Module 1

Module 1—The Nervous System

Module Glossary

 

Consult the glossary in the textbook for other definitions that you may need to complete your work.

 

accommodation: the process of changing the shape of the lens from round and fat to thin and flat, and vice versa, so that light can be focused on the retina to accommodate vision of objects near and far away

 

acetylcholine: one of the most common neurotransmitters of both the somatic nervous system and the parasympathetic nervous system

 

Acetylcholine functions by binding to receptors on the postsynaptic membrane and either depolarizing or hyperpolarizing the membrane.

 

action potential: the change in charge that occurs when the gates of the potassium ion channels close and the gates of the sodium ion channels open; a large depolarization event that is conducted along the membrane of a nerve cell or a muscle cell

 

adaptation: the process by which the iris adjusts the diameter of the pupil based on light conditions, thus controlling the amount of light that enters the eye and strikes the retina

 

all-or-none response: action that occurs either completely or not at all, such as the generation of an action potential by a neuron

 

amplify: to increase the amplitude or the extent of a vibration; see intensity

 

anterior chamber: the space in front of the iris and behind the cornea that is filled with aqueous humour

 

aqueous humour: a clear, watery fluid in the anterior chamber of the eye that maintains the shape of the cornea and provides oxygen and nutrients for the surrounding cells, including those of the lens and the cornea

 

astigmatism: an uneven curvature of the cornea or lens, resulting in uneven focusing, which results in poor vision

 

auditory canal: a short channel that funnels sound waves from outside the ear to the tympanum and amplifies sounds

 

auditory nerve: a nerve composed of sensory fibres from the organ of Corti, the vestibular apparatus that conducts impulses to the temporal lobe of the cerebrum

 

autonomic nervous system (ANS): a division of the peripheral nervous system that conducts nerve impulses to cardiac and smooth muscles, as well as to glands; may also be called the involuntary motor system

 

axon: the long extension that emerges from the cell body and conducts the nerve impulse away from the cell body

 

The axon may be up to 1 m long in motor neurons.

 

axon terminal: numerous endings found at the end of an axon

 

Axon terminals are enlarged into knobs that are specialized for producing, storing, and releasing neurotransmitters.

 

basilar membrane: one of two parallel membranes that comprise the organ of Corti in the inner ear

 

Hair cells are attached to the basilar membrane.

 

bipolar cells: specialized sensory nerve cells located in the retina that are stimulated by either rods or cones

 

Cones mostly have a one-to-one ratio with bipolar cells, whereas several rod cells may communicate with one bipolar cell.

 

blind spot: the area at the back of the eyeball that is deficient in rods and cones; the area where the sensory fibres come together to form the optic nerve

 

cataract: a cloudy or grey-white area on the lens caused by deterioration of the protein composing the lens

 

Cataracts prevent the passing of light to the photoreceptors of the retina.

 

cell body: the part of a neuron that contains the nucleus and other cell organelles for carrying out the metabolic reactions of the nerve cell; relays the nerve impulse from the dendrites to the axon

 

central nervous system (CNS): the part of the nervous system that includes the brain and spinal cord

 

chemoreceptor: a sensory receptor that transmits information about the solute concentration in a solution or about individual kinds of molecules in solution

 

cholinesterase: an enzyme necessary to decompose acetylcholine in the synaptic cleft so that the products (choline and ethanoic acid) can be reabsorbed by the presynaptic membrane; also called acetylcholinesterase

 

choroid: the middle layer of the eyeball that lies between the sclera and retina and is highly vascular and heavily pigmented

 

The choroid absorbs stray light rays not detected by the photoreceptors of the retina.

