The senses are olfaction (smell), gustation (taste), somatosensation (sensations associated with the skin and body), audition (hearing), equilibrium (balance), and vision. With the exception of somatosensation, this list represents the special senses, or those systems of the body that are associated with specific organs such as the tongue or eye. Somatosensation belongs to the general senses, which are those sensory structures that are distributed throughout the body and in the walls of various organs. The special senses are all primarily part of the somatic nervous system in that they are consciously perceived through cerebral processes, though some special senses contribute to autonomic function. The general senses can be divided into somatosensation, which is commonly considered touch, but includes tactile, pressure, vibration, temperature, and pain perception. The general senses also include the visceral senses, which are separate from the somatic nervous system function in that they do not normally rise to the level of conscious perception.
The cells that transduce sensory stimuli into the electrochemical signals of the nervous system are classified on the basis of structural or functional aspects of the cells. The structural classifications are either based on the anatomy of the cell that is interacting with the stimulus (free nerve endings, encapsulated endings, or specialized receptor cell), or where the cell is located relative to the stimulus (interoceptor, exteroceptor, proprioceptor). Thirdly, the functional classification is based on how the cell transduces the stimulus into a neural signal. Chemoreceptors respond to chemical stimuli and are the basis for olfaction and gustation. Related to chemoreceptors are osmoreceptors and nociceptors for fluid balance and pain reception, respectively. Mechanoreceptors respond to mechanical stimuli and are the basis for most aspects of somatosensation, as well as being the basis of audition and equilibrium in the inner ear. Thermoreceptors are sensitive to temperature changes, and photoreceptors are sensitive to light energy.
The nerves that convey sensory information from the periphery to the CNS are either spinal nerves, connected to the spinal cord, or cranial nerves, connected to the brain. Spinal nerves have mixed populations of fibers; some are motor fibers and some are sensory. The sensory fibers connect to the spinal cord through the dorsal root, which is attached to the dorsal root ganglion. Sensory information from the body that is conveyed through spinal nerves will project to the opposite side of the brain to be processed by the cerebral cortex. The cranial nerves can be strictly sensory fibers, such as the olfactory, optic, and vestibulocochlear nerves, or mixed sensory and motor nerves, such as the trigeminal, facial, glossopharyngeal, and vagus nerves. The cranial nerves are connected to the same side of the brain from which the sensory information originates.
amacrine cell
type of cell in the retina that connects to the bipolar cells near the outer synaptic layer and provides the basis for early image processing within the retina
aqueous humor
watery fluid that fills the anterior chamber containing the cornea, iris, ciliary body, and lens of the eye
bipolar cell
cell type in the retina that connects the photoreceptors to the RGCs
choroid
highly vascular tissue in the wall of the eye that supplies the outer retina with blood
ciliary body
smooth muscle structure on the interior surface of the iris that controls the shape of the lens through the zonule fibers
cone photoreceptor
one of the two types of retinal receptor cell that is specialized for color vision through the use of three photopigments distributed through three separate populations of cells
cornea
fibrous covering of the anterior region of the eye that is transparent so that light can pass through it
extraocular muscle
one of six muscles originating out of the bones of the orbit and inserting into the surface of the eye which are responsible for moving the eye
fibrous tunic
outer layer of the eye primarily composed of connective tissue known as the sclera and cornea
fovea
exact center of the retina at which visual stimuli are focused for maximal acuity, where the retina is thinnest, at which there is nothing but photoreceptors
inferior oblique
extraocular muscle responsible for lateral rotation of the eye
inferior rectus
extraocular muscle responsible for looking down
inner segment
in the eye, the section of a photoreceptor that contains the nucleus and other major organelles for normal cellular functions
inner synaptic layer
layer in the retina where bipolar cells connect to RGCs
iris
colored portion of the anterior eye that surrounds the pupil
lacrimal duct
duct in the medial corner of the orbit that drains tears into the nasal cavity
lacrimal gland
