The Human Nervous System: 100 Plus Billion Cells

The human nervous system contains roughly 100 billion nerve cells.  Worth pausing for an instant… and read it again.  That’s right, 100 billions!  To give an idea of the scale, the Milky Way, our own galaxy, contains roughly 100 billion stars.  And although human beings are way smaller than galaxies, we begin to appreciate how each one of us is as complex, as mysterious, and as magnificent in its own right, as any large astronomical entity in the physical Universe. 

The human nervous system consists of the central and peripheral nervous systems, although in reality the two systems are inextricably intertwined:

• The central nervous system (CNS) includes the brain and the spinal cord.  Perception, movement control, learning, memory and other higher neural functions are carried out in the brain.  The spinal cord contains the nerves for rhythmic motor behaviour, mediates reflexes and conducts sensory information to the brain.

 

• The peripheral nervous system (PNS) is made up of all the nerves that lie outside the brain and spinal cord.  The PNS can be subdivided into two main functional branches:
  • The somatic (or voluntary) nervous system controls the skeletal muscles, and provides sensory information from the body and the outside environment.  The information is conveyed along a neural pathway that terminates in the brain, and lets us experience various stimuli consciously.
  • The autonomic (or involuntary) nervous system regulates bodily activities that involve smooth muscles, but not skeletal muscles.  These activities proceed without any intervention of our conscious will, and whether we are asleep or awake.  The autonomic nervous system (ANS) is further divided into:
    • enteric system,
    • sympathetic system,
    • parasympathetic system.

In the somatic nervous system, single motor neurons whose cell bodies are in the spinal cord link the CNS to the skeletal muscles.  But, in the case of the autonomic system, combinations of two neurons span the pathway from the CNS to the organ that is the site of the action.

Generally speaking, each branch of the ANS, that is the sympathetic and parasympathetic systems, exerts opposite effects on the structure or organ they innervate.

Sympathetic activity causes the heart to increase its pumping activity, while parasympathetic activity will tend to reduce it.  In times of emergency, the well-known “fight or flight” reaction does remind us of ANS activity, whereby the sympathetic branch tends to be excited, while the parasympathetic is inhibited.  At rest, the converse happens and the parasympathetic dominates.

To maintain a state of homeostasis within the body, the influence of the sympathetic and parasympathetic nervous systems are regulated via feedback mechanisms.  Each one exerts an effect over the other to maintain the correct balance.

This way, the ANS controls the beating of the heart – brought about by contraction of the cardiac muscle, and the peristaltic activity of the digestive system, including the production of saliva and the constriction of blood vessels.

 

Sensory Perception, Electrical Signals and Pain

The organisation, identification and interpretation of sensory inputs and stimuli from the environment is one mighty multi-tasking feat for the human nervous system.

All perception involves signals in the nervous system, which in turn result from physical or chemical stimulation of the sense organs.  Vision involves light striking the retina of the eye, smell is mediated by odour molecules, and hearing involves pressure waves.

Perception is not the passive receipt of these signals.  Rather it is the opposite.  Perception is shaped by learning, memory, expectation, attention and emotion.

Perception involves “top-down” effects, as well as the “bottom-up” process of processing sensory input.  The “bottom-up” processing transforms low-level information to higher-level information

• shape identification for object recognition
• colour perception
• motion detection
• heights/distances estimation
• temperature information.

The “top-down” processing refers to a person’s concept and expectations (knowledge), and selective mechanisms (attention) that influence perception.

Perception depends on very complex functions of the nervous system, but subjectively it seems mostly effortless because this processing happens outside the bounds of our conscious awareness.

Ultimately, our nervous system guides and interprets out sensory perception.  Information about the surrounding environment is acquired through sensory cells that are specialised to respond to a particular external stimulus.  In any case, the sensory cell generates an electrical signal in response to the stimulus.

When tissue is injured, there is a rapid release of chemical messengers that stimulate the nerve endings.  Electrical impulses are relayed through the nerves to the spinal cord and the brain, which registers the sensation of pain.  This is a fundamental mechanism which forms part of the defence system that enable us to avoid life-threatening injuries. 

 

Neurons – The Basic Units of the Nervous System

The basic signalling unit of the nervous system is the nerve cell, or neuron, which comes in many different shapes, sizes and chemical content.  Neurons are the core components of the nervous system.

A neuron is an electrically excitable cell that processes and transmits information through electrical and chemical signals.  These signals between neurons occur via synapses – specialised connections with other cells.

