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.