Biology 102 Chapter 44
Neurons and Nervous Systems
1. Describe the three major functions of the nervous system.
---sensory input is the conduction of signals from
sensory receptors to integration centers
---integration is process by which information from
environmental stimulation of sensory receptors is
interpreted and associated with appropriate responses
of the body
--mostly carried out in central nervous system (CNS)
-brain and spinal cord (vertebrates)
---motor output is conduction of signals from integration
center (CNS) to effector cells, the muscle cells or
glands that actually carry out the body’s responses
to stimuli
---**nerves that communicate motor/sensory signals between
CNS and rest of body are collectively called the
peripheral nervous system (PNS)
---nerve defined as ropelike bundles of extensions
of neurons tightly wrapped in connective tissue
2. List and describe the three major parts of a neuron and
explain the function of each.
---neuron: cells specialized for transmitting chemical and
electrical signals from one location in body to another
---(1)large cell body; contains most of cytoplasm, nucleus,
and other organelles
--cell bodies of most neurons located in CNS
--some neurons have cell body located in ganglia outside
the CNS
---dendrites, one type of fiberlike extension (process)
that convey impulses TO the cell body
--short, numerous, & extensively branched to increase
surface area where cell most likely to be stimulated
---axons, other type of extension (process), conduct
signals AWAY from the cell body
--long, simple processes
--vertebrate axons in PNS wrapped in concentric layers
of Schwann cells, which form insulating myelin sheath
-in CNS, myelin sheath formed by oligodendrocytes
--axons extend from axon hillock (where signals are
generated) to many branches which are tipped with
synaptic terminals that release neurotransmitters
--synapse is gap between synaptic terminal and target
cell—dendrites of another neuron or an effector cell
--neurotransmitters are chemicals that cross the synapse
to relay the impulse
3. Describe the function and location of glial cells.
---supporting cells (glia cells) structurally reinforce,
protect, insulate, and generally assist neurons
--do not conduct impulses
--outnumber neurons 10 to 50 fold
---several types of glia are present
--astrocytes encircle capillaries in brain
--contribute to blood-brain barrier (restricts passage
of most substances into CNS
--communicate with each other/other neurons via
chemical signals
--oligodendrocytes form myelin sheaths that insulate
CNS nerve processes
--Schwann cells form insulating myelin sheath around
axons in PNS
---myelination provides electrical insulation & increases
speed of nerve impulse propagation
4. Explain what a resting potential is.
---all living cells have electrical charge difference
across their plasma membrane
---this difference gives rise to electrical voltage
gradient across the membrane
---voltage measured across plasma membrane is called
membrane potential
--typically in range of –50 to –100 mV in animal cell
---voltage outside cell is called “zero”
--minus sign indicates inside of cell is negative in
charge with respect to the outside
---for neuron in its resting state (NOT transmitting
an impulse), membrane potential of –70 mV is typical
---membrane potential of excitable cell at rest (unexcited
state) is called a resting potential
5. Describe the characteristics of an action potential and
explain the role of membrane permeability changes and ion gates
play in the generation of an action potential.
---action potential is rapid change in membrane potential
of excitable cell, caused by stimulus-triggered
selective opening and closing of voltage-gated ion
channels
---action potential is a ALL-OR-NONE EVENT
---selective permeability of plasma membrane maintains
ionic differences
--as charged molecules, ions cannot readily diffuse
through hydrophobic core of PM’s phospholipid bilayer
--ions can cross membranes by carrier-mediated transport
or by passing through ion channels
---ion channel is integral transmembrane protein that
allows specific ion to cross the membrane
--passive (open all time) or gated (requires stimulus)
---presence of gated ion channels in neurons permits
excitable cells to change PM’s permeability and alter
membrane potential in response to stimuli received by
cell
---K+ and Na+
---depolarizing threshold potential usually around –50 to
-55 mV
6. Explain how the action potential is propagated along a neuron.
---action potential is localized electrical event,
a membrane depolarization at a specific point of
stimulation
---neuron usually stimulated at dendrites or cell body
---for action potential to function as signal, it must
somehow “travel” along the axon to the other end of
the cell
---AP is regenerated ANEW in sequence along axon
---like tipping over row of standing dominoes
---strong depolarization of one AP assures the
neighboring region of neuron will be depolarized
above threshold, triggering new AP at that location
and so on to end of axon
7. Describe synaptic transmission across an electrical synapse
and a chemical synapse.
