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CHAPTER TWELVE
The Nervous System and Neural Tissue
12.1 Basic Structure and Function of The Nervous System
A. Functions of Nervous Tissue
1. Use sensory receptor to monitor changes both inside and outside the body (INPUT)
2. Process and interpret sensory input (INTEGRATION)
3. Effects a response appropriate to the stimulus (MOTOR OUTPUT)
4. Maintain homeostasis by acting as a regulatory or control center.
B. Divisions of the Nervous System
1. Central Nervous System or CNS
a.
Composed of the brain and spinal cord
b.
This is the “seat of all mental activity”: It interprets sensory input and dictates motor
responses based on past experiences, reflexes, and current conditions.
2. Peripheral Nervous System or PNS
a.
Composed of the cranial and spinal nerves.
b.
These are the communication lines between the CNS and the rest of the body.
c.
There are two divisions of the peripheral nervous system:
i. Sensory Division (afferent)
A. Conducts impulses from sensory receptors to the CNS.
B. Sensory receptors are within somatic (skin, muscle, joints) and visceral
(organs) systems.
C. The sensory division is considered the input region.
ii. Motor Division (efferent)
A. Consists of the motor neurons which conduct impulses from the central
nervous system to the effectors (muscles and glands).
B. The motor division is considered the output region.
C. The motor division is further subdivided into:
1.
Somatic nervous system which is a voluntary system that
conducts impulses from the CNS to skeletal muscles
2.
Autonomic nervous system which is involuntary because it
conducts impulses from the CNS to cardiac muscles, smooth
muscle, and glands.
a. Sympathetic nervous system =mobilizes the body during
emergency situations “fight or flight”.
b. Parasympathetic nervous system = conserves energy and
promotes non-emergency functions such as during “rest
and digest”.
12.2 Neural Tissue
A. Nervous tissue is composed of densely packed, intertwined cells of two specific types:
B. Neurons = the functional cells of nervous tissue responsible for receiving, interpreting, and sending
stimuli.
1.
Neurons exhibit several unique characteristics:
a. Excitable=neurons possess a polarized membrane which allows them to conducts
messages in the form of a nerve impulse from one part of the body to another.
b. Longevity=neurons can function for 100+ years. The neurons you’re born with are
essentially the same neurons you die with.
c. High metabolic rate=neurons cannot survive more than a few minutes without oxygen,
glucose, or ATP therefore they possess a large number of mitochondria and tremendous
vascularity.
2.
d. Can be large=neurons are among some of the largest cells in the body.
e. Amitotic=most CNS neurons lose their ability to divide after they assume the role as
communication lines. They do not possess centrioles. PNS nerves may regenerate.
Parts of the typical neuron:
a. Cell body (also called soma) is the enlarged metabolic region of the cell where the
nucleus is located.
i. Clusters of cell bodies in the CNS are often called nuclei.
ii. Clusters of cell bodies within the PNS are called ganglia.
iii. Neurons possess large numbers of rough ER clustered within the cell body to
form Nissl bodies.
iv. The cytoplasm surrounding the nucleus is called the perikaryon and it contains
numerous neurofilaments that make up the cytoskeleton of the neuron.
b. Processes=cellular processes are either called tracts (in CNS) or nerves (in PNS).
i. Dendrites
A. A neuron process which possess large surface area because of its
numerous branches known as dendritic spines.
B. Receive chemical signals as well as conduct electrical signals towards the
cell body.
C. These electrical signals are not nerve impulses; they are called graded
potentials.
ii. Axons
A. Axons are capable of generating action potentials and transmit nerve
impulses away from the cell body during axoplasmic transport.
Whenever the signal travels from the axon terminals back toward the cell
body, it is called retrograde flow.
B. The plasma membrane surrounding the axon is called the axolemma while
the cytoplasm is called the axoplasm.
C. The axon forms at a tapered area called the axon hillock. This is the
“trigger zone” because graded potentials must reach this area of the
neuron before they can be converted into action potentials.
