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Spinal Cord. The oldest part of the brain is the spinal cord,stretching from the neck down the center of the vertebrae to the bottom of the back.
There being a thin hollow down it's length filled with cerebrospinal fluid.There are two principle functions associated with the spinal cord:
It performs very simple reflexes,such as the knee jerk reflex,and it acts as the principal communication channel between the head and the rest of the body.The control of the body is conveyed via the spinal cord,and all bodily sensations reach the brain through the spinal cord.All that is,except those relating to the head itself,which enter and leave through the brain stem.
The Brain Stem. This is situated on top of the spinal cord.It still possesses the tubular structure of the spinal cord,and in some respects can be thought of as an extension of it.In the brain stem is a very intricate network of nerves about the size of your little finger called the reticular formation.It recieves nerves from all areas of the brain and like wise sends out nerves in every directionThe reticular formation plays an important role in maintaining wakefulness,and if it is isolated from the rest of the brain,the organism goes into permanent sleep.It also monitors and filters the the information coming in through the senses.
If for example,you are in a room with a clock that is ticking quietly,you will quickly habituate to the sound so that after a short while you will no longer hear it.But the sound is still being continually monitored by the brain,and if the clock were to stop,or change speed or volume,you would immediately notice it.The reticular formation would have alerted you.
Cerebellum. Connected to the brain stem is the cerebellum,which somewhat resembles the cortex in terms of neuronal structure,though it is very much older than the cortex.Although it is undoubtedly involved in a variety of functions,it is primarily concerned with coordination of movements.It seems to integrate the information coming from all the senses with all the muscles so as to produce smooth,finely tuned movements,rather than jerky uncordinated movements.
Midbrain. On top of the brain stem is the thalamus,a large region containing many nuclei,some relaying sensory information from the organs to the cortex,others relaying information from one area of the cortex to another and interacting with the reticular formation and the limbic system.The Limbic system is a group of structures in the middle of the brain that play an important role in emotion and motivation.
Just below the thalamus is the hypothalamus,a tiny structure about the size of a pea,yet a crucial part of the brain.This little organ is largely responsible for the maintenance of homeostasis,ensuring that all the parameters of bodily function are in balance and function at their optimum.The hypothalamus continually monitors the blood.If there is too little ot too much carbon dioxide,it reduces or increases breathing;if blood sugar is low,it makes you feel hungry;if your temperature is too low or too high,it initiates shivering or sweating;if the blood is too salty it makes you feel thirsty.It also plays a major role in the control of sleep,sexual behaviour,and the emotions.