Chapter 2 Terms

Optic Nerve

Bundle of nerve fibers that carry impulses from the retina to the lateral geniculate nucleus and other structures. Each optic nerve contains about 1 million ganglion cell fibers.

Sensory receptors

Cells specialized to respond to environmental energy, with each sensory system's receptors specialized to respond to a specific type of energy.

Isomerization

Change in shape of the retinal part of the visual pigment molecule that occurs when the molecule absorbs a quantum of light. Isomerization triggers the enzyme cascade that results in transduction from light energy to electrical energy in the retinal receptors.

Ions

Charged molecules. Sodium (Na+), potassium (K+), and chlorine (Cl-) are the main ions found within nerve fibers and in the liquid that surrounds nerve fiber

Receptor Site

Small area on the postsynaptic neuron that is sensitive to specific neurotransmitters

Outer Segment

Part of the rod and cone visual receptors that contain the light-sensitive visual pigment molecules.

Hyperpolarization

When the inside of a neuron becomes more negative. Hyperpolarization is often associated with the action of inhibitory neurotransmitters.

Fovea

=A small area in the human retina that contains only cone receptors. The fovea is located on the line of sight, so that when a person looks at an object, the center of its image falls on the fovea.

Neurotransmitter

A chemical stored in synaptic vesicles that is released in response to a nerve impulse and has an excitatory or inhibitory effect on another neuron.

Macular degeneration

A clinical condition that causes degeneration of the macula, an area of the retina that includes the fovea and a small surrounding area.

Retina

A complex network of cells that covers the inside back of the eye. These cells include the receptors, which generate an electrical signal in response to light, as well as the horizontal, bipolar, amacrine, and ganglion cells.

Hyperopia

A condition causing poor vision in which people can see objects that are far away but do not see near objects clearly. Also called farsightedness.

Farsightedness

A condition causing poor vision in which people can see objects that are far away but do not see near objects clearly. Also called hyperopia.

Detached retina

A condition in which the retina is detached from the back of the eye.

Rod

A cylinder-shaped receptor in the retina that is responsible for vision at low levels of illumination.

Visual pigment

A light-sensitive molecule contained in the rod and cone outer segments. The reaction of this molecule to light results in the generation of an electrical response in the receptors.

Ganglion cell

A neuron in the retina that receives inputs from bipolar and amacrine cells. The axons of the ganglion cells are the nerve fibers that travel out of the eye in the optic nerve.

Horizontal cell

A neuron that transmits signals laterally across the retina. Horizontal cells synapse with receptors and bipolar cells.

Amacrine cell

A neuron that transmits signals laterally in the retina. Amacrine cells synapse with bipolar cells and ganglion cells.

Neural Circuit

A number of neurons that are connected by synapses.

Rod monochromat

A person who has a retina in which the only functioning receptors are rods.

Absorption spectrum

A plot of the amount of light absorbed by a visual pigment versus the wavelength of light.

Laser-assisted in situ keratomileusis (LASIK)

A process in which the cornea is sculpted with a laser in order to achieve clear vision by adjusting the focusing power of the cornea so it focuses light onto the retina.

Permeability

A property of a membrane that refers to the ability of molecules to pass through it. If the permeability to a molecule is high, the molecule can easily pass through the membrane.

Propagated Response

A response, such as a nerve impulse, that travels all the way down the nerve fiber without decreasing in amplitude.

Retinitis pigmentosa

A retinal disease that causes a gradual loss of vision, beginning in the peripheral retina.

Bipolar cell

A retinal neuron that receives inputs from the visual receptors and sends signals to the retinal ganglion cells.

Synapse

A small space between the end of one neuron (the presynaptic neuron) and the cell body of another neuron (the postsynaptic neuron)

Preferential Looking (PL) technique

A technique used to measure perception in infants. Two stimuli are presented, and the infant's looking behavior is monitored for the amount of time the infant spends viewing each stimulus.

Far point

As a light is moved toward the eye, the distance at which the light becomes focused on the retina.

Visual evoked potential (VEP)

An electrical response to visual stimulation recorded by the placement of disk electrodes on the back of the head. This potential reflects the activity of a large population of neurons in the visual cortex.

Nearsightedness

An inability to see distant objects clearly. Also called myopia.

Myopia

An inability to see distant objects clearly. Also called nearsightedness.

Cone

Cone-shaped receptors in the retina that are primarily responsible for vision in high levels of illumination and for color vision and detail vision.

