12-1

Chapter 12 - NERVOUS TISSUE

(figures relate to Tortora/Grabowski 9th edition of Principles of Anatomy and Physiology)

 

I.          OVERVIEW OF THE NERVOUS SYSTEM

A.  Structures and Functions of the Nervous System

1 .   The nervous system is made up of the brain, cranial nerves, spinal cord, spinal nerves, ganglia, enteric plexus, and sensory receptors (Figure 12. la).

a.    The brain is housed within the skull.

b.    Twelve pairs of cranial nerves emerge from the base of the brain
through foramina of the skull.

c.    A nerve is a bundle of hundreds or thousands of axons. each of which
courses along a defined path and serves a specific region of the body.

d.    The spinal cord connects to the brain through the foramen magnum of
the skull and is encircled by the bones of the vertebral column.

e.    Thirty-one pairs of spinal nerves emerge from the spinal cord, each

serving a specific region of the body.

f.    Ganglia, located outside the brain and spinal cord (PNS), are small masses ofnervous tissue, containing primarily cell bodies of neurons.(unmyelinated)

g.    Enteric plexuses help regulate the digestive system.

h.   Sensory receptors are either parts of neurons or specialized cells that

monitor changes in the internal or external environment.

2.   The three basic functions of the nervous system are sensory, integrative, and

motor (see Figure 12. 1b).

a.   The sensory function(PNS) - Sensory neurons sense changes in the internal and external environment through sensory receptors

b.   The integrative function(Spinal Cord) - Association or interneurons analyze and store the sensory information, and make decisions regarding

appropriate behaviors.

c.   The motor function - Motor neurons respond to stimuli by initiating

action.

B   Organization of the Nervous System

1 .   The central nervous system (CNS) consists of the brain and spinal cord

(Figure 12.2).

 

 

12-2

2.              The peripheral nervous system (PNS) consists of cranial and spinal nerves
with sensory (afferent) and motor (efferent) components, ganglia, and sensory
receptors.

3.              The PNS is also subdivided into somatic (voluntary), autonomic (involuntary),
and enteric nervous systems.

a.    The somatic nervous system (SNS) consists of sensory neurons that
conduct impulses from cutaneous and special sense receptors to the
CNS, and motor neurons that conduct impulses from the CNS to
skeletal muscle tissue.

b.    The autonomic nervous system (ANS) contains sensory neurons from
visceral organs and motor neurons that convey impulses from the CNS
to smooth muscle tissue, cardiac muscle tissue, and glands.

1)  The motor part of the ANS consists of the sympathetic division
and the parasympathetic division.

2)           Usually, the two divisions have opposing actions.

c.    The enteric nervous system (ENS) consists of neurons in enteric
plexuses that extend the length of the GI tract.

1)  Many neurons of the enteric plexuses function independently,
of the ANS and CNS.

2)           Sensory neurons of the ENS monitor chemical changes within
the GI tract and stretching of its walls, whereas enteric motor
neurons govern contraction of GI tract organs, and activity of
the GI tract endocrine cells.

II.        HISTOLOGY OF THE NERVOUS SYSTEM

A. Neurons

1.  Neurons have the property of electrical excitability.

2.              Most neurons, or nerve cells, consist of a cell body (soma), many dendrites, and usually a single axon (Figure 12.3).

a.   The cell body contains a nucleus, lysosomes, mitochondria, a Golgi complex, cytoplasmic inclusions such as lipofuscin, chromatophilic substances( Nissl bodies = rough ER), and neurofibrils (cytoskeleton).

 

12-3

b.    The dendrites conduct impulses from receptors or other neurons to the
cell body.                                                                  

 

c.   The axon conducts nerve impulses from the cell body to the dendrites or
cell body of another neuron or to an effector organ of the body (muscle
or gland).

d.    The site of functional contact between two neurons or between a
neuron and an effector cell is called a synapse.

3.            Axonal transport, a natural mechanism of intracellular transport in neurons, is
exploited by certain microorganisms to reach other parts of the nervous
system. (Herpes Virus, Chicken Pox(Shingles))

4.  Fast axonal transport is the route by which some toxins (such as toxins
produced by Clostridium tetani bacteria) and disease causing viruses make
their way from axon terminals near skin cuts to cell bodies, where they cause
damage. (Clinical Application)

5.            Structural and Functional Variations in Neurons

a.    Both structural and functional features are used to classify the various
neurons in the body.

b.   On the basis of the number of processes extending from the cell body

(structure), neurons are classified as multipolar, bipolar, and unipolar (Figure 12.4).

c.   Most neurons in the body are interneurons and are often named for the
histologist who first described them. Examples are Purkinje cells

(Figure12.5a) or Renshaw cells. Interneurons named for the shape or appearance include pyramidal cells (Figure 12.5b).

