Physical nervous system

In large multi-cellular organisms like us, the cells needing to respond to the stimuli are often in completely different parts of the body to where the stimulus was detected.  In order to co-ordinate the response to stimuli, a system of communication is used to transmit messages about the stimulus to our command and control centre housed in the brain and from there to send messages back to the cells directing a course of action. 

This communication system is known as the nervous system and it consists of millions of nerve cells which act as the detectors and transmitters of messages. Nerve cells are also known as neurons.  Neurons connect to each other to form networks.

Neurons 

A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling between neurons occurs via synapses, specialized connections with other cells, whereas within neuron signalling is electrical.

Neurons connect to each other to form networks and are the core components of the nervous system.  The nervous system of the brain and spinal cord is called the Central Nervous system [CNS].  The nervous system of the rest of the body is called the Peripheral Nervous system [PNS].

The number of neurons in the human brain is estimated to be about  100 billion (10¹¹) neurons and 100 trillion (10¹⁴) synapses. Another estimate which puts the figure at  86 billion neurons, estimates that there are 16.3 billion in the cerebral cortex, and 69 billion in the cerebellum.

A neuron consists of the soma with its dendrites and nucleus and an axon covered by a protective myelin sheath leading to the axon terminal.  The axon is composed of various nodes connected by the nodes of Ranvier.

Neuron

Signalling within the nerve cell

A typical neuron is divided into three parts: the soma or cell body, dendrites, and axon. Axons carry signals away from the cell body, whilst dendrites carry signals to the cell body. 

The soma is usually compact; the axon and dendrites are filaments that extrude from it. Dendrites extend their farthest branches a few hundred micrometres from the soma. The axon on the other hand can extend for great distances, giving rise to hundreds of branches. The soma may give rise to numerous dendrites, but never to more than one axon.

Thus messages are received via the dendrites to the soma which generates an electrical signal that travels along the axon.

Axons and dendrites in the central nervous system are typically only about one micrometer thick, while some in the peripheral nervous system are much thicker. The soma is usually about 10–25 micrometers in diameter and often is not much larger than the cell nucleus it contains. The longest axon of a human motoneurone can be over a meter long, reaching from the base of the spine to the toes. Sensory neurons have axons that run from the toes to the dorsal columns, over 1.5 meters in adults.  The electrical charge always runs from the soma to the axon terminal.

Within the neuron, processing is electrical and the electrical part depends upon the neuron’s membrane.  Every neuron is surrounded by a plasma membrane, which is a powerful electrical insulator, but in neurons, many of the protein structures embedded in the membrane are electrically active. These include:

• ion channels that permit electrically charged ions to flow across the membrane

• and ion pumps that actively transport ions from one side of the membrane to the other

Most ion channels are permeable only to specific types of ions. Charge-carrying ions include sodium (Na⁺), potassium (K⁺), chloride (Cl^-), and calcium (Ca²⁺). 

Orders are also ‘addressed’ because they always go to the specific points in the body that need to act, so it is very like a computer communications network from this point of view.

The network

This handy picture shows us this vast network of nerves all linking up via the spinal cord and heading to the brain.  The effect is not unlike an upended tree with the roots in the brain and all the branches going out into our body.   If we now imagine the flow of signals along these fibres some going out to the muscles and organs and  some coming in to the brain, you can see there is a sort of constant flow of communication taking place up and down the  spinal cord to the brain.

In the physical system, a number of specialized types of neurons exist:

• Sensory neurons respond to touch, sound, light and numerous other stimuli affecting cells of the sensory organs that then send signals to the spinal cord and brain.
• Motor neurons receive signals from the brain and spinal cord, cause muscle contractions, and affect glands.

How it works to send a signal 

A resting neurone does not convey nerve impulses, but it maintains a potential difference across its membrane called the resting potential and measures about –70 millivolts.  During the resting potential the inside of the neuron is negative relative to the outside because of the unequal distribution of charged ions.  On the outside, sodium ions, chloride ions and calcium ions are present in higher concentrations than inside the cell.  By contrast inside the cell there is a high concentration of potassium ions.