 

ciliary muscle: a ring of muscle behind the iris that is attached to the lens by suspensory ligaments and is involved in changing the shape of the lens

 

cochlea: contains the organ of Corti, which functions to convert the mechanical energy of sound waves into a nerve impulse

 

colourblindness: an x-linked inherited disorder that results in nonfunctional or deficient cone function; inability to see certain colours such as red, green, or blue

 

cones: one of two types of photoreceptors in the retina of the eye that is sensitive to different wavelengths of light and are, thus, responsible for distinguishing colour

 

There are three types of cones: one sensitive to red light, one sensitive to blue light, and one sensitive to green light. The cones are responsible for acute vision, or distinguishing detail.

 

cornea: a transparent portion of the sclera (located at the front of the eye) that allows light to enter the eye and, in the process, refracts or bends the light rays so that they can be focused on the retina

 

dendrite: a short, branching terminal of a neuron that receives input from other neurons or sensory receptors and transmits a nerve impulse toward the cell body

 

depolarization: the loss or reduction of the negative resting membrane potential

 

depth perception: the ability to see in three dimensions

 

dopamine: generally an excitatory neurotransmitter in the CNS

 

dynamic balance (rotational equilibrium): balance resulting when the head and body are moved or rotated

 

effector: one of the three types of muscle or a gland that responds to a nerve impulse

 

Eustachian tube: a tiny passageway extending from the middle ear to the throat (pharynx); a passageway through which viruses and bacteria can travel from the nose and mouth, resulting in an inner ear infection

 

The Eustachian tube plays an important role in equalizing air pressure on both sides of the tympanum.

 

excitatory neurotransmitter: a neurotransmitter that promotes nerve impulse transmission in the postsynaptic membrane by opening sodium channels

 

fovea centralis: an area of the retina that is located directly behind the centre of the lens and has a very high concentration of cones, which makes this part of the eye responsible for great visual acuity

 

frequency: number of wavelengths per given time; number of waves that pass a given point in a given time

 

GABA (gamma aminobutyric acid): an inhibitory neurotransmitter in the CNS

 

ganglion cells: special sensory neurons that communicate with bipolar cells in the retina to transmit a nerve impulse to the brain

 

These cells have long axons that converge at the back of the eye to form the optic nerve.

 

glaucoma: a disorder caused by the malfunction of ducts that drain excess aqueous humour from the anterior chamber

 

The resulting pressure created by excess aqueous humour ruptures delicate blood vessels in the eye and causes deterioration of cells in the eye due to lack of nutrients. This can result in blindness if left untreated.

 

glutamate: an excitatory neurotransmitter in the CNS

 

hair cells: sensory mechanoreceptors attached to the basilar membrane in the organ of Corti

 

homeostasis: a state of body equilibrium or a stable internal environment of the body

 

hyperopia: farsightedness, or the inability to focus objects that are close, caused by an eyeball that is too short, which causes light to be focused behind the retina

 

hyperpolarization: the process of generating a membrane potential that is more negative than the normal resting membrane potential

 

incus (anvil): the middle in the sequence of the three ossicles located in the middle ear; connected to the malleus and the third ossicle (stapes); part of a lever system that amplifies sound waves

 

inhibitory neurotransmitter: a neurotransmitter that hinders nerve impulse transmission in the postsynaptic neuron by hyperpolarizing it

 

inner ear: one of the three regions of the ear that is located deepest into the head and consists of a fluid-filled chamber containing the semicircular canals, the utricle and saccule, and the cochlea

 

intensity: measurement of the difference between compressed and rarefied areas of a sound wave used in physics; corresponds to amplitude of a wave

 

interneuron: a type of nerve cell found either in the brain or spinal cord that transmits nerve impulses from sensory neurons within the various parts of the brain or to motor neurons

 

iodopsin: the general name of any of the three visual pigments found in cone cells that is stimulated by light to initiate a nerve impulse

 

loudness: subjective interpretation of sound intensity

 

malleus (hammer): the first in the sequence of the three ossicles located in the middle ear; connected to the tympanum and the second ossicle (incus); part of a lever system that amplifies sound waves

 

mechanoreceptor: a sensory receptor that detects physical deformations in the body’s environment associated with pressure, touch, stretch, motion, and sound

