Nervous Coordination

What makes a plant a plant and an animal, an animal? An animal feeds on already made (organic matter) food by the producers such as plants. Animals have a specialised nervous system to respond to stimuli (e.g. temperature change) rapidly.

Arguably, some plants also feed on organic matter in the soil or trap insects and have specialised sense organs in their root tips. Plants also react to stimuli in their environment, sometimes even rapidly, like the mimosa pudica plant. Trees, for example, send more resources to their neighbours when they are hurt and communicate via vast underground root networks.

Fig. 1 - An underground root network of wheatgrass

One aspect distinguishing animals from plants is the nervous system. In animals, the nervous system is based on the smallest functional unit, the neurone, which is in charge of the nervous coordination, making fast responses to stimuli in animals the rule rather than the exception (like in plants).

What is the definition of nervous coordination?

Nervous coordination is when all the parts of an organism work together smoothly. There are different types of nervous coordination, such as reflexes (involuntary) and voluntary movement.

Reflexes are an automatic reaction to stimuli entering the body through sensory neurones, which causes muscle cells to contract. You have probably had your reflexes checked by a doctor. A small hammer is used to hit the tendon just below the knee cap (Figure 2) to check the integrity of the nervous system.

Fig. 2 - Checking the knee reflex

Voluntary movement is also a reaction to a stimulus - the difference being that voluntary movement requires conscious control of the muscles.

The interplay between the different parts of the body - the eyes, the nerve fibres, the spinal cord and muscles- is referred to as nervous coordination.

What is the role of the nervous system in nervous coordination?

Nervous coordination is ensured by the nervous system, a network in the body of animals that is in charge of communication. All activity in the body is controlled by passing on information via its specialised neurone cells. Nerves are bundles of neurones grouped together.

The two main functions of the nervous system are:

1. To receive sensory input, process it, and react to it.

2. To coordinate all the different elements in the body (cells, glands, etc.).

Neurones communicate with each other using synaptic transmission, the majority of which is electrochemical. Most neurones have a gap between them which is bridged via neurotransmitters (e.g., acetylcholine).

Depending on the location or the function of the different nerves, the nervous system can be subdivided again.

What are the two main subdivisions of the nervous system?

There are two main subdivisions of the nervous system - the central nervous system (CNS) and the peripheral nervous system (PNS) (Figure 3).

The CNS is the centre of control for the entire organism. The central nervous system includes the brain and spinal cord and is responsible for conscious decisions and automatic reactions (reflexes) to stimuli.

The nervous system sends impulses from the peripherals to and from the CNS. The peripheral nervous system then is subdivided by function into the autonomic nervous system and the somatic nervous system. The autonomic nervous system can either be aroused or calmed depending on the response; it is overseen by the sympathetic nervous system (fight-or-flight response) and the parasympathetic nervous system (rest and digest response).

The components of the central nervous system

The CNS includes the brain and the spinal cord.

Defending from the toxins

The CNS has physiological measures to prevent harmful toxins from entering the central nervous system. A specific plasma-like fluid circulates in and around the central nervous system called the cerebrospinal fluid (CSF). CSF has several molecular structures and membranes functioning as security gates, preventing toxins from entering the brain even if they're already circulating in the body in substances such as blood. This means that although the brain and spinal cord connect to the other nerves, the central nervous system is a closed system in itself.

The brain

If you compare the size of other mammals to human brains, the human brain-to-body ratio is the same as that of a mouse or monkey. In other words, if a rat or mouse were as tall as a human, their brains would be the same size as the human brain.

Brains vary from organism to organism - some animals don't have a brain - such as jellyfish (only a loose network of neurones). In contrast, some animals, such as octopuses, have much larger brain-to-body ratios than humans.

However, the major structural difference between humans and other animals is that the brain's surface area, called the cerebral cortex is much larger than other mammals'. The human cortex is folded up, different from a rat's smooth brain. The cerebral cortex's increased surface area makes humans better at integrating information and planning than other animals. Conscious and unconscious decisions are made in the brain.

The spinal cord

The spinal cord is a tubular structure of nerves that extend from the brain into the peripheral nervous system. It reaches from the base of the brain, called the hindbrain, to the second lumbar vertebra in the lower back, about 5 cm above the pelvis.