D. Although each neuron possesses only one axon it may branch to form
collateral axons.
E. As the axon approaches the next cell, many fine extensions branch from
its end forming telodendria each ending in an axon terminal (also known
as synaptic terminal).
F. The synaptic terminals possess many synaptic vesicles that contain
neurotransmitters which are used to cross the synapse or synaptic cleft
found between the presynaptic cell and the postsynaptic cell.
G. There are several types of synapses:
1. Synapses with another neuron.
2. Synapses with a skeletal muscle cell forming a neuromuscular
junction.
3. Synapses with a gland or glandular cells.
c. Myelin sheath
i. Axons are often covered with a whitish, fatty (protein-lipoid), and segmented
myelin sheath. Myelin protects and electrically insulates fibers from one
another.
A.
Myelinated fibers conduct impulses rapidly. These form the white
matter of the nervous tissue.
B.
Unmyelinated fibers conduct impulse quite slowly. This type forms the
gray matter of the nervous tissue.
ii. Myelin sheaths are formed by Schwann cells in the PNS which envelop and then
rotate around the nerve fibers.
A.
The exposed portion of the Schwann cell is called the neurilemma.
B.
Gaps between each Schwann cell are called Nodes of Ranvier which also
aids in impulse transmission.
iii. Within the CNS, the myelin sheath is formed by oligodendrocytes.
3.
Classification of Neurons
a. Neurons are classified by their structure (number of cell processes):
i. Anaxonic neurons = lack the features that typically distinguish dendrites from
axons; all the cell processes look alike. These are found in the brain and in
special sense organs. Their functions are poorly understood.
ii. Bipolar neurons = have two cell processes – one dendrite and one axon. Bipolar
neurons are rare but occur in the retina of the eye and within the nasal mucosa.
iii. Unipolar neurons = the dendrites and axon are continuous and basically fuse
with the cell body located off to the side. Most sensory neurons of the PNS are
unipolar.
iv. Multipolar neurons = have one axon and two or more dendrites. These are the
most common neurons of the CNS and all motor neurons that control skeletal
muscles are multipolar neurons.
b. Neurons are also classified by their functions:
i. Sensory (afferent) neurons=transmit impulses from sensory receptors towards
the CNS.
A.
Unipolar neurons-skin or internal organs to the CNS for interpretation
B.
Bipolar neurons-special sense organs; retina
ii. Association (interneurons) neurons=transmit impulses within CNS (usually
between sensory and motor)
A.
Found in CNS (brain and spinal cord) only
B.
Mostly multipolar and 99% of all neurons
iii. Motor (efferent) neurons=transmit impulses away from CNS to the organs and
glands
A.
Almost all motor neurons are multipolar.
B.
Multipolar neurons have the cell body located within the CNS and the
axons form the neuromuscular junctions with effector cells.
C. Neuroglial cells (which are often simply called glial cells). Neuroglial cells feed, protect, and insulate
neurons. It is estimated that there are 700-900 neuroglial cells per neuron. Neuroglia differ within the
CNS and PNS.
1.
Neuroglial cells of the CNS:
a. Astrocytes
i. Make up half of all neural volume and are star-shaped.
ii. They possess numerous projections with bulbous ends that cling to neurons and
capillaries therefore they serve as connections between neurons and
blood/nutrient supply. This is called the blood-brain barrier.
iii. Astrocytes control the chemical environment around neurons by regulating ions,
nutrients, dissolved gas concentrations, and hormones. They also absorb and
recycle neurotransmitters that cannot be broken down and they form scar tissue
after injury.
b. Microglia
2.
i. Ovoid cells with highly branched processes
ii. These act as macrophages that engulf microbes and dead neural cells as well as
remove cellular debris, waste products, and pathogens.
c. Oligodendrocytes
i. Few branches (at least compared to Astrocytes and Microglia).
ii. These cells line up along thicker neuron fibers in the CNS and wrap their
extensions around nerve fibers forming the myelin sheath to insulate the neurons
from each other.
d. Ependymal cells
i. Line the central cavities of brain and spinal cord, creating a barrier between the
CNS cavities and the tissues surrounding the cavities.
ii. Ependymal cells assist in producing, monitoring, and circulating cerebrospinal
fluid, CSF.
iii. They use their cilia to circulate the CSF within the cavities of the CNS.
