Nervous Tissue
A. Nervous tissue is divided anatomically into
the central nervous system (CNS), which
includes the brain and spinal cord,
and the peripheral nervous system (PNS), which
includes the nerves outside the CNS and their
associated ganglia.
• B. Nervous tissue is divided functionally into
• Somatic nervous system (SNS)
• Autonomic nervous system (ANS)
• Somatic- provides sensory and motor
innervation to ALL parts of the body
• EXCEPT: viscera, smooth muscle and glands
• Autonomic- provides efferent Involuntary
motor innervation to smooth muscle,
conducting system of the heart and glands.
• It provides afferent sensory innervation from
viscera ( PAIN and REFLEXES ).
• Autonomic is subdivided into Sympathetic
and Parasympathetic division.
• C. Nervous tissue contains two types of cells:
nerve cells (neurons), which conduct electrical
impulses.
• Supporting cells:
•
Neuroglia (glia) in the CNS
•
Schwann , satellite cells in the PNS
• 1. Morphologic classification of neurons
• a. Unipolar neurons possess a single process but are rare in
vertebrates
• b. Bipolar neurons possess a single axon and a single dendrite.
These neurons are present in some sense organs (e.g., the
vestibular/cochlear mechanism).
• c. Multipolar neurons possess a single axon and more than one
dendrite.
• These neurons are the most common type of neuron in vertebrates.
• d. Pseudounipolar neurons possess a single process that extends
• from the cell body and subsequently branches into an axon and
dendrite.
• They are present in spinal and cranial ganglia.
• 2. Functional classification of neurons
• a. Sensory neurons receive stimuli from the internal and
external environment.
• They conduct impulses to the CNS for processing and
analysis.
• b. Interneurons connect other neurons in a chain or
sequence. They most commonly connect sensory and
motor neurons; they also regulate signals transmitted to
neurons.
• c. Motor neurons conduct impulses from the CNS to
other neurons, muscles, and glands.
Neuron structure
• 1.The nucleus is large, spherical, is centrally
located in the soma of most neurons.
• 2.Nissl bodies are composed of polysomes and rough
endoplasmic reticulum (RER).
• 3.The Golgi complex is located near the nucleus, and
mitochondria are scattered throughout the
cytoplasm
• Dendrites receive stimuli (signals) from sensory
cells, axons, or other neurons and convert these
signals into small electrical impulses (action
potentials) that are transmitted toward the soma.
• The dendrite cytoplasm is similar to that of the
soma except that it lacks a Golgi complex.
• Organelles become reduced or absent near the
terminals except for mitochondria, which are
abundant.
• Spines located on the surface of dendrites
increase the area available for synapse formation
with other neurons.
• Axons conduct impulses away from the soma
to the axon terminals without any diminution
in their strength.
• (1) The diameter and length of axons in
different types of neurons vary. Some axons
are as long as 100 centimeters (cm).
• • (2) Axons originate from the axon hillock, a
specialized region of the soma that lacks RER
and ribosomes but contains many
microtubules and neurofilaments; abundance
of the latter may regulate neuron diameter.
• (3) Axons may have collaterals, branching at
right angles from the main trunk.
• (4) Axon cytoplasm (axoplasm) lacks a Golgi
complex but contains smooth endoplasmic
reticulum (SER), RER, and elongated
mitochondria.
• (5) A plasma membrane surrounds the axon
and is called the axolemma.
• (6) Axons terminate in many small branches
(axon terminals) from which impulses are
passed to another neuron or other type of
cell.
•
•
•
•
Neuroglial cells
Astrocytes
Astrocytes are the largest of the neuroglial cells
Functions:
(a) Astrocytes scavenge ions and debris from neuron
metabolism and supply energy for metabolism.
• (b) Form a protective sealed barrier between the pia
mater and the nervous tissue of the brain and spinal
cord.
• (c) They provide structural support for nervous
tissue.
Types of astrocytes
• A. Protoplasmic astrocytes reside mostly in
gray matter and have branched processes that
envelop blood vessels, neurons, and synaptic
areas.
• B. Fibrous astrocytes reside mostly in white
matter and have long, slender processes with
few branches
• Oligodendrocytes
• Oligodendrocytes are neuroglial cells that live
symbiotically with neurons. They are
necessary for the survival of neurons
• They are located in both gray matter and
white matter.
• They possess a small, round, condensed
nucleus and only a few short processes.
• Their electron-dense cytoplasm contains
ribosomes, numerous microtubules, many
mitochondria, RER, and a large Golgi complex.
• Oligodendrocytes produce myelin, a
lipoprotein material organized into a sheath
that insulates and protects axons in the CNS.
Each oligodendrocytes produces myelin for
several axons.
Schwann cells
• Schwann cells are flattened cells with only a
few mitochondria and a small Golgi region.
• These cells perform the same function in the
PNS as oligodendrocytes in the CNS: they
protect and insulate neurons. Schwann cells
form either unmyelinated or myelinated
coverings over neurons.
• A myelin sheath consists of several Schwann
cell plasmalemmae wrapped around a single
axon
Synapses
• 1. Site of synaptic contact
• a. Axodemiritic synapses are located between an
axon and a dendrite.
• b. Axosomatic synapses are located between an
axon and a soma.
• The CNS primarily contains axodendritic and
axosomatic synapses.
• c. Axoaxonic synapses are located between axons.
• d. Dendrodendritic synapses are located between
dendrites
• 2. Method of signal transmission
• a. Chemical synapses
• These synapses involve the release of a
chemical substance (neurotransmitter
• or neuromodulator) by the presynaptic cell,
• b. Electrical synapses
• These synapses involve movement of ions
from one neuron to another via gap junctions
Nerve Fibers
• Nerve fibers are individual axons enveloped by a
myelin sheath or by Schwann cells in the PNS (or
oligodendrocytes in the CNS).
• A. Myelin sheath
• 1. The myelin sheath is produced by oligodendrocytes
in the CNS and by Schwann cells in the PNS.
• 2. It consists of several spiral layers of the plasma
membrane of an oligodendrocyte or Schwann cell
wrapping around the axon.
• 3. It is not continuous along the length of the axon but
is interrupted by gaps called nodes of Ranvier.
• B. Nodes of Ranvier are regions along the axon
that lack myelin and represent discontinuities
between adjacent Schwann cells (or
oligodendrocytes).
• 1. In the PNS, the axon at the nodes is covered
by Schwann cells. In the CNS, however, the
axon may not be covered by oligodendrocytes.
• 2. The axolemma at the nodes contains many
Na+ pumps and displays, in electron
micrographs, a characteristic electron density.
Ganglia
• Ganglia are encapsulated aggregations of
neuronal cell bodies (soma) located outside the
CNS.
• A. Autonomic ganglia are motor ganglia in which
axons of preganglionic neurons synapse on
postganglionic neurons.
• B. Craniospinal ganglia are sensory ganglia
associated with most cranial nerves and the
dorsal roots of spinal nerves (dorsal root ganglia).
Unlike autonomic ganglia, craniospinal ganglia do
not have synapses.
• neuron