 

membrane potential: a form of potential energy resulting from the separation of charges between the inside and the outside of a cell membrane; voltage across the cell membrane

 

middle ear: one of the three main regions of the ear, which begins just past the tympanum and consists of a chamber containing three tiny bones (ossicles) called the malleus (hammer), the incus (anvil), and the stapes (stirrup); leads into a minute opening called the Eustachian tube

 

motor neuron: a type of nerve cell that transmits nerve impulses toward an effector, which can be a muscle or a gland

 

myelinated neuron: a neuron whose axon is wrapped by Schwann cells, which produces a myelin sheath

 

Myelinated neurons make up the white matter of the brain and the spinal cord and transmit nerve impulses very quickly.

 

myelin sheath: a fatty insulating layer that surrounds axons and greatly increases the rate of impulse transmission and maintains the strength of the impulse by preventing the loss of ions along the length of the axon

 

myopia: nearsightedness, or the inability to focus objects that are far away, caused by an eyeball that is elongated, which causes light to be focused in front of retina rather than directly on it

 

nerve: a message pathway of the nervous system; made up of many neurons grouped into bundles and surrounded by protective tissue

 

There are 12 pairs of cranial nerves that insert into the brain and 31 pairs of spinal nerves that emanate from the spinal cord.

nervous system: an elaborate communication system that receives input; processes, integrates, and stores information; and triggers muscle contraction or glandular secretion

 

neuromuscular junction: a tiny gap located between an axon terminal and a muscle cell

 

neuron: a cell in the nervous system that generates and transmits nerve impulses; consists of dendrites, a cell body containing the nucleus, and an axon that may or may not have a myelin sheath

 

neurotransmitter: a chemical messenger released from the synaptic knob of a neuron at a synapse that diffuses across the synaptic cleft, binds to specially shaped protein receptors on the postsynaptic membrane, and stimulates the postsynaptic neuron

 

node of Ranvier: a tiny gap in the myelin sheath surrounding the axon of myelinated neurons

 

Nerve impulse transmission occurs between nodes of Ranvier in what is called salutatory conduction, which causes the increase in the speed of impulse transmission.

 

norepinephrine: a neurotransmitter released by sympathetic neurons of the autonomic system to produce an excitatory effect on target muscles; also called noradrenalin

 

occipital lobe: one of the four lobes of the cerebrum that receives and analyzes visual information that is sent to association centres for recognition of what is being seen

 

olfactory (receptor) cell: a neuron located in the olfactory epithelium that is specialized to receive chemical stimuli and to initiate a nerve impulse

 

olfactory epithelium: a patch of tissue located in the upper part of the nasal cavity that contains mucous-secreting cells and olfactory cells, or smell cells, that can detect different smells; a sense organ for smell

 

opsin: a protein that is the result of the decomposition of rhodopsin

 

optic nerve: a collection of sensory neurons that carries sensory information from the photoreceptors to the brain

 

organ of Corti: a structure of the inner ear that contains hair cells that detect vibrations in the fluid of the inner ear and initiates a nerve impulse that is transmitted to the auditory nerve

 

osmoreceptor: a sensory receptor that detects changes in osmotic pressure, pressure due to water movement

 

ossicles: three tiny bones located in the middle ear that are connected to each other, to the tympanum, and to the oval window; involved in amplifying sound waves

 

otolith: a tiny particle of calcium carbonate found in the utricle and saccule that contacts the hair cells of these structures and stimulates them

 

outer ear: one of the three main regions of the ear, which consists of the pinna and the auditory canal and ends at the tympanum or eardrum

 

oval window: a membrane-covered opening located between the chamber of the middle ear and the wall of the inner ear

 

The stapes (stirrup) is attached to it and transmits sound waves by it to the inner ear.

