Nerve and Muscle Tissues
Nerve Tissue
These will be discussed in more detail later in the course. For now, nervous tissue is
divided into two types:
o Neurons
o Supporting cells
A) Neurons
These generate and conduct the nerve impulses. The dendrites respond to stimuli. The
nerve impulses are transmitted overlong distances by the axons.
The axon is covered by a myelin sheath which acts as an insulator. The axon can vary in
length from inches to feet.
B) Support Cells
These are a mixed group of cells which support and insulate the neurons. Collectively
they are called neuralgia (glial cells). There are four types:
o
o
o
o
Astrocytes
Microglia
Ependymal cells
Oligodendrocytes
Muscle Tissue
These cells are responsible for movement. They are highly vascularized and cellular.
Like the nervous tissue they will be discussed in greater detail later. Muscle cells are
filled with myofilaments; these are variants of actin and myosin and are responsible for
movement. Muscle tissue is divided into three types:
o Skeletal
o Cardiac
o Smooth
A) Skeletal Muscle
This attaches to bone and is responsible for voluntary motion. The muscle cells (muscle
fibers) are cylindrical cells that contain many nuclei. The skeletal muscles are called
striated because of the arrangement of the myofibrils.
B) Cardiac Muscle
This type is found only in the heart where its contractions propel blood through the body.
Like the skeletal muscle, it is striated but the cells are uninucleated, however, this muscle
in involuntary. The cells are branched and connect to each other with unique junctions
called intercalated disks.
C) Smooth Muscle Tissue
These cells have no visible striations. They are uninucleated and spindle shaped. They
are found lining the hollow organs such as the intestines, respiratory and urinary tracts
and blood vessels. Like the cardiac muscle they are involuntary.
Coverings and Membrane Linings
The four tissues are incorporated into forming membranes which cover the body and its
cavities. There are three types:
o Cutaneous
o Mucous
o Serous
All have an epithelial tissue layer and a connective tissue layer.
A) Cutaneous Membrane
This covers the exterior of the body and is commonly known as the skin. It has a layer of
keratinized stratified squamous epithelium (epidermis) with a layer of dense irregular
connective tissue below it. Because it is exposed to the air it is considered a dry
membrane.
B) Mucous Membrane
These line the body cavities that are open to the exterior; examples include the digestive,
respiratory and urinary tracts. These are moist membranes. The epithelium can be
stratified squamous or simple columnar epithelium. Underneath this is a layer of loose
Areolar connective tissue forming the lamina propria. Below this can be a layer of
smooth muscle.
3) Serous Membranes
These are the moist membranes that cover the body’s cavities. The membrane consists of
a simple squamous epithelium (Mesothelium) resting on a layer of Areolar tissue. The
Mesothelial cells secrete a fluid which lubricates the surfaces.
Tissue Repair
Tissue injury is followed by a predictable series of steps which result in healing of the
wound. These steps involve a nonspecific inflammatory response followed by repair
which can result in either replacement of the tissue or fibrosis (scarring). Which path
taken depends on the type of tissue injured.
Steps
1. Inflammation
This occurs right after the injury and involves cells such as the macrophages, mast cells
and lymphocytes (normal residents of the Areolar tissue) releasing factors which lead to
increased vascular permeability and stimulation of cell division. The plasma releases
factors which begin forming a blood clot.
2. Organization
Once the blood clot is formed, it is replaced by granulation tissue. This is a collection of
capillaries and fibroblasts which lay down a new matrix.
3. Regeneration or Fibrosis
This depends on the tissue type but some tissues will regenerate while others will be
replaced by scar tissue. Epithelial, bone, areolar, dense irregular and blood tissues
regenerate. Smooth muscle and dense regular connective tissue have a moderate capacity
to regenerate while skeletal muscle and cartilage are weak. Cardiac and nervous tissue
has none.
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