Download Chapter 12: Neural Tissue

Chapter 12: Neural Tissue
I. An Overview of the Nervous System, p. 380
Objective
1. Describe the anatomical and functional divisions of the nervous system.
•
The nervous system includes all the neural tissue in the body.
•
Neural tissue contains 2 kinds of cells:
1. neurons: the cells that send and receive signals
2. neuroglia (glial cells): the cells that support and protect the neurons
•
The organs of the nervous system include:
- the brain
- the spinal cord
- sensory receptors of sense organs (eye, ears, etc.)
- the nerves that connect the nervous system with other systems
•
The nervous system has 2 major anatomical divisions:
1. the central nervous system (CNS)
2. the peripheral nervous system (PNS)
The Anatomical Divisions of the Nervous System, p. 380
•
The central nervous system (CNS) consists of the spinal cord and brain, which
contain neural tissue, connective tissues and blood vessels.
•
The CNS is responsible for processing and coordinating:
1. sensory data from inside and outside the body.
2. motor commands that control activities of peripheral organs such as the
skeletal muscles.
3. higher functions of the brain such as intelligence, memory, learning and
emotion.
•
The peripheral nervous system (PNS) includes all neural tissue outside the CNS.
•
The PNS is responsible for:
1. delivering sensory information to the CNS
2. carrying motor command to peripheral tissues and systems
•
Sensory information and motor commands in the PNS are carried by bundles of
axons (with their associated connective tissues and blood vessels) called
peripheral nerves (nerves):
1. cranial nerves are connected to the brain
2. spinal nerves are attached to the spinal cord
The Functional Divisions of the Nervous System, p. 380
•
The PNS is separated into 2 divisions:
1. the afferent division, which carries sensory information from sensory
receptors of the PNS to the CNS. Receptors include neurons or specialized cells
that detect changes or respond to stimuli, and complex sensory organs such as the
eyes and ears.
2. the efferent division, which carries motor commands from the CNS to
muscles and glands of the PNS. The cells or organs that respond to efferent
signals by doing something are called effectors.
•
The efferent division is divided into 2 parts:
1. the somatic nervous system (SNS), which controls skeletal muscle
contractions
a. voluntary muscle contractions
b. involuntary muscle contractions (reflexes)
2. the autonomic nervous system (ANS), which controls subconscious
actions such as contractions of smooth muscle and cardiac muscle, and glandular
secretions.
•
The ANS is separated into 2 divisions:
1. the sympathetic division, which has a stimulating effect
2. the parasympathetic division, which has a relaxing effect
•
Neurons are the basic functional units of the nervous system.
II. Neurons, p. 380
Objectives
1. Sketch and label the structure of a typical neuron and describe the functions of
each component.
2. Classify neurons on the basis of their structure and function.
The Structure of Neurons, p. 381
Figure 12-1
• The multipolar neuron is a common type of neuron in the central nervous system.
It consists of:
1. a cell body (soma)
2. several short, branched dendrites, and
3. a long, single axon
1. The cell body includes:
- a relatively large nucleus and nucleolus
- the cytoplasm, called the perikaryon, which contains mitochondria that
provide energy, and dense areas of RER and ribosomes that produce
neurotransmitters. These dense areas, called Nissl bodies, make neural tissues
appear gray (the gray matter).
- the cytoskeleton with neurofilaments and neurotubules (in place of
microfilaments and microtubules) Bundles of neurofilaments called
neurofibrils support the dendrites and axon.
- most nerve cells do not contain centrioles and cannot divide
2. Dendrites are highly branched, with many fine dendritic spines that receive
information from other neurons. Dendritic spines make up 80-90% of the
neuron’s surface area.
3. The long axon carries the electrical signal (action potential) to its target. The
structure of an axon is critical to its function.
- axoplasm: the cytoplasm of the axon, which contains neurotubules,
neurofibrils, enzymes and various organelles
- axolemma: a specialized cell membrane, covers the axoplasm
- the initial segment of the axon attaches to the cell body at a thick section
called the axon hillock
- collaterals are branches of a single axon
- telodendria are the fine extensions at the synaptic terminal of the axon
Figure 12-2
• The synapse is the critical area where one neuron communicates with another cell
or neuron. The neuron that sends the message is the presynaptic cell, and the cell
that receives the message is the postsynaptic cell.
