Basic Structure of the Human Nervous System
The Neuron: Simple, Yet Complex
The neuron is the basic functional unit of the nervous system. The neuron is quite a basic cell, but its specialized structures enable it to perform complex processes. The axon is the part of the neuron along which a message (in the form of an electrical signal) is sent to the dendrite, the receiving end of the next neuron. Axons send electrical messages that are fired when neurotransmitters, chemicals specific to the nervous system, reach the dendrite of that neuron. There are over 60 different identified neurotransmitters, each with a different role and associated issues, from acetylcholine to serotonin. Neurons exist throughout the body, both along nerve pathways and in the brain.
The most important role of the neuron is the change from chemical to electrical signal. A neurotransmitter that diffuses across the synapse (space between axon and dendrite) will reach the dendrite. In response, the neuron will "fire" if the enough of the neurotransmitter reaches it, sending an electrical signal down the next axon, and so forth until the message reaches the proper cells. The neuron can only fire if the electrical charges are in correct balance. At resting potential, the inside of the axon remains negative, while positive sodium and potassium ions line the outside. Upon firing, the positive sodium and potassium ions upset the balance, sending an electrical impulse. After firing, the neuron will undergo a refractory period before it can fire again, during which the ions diffuse back out of the axon and the neuron regains its negative charge. This switching between chemical and electrical signals occurs at lightning speed, allowing very efficient and fast transactions to occur in the brain. Myelin sheath, or fatty patches that line the axon, serve to insulate and thus speed up an axon's firing.
The most important role of the neuron is the change from chemical to electrical signal. A neurotransmitter that diffuses across the synapse (space between axon and dendrite) will reach the dendrite. In response, the neuron will "fire" if the enough of the neurotransmitter reaches it, sending an electrical signal down the next axon, and so forth until the message reaches the proper cells. The neuron can only fire if the electrical charges are in correct balance. At resting potential, the inside of the axon remains negative, while positive sodium and potassium ions line the outside. Upon firing, the positive sodium and potassium ions upset the balance, sending an electrical impulse. After firing, the neuron will undergo a refractory period before it can fire again, during which the ions diffuse back out of the axon and the neuron regains its negative charge. This switching between chemical and electrical signals occurs at lightning speed, allowing very efficient and fast transactions to occur in the brain. Myelin sheath, or fatty patches that line the axon, serve to insulate and thus speed up an axon's firing.
The Brain: The Command Center
Perhaps the absolute most important human processes originate in the brain. The brain consists of the cerebral cortex, the midbrain, and the hindbrain. The cerebral cortex, pictured above, is broken down into the frontal lobe (thinking, cognition, reasoning), the parietal lobe (somatosensory/motor cortices), the occipital lobe (vision), and temporal lobes (hearing). The midbrain contains specialized regions such as the hippocampus (memory) and the amygdala (emotion/fear). Last, but certainly not least, the hindbrain contains the vital-for-life medulla (cardiac/respiratory functions) and the pons (sleep, posture, etc.). Needless to say, the brain controls everything we do; without it, we are not human.
Micro/Macro
Although the nervous system (NS) can be greatly studied at the micro-level, the macro-level organization is also an important aspect to consider. The first major division is that between the central and peripheral nervous systems. The central nervous system, or CNS, describes the structures within the NS. The major entities include the brain and all of its lobes and the spinal cord. Much more detail is accounted for in the brain, which has very specific structures and functions (more can be found below). The peripheral nervous system is the responsible for the processes that occur throughout the entire body, and can be split into the autonomic NS (essentially automatic) and somatic NS (essentially voluntary). Autonomic processes include the fight or flight response (sympathetic) and the resting state (parasympathetic), whereas areas such as movement are included under the somatic NS.