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Medical Biology
Dr:Fadia Al-khayat
Muscular and Nervous Tissues
Muscular Tissues
Characteristics of the Muscular Tissue
Muscle has four major functional characteristics: contractility, excitability, extensibility, and
elasticity. Contractility refers to the capacity of muscle to contract or shorten forcefully.
Excitability means that muscle responds to stimulation by nerves and hormones, making it
possible for the nervous system and (in some muscle types) the endocrine system, to regulate
muscle activity. Extensibility means that muscles can be stretched to their normal resting length
and beyond to a limited degree. Elasticity means that if muscles are stretched, they recoil to
their original resting length.
Function of the Muscular Tissue
The muscles are responsible for the movement of the body, either partial movement
(locomotion) or the movement of the whole body.
Origin of the Muscular Tissue
The muscular tissues are derived from the mesodermal germ layer.
Basic Elements of the Muscular Tissue
Three basic elements are involved:
1- Muscle fibers.
2- Rich network of capillaries and blood vessels.
3- Connective tissue participating in transmission of contraction.
Morphology
Muscular tissue has very long cells. Their lengths range from 1-40 mm. For this
reason, these cells are called muscle fibers.
Terminology
The following special terminology is associated with muscle tissues:
Cell membrane is called sarcolemma.
Cytoplasm is called sarcoplasm.
Endoplasmic reticulum is called sarcoplasmic reticulum.
Mitochondrium is called sarcosome.
Functional unit is called sarcomere.
The contractile elements are myofibrils and myofilaments.
Structure
Sarcoplasm of the muscle fiber contains numerous very long fine fibers known as myofibrils.
Each myofibril consists of numerous smaller fibrils known as myofilaments. Each
myofilament consists of two elements: thick and thin elements. Thick element consists of
myosin protein, while the thin element consists of actin.
Classification
According to their structure, muscles are either striated (having regular transverse bands, called
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striae, along the length of the fiber) or smooth (with no such bands).
According to their function, muscles are either voluntary (under the control of the will) or
involuntary (not under the control of the will).
According to their structure and function, three types of muscles are found:
1 - Striated voluntary or skeletal muscles.
2- Striated involuntary or cardiac muscles.
3-Smooth involuntary muscles.
Skeletal Muscles (Fig. 1):
 Bundles of cylindrically- shaped muscular fibers.
 Muscle fibers are parallel to each other in regular manner.
 Muscle fibers are multinucleated or syncytial. Nuclei are located peripherally and
elongated or oval in shape.
 The whole muscle is usually tapered at both ends.
Each muscle fiber is enclosed with a thin layer of connective tissue known as the
endomysium. This connective tissue enables each muscle fiber to react
independently of the other fibers when stimulated by nerve impulse. A group of
neighboring muscle fibers, making muscle fasciculus or fascicle or bundle, is
surrounded by a tough sheath of fibrous connective tissue known as the
perimysium. The fascicle of the muscle is surrounded by a coarse connective
tissue known as epimysium. Exterior to the epimysium is a deep fascia which
covers the entire muscle.
The connective tissue in the muscle functions as:
1- Protective and supportive.
2- Connective tissue resists excessive stretching of the muscle to minimize damage. Also
enable muscle to retain to normal shape when external force is remove.
3- Carrier of blood vessels, lymphatics and nerve fibers.
In connection with myofibrils and myofilaments, muscle fibers show alternating dark
anisotropic bands (A bands) and light isotropic bands (I band). Each A band shows a central
area known as the H band. Each I band is bisected by a distinct Z line or Z band.
The segment between two adjacent Z lines is called sarcomere which is longer than A and I
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bands. The sarcomere is not only a structural unit but also the basic functional (contractile)
unit.
Myofibrils are composed of smaller units called myofilaments. These myofilaments are of
two main types of sizes (thick and thin) and chemical composition.
The thick filaments (12-15 nm in diameter) contain mainly myosin. They lie in the center of
the sarcomere occupying the A band.
The thin filaments (5 nm in diameter) contain actin and extend from each side of Z line
through the adjacent I bands and part way into A bands, interdigitating there with thick
filaments. The H band (the central area of the A band) is free of thin filaments. At the M line
(the center of the H band), thick filaments are interconnected by radially arranged fine
filaments. At the Z line, thin filaments of adjacent sarcomere give a characteristic zigzig
appearance to the Z line.