 

overshoot: the situation that results when more potassium ions leak out of the neuron than should because the potassium gates are slow to close; results in hyperpolarization

 

parasympathetic nervous system: the division of the autonomic nervous system that oversees digestion, elimination, and glandular function; often works opposite the sympathetic nervous system to bring the body back to normal

 

perception: the interpretation of sensory information by the cerebral cortex

peripheral nervous system (PNS): the portion of the nervous system consisting of nerves and ganglia (collections of nerve cell bodies) that are outside the brain and spinal cord

 

photoreceptor: a sensory receptor that responds to light stimuli, allowing people to see images and colours

 

pinna: the outer flap of the ear that is composed of skin and cartilage and shaped so that it enhances sound vibrations and funnels them into the auditory canal

 

pitch: corresponds to the frequency of a sound wave

 

polarization: the process of generating a resting membrane potential averaging approximately – 70 mV

 

polarized membrane: the state of the cell membrane in an unstimulated neuron in which the inside of the neuron is negatively charged in comparison to the outside of the neuron; the resting state of a membrane averaging approximately – 70 mV

 

postsynaptic membrane: the surface membrane on the dendrites of a receiving neuron across the synaptic cleft; contains receptor sites for neurotransmitters

 

Note the role of “shape” in controlling biological processes.

 

postsynaptic neuron: the receiving neuron

 

presynaptic membrane: the surface membrane surrounding the synaptic knob and facing the synaptic cleft

 

presynaptic neuron: the sending neuron

 

reflex: an inborn, unlearned behaviour that results from the stimulation of a special neural pathway called the reflex arc

 

reflex arc: an involuntary neural pathway that consists of a sensory receptor, a sensory neuron, a control centre that can be either the brain or spinal cord, a motor neuron, and an effector that results in a reflex behaviour that usually has survival value

 

refract: to bend light as it passes through a substance with a different desnsity

 

refractory period: the short time immediately after an action potential in which the neuron cannot respond to another stimulus; period of time it takes to re-establish the net positive charge on the outside of the neuron and the net negative charge on the inside of the neuron, where there are more sodium ions on the outside and more potassium ions on the inside of the neuron

 

repolarization: restoring the resting membrane potential (– 70 mV) from the depolarized state

 

resting membrane potential: the voltage that exists across a cell membrane during the resting state of an excitable cell, such as the neuron; averages around – 70 mV, but may range from – 50 to – 200 mV depending on the cell

 

retina: the innermost layer of the eye that contains the photoreceptors

 

retinal: a derivative of Vitamin A (retinol) that is the result of the decomposition of rhodopsin and is instrumental in initiating a nerve impulse

 

rhodopsin: a visual pigment found in rod cells that is decomposed by light into opsin and retinal

 

The change from rhodopsin to opsin and retinal initiates a nerve impulse.

 

rods: one of two types of photoreceptors in the retina of the eye that is sensitive to light intensity and detect movement

 

Rods do not distinguish colour.

 

round window: a membrane-covered opening between the chamber of the middle ear and the inner ear located below the oval window

 

The round window functions in dissipating sound waves in the inner ear.

 

saltatory conduction: rapid transmission of a nerve impulse along an axon resulting from the action potential jumping from one node of Ranvier to another, skipping the myelinated regions of the membrane

 

Schwann cell: a type of supporting nerve cell that is found in the peripheral nervous system and wraps around axons of neurons and produces the myelin sheath

 

sclera: the white, tough, protective outer layer of the eye that helps gives the eyeball its shape; sometimes called the white of the eye

 

semicircular canals: three tubes that are situated at right angles to one another and contain mechanoreceptors that detect head and body rotation; responsible for dynamic balance or rotational equilibrium (rotational equilibrium dynamic balance), which is the balance that is established in response to the head and body being moved or rotated

 

sensation: the reception and processing by the brain of a nerve impulse sent by an activated sensory receptor

 

senses: specialized mechanisms or functions by which an organism is receptive and responsive to a certain class of stimuli, which are typically external (as in the senses of sight, hearing, touch, and pain) but also may be internal (as in sensing the temperature of the blood or the levels of carbon dioxide)