•
Within the synapse, the 2 cells do not actually touch. A small gap called the
synaptic cleft separates the presynaptic membrane and the postsynaptic
membrane. The message carried by the action potential is carried between the
membranes by chemicals.
•
The expanded area of the axon, called the synaptic knob, contains synaptic
vesicles filled with chemical messengers called neurotransmitters, which affect
receptors on the postsynaptic membrane.
•
The neurotransmitter chemical is then broken down and reassembled at the
synaptic knob. The transport of raw materials between the cell body and the
synaptic knob by neurotubules within the axon is called axoplasmic transport
(powered by mitochondria and kinesins).
•
The postsynaptic cell can be a neuron, or another type of cell. A synapse between
a neuron and a muscle is a neuromuscular junction. A neuroglandular junction is a
synapse between a neuron and a gland.
The Classification of Neurons, p. 383
• Neurons can be classified by structure or by function.
Figure 12-3
• There are 4 classifications of neurons based on structure:
1. Anaxonic neurons:
- small
- all cell processes look alike
- found in brain and sense organs
2. Bipolar neurons:
- small
- one dendrite and one axon
- found in special sensory organs (sight, smell, hearing)
3. Unipolar neurons:
- very long axons
- dendrites and axon are fused, with cell body to one side
- found in sensory neurons of the peripheral nervous system
4. Multipolar neurons:
- very long axons
- 2 or more dendrites and 1 axon
- common in the CNS
- includes all motor neurons of skeletal muscles
•
There are 3 classifications of neurons based on function:
1. Sensory neurons or afferent neurons, (the afferent division of the PNS):
- Cell bodies of sensory neurons are grouped in sensory ganglia.
- Sensory neurons collect information about our internal
environment (visceral sensory neurons) and our relationship to the
external environment (somatic sensory neurons).
- Sensory neurons are unipolar. Their processes, called afferent
fibers, extend (deliver messages) from sensory receptors to the
CNS.
-
Sensory receptors are categorized as:
a. interoceptors:
- monitor digestive, respiratory, cardiovascular,
urinary and reproductive systems
- provide internal senses of taste, deep pressure and
pain
b. exteroceptors:
- external senses of touch, temperature, and pressure
- distance senses of sight, smell and hearing
c. proprioceptors:
- monitor position and movement of skeletal
muscles and joints
2. Motor neurons or efferent neurons (the efferent division of the PNS):
- carry instructions from the CNS to peripheral effectors of tissues and
organs via axons called efferent fibers.
- the 2 major efferent systems are:
1. the somatic nervous system (SNS), including all the
somatic motor neurons that innervate skeletal muscles.
2. the autonomic nervous system (ANS), including the
visceral motor neurons that innervate all other peripheral
effectors (smooth muscle, cardiac muscle, glands and
adipose tissue).
- signals from CNS motor neurons to visceral effectors pass through
synapses at autonomic ganglia, dividing efferent axons into 2 groups:
1. preganglionic fibers
2. postganglionic fibers
3. Interneurons or association neurons:
- located in the brain, spinal cord and some autonomic ganglia, between
sensory neurons and motor neurons
- responsible for distribution of sensory information and coordination of
motor activity
- involved in higher functions such as memory, planning and learning
III. Neuroglia, p. 384
Objective
1. Describe the locations and functions of neuroglia.
•
Neuroglia make up half the volume of the nervous system. There are many types
of neuroglia, with more variety in the CNS than in the PNS.
Neuroglia of the Central Nervous System, p. 384
Figure 12-4
• The central nervous system has 4 types of neuroglia:
1. ependymal cells
2. astrocytes
3. oligodendrocytes
4. microglia
1. Ependymal Cells
- The central canal of the spinal cord and ventricles of the brain, filled with
circulating cerebrospinal fluid (CSF), are lined with ependymal cells
which form an epithelium called the ependyma.
- Some ependymal cells secrete cerebrospinal fluid, and some have cilia or
microvilli that help circulate CSF. Others monitor the CSF or contain stem
cells for repair. Processes of ependymal cells are highly branched and
contact neuroglia directly.
2. Astrocytes
- Astrocytes are large and have many functions, including:
• maintaining the blood-brain barrier that isolates the CNS
• creating a 3-dimensional framework for the CNS
• repairing damaged neural tissue
• guiding neuron development
• controlling the interstitial environment
3. Oligodendrocytes
- Oligodendrocytes have smaller cell bodies and fewer processes than
astrocytes. Processes may contact other neuron cell bodies, or wrap around
axons to form insulating myelin sheaths. An axon covered with myelin
(myelinated) increases the speed of action potentials.
