to mediate different responses

The Auditory System

• The outer ear collects sound waves and directs them into the external ear canal
• The middle ear contains the eardrum, which vibrates and transfers the vibrations to the ossicles
• The inner ear contains the cochlea, which converts sound waves into action potentials

Transduction of Sound

• Sound waves cause the basilar membrane to bend, stimulating hair cells to produce action potentials
• Frequency of sound is transduced by the longitudinal structure of the basilar membrane

Auditory Pathways

• Axons of the hair cells form the auditory nerve, which projects to the cochlear nucleus
• The auditory pathway includes the ventral and dorsal cochlear nuclei, the lateral lemniscus, and the inferior colliculus
• The pathway terminates in the ventral medial geniculate body and Area 41 of the cortex### Body Sense and Submodalities
• The body has cells that sense blood flow, air inflation, and fullness of organs.
• Hearing is based on cells in the inner ear detecting sound waves.
• There are distinct submodalities:
  • Nocioception: perception of pain and temperature.
  • Hapsis: perception of objects using fine touch and pressure receptors.
  • Proprioception: perception of body awareness and balance.

Receptors and Pathways

• Nocioceptive receptors:
  • Damage to the dendrite or surrounding cells releases chemicals that stimulate the dendrite to produce action potentials.
• Haptic receptors:
  • Pressure on tissue capsules mechanically stimulates the dendrites to produce action potentials.
• Proprioception receptors:
  • Movements stretch the receptors to mechanically stimulate the dendrites to produce an action potential.
• Somatosensory pathways:
  • Two major pathways extend from the spinal cord to the brain: touch and proprioception, and pain and temperature.
  • Neurons for touch and proprioception:
    • Fibers are large and heavily myelinated.
    • Cell bodies are located in the dorsal root ganglia.
    • Dendrites project to sensory receptors in the body.
    • Axons project into the spinal cord.
  • Pathways for touch and proprioception:
    • Axons ascend through the dorsal columns to synapse in the dorsal column nuclei.
    • Then, they cross over to the contralateral medial lemniscus and ascend to synapse in the ventrobasal thalamus.
    • Finally, they project to the primary somatosensory region and primary motor area.
• Neurons for pain and temperature:
  • Fibers are smaller and less myelinated than those for touch and proprioception.
  • They follow the same course as touch and proprioception fibers to enter the spinal cord.
  • They project to neurons in the central region of the spinal cord, the substantia gelatinosa.
  • Then, they cross over to the other side of the cord, form the ventral spinothalamic tract, and terminate in the ventrobasal thalamus.

Taste and Smell

• Taste receptors:
  • Five different types, each responding to a different chemical component of food.
  • The specificity of a given taste receptor is not absolute.
  • Single fibers can respond to a variety of chemical stimuli.
• Anatomy of a taste bud:
  • Individual differences in taste preferences both within and between species.
  • Taste preference relates to the state of the organism.
  • Children's sense of taste is stronger.
• Olfactory receptors:
  • The receptor surface is the olfactory epithelium in the nasal cavity.
  • Composed of three cell types: receptor cells, supporting cells, and basal cells.
  • Axons projecting from the olfactory receptors relay onto the glomeruli in the olfactory bulb.
  • From the glomeruli, mitral cells form the olfactory tract.
• Olfactory epithelium:
  • The outer surface is covered by a layer of mucus.
  • Odors must pass through the mucus to reach the receptors.
  • Changes in the properties of the mucus can influence how easily an odor can be detected.
• Interspecies differences in the size of the olfactory epithelium:
  • Relates to sensitivity to odors.
• Mechanisms of smell discrimination:
  • The large family of genes gives rise to an equivalent number of odorant receptor types.
  • Each receptor is located on one receptor cell and is sensitive to only a few odors.
  • Receptors of like types project to one of the glomeruli.
  • The pattern of activation produced in the glomeruli cells allows us to distinguish as many as 10,000 odors.

Cortical Processing

• The chemical senses, like all the other senses, employ dual pathways to primary and secondary areas of the cortex.
• Gustatory pathways:
  • Three cranial nerves carry information from the tongue: Glossopharyngeal, Vagus, and the chorda tympani branch of the Facial nerve.
  • All three nerves enter the solitary tract, which forms the main gustatory nerve.
  • At that point, the pathway divides into two:
    • One route to the ventroposterior medial nucleus of the thalamus.
    • The second route leads to the pontine taste area, which then projects to the lateral hypothalamus and amygdala.
• Gustatory cortex:
  • SI region is sensitive to tactile stimuli and is probably responsible for the localization of tastes on the tongue.
  • The insular cortex is dedicated entirely to taste but is not responsive to tactile stimulation.
  • Insular cortex and SI project to the orbital frontal cortex, which may be a secondary taste area.
• Olfactory pathways:
  • Axons of the olfactory receptor cells synapse in the olfactory bulb.
  • The major output of the bulb is the lateral olfactory tract, which passes ipsilaterally to the pyriform cortex, the amygdala, and the entorhinal cortex.
  • Pyriform cortex goes to the central part of the dorsal medial nucleus of the thalamus.
  • Thalamus to the orbital frontal cortex, which can be considered the primary olfactory neocortex.
• Role of right orbital frontal cortex for taste and smell:
  • Plays a special role in the perception of odors and taste.
• Neuroimaging of olfactory and gustatory centers:
  • The orbital frontal cortex is activated by both taste and smell stimuli.
  • The right hemisphere response is stronger and more localized.

Integration and Organization of Sensory Information

• The role of secondary and tertiary association areas:
  • The information is integrated and organized in a meaningful manner.
  • Association of information within and among sensory submodalities:
    • Discrete stimuli experienced repeatedly become associated in the temporal and simultaneous patterns in which they are presented.
    • The association of discrete stimuli confers meaning upon the experience.
  • Hebbian learning:
    • The strengths of the connections between two neurons can alter with time.
    • The idea was expanded later to include the strength of the synapse weakening if one neuron was active and the other was not.
• Cortical layers and association areas:
  • The type of associations are molded by the patterns of sensory input to which one is exposed.
  • Soft-wired and malleable versus the hard wiring of the primary regions.
• Hebb's cell assemblies:
  • The connections between large groups of neurons would alter themselves thanks to Hebbian learning.
  • Loops would form, and the synapses linking these neurons would all strengthen and increase the possibility of the whole thing happening again in the future.

This quiz covers the tectopulvinar pathway, its route to various brain regions, and its role in the visual system of fish, amphibians, and reptiles. It also compares it to the geniculostriate pathway. Test your knowledge of the neural pathways involved in vision!