The modern human's brain is quite possibly the most complex system found in organisms and this is not surprising, given the millions of years it has taken to evolve as such. Even with today's highly advanced technology, much of the brain remains a mystery. However, discoveries are being made every day that slowly unfold the mystery that is our very own brain.
The human brain consists of many highly-specialized cells referred to as neurons. Neurons essentially transmit signals using an electrochemical process. In detail, electricity travels within the cell and chemicals travel between the cells by way of the synaptic gap. The body of the neuron is referred to as the soma, which is a conductive substance containing the required organs that sustain the cells life. Extending from the soma like branches are dendrites and axons. The path of the signal flow is from dendrites to axons, therefore dendrites receive the signal and the axon transmits it. The dendrites are on average much shorter than the axons and the axons are sheathed in myelin, a protein-like substance that speeds neural impulses. At the end of the axon is terminal buttons, which contain neurotransmitters that cross the synaptic gap between neurons. The synapse or synaptic gap is merely the space between the dendrites of one neuron and the axon of another.
The "firing" of a neuron is a rather complex process. In a resting state, the neuron possesses a slightly negative charge due to the fact that the cell contains mostly negative ions, while positive ions surround it. The cell membrane prevents mixing of these ions selectively in order to maintain stability. A reaction between two neurons occurs when the terminal buttons of a neuron are stimulated and release neurotransmitters. The neurotransmitters cross the synaptic gap and bond with receptor sites on the dendrites of the second neuron. When enough neurotransmitters are collected, the membrane of the second cell allows positive ions to enter. The resulting change in the electrical charge quickly goes down the neuron and activates the terminal buttons on the second neuron. The chain reaction continues down the line of neurons, until a lack of neurotransmitters halts it. The fact that the neurons can not halfway fire is important since the process does not happen until a certain amount of neurotransmitters is collected (this point is known as the threshold).
Neurotransmitters have several different types, each of which has a specific function. There are far too many to list here, so I will only list the ones that are most important in psychology. Acetylcholine controls motor movement and a lack of this neurotransmitter seems to be a contributing factor to Alzheimer's disease. Dopamine controls motor movement and alertness, but a lack of it seems to contribute to Parkinson's disease, while an excessive amount is associated with schizophrenia. Endorphins are for control of pain and endorphins are the cause of addictions. Serotonin is involved in mood control and a lack of it is a contributing factor in depression.
The nervous system is divided into several parts. The somatic nervous system controls all of our voluntary muscle movements. The autonomic nervous system controls the automatic functions of our body and is divided into two parts. The sympathetic nervous system motivates our body to respond to stress. This is associated with adrenaline and the fight or flight response. The parasympathetic nervous system counters the sympathetic nervous system by slowing down our body after just such a response.
The brain itself has a structure that is divided into three main parts: the cerebral cortex, hindbrain, midbrain, and forebrain.
The hindbrain contains the medulla, which controls our blood pressure, heart rate, and breathing. The pons connects the hindbrain with the midbrain and forebrain and is also involved in the control of facial expressions. The cerebellum controls very precise muscle movements, used during activities such as the playing of a musical instrument or sewing.
The midbrain of humans is small in proportion, when compared to other species. However, this section controls very important things such as simple movements. It also controls things such as arousal, our attention, and coordination. Our body would not be able to move without existence of the midbrain.
The forebrain is where most thought and logic operations occur. The thalamus receives sensory signals from the spinal cord and sends them to the right section of the brain. The hypothalamus controls metabolic functions such as temperature, sexual arousal, hunger, and thirst. The amygdala and hippocampus are very important in the processing of memories and emotion.
The cerebral cortex is the gray wrinkled part of the brain that is the most easily seen. The wrinkling occurs in order to increase the surface area, which allows more neural connections without increasing the size of the brain itself. The cerebral cortex has two hemispheres, which are connected by the corpus callosum. Surprisingly, the two hemispheres specialize in function, as is evidenced by patients lacking the corpus callosum.
The frontal lobes of the cerebral cortex and are believed to control abstract thought and emotional control. They are also very important in the production of speech and the use of language. A small area of the frontal lobes known as the motor cortex is very important in voluntary muscular movements.
The parietal lobes contain the sensory cortex, which receives touch sensations from the body. The senses are localized in this cortex as the top of the body's sensations are processed in the top of the cortex, while the bottom is processed in the bottom of the cortex.
The occipital lobes are very important in the processing of visual information from the eyes. Impulses from the right half of each eye are processed in the left occipital lobe and vice versa.
The temporal lobes process sound from the ears. The processing here is done in both lobes, unlike the occipital lobes. The lobes are also important in the processing of language.