Neuroglial cells of the PNS:
a. Schwann cells
i. Forms the myelin sheath around large nerve fibers in the PNS.
ii. Can also act as phagocytic cells that engulf damaged or dying nerve cells and are
important in directing the process of regeneration.
iii. The myelin sheath forms by the Schwann cells wrapping around and around the
axon. The inner compressed layers are the myelin while the outermost
metabolically active layer is called the neurilemma. Gaps between the Schwann
cells are called the Nodes of Ranvier.
b. Satellite cells
i. These surround the nerve cell body.
ii. May aid in controlling chemical environment about the neuron much like the
astrocytes of the CNS.
12.3 Neurophysiology: Function of the Nervous Tissue
A. In the body, electrical currents correspond to the flow of ions across cellular membranes.
B. The transmission of a nerve impulse is very similar to the process of a muscle contraction that we
discussed in chapter nine and can be more or less summed up in the following steps.
1.
Resting membrane potential (polarized cell membrane).
2.
Depolarization and production of a graded potential.
3.
Conversion of the graded potential to an action potential and then the propagation of an Action
potential down the length of the axon to the synaptic terminals.
4.
Repolarization or re-establishing the resting membrane conditions.
C. Ion channels play a crucial role in establishing ion concentrations on either side of the neuron cell
membrane.
1.
Passive, or leakage, channels are protein channels that are always open allowing certain ions to
pass through. These channels are responsible for maintaining the resting membrane potential
and are located all over the surface of a neuron.
2.
Active, or gated, channels are proteins channels that open and close in response to various
signals:
a. Chemically-gated ion channel=open when the appropriate neurotransmitter (chemical)
binds to the receptor site on the protein. These are important for depolarization and
production of the graded potential and are located only on the dendrites and cell body.
b. Voltage-gated ion channels= open in response to changes in the membrane potential.
These are important in the generation and propagation of an action potential and are
located only on the axon.
c. Mechanical-gated ion channels=open in response to physical deformation of the
membrane surface caused by exposure to touch, pressure, or vibration.
12.4 Action Potential
A. RESTING MEMBRANE POTENTIAL (the cell membrane is polarized)
1.
The resting membrane potential exists only across the membrane; that is to say, the bulk
solutions inside and outside the cell are electrically neutral. The resting membrane potential is
approximately -70mV.
2.
The inside of the neuron’s membrane is negatively charged while the outside of the neuron’s
membrane is positively charged.
3.
Na+ is in highest concentration outside the cell while K+ is in highest concentration of the inside
of the cell.
4.
All voltage-gated Na+ and K+ ion channels are closed so that the neuron cell membrane is
relatively impermeable to the two ions. Passive gates for both ions remain open but movement is
minimal.
B. DEPOLARIZATION and formation of the GRADED POTENTIAL
1.
The presence of a neurotransmitter within the synaptic cleft opens the Na+ chemically-gated
channels and sodium begins to rush into the neuron down its concentration gradient.
2.
In that local patch of the membrane, the interior side of the membrane begins to change from a
negative charge to a more positive charge while the exterior changes from positive to negative.
Any change from -70 mV toward 0 is called depolarization. As depolarization occurs, the
membrane potential moves from -70 mV toward -60 mV.
3.
This switch in charge begins to spread across the dendrites and cell body and is now called a
graded potential.
C. PROPAGATION of an ACTION POTENTIAL
1.
If the graded potential reaches the axon hillock, voltage-gated channels within the axon hillock
open which cause the Na+ ions to flow into the axon switching the charge across the axolemma.
2.
This causes the voltage channels to start opening all the way down the axon and the action
potential now moves down the length of the axon. This is called the propagation of the action
potential and generates a change in charge from -60 mV to +30 mV.