- Myelinated segments of an axon are called internodes. The gaps between
internodes, where axons may branch, are called nodes (nodes of Ranvier).
- Because myelin is white, regions of the CNS that have many myelinated
nerves are called white matter, while unmyelinated areas are called gray
matter.
4. Microglia
- Microglia are small, with many fine-branched processes. They migrate
through neural tissue, cleaning up cellular debris, waste products and
pathogens.
•
The cell bodies of neurons in the PNS are clustered in masses called ganglia,
which are surrounded and protected by support cells called neuroglia.
Neuroglia of the Peripheral Nervous System, p. 387
•
There are 2 types of neuroglia in the PNS: satellite cells and Schwann cells.
1. Satellite cells (amphicytes) surround ganglia and regulate the environment
around the neuron.
Figure 12-5
2. Schwann cells (neurilemmacytes) form a myelin sheath called the neurilemma
around peripheral axons. One Schwann cell encloses only one segment of an
axon, so it takes many Schwann cells to sheath an entire axon.
Key
•
•
Neurons perform all communication, information processing, and control
functions of the nervous system.
Neuroglia preserve the physical and biochemical structure of neural tissue, and
are essential to the survival and function of neurons.
Neural Responses to Injuries, p. 387
Figure 12-6
• In the PNS, peripheral nerves can regenerate after injury. Scwann cells assist in a
process called Wallerian degeneration. As the axon distal to the injury site
degenerates, Schwann cells form a line along the path of the original axon, and
wrap the new axon as it grows.
•
In the CNS, nerve regeneration is limited because astrocytes block growth by
releasing chemicals and producing scar tissue.
IV. Ion Movements and Electrical Signals, p. 390
Objectives
1. Explain how the resting potential is created and maintained.
2. Describe the events involved in the generation and propagation of an action
potential.
3. Discuss the factors that affect the speed with which action potentials are
propagated.
•
While the membranes of all cells produce electrical signals by ion movements
(transmembrane potential), this function is particularly important to neurons.
Figure 12-7 (navigator)
• The main membrane processes involved in neural activities are:
1. resting potential: the transmembrane potential of a resting cell
2. graded potential: a temporary localized change in the resting potential,
caused by a stimulus
3. action potential: an electrical impulse (produced by the graded potential)
that propagates along the surface of an axon to a synapse.
4. synaptic activity: the release of neurotransmitters at the presynaptic
membrane, which produce graded potentials in a postsynaptic membrane.
5. information processing: the response (integration of stimuli) of a
postsynaptic cell.
The Transmembrane Potential, p. 390
Figure 12-8 (navigator)
• The 3 main requirements for a transmembrane potential (review chapter 3) are:
1. A concentration gradient of ions (Na+, K+) across the cell membrane
2. The membrane be selectively permeable through membrane channels
3. Passive and active transport mechanisms maintain a difference in charge
across the membrane (resting potential = -70 mV)
•
Passive forces acting across the membrane are chemical and electrical.
1. Chemical gradients:
- concentration gradients of ions (Na+, K+) across the membrane
2. Electrical gradients:
- the charges of positive and negative ions are separated across the
membrane, resulting in a potential difference.
- positive and negative charges attract one another
- if charges are not separated, they will move to eliminate potential
difference, resulting in an electrical current
- how much current a membrane can restrict is called its resistance
Figure 12-9
• Electrochemical gradient:
1. the sum of chemical and electrical forces acting on an ion (Na+, K+)
across a cell membrane is the electrochemical gradient for that ion.
2. chemical gradient of potassium tends to move potassium out of the cell,
but the electrical gradient of the cell membrane opposes this movement
(Figure 12-9a)
3. the transmembrane potential at which there is no net movement of a
particular ion across the cell membrane is the equilibrium potential for
that ion (K+ = -90 mV, Na+ = +66 mV).
4. the electrochemical gradient is a form of potential energy
•
Active forces maintain the cell membrane’s resting potential (-70 mV). The cell
actively pumps out sodium ions (Na+), and pumps in potassium ions (K+). The
sodium-potassium exchange pump (the carrier protein sodium-potassium
ATPase), powered by ATP, exchanges 3 Na+ for each 2 K+, balancing the
passive forces of diffusion.
Table 12-1 summarizes the important features of resting potential.