Neurons can connect to each other to form neural networks.

Dendrites are thin structures that arise from the cell body, often extending for hundreds of micrometres and branching multiple times, giving rise to a complex “dendritic tree”. 

An axon is a special cellular extension that arises from the cell body at a site called the axon hillock and extends for a distance, as far as 1 metre in humans, or more in the case of other species.  The cell body of a neuron frequently gives rise to multiple dendrites, but never more than one axon, although the axon may branch hundreds of times before it terminates.

Thus, information via electrochemical signals is received on the dendrites and passed on through an axon, from neuron to neuron.

At the majority of synapses, signals are sent from the axon of one neuron to a dendrite of another. However, there are many exceptions to these rules: neurons that lack dendrites, neurons with no axon, synapses that connect an axon to another axon, or a dendrite to another dendrite, and so on.

Specialised types of neurons include: sensory neurons which respond to touch, sound, light and all other stimuli affecting the cells of the sensory organs that then send signals to the spinal cord and brain, the motor neurons that receive signals from the brain and spinal cord to cause muscle contractions and affect glandular outputs, and interneurons which connect neurons to other neurons within the same region of the brain or spinal cord in neural networks.

 

Synapses, Action Potentials and Neurotransmission

Neurons convey information by means of short pulses of electricity that circulate along the length of an axon.

The electrical signal in axons is a brief voltage change called an action potential, or nerve impulse, which can travel long distances, sometimes at high speeds, without changing size or shape.

Like all cells in the human body, neurons have a cell membrane, which forms a barrier between the inside of the cell and the matrix that surrounds the cell.  This membrane is semi-permeable to some substances.  Now, intracellular fluid is of a different ionic composition to that of the fluid outside the cell.  There are more sodium Na⁺ ions outside, and more potassium K⁺ ions inside.  This difference in ion concentrations between the inside and the outside of cells is normally stable.

Action potentials are generated by the transitory rapid flow of sodium and potassium ions across the neuron cell membrane causing localised changes in voltage across the membrane.  When an action potential arrives at the ends of the axon, it interacts with up to thousands of neighbouring cells across synapses.  The electrochemical interactions at these synapses modify the intensity of the signals as they pass from cell to cell.

When a neuron is not transmitting an action potential, there is a small potential difference across its cell membrane – this small difference of – 70 millivolts (mV) is called the resting membrane potential.

A neuron receives signals from other neurons, or from sensory events around it, which can arise from the external environment (e.g. light) or from the internal environment of the body (e.g. blood sugar level, etc.).  If the excitatory strength of incoming signals is strong enough, it will cause the opening of ion channels that allow sodium ions across the membrane.

Movement of Na⁺ ions into the neuron makes the membrane potential less negative.  If the change in potential reaches a critical level or critical threshold of around – 55 mV, more voltage-dependent sodium ions channels open to allow more Na⁺ to enter the neuron.  At this point, a rapid alteration in electrical potential occurs until the membrane loses its potential, and momentarily becomes positive between + 20 mV to + 30 mV.  The signal peaks, then abruptly drops back to negative, as the $ Na^+$ ion channels suddenly close, and at the same time K⁺ ions channels open.

After each action potential has occurred, there is a refractory period, during which a second action potential cannot be generated  on precisely the same region of the membrane.  This prevents an indefinite number of action potentials moving backwards and forwards along the axon.

Action potentials normally move in only one direction along axons.  They can travel in rapid succession.  Neurons transmit information at speeds between 0.2 and 100 metres per second, and are then said to be ‘firing’.  The speed of transmission is increased where the axon diameter is large, and/or coated with myelin.

The junction between two neurons is called a synapse.  Neurons can also form synapses with muscle fibres, in which case the synapse is called a neuromuscular junction.  Information is carried across the synaptic gap by the release of neurotransmitter molecules that bind to specific receptors.

The effect of a neurotransmitter can be one of excitation or inhibition, depending upon the properties of the combination of neurotransmitter and its receptor.

 

Lifecycle of the Nervous System

The nervous system appears to be influenced by its own neurons-synapses activity and connections.   In infants and children, these connections seem to come in an over-abundance for the actual need.   If you use a particular pathway, it becomes solidified and stronger, and establishes itself. 

Pathways that are not used fall away, sometimes permanently.  And to some extent, you either…

Use it or lose it.

 

One of the latest regions of our brains to stop forming synapses is the prefrontal cortex, the outer region of the brain directly behind your forehead.   Here the process of synapse formation goes on into the mid-teen years…