 

sensory adaptation: the tendency of sensory neurons to become less sensitive when they are repeatedly stimulated

 

sensory neuron: a type of nerve cell that receives stimuli and conducts an impulse toward the brain and spinal cord (central nervous system); a nerve cell that is stimulated by hair cells in the organ of Corti to conduct messages toward the temporal lobe of the brain

 

sensory receptor: a cell or a group of cells that is specialized to receive stimuli that provide information about the body’s external conditions (through sight, hearing, taste, smell, or touch) and internal conditions (such as temperature, pH, glucose levels, and blood pressure)

 

serotonin: an inhibitory neurotransmitter in the CNS

somatic nervous system (SNS): a division of the peripheral nervous system that conducts nerve messages to the skeletal muscles; may sometimes be called the voluntary nervous system

 

sound: pressure disturbance beginning at a vibrating object (source) and spread out by a medium such as air

 

sound wave: a series of compressions and rarefactions resulting in an S-shaped curve or sine wave

 

stapes (stirrup): the third in the sequence of the three ossicles located in the middle ear; connected to the second ossicle (incus) and the oval window; part of a lever system that amplifies sound waves and causes vibrations in the fluid of the inner ear

 

static balance (gravitational equilibrium): balance resulting from changes in the position or the movement of the head in one direction; usually in response to gravity

sympathetic nervous system: the division of the autonomic nervous system that activates the body to cope with some stressor, such as danger, excitement, or fear; sometimes referred to as the fight, fright, and flight subdivision

 

synapse (also synaptic cleft and synaptic gap): the location or junction in a neural pathway where one neuron communicates with another neuron; a tiny gap between the synaptic terminal of an axon and the signal receiving dendrite or cell body of another neuron or an effector, such as a muscle

 

synaptic cleft: a tiny space separating the synaptic knob of a transmitting neuron from a receiving neuron or effector cell

 

synaptic knob: the tiny enlarged ending on an axon terminal

 

synaptic vesicle: a tiny membranous sac that, in this case, contains neurotransmitters

 

tapetum: a layer in the choroid that increases the absorption of light to stimulate photoreceptors in dim conditions

 

taste bud: a sensory organ composed of a taste pore, taste cells, and sensory fibres of a sensory neuron involved in initiating taste sensations

 

tectorial membrane: one of two parallel membranes found in the organ of Corti

 

During the transmission of sound waves, the basilar membrane vibrates and causes the sensory hairs to flex against the tectorial membrane.

 

thermoreceptor: a sensory receptor that detects heat or cold

 

threshold potential: the smallest change in the membrane potential of a cell membrane that is needed to initiate an action potential; approximately – 55 mV

 

threshold stimulus: weakest possible stimulus that is needed to initiate a nerve impulse

 

tympanum: a round elastic structure located in the middle ear that vibrates in response to sound waves; also called the eardrum or tympanic membrane

 

unmyelinated neuron: a neuron that does not have Schwann cells and, therefore, lacks a myelin sheath

 

Unmyelinated neurons make up the grey matter of the brain and spinal cord and transmit nerve impulses much more slowly than myelinated neurons.

 

utricle and saccule: tiny chambers in the inner ear that contain otoliths and hair cells, which respond to changes in head position with respect to gravity and movement in one direction; responsible for static balance or gravitational equilibrium

 

vestibule: the fluid-filled area of the inner ear located between the semicircular canals and the cochlea

 

The vestibule contains the utricle and saccule.

 

vestigial: small or imperfectly developed; often referring to a part or organ that was once more fully developed in an earlier evolutionary stage, but is now non-functional

 

vitreous humour: the transparent, amber-coloured, jelly-like fluid in the posterior chamber of the eye that helps to maintain the shape of the eyeball

 

voltage: electrical potential difference across a membrane as measured by a voltmeter

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