Changes in the Transmembrane Potential, p. 394
•
The transmembrane potential rises or falls in response to temporary changes in
membrane permeability resulting from opening or closing specific membrane
channels.
•
It is primarily the membrane’s permeability to sodium and potassium ions that
determines transmembrane potential. Sodium and potassium channels are either
passive or active.
•
Passive channels (leak channels) are always open, but their permeability changes
according to conditions.
•
Active channels (gated channels) open and close in response to stimuli. At the
resting potential, most gated channels are closed.
•
Gated channels can be in one of 3 conditions:
1. closed, but capable of opening
2. open (activated)
3. closed, and not capable of opening (inactivated)
Figure 12-10
• There are 3 classes of gated channels:
1. chemically regulated channels:
- open in response to the presence of specific chemicals (e.g. ACh)
at a binding site
- found on the dendrites and cell body of a neuron
2. voltage-regulated channels:
- respond to changes in the transmembrane potential
- characteristic of excitable membrane
- found in axons of neurons, sarcolemma of skeletal muscle fibers,
and cardiac muscle cells
- have an activation gate (opens) and an inactivation gate (closes)
3. mechanically regulated channels:
- open in response to distortion of the membrane
- found in sensory receptors for touch, pressure and vibration
Key
•
•
•
A transmembrane potential exists across the cell membrane.
It is there because (1) the cytosol differs from extracellular fluid in chemical and
ionic composition and (2) the cell membrane is selectively permeable.
The transmembrane potential can change from moment to moment, as the cell
membrane changes its permeability in response to chemical or physical stimuli.
Graded Potentials, p. 396
•
Graded potentials (local potentials) are changes in transmembrane potential that
cannot spread far from the site of stimulation.
•
Any stimulus that opens a gated channel produces a graded potential.
Figure 12-11 (navigator)
• Opening a sodium channel produces a graded potential:
1. The resting membrane is exposed to a chemical that opens the sodium
channel. Sodium ions enter the cell, raising the transmembrane potential.
A shift in transmembrane potential toward 0 mV is called depolarization.
2. The movement of sodium ions through the channel produces a local
current that depolarizes nearby parts of the cell membrane (the graded
potential). The change in transmembrane potential is proportional to the
stimulus.
Figure 12-12
• When the stimulus is removed, the transmembrane potential returns to normal
(repolarization).
•
Opening a potassium channel has the opposite effect of opening a sodium channel
(since positive ions move out of, instead of into the cell), increasing the negativity
of the resting potential (hyperpolarization).
Table 12-2 summarizes the basic characteristics of graded potentials.
•
Local graded potentials, originating on cell dendrites or cell bodies, trigger
specific cell functions (such as exocytosis of glandular secretions or ACh at a
motor end plate) at the synaptic terminals of axons. The link between the two
actions is the action potential.
Action Potentials, p. 398
•
Action potentials are propagated changes in transmembrane potential that affect
an entire excitable membrane.
•
The stimulus that initiates an action potential is a graded depolarization of the
axon hillock, large enough (10 to 15 mV) to reach the threshold level required to
open voltage-regulated sodium channels (usually -60 to -55 mV).
•
A smaller stimulus will produce only a local, graded depolarization.
•
As long as a stimulus exceeds the threshold amount, the action potential is the
same, no matter how large the stimulus may be (much like the pressure on the
trigger of a gun). Either an action potential is triggered, or it is not. This is called
the all-or-none principle.
Figure 12-13 (navigator)
• An action potential is generated in 4 steps:
1. Depolarization to threshold.
2. Activation of sodium channels and rapid depolarization:
- sodium ions rush into the cytoplasm
- the inner membrane surface changes from negative to positive
3. Inactivation of sodium channels and activation of potassium channels:
- at +30 mV, inactivation gates of sodium channels close (sodium
channel inactivation), and potassium channels open, beginning
repolarization
4. Return to normal permeability:
- potassium channels begin to close when the membrane reaches
normal resting potential (-70 mV)
- when potassium channels finish closing, the membrane is
hyperpolarized to -90 mV
- transmembrane potential then returns to resting level, and the
action potential is over
Table 12-3 summarizes the generation of action potentials
The Refractory Period:
• During the time period from the beginning of the action potential to the return to
resting state (the refractory period), the membrane will not respond normally to
additional stimuli.