B. Neuroglia

1.            Neuroglia (or glia) are specialized tissue cells that support neurons, attach
neurons to blood vessels, produce the myelin sheath around axons, and carry
out phagocytosis.

2.      The types of neuroglia include astrocytes, oligodendrocytes, microglia,
ependymal cells, neurolemmocytes, (Schwann cells), and satellite cells.

3.            Table 12.1 summarizes the types of neuroglia.

 

12-4

C.   Myelination

1 .   A multilayered lipid and protein covering called the myelin sheath and produced by Schwann cells and oligodendrocytes surrounds the axons of most neurons (Figure 12.6).

2.             The sheath electrically insulates the axon and increases the speed of nerve
impulse conduction.

3.             Schwann cells produce the myelin sheath in the PNS.

a.    These cells have neurolemma (sheath of Schwann).

b.    The neurolemma aids in regeneration in an injured axon.

c.    The myelin sheath has gaps called neurofibril nodes or nodes of
Ranvier (Figure 12.4) along the axon.

4.    Oligodendrocytes form myelin sheaths for CNS axons (Exhibit 12.1).

a.    No neurolemma is formed.

b.   No regrowth after injury occurs.

D.   Gray and White Matter

1.  White matter is composed of aggregations of myelinated processes whereas
gray matter contains nerve cell bodies, dendrites, and axon terminals or
bundles of unmyelinated axons and neuroglia (Figure 12.7).

2.            In the spinal cord, gray maner forms an H-shaped inner core, surrounded by
white matter; in the brain a thin outer shell of gray matter covers the cerebral
hemispheres.

3.            A nucleus is a mass of nerve cell bodies and dendrites inside the CNS.

 

III      ELECTRICAL SIGNALS IN NEURONS

A. Excitable cells communicate with each other by action potentials or graded potentials.

1.   Action potentials allow communication over short and long distances whereas

graded potentials allow communication over short distances only. 2.   Production of both types of potentials depend upon the existence of a resting membrane potential and the presence of certain types of ion channels.

a.    The membrane potential is an electrical voltage across the membrane.

b.    Graded and action potentials occur because of ion channels in the
membrane that allow ion movement across the membrane that can
change the membrane potential.

12-5

B.   Ion Channels

1.   The two basic types of ion channels are leakage (nongated) and gated.

a.    Leakage (nongated) channels are always open.

b.    Gated channels open and close in response to some sort of stimulus:
voltage changes, ligands (chemicals), mechanical pressure, membrane
potential (Figure 12.8a), chemical stimuli (Figure 12.8b), mechanical
vibration or pressure.

C.   Resting Membrane Potential - typical value is -70mV and is said to be polarized.

1.  The membrane of a nonconducting neuron is positive outside and negative
inside owing to the distribution of different ions across the membrane and the
relative permeability of the membrane toward Na+ and K+ (Figure 12.9a).

2.              The resting membrane potential is determined by the unequal distribution of
ions across the plasma membrane and the selective permeability of the
membrane to Na+ and K+ .

3.              The sodium-potassium pumps compensate for slow leakage of Na+ into the
cell by pumping it back out.

D.   Graded Potentials - short distance only

1.  A graded potential is a small deviation from the resting membrane potential
that makes the membrane either more polarized (hyperpolarizatio) or less
polarized (depolarization) (Figure 12.10).

2.              Graded potentials occur most often in the dendrites and cell body of a neuron.

3.              The signals are graded, meaning they vary in amplitude (size), depending on
the strength of the stimulus and localized.

E.   Action Potential - long or short distances

 1. An action potential (AP) or impulse is a sequence of rapidly occurring events that decrease and eventually reverse the membrane potential (depolarization) and then restore it to the resting state (repolarization).

2.              During an action potential, voltage-gated Na+ and K+ channels open in
sequence (Figure 12.11).

3.              Rapid opening cf voltage-gated Na+ channels causes depolarization. If the
depolarization is to threshold, the membrane potential reverses.

 

4.              The slower opening of voltage-gated K+ channels and closing of previously
open Na+ channels leads to repolarization, the recovery of the resting
membrane potential.

5.              During the refractory period (Figure 12.11), another impulse cannot be
generated at all or can be triggered only by a suprathreshold stimulus.

6.              An action potential conducts or propagates (travels) from point to point along
the membrane; the traveling action potential is called a nerve impulse.

7.              Local anesthetics prevent opening of voltage-gated Na+ channels so nerve

impulses cannot pass the obstructed region.

12-6

8.   According to the all-or-none principle, if a stimulus is strong enough to
generate an action potential, the impulse travels at a constant and maximum
strength for the existing conditions; a stronger stimulus will not cause a larger
impulse.

