**2. Neurons**

the task is given. The idea of storing for long‐term retrieval is supported by the long‐term memories in the human brain, where the retrieval of results from the memory reduces the

The ability of the brain to process computational operations in real time is reflective of an active working memory. In many of the mental calculations, one makes on a day‐to‐day basis can be analysed by looking into the activities in the prefrontal cortex (see **Figure 1**). The stud‐ ies using neuroimages indicate 10 separate regions in the brain that contribute to even simple task of subtraction of two numbers. The main areas of activation for this simple task include fusiform gyrus, parietal cortices, lateral and medial parts of the temporal lobe and inferior

The interconnections between the modules and the way they interact with each other for different set of arithmetic operations are different. It is also found that there is a separate net‐ work for estimation (bilateral inferior parietal cortex) as opposed to computation (left parietal and frontal cortices). These features point out the fact that there is one specific unit for per‐ forming computation; instead it is a collaborative effort between various regions in the brain.

effort on cognitive processes.

96 Fourier Transforms - High-tech Application and Current Trends

parts of the frontal lobes.

**Figure 1.** Functional units in brain.

Neurons are the basic building blocks of the nervous system, which includes brain, spinal cord and peripheral ganglia. Neurons are electrically excitable cells and they process and transmit information through electrochemical signals. Neurons connect together to form what is known as neural networks.

The basic structure of a biological neuron is shown in **Figure 2**. It consists of a cell body, den‐ drites and axons. Cell body or the soma is bulbous in shape and contains the nucleus. The cell body or the soma contains many cell organelles, including Nissl granules that are the site of protein synthesis. Nissl granules contain endoplasmic reticulum and free polyribosomes.

Dendrites arise from the cell body, branches into what is known as the 'dendritic tree'. Dendrites are the branched projections of the neuron arising from the cell body and its func‐ tion is to receive the electrochemical simulations from other neurons and to conduct it to

**Figure 2.** Biological neurons.

the cell body. The electrical simulations are transmitted from one neuron to the dendrite of another neuron at the synaptic terminals.

Another important part of the neuron is the axon. Axon arises from the cell body at a site called axon hillock and extends to over 1 m in length. A neuron can have multiple dendrites, but only one axon. Axon is covered by a layer of dielectric material myelin, known as myelin sheath. Before termination, the axon gets divided into a large number of branches.

The axon terminals of one neuron connect to the dendrites of another neuron through syn‐ apses. Electrochemical signals are transmitted from one neuron to another through synapses. Chemicals known as neurotransmitters are released from the presynaptic neuron, which binds to the receptors located at the dendrites of the postsynaptic neurons. These neurotrans‐ mitters are initially present in small bag‐like structures known as synaptic vesicles that are found at the axonic terminals of the neurons. These synaptic vesicles, when excited, migrate towards the synapse and get attached to the synapse and release the chemical ions through the semipermeable membrane of the synapses.

The major ions that are involved in the process are sodium, potassium, chlorine and calcium. Once released, these ions diffuse through the semipermeable membrane and binds to the receptors which are present on the dendrites of the post‐synaptic neurons. The basic structure of a synapse is shown in **Figure 3**.

Due to the ion exchange between neurons, a gradient in the ion concentration arises on either side of the semipermeable membrane. Due to this ion concentration difference, a potential will be generated, known as Nernst potential. Changes in the cross‐membrane voltage between the intra‐cellular and extra‐cellular potential will alter the function of the voltage‐dependention

**Figure 3.** Structure of synapse.

channels. As the difference in ion concentration increases, the resultant Nernst potential also increases and when this potential reaches a particular threshold value, the post‐synaptic neu‐ ron fires and an action potential is generated which moves from cell body to the next neuron through the axon. This is how a biological neuron transmits signals.

There are several differences between the processing in human brain and processing in a com‐ puter. One of the most important differences is that brain is analogue whereas the computers are digital. The computers work with 0's and 1's whereas neuron signals are not bi‐state. But we can find a superficial similarity between neurons and digital circuits in the aspect that neu‐ rons fire an action potential when they reach a threshold value. In computers, information in memory is accessed by polling its precise memory address. This is known as byte‐addressable memory whereas brain uses content‐addressable memory.

Human brain can be considered as a massively parallel machine, where different functions are carried out simultaneously in different parts of the brain. Brain has got several dedicated modules for carrying out different functions. But if we consider the case of computers, the processing is in modular and serial in nature.

The brain has got a body at its disposal. This may seem to be trivial, but this is a major differ‐ ence which gives the humans a clear advantage over the computers. Once the brain takes a decision based on the input signals, the brain directs the body to respond to the signals. But in computers, although it can take decisions based on the input signals, there is no body so that it can respond to the stimulus.

Although there are several other differences, one of the most important differences between brain and computing processors is that there exists no distinction between memory and pro‐ cessing architecture in brain. These two important activities are not separable. As the neurons process information, they also modify their synapses that are the substrate of memory. But in computers there exist a clear distinction between processor and memory.