•
The refractory period is divided into:
1. the absolute refractory period, when sodium channels are either open or
inactivated and no action potential is possible
2. the relative refractory period, when the membrane potential is almost
normal, and a very large stimulus can initiate an action potential.
The Sodium-Potassium Exchange Pump:
• A stimulated neuron can produce 1000 action potentials per second. Maintaining
the concentration gradients of Na+ and K+ over time requires the sodium
potassium pump, which in turn requires energy in the form of ATP (1 ATP for
each exchange of 2K+ for 3 Na+). If a cell ran out of ATP, neurons would stop
functioning.
Propagation of Action Potentials:
• Once an action potential has been generated in the axon hillock, it must be
propagated along the entire length of the axon. The term propagation is used
because it is a series of repeated actions rather than a passive flow.
•
Action potentials travel along axons by:
1. continuous propagation (unmyelinated axons)
2. saltatory propagation (myelinated axons)
Figure 12-14
• Action potentials move along an unmyelinated axon by continuous propagation,
in which the moving action potential affects one segment of the axon at a time:
1. The action potential in segment 1 depolarizes the membrane to +30 mV.
2. A local current depolarizes the 2nd segment to threshold.
3. The 2nd segment develops an action potential, and the 1st segment enters
the refractory period.
4. A local current depolarizes the next segment to threshold, and the cycle
repeats, propagating the action potential along the axon in 1 direction
only, at a speed of about 1 meter/sec.
Figure 12-15
•
An action potential moves along a myelinated axon by saltatory propagation,
which is faster and uses less energy than continuous propagation. (Continuous
propagation cannot occur in myelinated axons because of the insulating effect of
the myelin.) In saltatory propagation, the local current produced by the action
potential “jumps” from node to node along the axon, so depolarization occurs
only at the nodes.
Table 12-4 reviews the key differences between graded potentials and action potentials.
Axon Diameter and Propagation Speed
• Since ion movement is related to the cytoplasm, the diameter of an axon affects
the speed of an action potential -- the larger the diameter, the lower the resistance.
•
There are 3 groups of axons, depending on diameter, myelination and speed of
their action potentials:
1. Type A fibers:
- myelinated
- large diameter
- high speed (140 m/sec)
2. Type B fibers:
- myelinated
- medium diameter
- medium speed (18 m/sec)
3. Type C fibers:
- unmyelinated
- small diameter
- slow speed (1 m/sec)
•
Type A fibers carry the most time-sensitive information to and from the CNS
(senses of position, balance and touch; and motor impulses to skeletal muscles).
All other signals (sensory information, peripheral effectors, and involuntary
muscle and gland controls) travel by Type B and Type C fibers.
Key
•
•
“Information” travels within the nervous system primarily in the form of
propagated electrical signals known as action potentials.
The most important information -- including vision and balance sensations, and
the motor commands to skeletal muscles -- is carried by large-diameter
myelinated axons.
V. Synaptic Activity, p. 404
Objectives
1. Describe the general structure of synapses in the CNS and PNS.
2. Discuss the events that occur at a chemical synapse.
3. Describe the major types of neurotransmitters and neuromodulators, and discuss
their effects on postsynaptic membranes.
•
To be effective, action potentials (also called nerve impulses) must be transmitted
across a synapse, from a presynaptic neuron to a postsynaptic neuron or other type
of postsynaptic cell.
•
There are 2 types of synapses:
1. electrical synapses in which there is direct physical contact between cells,
and
2. chemical synapses in which the signal is transmitted across a gap by a
chemical neurotransmitter.
Electrical Synapses
• Electrical synapses are locked together at gap junctions held together by
connexons which allow ions to pass between the cells, producing a continuous
local current and action potential propagation (Figure 4-2). Electrical synapses
are found in some areas of the brain, eye, and ciliary ganglia.
Chemical Synapses
• Most synapses between neurons and all synapses between neurons and other types
of cells are chemical synapses.
•
At a chemical synapse, the cells are not in direct contact. An arriving action
potential may or may not be propagated to the postsynaptic cell, depending on the
amount of neurotransmitter released and the sensitivity of the postsynaptic cell.
•
There are 2 classes of neurotransmitters, based on their effects on postsynaptic
membranes:
1. Excitatory neurotransmitters cause depolarization and promote action
potentials.
2. Inhibitory neurotransmitters cause hyperpolarization and suppress action
potentials.
•
However, the effect of a neurotransmitter on a postsynaptic membrane depends on
the properties of the receptor, not on the nature of the neurotransmitter.