gland lateral to the orbit that produces tears to wash across the surface of the eye
lateral rectus
extraocular muscle responsible for abduction of the eye
lens
component of the eye that focuses light on the retina
levator palpebrae superioris
muscle that causes elevation of the upper eyelid, controlled by fibers in the oculomotor nerve
medial rectus
extraocular muscle responsible for adduction of the eye
neural tunic
layer of the eye that contains nervous tissue, namely the retina
opsin
protein that contains the photosensitive cofactor retinal for phototransduction
optic disc
spot on the retina at which RGC axons leave the eye and blood vessels of the inner retina pass
optic nerve
second cranial nerve, which is responsible visual sensation
outer segment
in the eye, the section of a photoreceptor that contains opsin molecules that transduce light stimuli
outer synaptic layer
layer in the retina at which photoreceptors connect to bipolar cells
palpebral conjunctiva
membrane attached to the inner surface of the eyelids that covers the anterior surface of the cornea
photoisomerization
chemical change in the retinal molecule that alters the bonding so that it switches from the 11-cis-retinal isomer to the all-trans-retinal isomer
photon
individual “packet” of light
photoreceptor
receptor cell specialized to respond to light stimuli
pupil
open hole at the center of the iris that light passes through into the eye
retina
nervous tissue of the eye at which phototransduction takes place
retinal
cofactor in an opsin molecule that undergoes a biochemical change when struck by a photon (pronounced with a stress on the last syllable)
retinal ganglion cell (RGC)
neuron of the retina that projects along the second cranial nerve
rhodopsin
photopigment molecule found in the rod photoreceptors
rod photoreceptor
one of the two types of retinal receptor cell that is specialized for low-light vision
sclera
white of the eye
superior oblique
extraocular muscle responsible for medial rotation of the eye
superior rectus
extraocular muscle responsible for looking up
trochlea
cartilaginous structure that acts like a pulley for the superior oblique muscle
vascular tunic
middle layer of the eye primarily composed of connective tissue with a rich blood supply
vision
special sense of sight based on transduction of light stimuli
visual acuity
property of vision related to the sharpness of focus, which varies in relation to retinal position
vitreous humor
viscous fluid that fills the posterior chamber of the eye
zonule fibers
fibrous connections between the ciliary body and the lens
Watch this video to learn more about a transverse section through the brain that depicts the visual pathway from the eye to the occipital cortex. The first half of the pathway is the projection from the RGCs through the optic nerve to the lateral geniculate nucleus in the thalamus on either side. This first fiber in the pathway synapses on a thalamic cell that then projects to the visual cortex in the occipital lobe where “seeing,” or visual perception, takes place. This video gives an abbreviated overview of the visual system by concentrating on the pathway from the eyes to the occipital lobe. The video makes the statement (at 0:45) that “specialized cells in the retina called ganglion cells convert the light rays into electrical signals.” What aspect of retinal processing is simplified by that statement? Explain your answer.
Photoreceptors convert light energy, or photons, into an electrochemical signal. The retina contains bipolar cells and the RGCs that finally convert it into action potentials that are sent from the retina to the CNS. It is important to recognize when popular media and online sources oversimplify complex physiological processes so that misunderstandings are not generated. This video was created by a medical device manufacturer who might be trying to highlight other aspects of the visual system than retinal processing. The statement they make is not incorrect, it just bundles together several steps, which makes it sound like RGCs are the transducers, rather than photoreceptors.
1. Axons from which neuron in the retina make up the optic nerve?
A) amacrine cells
B) photoreceptors
C) bipolar cells
D) retinal ganglion cells
D
1. Why does the blind spot from the optic disc in either eye not result in a blind spot in the visual field?
The visual field for each eye is projected onto the retina as light is focused by the lens. The visual information from the right visual field falls on the left side of the retina and vice versa. The optic disc in the right eye is on the medial side of the fovea, which would be the left side of the retina. However, the optic disc in the left eye would be on the right side of that fovea, so the right visual field falls on the side of the retina in the left field where there is no blind spot.