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There has been a lot of discussion around neurons and neuronal networks. I thought I should get together some basic information around these. This is part one of the series where I look at basic anatomy of the human nervous system.
The human nervous system, comprising of the brain (control center of the nervous system), the spinal cord and a complex neuronal network, is divided into two sub systems – the central (brain and spinal cord) and the peripheral nervous systems (complex neuronal network / nerve bundles). The Peripheral nervous system connects the central nervous system with the rest of the body. For example, the cranial nerves take impulses to/from the brain and the spinal nerves, from to/from the spinal cord.
The brain consists of the cerebrum (largest part, seat of consciousness, controls movements, sensory perception and plays an important role in memory, language and thinking), the cerebullum (responsible for coordination and balance) and the brain stem.
The cerebrum is broken up into two hemispheres – the left and the right (see also left-right brain). These are covered by a thin layer of grey matter called the cerebral cortex. The cortex itself is divided into 4 parts or lobes – occipital (visual), temporal (auditory), parietal (touch, taste, pressure, pan, heat and cold) and frontal (motor activity and integration of muscle activity, speech, thought processes).
The brain stem is broken down into five sections – thalamus (which is the central relay point for all incoming messages), the hypothalamus (that regulates blood pressure, temperature, hunger, thirst etc), midbrain (that contains some areas of sight and hearing), the pons (controls the nerve centers of the muscles of the face) and the medulla oblangata (which is closest to the spinal cord and controls breathing, heartbeat etc).
The spinal cord serves as the pathway between the brain and the peripheral nervous system. The bones of the vertebral column are designed to protect the spinal cord.
Neurons also called nerve cells are the oldest cells in our bodies and send/receive information by the means of electro-chemical signals. Each neuron is connected to as many as 80,000 other neurons. There are approximately a 100 billion in the brain. They measure 4-100 mm microns wide (1 micron = 1/1000 mm) and some neurons can be many feet long.
The neuron has a cell body and nerve processes (that are able to conduct and transmit signals at speeds of as much as 360 feet per second). The two types of nerve processes are Axons (carry signals away from the cell body, typically one per neuron) and Dendrites (that carry signals towards the cell body, many per neuron). A synapse is the space between the axon of one neuron and the dendrite of another. The signal comes in through the dendrite, passes through the axon and is passed on to the dendrite of the other neuron through the use of neurotransmitters that are chemicals in the synapse.
Neurons can be classified in many ways. By structure, there are many types of neurons – unipolar, bipolar and multipolar. Some types of neurons have also been identified based on their location and distinct shape such as Betz cells and Pyramidal cells. Functionally, they can be separated into Afferent (peripheral nervous system, sensory nervous cells or sensory neurons carry information from internal organs and external stimuli to the Central nervous system, CNS); Efferent (peripheral nervous system, motor neurons, carry signals from CNS to organs, muscles and glands) and Interneurons (that conduct signals between sensory and motor neurons).
Neurons work by conducting electrical impulses using the action potential. Potential is defined by the diferent in electrical state outside and inside the neuron (which has ions such as Sodium, Potassium, Chloride etc). In the resting state, this difference is about 70 mV (inside of neuron is 70 mV less than outside) and this is called Resting Potential. In a state of excitation i.e. when a signal moves down the axon, depolarization occurs that lowers the potential. When the potential goes down to a critical threshold level, an action potential is created and the neuron “fires” or gets activated. A repolarization occurs to bring the neuron back to it’s resting potential subsequently. The polarization happens because of the movement of Sodium and Potassium positively charged ions through channels that are semi-permeable – i.e. only allow specific ions to pass through.
When the action potential reaches the synapse, the neurotransmitters take it up and the signal is passed to the receptor neuron. The neurotransmitters are made in the cell body of the neuron and passed down the axon. Many things could happen when the neurotransmitter is released.
The incoming messages are processed by the appropriate part of the brain and corresponding output signals are generated that represent human response.