•
For example, the neurotransmitter acetylcholine (ACh) usually promotes action
potentials, but inhibits neuromuscular junctions in the heart.
Figure 12-16
• Synapses that release acetylcholine are cholinergic synapses. ACH is released at:
1. all neuromuscular junctions involving skeletal muscle fibers
2. many synapses in the CNS
3. all neuron-to-neuron synapses in the PNS
4. all neuromuscular and neuroglandular junctions within the parasympathetic
division of the ANS
•
Events at a Cholinergic Synapse
1. An action potential arrives and depolarizes the synaptic knob.
2. Extracellular calcium ions enter the synaptic knob, triggering the exocytosis
of ACh.
3. ACH binds to receptors and depolarizes the postsynaptic membrane.
4. AChE breaks down ACh into molecules of acetate and choline
Table 12-5 summarizes the events that occur at a cholinergic synapse
Synaptic Delay
• A 0.2 - 0.5 msec synaptic delay occurs between the arrival of the action potential
at the synaptic knob and the effect on the postsynaptic membrane -- a long time
relative to the speed of the action potential along the axon. The fewer synapses
involved in relaying a message, the faster the response. Reflexes are important to
survival because they may involve only one synapse.
Synaptic Fatigue
• When a neurotransmitter cannot be recycled fast enough to meet the demand of an
intense stimulus, synaptic fatigue occurs. The synapse becomes inactive until
ACh is replenished.
The Activities of Other Neurotransmitters, p. 408
•
Key
•
At least 50 neurotransmitters have been identified so far, including certain amino
acids, peptides, prostaglandins, ATP, and some dissolved gases. Other than
acetylcholine, some of the most important neurotransmitters are:
- norepinephrine (NE):
- found in the brain and portions of the ANS
- effect is excitatory and depolarizing
- synapses that release NE are called adrenergic synapses
- dopamine:
- a CNS neurotransmitter
- may be excitatory or inhibitory
- involved in Parkinson’s disease and cocaine use
- serotonin:
- a CNS neurotransmitter
- affects attention and emotional states
- gamma aminobutyric acid (GABA):
- has an inhibitory effect
- its functions in the CNS are not well understood
At a chemical synapse, a synaptic terminal releases a neurotransmitter that binds
to the postsynaptic cell membrane.
•
•
•
•
The result is a temporary, localized change in the permeability or function of the
postsynaptic cell.
This change may have broader effects on the cell, depending on the nature and
number of stimulated receptors.
Many drugs affect the nervous system by stimulating receptors that otherwise
respond only to neurotransmitters.
These drugs can have complex effects on perception, motor control and emotional
states.
Neuromodulators, p. 408
•
In addition to neurotransmitters, synaptic knobs may release other chemicals,
called neuromodulators. (There may be little functional difference between
neurotransmitters and neuromodulators.) Some characteristics of neuromodulators
are:
1. Their effects are long-term and slow to appear.
2. They trigger responses that involve multiple steps and intermediary
compounds.
3. They may affect the presynaptic membrane, postsynaptic membrane, or
both.
4. They can be released alone or with a neurotransmitter.
•
Neuropeptides are neuromodulators that act by binding to receptors and activating
enzymes.
•
Opioids are neuromodulators that bind to the same receptors as opium or
morphine, and probably function to relieve pain. There are 4 classes of opioids in
the CNS:
1. endorphins
2. enkephalins
3. endomorphins
4. dynorphins
Table 12-6 lists the major neurotransmitters and neuromodulators of the brain and spinal
cord, and their primary effects.
How Neurotransmitters and Neuromodulators Work, p. 409
•
There are 3 groups of neurotransmitters and neuromodulators:
Figure 12-17a
1. Compounds that have a direct effect on membrane potential:
- ionotropic effects
- open or close gated ion channels
Figure 12-17b
2. Compounds that have an indirect effect on membrane potential:
- work through second messengers
- the link between the neurotransmitter (first messenger) and the second
messenger may be a G protein (enzyme complex) which binds GTP.
- binding may activate the enzyme adenylate cyclase, which produces the
second messenger cyclic AMP.
Figure 12-17c
3. Lipid soluble gases that affect the inside of the cell:
- nitric oxide (NO) and carbon monoxide (CO)
- bind to enzymes in brain cells
VI. Information Processing by Individual Neurons, p. 412
Objectives
1. Discuss the interactions that make possible the processing of information
in neural tissue.