Electromagnetic spectrum

Continuum of electromagnetic energy that extends from very-short-wavelength gamma rays to longwavelength radio waves. Visible light is a narrow band within this spectrum.

Nerve Fiber

In most sensory neurons, the long part of the neuron that transmits electrical impulses from one point to another. Also called the axon.

Rising phase of the action potential

In the axon, or nerve fiber, the decrease in negativity from -70 mV to +40 mV (the peak action potential level) that occurs during the action potential. This increase is caused by an inflow of Na+ ions into the axon.

Falling phase of the action potential

In the axon, or nerve fiber, the increase in negativity from +40 mV back to -70 mV (the resting potential level) that occurs during the action potential. This increase in negativity is associated with the flow of positively charged potassium ions (K+) out of the axon.

Transduction

In the senses, the transformation of environmental energy into electrical energy. For example, the retinal receptors transduce light energy into electrical energy.

Accommodation

In vision, bringing objects located at different distances into focus by changing the shape of the lens.

Monochromatic light

Light that contains only a single wavelength.

Inhibitory response

Occurs when a neuron's firing rate decreases due to inhibition from another neuron.

Refractive Myopia

Myopia (nearsightedness) in which the cornea and/or the lens bends the light too much.

Axial myopia

Myopia (nearsightedness) in which the eyeball is too long. See also refractive myopia

Spontaneous activity

Nerve firing that occurs in the absence of environmental stimulation.

Dendrites

Nerve processes on the cell body that receive stimulation from other neurons.

Action potential

Rapid increase in positive charge in a nerve fiber that travels down the fiber. Also called the nerve impulse.

Neural Convergence

Synapsing of a number of neurons onto one neuron.

Visual acuity

The ability to resolve small details.

Peripheral Retina

The area of retina outside the fovea.

Visible light

The band of electromagnetic energy that activates the visual system and that, therefore, can be perceived. For humans, visible light has wavelengths between 400 and 700 nanometers.

Resting Potential

The difference in charge between the inside and the outside of the nerve fiber when the fi ber is not conducting electrical signals. Most nerve fi bers have resting potentials of about 70 mV, which means the inside of the fiber is negative relative to the outside.

Near Point

The distance at which the lens can no longer accommodate enough to bring close objects into focus. Objects nearer than the near point can be brought into focus only by corrective lenses.

Eyes

The eyeball and its contents, which include focusing elements, the retina, and supporting structures.

Presbyopia

The inability of the eye to accommodate due to a hardening of the lens and a weakening of the ciliary muscles. It occurs as people get older.

Pupil

The opening through which light reflected from objects in the environment enters the eye.

Cell body

The part of a neuron that contains the neuron's metabolic machinery and that receives stimulation from other neurons.

Axon

The part of the neuron that conducts nerve impulses over distances. Also called the nerve fiber.

Excitatory response

The response of a nerve fiber in which the firing rate increases.

Spectral sensitivity

The sensitivity of visual receptors to different parts of the visible spectrum. See also spectral sensitivity curve.

Blind spot

The small area where the optic nerve leaves the back of the eye. There are no visual receptors in this area, so small images falling directly on the blind spot cannot be seen.

Neuron

The structure that transmits electrical signals in the body. Key components of neurons are the cell body, dendrites, and the axon or nerve fiber.

Refractory Period

The time period of about 1/1,000th of a second that a nerve fiber needs to recover from conducting a nerve impulse. No new nerve impulses can be generated in the fiber until the refractory period is over.

Cornea

The transparent focusing element of the eye that is the first structure through which light passes as it enters the eye. The cornea is the eye's major focusing element.

Lens

The transparent focusing element of the eye through which light passes after passing through the cornea and the aqueous humor. The lens's change in shape to focus at different distances is called accommodation.

Convergence (neural)

When many neurons synapse onto a single neuron.

Depolarization

When the inside of a neuron becomes more positive, as occurs during the initial phases of the action potential. Depolarization is often associated with the action of excitatory neurotransmitters.

Martin Banks and Patrick Bennett

calculated that the cone receptors' outer segments effectively cover 68% of the adult fovea, but only 2% of the newborn fovea

Wavelength

the distance between the peaks of the electromagnetic waves

Visible Light

the energy within the electromagnetic spectrum that humans can perceive, has wavelengths from about 400 to 700 nanometers (nm)