Although striated muscle unusually thought of as being under voluntary control and always
attached to the skeleton, there are exceptions. (1) striated muscle of the tongue may not be
attached to bones; and (2) striated muscles of the pharynx and upper esophagus have no
skeletal attachment and are not voluntarily controlled.
Cardiac Muscles (Fig. 4):
• Cross striated banding pattern is identical to that found in the skeletal muscles.
• One centrally located pale nucleus is present.
• Intercalated discs are present. They represent junctional complex.
• Cardiac fibers often bifurcate or branch
• Cardiac muscles are found only in the walls of the heart (myocardium)
and in the walls of the vena cavae where they enter the right atrium of the heart.
 Cardiac tissue is unique in its ability to contract rhythmically and continuously as a result of
intrinsic cellular activity. Unlike skeletal muscles, cardiac muscle is not normally subject to
voluntary control.
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(Fig. 4): cardiac muscles
Smooth Muscles (Fig. 5):
Elongated, non-striated fibers.
These fibers are fusiform (spindle-like), range from 20 mm in small blood vessels to 500
mm in pregnant uteri.
 The narrowest part lies adjacent to the broadest part of neighboring fiber.
 Fibers with single, centrally located nucleus
 Smooth muscle fibers are found in the walls of blood vessels, walls of
organs of the digestive tract, the urinary bladder and other internal
organs.
 Smooth muscles are innervated by the autonomic nervous system and thus they are under
involuntary nervous control.


(Fig. 5): Smooth muscles
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The Nervous Tissue
Components of the Nervous Tissue
The nervous tissue is composed of two kinds of cells:
1- Neurons or nerve cells are the functional unit of the nervous tissue. They transmit nerve
impulses.
2- Neuroglia or glia or neurolgial cells are connective, supportive and
nutritive but not excitable.
Function of the Nervous Tissue
The nervous system receives stimuli from outside or inside and transfers them toward highly
specified central parts “ brain and spinal cord”.
Location of the Nervous tissue
Anatomically, the nervous system is divided into:
1-Central nervous system (CNS) which is represented by the brain and the spinal cord.
2-Peripheral nervous system (PNS) which is represented by the nerve fibers and ganglia.
Neuron
The cell body of the neuron is called perikaryon. It usually contains a rather large nucleus
surrounded by Nissil bodies and neurofibrils in the cytoplasm (Fig. 6).
Nissil bodies are dense aggregations of rough endoplasmic reticulum. They are present in all
cell processes except the axon and axon hillock. They represent sites of protein synthesis.
Neurofibrils are slender protein filaments that extend throughout cytoplasm. These minute
fibrils converge into axon to form the core of the nerve fiber.
The most remarkable features of the neurons are their cytoplasmic processes: the axon and the
dendrites.
The axon is a single elongated extension of the cytoplasm. It gives no branches along its coarse
except at its termination where it breaks up into the end bulb. It transmits impulses away from
the dendritic zone.
The dendrites function in receiving signals from receptors or other neurons and play an
important part in integration of the information. The dendrites are the short branching
processes of the perikaryon, while the axon is the long single process. Although a neuron
usually has several dendrites, it has only one axon. Some neurons, however, have no axon at
all as in the amacrine cells of the retina.
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Schwann cell (neurolemmocytes)
Special cells in the peripheral nerve system that form myelin sheath around a neuron’s axon ,
these cells do not cover the entire axon but there are unmyelinated gaps between myelin
sheaths surrounding an axon known as nodes of Ranvier .,these nodes play a significant role in
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facilitating the speed of nerve impulse transmission Not all axons covered with myelin sheath
, these are called unmyelinated axon
Function
1 provides insulation by producing myelin (Myelin, which is a fatty layer that insulates the
axon)
2- helps to increase the saltatory conduction of the neuron
3- Non-myelinating Schwann cells are involved in maintenance of axons and are crucial for
neuronal survival
1-Functional classification:
A-Sensory neurons (afferent) : Relay sensory information in the form of an action nerve
impulse from the PNS to the CNS
B-Motor neurons (efferent) : Relay an action potential out of the CNS to the proper effector
(muscles, glands)
C-Interneuron : create neural circuits, enabling communication between sensory and motor
neurons in the central nervous system (CNS). They have been found to function in
reflexes.These cells found only in the brain or spinal cord
2- Structural classification
Four types of neurons are known according to the number of processes they posses (Fig. 7):
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1-Unipolar:that have a single short dendrite terminating in a brush-like tuft of dendrioles.