The unequal distribution of ions results from a combination of active transport and diffusion of sodium and potassium ions across the cell membrane.  A sodium potassium pump actively transports sodium ions out of the neuron and potassium ions in.  For every three sodium ions pumped out, only two potassium ions are pumped inwards.  On its own, this would result in only a slight potential difference across the membrane, however, this difference is amplified by the membrane being about 50 times more permeable to potassium ions than to sodium ions.  Potassium ions are able to diffuse freely back out of the cell down their concentration gradient, but the sodium ions diffuse back into the cell only very slowly.  This creates a negative electrical charge inside compared with outside.

A nerve impulse occurs when the resting potential across the membrane of a neurone has a sufficiently high stimulus.  The stimulus can be chemical, mechanical, thermal,  electrical or magnetic or it can be simply a change in light intensity. 

When the stimulus is applied, the axon becomes depolarised, that is, the inside becomes temporarily less negative.  If the stimulus is strong enough – it exceeds the threshold level – an action potential occurs.  There is a complete reversal of the charge across the nerve cell, the interior becomes positively charged relative to the outside reaching a normal peak of about 35 millivolts.

The potential difference then drops back down, undershoots the resting potential and finally returns to it.  The entire sequence of action takes about 7 milliseconds.

These localised action potentials are converted into nerve impulses which then travel along the neurone

Strength of signal

In the nervous system, the information about the strength of a stimulus is not carried in the form of the size or amplitude of the nerve impulses, but by changes in their frequency. 

Nerve fibres are protected by a thin layer called a myelin sheath that is like an insulator, ensuring no leakage in or out.  So any indication of strength of signal is always targeted along the appropriate nerve fibres to the appropriate target. 

So if we take a very dramatic example, a function like emotional shock, can send a burst of electrical signals to the heart and we can literally die of heart failure from the enormous burst of electrical signals received.  But the burst is not, as I said, amplitude, but frequency.  The greater the strength of signal the greater the frequency of impulses.  Neurones can generate nerve impulses over a wide range of frequencies from one or a few per second to more than a hundred per second

Synapses

Once at the end of the axons, the end may connect to the next nerve cell so that the message keeps on going to other nerve cells and it does this via synapses.  Here the connection is chemical.  Thus, within nerve cells the signalling is electrical, however, between nerve cells there is chemical signaling that occurs via the  ‘synapses’ between neurons.  It is a bit like a computer network, with routers and channels and fibres, but of course cleverer.

The speed of communication is extremely fast – a single nerve impulse can be transmitted from the spinal cord to the feet in a few milliseconds.  We need this fast transmission to enable us to respond almost instantaneously to stimuli.

See also Neurotransmitters.

The final destination

Exactly the same sequence of actions occurs if the neuron is connected with an ordinary cell of the body – for example, a muscle cell or a  gland cell.  The final destination of the message.

Thus for example, the mind releases, via a neuron, an electrical signal that is intended to stimulate the muscles of the  legs.  The message is targeted to a location and a type of activity and it travels along the nerve fibres electrically and chemically – the targeting being achieved partly via the chemical messages.  If we use the analogy of a computer network, this is where the router action takes place.  Eventually it will get to the targeted muscles and the final action is to send the chemical messenger to the muscle cells telling them to trigger the function of ‘contraction’ say.

Thus all activity in the end is determined by the final stage of action, when the message gets to the cell and its receptors.  Only then does something actually happen functionally.

Overall

Let me now recap this section as it has described some quite complex biological activity.

Our nervous system uses neurons consisting of axons and dendrites to transmit information.  Within the neuron, the signal is an electrical one, but between neurons the signal is chemical.

What has this to do with spiritual experience? 

One of the ways in which spiritual experience can be obtained is electrical.  As the nervous system is an electrical system, it should come as no surprise that we can use electricity to affect various cells and functions. 

Epilepsy is an illness which is electrical in nature and through which spiritual experience is possible, an electric shock can cause hallucinations, being hit by lightning can result in profound spiritual experience.  There is even a device called a Transcranial Magnetic Stimulator, that uses electricity to provoke spiritual experience.  Furthermore, the descriptions of ‘kundalini energy’ found in numerous text books on the Hindu systems of yoga, are actually describing a huge burst of electrical energy that rises up the spine that appears to affect the neurons in the brain and thus the functions in the brain.  So the fact that the nervous system is electrical is quite key in this context. 

So the way the nervous system works is absolutely key.