Initially we should look at the different types of cell in the human body:
Then we can see how these contribute to the four types of tissue in the human body:
This covers and protects all surfaces within and without the body. It lines body cavities and specialises in moving substances into and out of the blood in the form of secretion, excretion and absorption and this tissue is used to form many glands.
- Epithelial tissue lies on a basement membrane this is a layer of acellular material, a sticky layer to keep the epithelial cells attached to whatever underlies them.
- Cells within this tissue readily divide to make more cells. This helps this tissue recover after any sort of abrasions occur.
- This tissue does not have any vasculature. This means that there are no blood vessels within it. This should make sense, since epithelial tissue is likely to get damaged by material moving against it– and you don’t want to bleed everytime something bangs into your skin or every time you swallow something rough.
- The cells within this tissue are firmly attached to each other. As a border-tissue, if the cells weren’t adherent to one another, it would be a leaky border. This would be no good– liquids from inside of us would drip out! So the cells all make the type of junctions with each other called tight junctions.
Connective tissue supports the body and its parts. It’s function is to connect and hold different parts together, to transport substances through the body and to protect it from foreign invaders.
The cells are often relatively far apart, separated by large quantities of non living matrix.
It is one of the most widespread tissues found in the body and is found in or around nearly every organ in the body. It can appear as:
- delicate tissue paper webs – (fibrous connective tissue of the arachnoid membrane in the meninges)
- tough resiliant cords (tendons in the arms and legs)
- rigid bones (skeleton)
- and blood (pumping through the circulatory system)
These types of tissue are the movement specialists of the body.
- Cardiac muscle tissue makes up the walls of the heart. Cardiac muscle tissue plays the most important role in the contraction of the atria and ventricles of the heart. It causes the rhythmical beating of the heart, circulating the blood and its contents throughout the body as a consequence.
- Skeletal muscle tissue attaches to the bones of the body. Skeletal muscles function in pairs to bring about the co-ordinated movements of the limbs, trunk, jaws, eyeballs, etc. Skeletal muscles are directly involved in the breathing process.
- Smooth muscle tissue lines the walls of the viscera that is the hollow organs such as the stomach intestines and blood vessels. Smooth muscle controls slow, involuntary movements such as the contraction of the smooth muscle tissue in the walls of the stomach and intestines. The muscle of the arteries contracts and relaxes to regulate the blood pressure and the flow of blood.
The autonomic nervous system can be divided into two divisions;
The parasympathetic division allows for functions of “rest and digest” or “feed and breed”, where the body can access a more relaxed state for digestion and arousal; there is increased saliva production the heart slows and the stomach, pancreas and intestines are stimulated.
The sympathetic nervous system drives our “fight or flight” responses where we see the heart rate accelerate, the pupils dilate and the release of epinephrine and norepinephrine along with glucose – digestion and urination are inhibited.
Nerve tissue rapidly regulates and integrates the activities of different parts of the body. It has more developed excitability and conductivity characteristics.
Nervous tissue is responsible for many of the body’s activities and processes, including memory, reasoning and emotions. Signals from this tissue also cause muscle contractions.
The two main functions of nerve tissue are:
To sense stimuli – analyse and integrate the stimuli
To respond to this stimuli with an appropriate, coordinated impulse.
Nervous tissue is made up of neurons and glial cells.
Neurons contain a nucleus and mitochondria and nerve processes axons and dendrites made of cytoplasm. The nerve processes transmit signals to the neuron (dendrites) and away from it, (axon) – these two form the nerves
Glial cells are known as neuroglia when located in the central nervous system. They are smaller than neurons and are often found clustered in bunches around neurons.
There are different types of glial cell; Schwann cells, despite not transmitting neurological signals, provide support, nutrition and protection against bacteria to the neurons in the peripheral nervous system .Other glial cells also play a protective role; microglia help to repair damage to the neurons and ogliodendrocytes myelinate the axons in the central nervous system.
As can be seen from the diagram the dendrites from the afferent or sensory neurons conduct nerve impulses from the sense organs (here the skin) to the axon terminals where they meet with an interneuron in the spinal cord and pass the impulse to the dendrites of the afferent motor neurons. The effect of the axons of these efferent motor neuron axons is an impulse that causes contraction in the muscle of the forearm and thus raises it.