These are found in the granular layer of the cerebellum .
2-Pseudounipolar: neurons that have one process that splits into two branches, these
neuron has one axon with two branches The axon has a peripheral branch (from the cell body to
the periphery: skin, joint and muscle) and a central branch (from the cell body to spinal cord).
This type is found in dorsal root ganglia.
3- Bipolar: One axon and one dendrite are available as in neurons of retina.
4-Multipolar: More than one dendrite in addition to the axon is present. This type is
commonly distributed in the nervous system.
Neuroglia
Neuroglia are supportive cells and important for the viability of
neurons. Four types are present (Fig. 8):
1-Astrocytes (Astroglia): Star-shaped cells represent the longest neuroglia. They have many
processes with branches.
Astrocytes are of two types: Protoplasmic and fibrous astrocytes.
A- Protoplasmic astrocytes: The protoplasmic glia are the most prevalent and are found in
grey matter tissue, possess a larger quantity of organelles, and exhibit short and highly
branched tertiary processes
B- Fibrous astrocytes: The fibrous glia are usually located within white matter, have
relatively few organelles , and exhibit long unbranched cellular processes
2-Oligodendrocytes (Oligodendroglia): They are arranged as rows between neural fibers. Few
amount of cytoplasm. Few small processes, with few branches and nodes. They have the ability
to produce a myelin sheath in CNS. They are found in white and gray matter but more abundant
in white matter.
3-Microglia: Nuclei are small and elongated. Few cytoplasm distributed in cell processes
which are branched. These are the smallest neuroglia. They are found in the CNS near the
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perikarya of neurons. They are considered as macrophages of the nervous tissue as they are
able to migrate.
4-Ependymal cells: These cells have an epithelial appearance (cuboidal or columnar) with
cilia. They line the cavities of CNS.
Meninges
The living nervous tissue of the CNS is soft and delicate and requires adequate protection and
nourishment. Protection is provided by a complete bony case covering the brain and the spinal
cord in the form of cranium and vertebral column. Within the bony case are three membranous
investments called meninges (Fig. 9).
 The outermost of the meninges, the dura mater is a fibrous, tough and relatively
inelastic and lines the cranium, being attached firmly to bone. Dura mater surrounding
the spinal cord within the vertebral canal but is separated from bone by an epidural or
extradural space.
 The middle layer is the arachnoid, composed of fine, cobweb-like strands of interlacing
reticular fibers.
 The most internal layer, which closely invests the brain and spinal cord, is the pia
mater. In this layer, blood vessels supplying CNS are found.
Pia and arachnoid have a similar structure and sometimes are regarded as a single layer called
the leptomeninges.
Spinal Cord
The spinal cord looks oval in cross section. Posteriorly, the cord is divided partially
into right and left halves by the dorsal median septum. Anteriorly, there is a deep longitudinal
cleft called the anterior (ventral) median fissure (Fig. 10). The entire cord is surrounded by pia
mater which extends into the anterior median fissure. Centrally, in cross section of the cord is
called the anterior and the posterior horns.
The central (spinal) canal is situated in the horizontal bar of the H. This canal is lined by
ependyma.
The white matter, formed by nerve fibers, surrounds the gray matter and is divided into
longitudinal columns or funiculi.
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Differences between grey and white matter
1. Grey matter is made up of nerve cell bodies, and white matter is made up of fibers.
2. Unlike the white matter, the neurons of grey matter do not have extended axons.
3. Grey matter occupies 40 percent of the brain, while white matter fills 60 percent of the
brain.
4. Grey matter has a grey color because of the grey nuclei that comprises the cells. Myelin is
responsible for the white appearance of the white matter.
5. Processing is concluded in the grey matter, while white matter allows communication to and
from grey matter areas, and between the grey matter and the other parts of the body.
6. Grey matter has no myelin sheath, while white matter is myelinated
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