The nervous system is a complex network of nerves and cells that carry messages to and from the brain and spinal cord to various parts of the body. The nervous system includes both the central nevous system and peripheral nervous system. The central nervous system is made up of the brain and spinal cord and the peripheral nervous system is made up of the somatic and the autonomic nervous system (figure 1).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Functions of the Nervous System

• Receive sensory input from internal and external environments

• Integrate the input

• Respond to stimuli

 

Feedback in the Nervous System

The nervous system monitors and controls almost every organ system in the body through a series of a positive and negative feedback loops. The peripheral nervous system (PNS) connects the central nervous system (brain and spinal cord) to other parts of the body, and is composed of nerves (bundles of nerurons).[2]

The PNS consists of:

 

• Sensory neurons running from stimulus receptors that inform the CNS of the stimuli

• Motor neurons running from the CNS to the muscles and glands – called effectors – that take action.

  • Somatic division nerves control skeletal muscles.

  • Autonomic division nerves control internal processes such as digestion and heart rate. [1]

 

 

Transmission of Nerve Impulses

A nerve impluse is essentially an electrical stimulus that travels over the cell’s membrane. It passes through the axons and dendrites of the neurons. It travels via the dendrites from the skin and then reaches the cell body, axon terminals and the synapse of the neuron. The synapse is the junction between two neurons where the impulse moves from one neuron to the next.

 

A synapse converts the activity from the axon into electrical effects that inhibit or excite activity on the contacted (target) neuron. At the synapse, neurotransmitters are present. These are chemical transmitters of messages that transmit the impluse. They include Acetylcholine and Noradrenaline.[1] When a neuron receives excitatory input (due to the action of neurotransmitters) that is sufficiently large compared with its inhibitory input, it sends a spike of electrical activity (an action potential) down its axon. The impluse continues to the next dendrite in a chain reaction till it reaches the brain, which in turn instructs the body to produce an output response (e.g. skeletal muscles to work).

 

Action Potentials

Nerve impulses in neurons are also known as “action potentials". Action potentials are caused when different ions cross the neuron membrane. The sodium (Na+) and potassium (K+) gated ion channels open and close as the membrane reaches the threshold potential, in response to a signal from another neuron.[3] At the beginning of the action potential, the Na+ channels open and Na+  moves into the axon because sodium ions are positively-charged, so the neuron becomes more positive and becomes depolarized. The impulse travels down the axon in one direction only, to the axon terminal where it signals other neurons. It takes longer for potassium channels to open. When they do open, potassium rushes out of the cell, reversing the depolarization. Also, it is at around this time that sodium channels start to close. This causes the action potential to return towards -70mV (repolarization). The action potential actually goes past -70mV (hyperpolrization) because the potassium channels stay open a bit too long. Gradually, the ion concentrations go back to resting levels and the charge over the neuronal membrane returns to -70mV. [3]

 

 

 

 

 

 

 

 

 

 

 

Neural Networks

In information technology, a neural network is a system of programs and data structures that approximates the operation of the human brain.[4] A neural network usually involves a large number of processors operating in parallel, each with its own small sphere of knowledge and access to data in its local memory. A program can then tell the network how to behave in response to an external stimulus (for example, an input from a computer user who is interacting with the network) or can initiate activity on its own (within the limits of its access to the external world).

 

Similarly, nerve cells in the brain never work alone. In a neural circuit, the activity of one cell directly influences many others. To gain insight into how these interactions control brain function, researchers are exploring the connections between nerve cells and how they change over time.[4] This insight could lead scientists to a better understanding of how the nervous system develops and the ways disease or injury disrupts the natural rhythms of brain cell communication. With new imaging technology, scientists are now better able to visualize circuits connecting brain regions (figure 3).

 

Artificial neural networks are typically composed of interconnected units which serve as model neurons. The synapse is modeled by a modifiable weight associated with each particular connection (figure 4). Most artificial networks do not reflect the detailed geometry of the dendrites and axons, and they express the electrical output of a neuron as a single number that represents the rate of firing.[4]

 

In addition, advances in genetic engineering, microscopy and computing are enabling scientists to begin to map the connections between individual nerve cells in animals better than ever before. These findings may one day shed light on disorders scientists suspect arise from errors in neural network development, including autism and schizophrenia.[4]

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

References

1. Cashin-Garbutt, A. (2010). What is the Nervous System?. [online] News-Medical.net. Available at: http://www.news-medical.net/health/What-is-the-Nervous-System.aspx

2. Uic.edu,(2016). The Body. [online] Available at: http://www.uic.edu/classes/bios/bios100/lectures/nervous.htm

3. Faculty.washington.edu, (2016). Neuroscience For Kids - action potential. [online] Available at: https://faculty.washington.edu/chudler/ap.html

4. Ifc.unam.mx,(2016). A Brief Introduction to the Brain:Neural Nets. [online] Available at: http://www.ifc.unam.mx/Brain/nenet.htm

5. https://en.wikipedia.org/wiki/Biological_neural_network#/media/File:Brain_network.png

6. https://commons.wikimedia.org/wiki/File:Neural_network_example.svg