•
A single neuron has many dendrites and can receive many neurotransmitter
messages at the same time -- some excitatory, some inhibitory. The net effect on
the axon hillock, which determines whether or not an action potential will be
produced, is the simplest level of information processing.
Postsynaptic Potentials, p. 412
•
The graded potentials that develop in a postsynaptic cell in response to
neurotransmitters are called postsynaptic potentials. The 2 major types of
postsynaptic potentials are:
1. excitatory postsynaptic potential (EPSP):
- a graded depolarization of the postsynaptic membrane
2. inhibitory postsynaptic potential (IPSP):
- a graded hyperpolarization of the postsynaptic membrane
•
A neuron that receives many IPSPs is inhibited from producing an action
potential, because it increases the amount of stimulation necessary to reach
threshold.
Figure 12-18 (navigator)
•
A single EPSP is not enough to trigger an action potential. EPSPs (and IPSPs)
must combine through a process called summation. If the resulting depolarization
is large enough, an action potential is triggered.
•
There are 2 forms of summation:
1. temporal summation (multiple times):
- rapid, repeated stimuli at a single synapse
2. spatial summation (multiple locations):
- many stimuli, arriving simultaneously at multiple synapses
•
As EPSPs accumulate, raising the transmembrane potential closer to threshold,
the neuron becomes facilitated, making it easier for even a small stimulus to
trigger an action potential.
Figure 12-19
• Neuromodulators and hormones can change a membrane’s sensitivity to
excitatory or inhibitory neurotransmitters, shifting the balance between EPSPs
and IPSPs.
Presynaptic Inhibition and Presynaptic Facilitation, p. 414
Figure 12-20
• Synapses may be found not only between cell bodies and dendrites, but also
between the axons of 2 neurons (an axoaxonal synapse).
•
An axoaxonal synapse at a synaptic knob may either decrease the amount of
neurotransmitter released by the presynaptic membrane (presynaptic inhibition) or
increase the amount of neurotransmitter released by the presynaptic membrane
(presynaptic facilitation).
The Rate of Generation of Action Potentials, p. 415
•
The information received by a postsynaptic cell is often interpreted only in terms
of the frequency or rate of action potentials received. The frequency of action
potentials in turn depends on the degree of depolarization above threshold.
Holding the membrane above threshold for an extended period of time has the
same effect as applying a second, larger stimulus, reducing the relative refractory
period.
Table 12-7 summarizes the basic principles of information processing.
Key
•
In the nervous system, the changes in transmembrane potential that determine
whether or not action potentials are generated represent the simplest form of
information processing.
SUMMARY
In chapter 12 we learned:
- about neural tissue and its basic functional unit, the neuron
- the anatomical divisions of the nervous system
- central nervous system and peripheral nervous system
- nerves and axons
- the functional divisions of the nervous system
- the afferent division and its receptors
- the efferent division and its effectors
- the somatic and autonomic nervous systems
- the structure of neurons
- organelles of the neuron
(neurofilaments, neurotubules, neurofibrils)
- structures of the axon
(axon hillock, initial segment, axoplasm)
- the synapse and neurotransmitters
- the classification of neurons
- structural classifications
(anaxonic, bipolar, unipolar and multipolar)
- functional classifications
(sensory, motor and interneurons)
- the 4 types of neuroglia
- ependymal, astrocytes, oligodendrocytes, microglia
- about ganglia and neurons of the PNS
- satellite cells, Schwann cells
- about repair of neurons in the PNS (Wallerian regeneration)
- the transmembrane potential
- electrochemical gradient
- passive and active channels
- gated channels
- chemically regulated, voltage-regulated, and mechanically regulated
- graded potentials
- depolarization and hyperpolarization
- action potentials
- threshold
- refractory period
- continuous and saltatory propagation
- 3 types of axons (A, B and C fibers)
- the transmission of a nerve impulse across a synapse
- presynaptic and postsynaptic neurons
- electrical and chemical synapses
- excitatory and inhibitory neurotransmitters
- cholinergic synapses (ACh)
- other neurotransmitters (NE, dopamine, seratonin, GABA)
- neuromodulators
- direct, indirect and lipid-soluble gases
- information processing
- integration of postsynaptic potentials
- EPSPs and IPSPs
- spatial and temporal summation
- presynaptic inhibition and facilitation
- the rate of generation of action potentials