2. Overview
3.2 The Neuron as the core component of the nervous system
The brain constitute a massively parallel computational system that, by a method not yet understood, gives rise to our consciousness as an emergent property of its operation. The brain exerts centralised control over the body with results similar to that of a Central Processing Unit (CPU) in a modern computer. While the results might be similar to a CPU the method by which control is achieved is radically different. Neuroscience has established the Neuronal Doctrine to explain how the biological neural network of the brain functions as an information processing system (Shepherd, 1991). The Neuronal Doctrine is an extension
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of ‘cell theory’ based on Golgi and Cajal’s work and has steadily evolved over the last few decades (and continues to evolve). The doctrine makes the following assentation’s (Finger, 2001):
i. Neural units: The brain is composed of individual units – Neurons.
ii. Neurons are cells: Neurons are biological cells with unique features (dendrites and axon’s).
iii. Specialisation: The neurons size, shape and structure vary according to its location or function.
iv. Nucleus is the cells core: The centre of the cell is the nucleus; this contains the genetic material of the cell that is always replicated before cell division.
v. Cell division: Nerve cells multiply through the process of cell division.
vi. Axons are cell processes: Axons are outgrowths of nerve cells (whether myelinated or not).
vii. Law of dynamic polarization: The axon can conduct in both directions BUT there is preferred direction of transmission from cell to cell. This preferred direction is created by a refractory period of 1-2ms in which the cell’s Na+ channels that originally opened to depolarize the membrane remain open. During this period the cell cannot respond to any stimulus.
viii. Synapse: A barrier to transmission exists at the site of contact between two neurons but it may permit transmission
ix. Unity of transmission: The contact between any two cells may be excitatory or inhibitory but will always be of the same type.
x. Dale’s law: Each nerve terminal releases the same types of neural transmitter (Connors & Long, 2004; Dale, 1935).
To fully understand the neuronal doctrine it is necessary to examine what is known about the structure and operation of neurons in the brain. Figure 3-1 details the structure of a typical neuron:
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Figure 3-1: Major structures of a neuron (amended). (Jarosz, 2009)
As the neural doctrine asserts that the neuron is a biological cell the key features of any body cell are present in the form of a nucleus and soma (cell body). The specialised structures of the rest of the cell provide the biological system to create and receive the electrochemical signals used to encode and exchange data between the neurons in a neural network.
3.2.1 Dendrites
The dendrites are branched projections that receive electrochemical stimulation from connected neurons. Their primary task is to transfer the received signal(s) to the soma where if the combined signal from all connected dendrites is ‘strong enough’ the neuron will ‘fire’, that is generate its own electrical signal (action potential) to be propagated through the axon to other connected neurons within its neural network. It is believed that the dendrite is considerably more than a simple transmission system, the total surface area of the dendrite places a limit on the amount of information a neuron may gather, chemicals within the dendrite may serve to enhance or suppress the strength of the received electrical signal. Over time a neuron may even vary these factors; dendritic spines are protrusions on each branch that may grow to increase the surface area allowing additional connections to a neuron to be formed, and chemical changes within the cell may moderate the number and strength of electrical signals received (Roo et al., 2008). The dendrite / dendritic spine forms the second part of a synapse (see below) and this ability to vary the strength and number of connections over time gives rise to synaptic plasticity in which the strength of a connection varies over time depending on activity level. This synaptic plasticity is believed to form the biological basis by which the neural network achieves learning.
3.2.2 Axon
The neurons axon performs the task of delivering an electrochemical stimulation or action potential to the dendrites of connected neurons. Unlike the dendrite branches, which can be numerous, neurons have only a single axon however at the end of the axon it divides
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into smaller branches called telodendria. Hence even though a neuron has a single axon it may connect too many dendrites and many other neurons within a neural network. The axon connects to the soma at the axon hillock. When a dendrite experiences an electrochemical stimulation at a synapse it transmits a signal to the neurons soma. The signals carried by dendrites are passive and they decrease with distance (much like signals in an electric cable). It is at the axon hillock that the signals received from all the neurons dendrites are summed over time. Should the combined sum of all signals from all dendrites exceed a threshold value that varies from neuron to neuron an action potential will be generated. For many years it was believed that creation of the neurons action potential occurred in the axon hillock but it has recently been shown that the initiation of action potentials usually begins in the adjacent (unmyelinated) segment of the axon proper (Clark, Goldberg & Rudy, 2009). The axon serves as the means to propagate the action potential to the next synapse. An axon may be myelinated or unmyelinated. An unmyelinated axon can be likened to a simple electrical wire with the action potential being transmitted through continuous conduction. In the case of myelinated axons the actual process of propagation is called ‘saltatory conduction’ meaning to ‘hop or leap’. The process of saltatory conduction is achieved by the electrochemical interactions of the axons myelin sheath and the unsheathed segments called the nodes of Ranvier after their discoverer Louis-Antone Ranvier. The cytoplasm of the axon is electrically conductive while the myelin sheath inhibits charge leakage. Depolarization at one node of Ranvier elevates the voltage at the next node of Ranvier. This causes the action potential to be regenerated at the next node of Ranvier. The result is an electrical signal that appears to hop or leap from one node of Ranvier to the next without diminishing in strength as it travels. This results in a significantly faster transmission of the nerve impulse down the length of the axon (Huxley & Stämpfli, 1949; Tasaki, 1939). The speed of propagation will be dependent on the diameter of the axon.
3.2.3 Synapse
The branches of an axon’s telodendria connect to the dendrites / dendritic spines of another neuron via a structure known as a synapse. The synapse transmits, usually via an electrochemical reaction, the action potential from the pre-synaptic neuron to the post- synaptic neurons dendrite. Figure 3-2 details the structure of a typical chemical synapse:
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Figure 3-2: The structure and operation of a chemical synapse. (Wikipedia, 2014)
The primary features of a synapse are, of course, the axon of the transmitting neuron and the dendrite of the post-synaptic neuron. These two primary features are not, however, directly connected a small space remains between the membranes of the axon terminal and the dendrite. This space is known as the synaptic cleft and is on average 0.2 micron wide. Information is carried across the gap using a chemical called a neurotransmitter. The sequence of events to transmit the information from the axon terminal to the dendrite of the connecting neuron is summarised as ('synapse,' 2014):
i. The arrival of an action potential at the axon terminal forces the movement of the synaptic vesicle(s) towards the axon terminals membrane
ii. A synaptic vesicle binds and fuses with the membrane of the axon terminal and releases a neurotransmitter into the synaptic cleft.
iii. The neurotransmitter diffuses across the synaptic cleft and binds to receptor molecules on the postsynaptic membrane.
iv. The binding action opens channel shaped protein molecules and electrically charged ions flow into or out of the neuron.
v. The abrupt shift in electrical charge across the postsynaptic membrane changes the electrical polarisation of the membrane creating a postsynaptic potential (PSP). vi. If the inflow of positively charged ions is large enough then the PSP is excitatory and
can lead to the generation of a new action potential in the post-synaptic neuron. Alternatively the response may be inhibitory, suppressing action potential generation. vii. After binding to a receptor the neurotransmitter is hydrolysed (broken down) by
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viii. The neurotransmitter is then re-absorbed by the presynaptic membrane of the axon terminal.