9.   The propagation speed of a nerve impulse is not related to stimulus strength.

a.    Larger-diameter fibers conduct impulses faster than those with smaller
diameters.

b.    Myelinated fibers conduct impulses faster than unmyelinated fibers.

c.    Nerve fibers conduct impulses faster when warmed and slower when cooled.

10. The intensity of a stimulus is measured by the rate of impulse production, i.e.,
the frequency of action potentials.

IV.       SIGNAL TRANSMISSION AT SYNAPSES

A.   A synapse is the functional junction between one neuron and another or between a
neuron and an effector such as a muscle or gland.

B.   Electrical Synapses

1.    Ionic current spreads directly from one cell to another through gap junctions
(Figure 4.1).

2.   Allow faster communication, can synchronize the activity of a group of
neurons or muscle fibers, and may set up two-way transmission of impulses.

C.   Chemical Synapses

1.    There is only one-way information transfer from a presynaptic neuron to a
postsynaptic neuron (Figure 12.14).

2.   Neurotransmitters at chemical synapses cause either an excitatory or
inhibitory graded potential.

a.    An excitatory neurotransmitter is one that can depolarize or make less

EPSP                                    negative the postsynaptic neuron's membrane, bringing the membrane

(excitatory post synaptic potential) potential closer to threshold (Figure 12.10a).

b.      An inhibitory- neurotransmitter hyperpolarizes the membrane of the

postsynaptic neuron, making the inside more negative and generation
of a nerve impulse more difficult.

IPSP (Inhibitory post synaptic potential)

12-7

3. A neurotransmitter is removed from the synaptic cleft in three ways: diffusion,
enzymatic degradation, and uptake into cells (neurons and glia).

4.  Table 12.3 summarizes the structural and functional elements of a neuron.

     5.   Strychnine poisoning demonstrates the importance of inhibitory neurons.

              (Clinical Application)

D. Neurotransmitters

1 .   Both excitatory and inhibitory neurotransmitters are present in the CNS and PNS: the same neurotransmitter may be excitatory in some locations and inhibitory in others.

 2.  Important neurotransmitters include acetylcholine, glutamate, aspartate, gamma aminobutyric acid, glycine, norepinephrine, epinephrine, and dopamine.

3.    Neurotransmitters can be modified by stimulating or inhibiting
neurotransmitter synthesis, blocking or enhancing neurotransmitter release, stimulating or inhibiting neurotransmitter removal, and/or blocking or activating the receptor site.

4.    Neurotransmitters can be divided into two classes: small-molecule
neurotransmitters and neuropeptides.

a.    Small-molecule neurotransmitters include acetylcholine, amino acids, biogenic amines. ATP and other purines, and gases.

b.    Neurotransmitters consisting of 3-40 amino acids linked by peptide bonds are called neuropeptides.

5.    Table 12.4 describes neuropeptides and some other neurotransmitters.
V.     REGENERATION AND REPAIR OF NERVOUS TISSUE

A. Throughout life, the nervous system exhibits plasticity, the capability for change with learning.

1 .   Despite plasticity, neurons have a limited capacity to repair or replicate themselves.

    2.    In the PNS, damage to dendrites and myelinated axons may be repaired if the cell body remains intact and if Schwann cells are active (Figure 12. 19b)

                                              3.  In the CNS, there is little or no repair of damage to neurons.

12-8

B.  Current research is going on to find ways to promote neurogenesis and to find ways

to encourage and promote regrowth in the CNS.

 

VI.      DISORDERS: HOMEOSTATIC IMBALANCES

A.   Multiple Sclerosis (MS)

1.  Multiple sclerosis is an autoimmune disease that results in the progressive
destruction of myelin sheaths in neurons in the CNS.

2.            Myelin sheaths deteriorate to scleroses, which are hardened scars or plaques,
in multiple regions.

3.            This is a progressive debilitating disease.

B.   Epilepsy

1.  The second most common neurological disorder after stroke is epilepsy, which
affects 1% of the population. It is characterized by short, recurrent, periodic
attacks of motor, sensory, or psychological malfunction called epileptic
seizures.

2.            These seizures are initiated by abnormal synchronous electrical discharges
from millions of neurons in the brain, perhaps resulting from abnormal
reverberating circuits.

3.            Epilepsy has many causes, including brain damage at birth, the most common
cause: metabolic disturbances, infections, toxins, vascular disturbances, head
injuries, and tumors and abscesses of the brain. Most epileptic seizures,
however, are idiopathic (i.e., they have no demonstrable cause).

4.            Epileptic seizures can be eliminated or alleviated by drugs that depress
neuronal excitability.