Did you know..
..out of all mammals, humans have the largest brain? In relation to their body size, all mammals have rather large brains, but the human brain is the largest. This means that, for example, even though a whale is a massive sea mammal with a massive brain, its body to brain ratio is smaller than a human's.
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The brain is a very delicate organ as it is the control centre for the rest of the body. It makes sure that the rest of the body is working at optimum levels, and is therefore heavily protected by three main protective structures: |
the skull, or cranium; three layers of connective tissue called meninges; and cerebrospinal fluid, which occupies the space between the brain and meninges.
Cerebrum: This is the biggest part of the brain. The most obvious part is the outer surface of grey matter, which actually looks pinkish in colour. This part is known as the cerebral cortex, and is involved in such mental activities as thinking, learning, memory, reasoning, intelligence and sense of responsibility. The cortex is also involved in three functional areas:
- Sensory areas, which interpret impulses from receptors in the body (for example, temperature receptors found in the skin).
- Motor areas, which control the movements of muscles (when walking, eating, etc.).
- Association areas, which are concerned with intellectual and emotional processes (such as thinking, reasoning, etc.).
Diencephalon: This part of the brain is made up of the thalamus and hypothalamus.
Thalamus: All sensory impulses (except for smell) pass through the thalamus. From here these impulses are relayed to the specific part of the cerebrum that is concerned with that particular sense. For example, when we look at something, the image passes through the eye. The eye receives the image as an impulse and this impulse passes through the thalamus. From there it is sent to the occipital lobe at the back of the brain, which interprets the impulse as an image.
Hypothalamus: Although small, the hypothalamus controls many body activities. It is mainly concerned with maintaining a constant environment for bodily cells. The main functions of the hypothalamus include the regulation of:
- heart rate
- body temperature
- movement of food through the alimentary canal
- food and water intake
- patterns of waking and sleeping
- contraction of the urinary bladder
- sexual cylces
- release of hormones from the pituitary gland
The hypothalamus is also associated with fear and anger, and receives sensory impulses from internal organs.
Pituitary gland: This organ only measures about 13 millimetres in diameter, yet it is vital to the normal functioning of the body. The hormones secreted from the pituitary gland regulate the activity of other glands in the body. There are two parts to the pituitary gland, both of which function differently:
Anterior lobe of the pituitary: This produces a number of hormones that regulate a range of bodily activities. First, the hypothalamus receives a message from the body and thus, secretes a hormone that tells the anterior lobe of the pituitary to secrete another hormone for that particular part of the body. Such hormones are:
- Gonadotrophins, which affect the ovaries in women, and testes in men.
- Growth hormone (GH), which stimulates growth of the body, in particular, the growth of the skeleton. It also maintains the size of body organs once physical maturity is reached.
- Thyroid stimulating hormone (TSH), which stimulates the production and release of hormones from the thyroid gland.
- Adrenocorticotrophic hormone (ACTH), which controls the production and release of some hormones from glands associated with the kidneys.
- Prolactin, which helps initiate and maintain milk secretion in breast-feeding females.
Posterior lobe of the pituitary: This releases the hormone oxytocin and antidiuretic hormone. These hormones are produced in the hypothalamus, however their release into the body is controlled by the posterior lobe of the pituitary.
- Oxytocin: stimulates the contraction of muscles in the uterus, and is released in large quantities during labour. This hormone also stimulates the release of milk during breast-feeding.
- Antidiuretic hormone (ADH): This hormone helps to keep fluid within the body. It does this by causing the kidneys to remove water from urine, and this water is returned to the bloodstream.
Cerebellum: This is the second largest part of the brain, and is mainly concerned with the fine co-ordination of voluntary muscle movements. All functions of the cerebellum take place below the conscious level, meaning we aren't necessarily aware of what it is doing when it is doing it. Without this particular part of the brain, we would still be able to move our muscles, however the movement would be jerky, spasmodic and uncontrolled. Such things as playing an instrument, writing, or typing on a keyboard would be impossible.
Pons varolii: "Pons" is Latin for "bridge", and that is precisely what the pons varolii is: a bridge of nerve cells and fibres which connect the spinal cord with the brain, and also various parts of the brain with other parts of the brain.
Medulla: Also known as "medulla oblongata", it is basically a continuation of the spinal cord. Many nerve fibres pass through this part of the brain, but it also is responsible for the regulation of such reflexes as swallowing, sneezing, coughing and vomiting. Situated in the medulla are the following:
- Cardiac centre, which regulates heartbeat
- Respiratory centre, which controls breathing rhythm
- Vasomotor centre, which regulates diameter of blood vessels.
Summary
So, how does the brain work exactly?
Here is an example of homeostasis, one of the many functions of the brain. When you feel cold, what happens to your body? You get goosebumps, the hairs on your arms and legs stand up, you start to shiver and eventually you do something about it, like put a jumper on.
But this is what you don't see happening: the brain receives a message from heat receptors in the skin, saying, "hey man, its cold!" The brain therefore says, "ok, I'll see what I can do," and sends impulses to the little muscles that make the hairs stand up on your skin. This is to trap a layer of warmth between the skin and the hairs. If this isn't working, the brain then sends messages to all muscles in the body causing them to rapidly contract and relax, to raise the temperature of the body. If body temperature still hasn't returned to normal, you either become delirious, or go and put a jumper on. Once the brain sees that body temperature has returned to normal, it stops the impulses to the muscles and the little hairs on you skin.