---synapse is tiny gap between synaptic terminal of axon
and signal-receiving portion of another neuron/effector
cell
---presynaptic cell is transmitting cell
---postsynaptic cell is receiving cell
---two types of synapses: electrical and chemical
ELECTRICAL SYNAPSE
---allow APs to spread directly from pre-to postsynaptic
cells via gap junctions (intercellular channels)
--allows impulses to travel from one cell to next without
delay or loss of signal strength
--much less common than chemical synapses
CHEMICAL SYNAPSE
---at chemical synapse, synaptic cleft separates pre-and
postsynaptic cells
--NOT electrically coupled
---synaptic vesiclesrelease neurotransmitter molecules
released into synaptic cleft by exocytosisdiffuse
to postsynaptic membrane where binds to specific
receptors, causing ion gates to open
---neurotransmitter may excite or inhibit postsynaptic cell
--excite = depolarization
--inhibit = hyperpolarization
---neurotransmitter quickly degraded by enzymes
---allows transmission in only ONE direction
8. Describe the role of cholinesterase and explain what would
happen if acetylcholine was not destroyed.
---anyone care to spray a “bug?” (organophosphates)
---enough said
9. List some other possible neurotransmitters.
---biogenic amines; epinephrine, norepinephrine, and
dopamine; glycine, glutamate, aspartate, and gamma
aminobutyric acid (GABA); neuropeptides (Substance P and
endorphins); nitric oxide and carbon monoxide.
10. Explain how excitatory postsynaptic potentials (EPSP) and
inhibitory postsynaptic potentials (IPSP) affect the postsynaptic
membrane potential.
---excitatory postsynaptic potentials (EPSP) occur when excitatory
synapses release a neurotransmitter that opens gated channels
allowing Na+ to enter cell and K+ to leave (depolarization)
---inhibitory postsynaptic potentials (IPSP) occur when
neurotransmitters released from inhibitory synapses bind to
receptors that open ion gates, which make membrane MORE
permeable to K+ (which leaves the cell) and/or to Cl- (which
enters the cell) causing hyperpolarization
---EPSPs and IPSPs are graded potentials
--vary in magnitude with number of neurotransmitter molecules
binding to postsynaptic receptors
11. Explain how a neuron integrates incoming information,
including a description of summation.
---single EPSP rarely strong enough to trigger an action potential
---additive effect (summation) from several terminals/repeated
firing of terminals can change membrane potential
---temporal summation: chemical transmissions from 1 or more
synaptic terminals occurs so close in time that each affects
membrane while it is partially depolarized and before it has
returned to resting potential
---spatial summation: several different synaptic terminals,
usually from different presynaptic neurons, stimulate
postsynaptic cell at same time and have additive effect on
membrane potential
---EPSPs and IPSPs can summate, each countering the effects of the
other
---at any instant, axon hillock’s membrane potential is average of
summated depolarizations due to all EPSPs and the summated
hyperpolarizations due to all IPSPs
---AP is generated when EPSP summation exceeds IPSP summation to
point where membrane potential of axon hillock reaches
threshold voltage
12. List the two classes of neuropeptides and explain how they
illustrate overlap between endocrine and nervous control.
---often operate via signal-transduction pathways
---substance P is key excitatory signal that mediates perception
of pain (humans)
---endorphins are neuropeptides that function as natural
analgesics, decreasing the perception of pain by the CNS
--also decrease urine output, depress respiration, produce
euphoria, and have other emotional effects
---endorphin also released from anterior pituitary that serves as
hormone affecting specific regions of brain
---again, overlap between endocrine and nervous system