3.
Once generated, an action potential cannot be stopped!! This is referred to as the All or None
Principle.
a. Myelin on the myelinated nerves causes the local depolarization to jump to the next Node
of Ranvier and then from node to node. This type of propagation is called saltatory
conduction which is very rapid.
b. On unmyelinated nerves, local depolarizations must spread to sites immediately adjacent
to each other creating a continuous conduction pattern. This type of propagation is
relatively slow.
D. REPOLARIZATION
1.
The removal of the neurotransmitter from the synaptic cleft causes the Na+ channels to close so
that no additional sodium enters the cell. HOWEVER, potassium ions are allowed to flow out of
the cell.
2.
The rapid outflow reduces the total number of positive charges within the cell causing the charge
to switch back across the membrane (from +30mV to -70 mV).
3.
The membrane now goes back to positive outside and negative inside. Because the K+ channels
stay open longer than the Na+ channels did, the membrane may become hyperpolarized (90mV).
4.
There is another problem though. Na+ is stuck inside (supposed to be outside) and K+ is stuck
outside (supposed to be inside).
5.
6.
At this point, the sodium-potassium pump turns on and pumps 3 Na+ ions to the outside for
every two K+ ions pumped to the inside. This reestablishes the resting location of the ions while
also reestablishing the -70mV.
While the cell is repolarizing, the cell is insensitive to another stimulus and is called the
refractory period.
12.5 Communication Between Neurons
A. Graded potentials – associated with the dentrites of a neuron: depolarizating or hyperpolarizing.
1. Generator potential – generates an action potential within that neuron.
2. Receptor potential – found in sensory receptor neurons, result in the release of
neurotransmitters at the synapse.
B. Synapses - many electrical impulses must travel from neuron to neuron or from neuron to muscles or
glands. When the message has to be sent from one cell to another, the cells are connected by a synapse.
1. The cell conducting the impulse towards the synapse is called the Presynaptic neuron.
2. The cell transmitting the impulse away from the synapse is the Postsynaptic Neuron.
3. The Presynaptic and Postsynaptic Neurons communicate via either an electrical synapse or a
chemical synapse.
a. Electrical synapses are where the cell membranes of the two neurons are actually
connected by gap junctions that allow ions from the Presynaptic neuron to flow into the
postsynaptic neuron so that the propagation of the action potential is continued in the next
neuron. This type of synapse can be found in both the CNS and PNS but are relatively
rare.
b. Chemical synapses are where a synaptic cleft is formed between the two neurons and the
Presynaptic neuron releases neurotransmitters that stimulate an action potential in the
postsynaptic neuron.
4. Postsynaptic neurons receive either depolarization (excitatory postsynaptic potential EPSP) or
hyperpolarization signals (inhibitory postsynaptic potentials IPSP).
5. Summation is the sum of all of the postsynaptic potentials added together.
a. Spatial summation – associating the activity of multiple inputs to a neuron with each
other.
b. Temporal summation – multiple action potentials from a single cells resulting in
significant change in the membrane potential.
C. Neurotransmitters
1. Neurotransmitters released at a synapse may have an excitatory or inhibitory effect. The effect
on the axon’s initial segment reflects a summation of the stimuli arriving at any moment. The
frequency of action potentials generated is an indication of the degree of sustained depolarization
at the axon hillock.
2. Neurons may be facilitated or inhibited by extracellular chemicals other than neurotransmitters.
3. The responses of postsynaptic neurons to the activation of a presynaptic neuron can be altered by
1) the presence of chemicals that cause facilitation or inhibition at the synapse, 2) activity under
way at other synapses affecting the postsynaptic cell, and 3) modification of the rate of
neurotransmitter release through stimulation or inhibition by regulatory neurons.
4. Information is relayed in the form of action potentials. In general, the degree of sensory
stimulation or the strength of the motor response is proportional to the frequency of the action
potentials.
5. Although there are more than 100 different neurotransmitters that each work in different ways,
the following table introduces just a few of these:
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