To enable the body to react quickly, specialised neurones, called relay neurons, carry out unconscious reactions to stimuli known as reflexes. Pulling your hand away from a hot plate, jumping when startled, and your knee jerking up when a doctor hits it are all examples of reflexes. The spinal cord includes the nerve endings that connect to the peripheral nervous system.

The components of the peripheral nervous system

In the PNS, nerves extend from clusters of neurone cell bodies called ganglions into all the muscles and senses in the body. In the PNS, information gets passed to the CNS and from the CNS to muscles and organs. Here, the information processed by the senses (smell, taste, sight) and other receptors (touch, heat, pain) are passed to the CNS for integration.

These two aspects are subdivided into the somatic nervous system and the autonomic nervous system. The divisions of the nervous system run parallel to each other.

Somatic nervous system: This part of the peripheral nervous system communicates with your senses ("soma"). It also is responsible for the voluntary control of your muscles. Any activity that you consciously control, such as moving fingers or speaking, fall under the banner of the somatic nervous system.

Autonomic nervous system: This is the part of the peripheral nervous system in charge of the involuntary and unconscious control of processes of the body such as heart rate, blinking, digestion, relaxation and arousal. It works autonomously and is controlled by a specific part of the brain called the hypothalamus. The autonomic nervous system is divided into two functional units - the sympathetic and parasympathetic nervous systems.

What is a neurone?

Neurones are specialised cells found in the nervous systems of all animals. Their job is to transmit nerve impulses. A mind-boggling 86,000,000,000 (yes, that's 86 billion!) of them form a dense network in the brain, which is why in everyday language, called "brain cells".

They're not only found passing on information in the brain but also throughout the body. Neurones collect information from the outside world through the senses and pass information from the brain to the muscles, which makes all movement and communication possible - for instance, without your brain controlling the movement of your eye muscles, you wouldn't be able to read this text right now.

What's the structure and function of a neurone?

All cells start as embryonic stem cells. Later, these begin to differentiate, meaning they develop different shapes according to their function in the body. Like other animal cells, the neurone has a membrane, nucleus and cytoplasm. But what sets a neurone's cell structure apart is that its form specialises in transmitting information - it generally has an input (the dendrites) and an output (the axon).

Dendrites

Dendrites are branch-like structures that grow from the cell body. The word comes from the Greek "dendritos", meaning tree-like. And like in trees, usually the centre stem develops first, and the newest growth is at the tips. It's here that information is taken in from neighbouring cells.

Axon

The axon is the long part of a neurone along which impulses travel from the cell body to other cells. An axon can be anywhere from a few micrometres to one metre long in humans (in the leg) and up to a whopping 25 metres long in whales.

The nerve impulse always travels away from the cell body via the axon to the thick bits at the end of the axon. They are called terminal buttons or axon terminals. Nerve impulses are unidirectional because of how nerve impulses travel, called the action potential.

The axons can also branch out, but not as much as the dendrites. These branches are called collaterals. Where the axon terminals or buttons meet another cell is a synapse. Nerve impulses are passed on from one neuron to the next via the synapse.

Myelin sheaths

Often, axons are wrapped with a protein and fat compound called myelin. Myelin sheaths insulate the electrical activity of the axon to prevent electrical interference with the other nerve impulses in the closely packed neurone network of the central nervous system. It's similar to the rubber insulation that wraps around the wires in your phone charger cable.

Myelin also speeds up the transmission of nerve impulses, so the more myelin is wrapped around an axon, the faster the electrical impulse is sent to the next cell. It will provide more insulation. Myelin is made up of glial cells wrapping around the axon. The bits of the axon where there are no myelin sheaths are called the nodes of Ranvier.

Types of neurones

There are three classifications of neurones:

1. Sensory neurones gather information and send it to the brain and spinal cord.

2. Interneurons (relay neurones) connect one neurone with another in the brain and spinal cord.

3. Motor neurones send the information back from the brain and spinal cord to the muscles.

Ideally, the three types of neurones work together smoothly. If one of the elements is affected, this will lead to severe illnesses in the organism. The conduction of nerve impulses through these three types of neurones is the process that takes place in all actions, including reflexes.

What's the function of motor neurones?

Motor neurons make the body move (the muscles) - like a motor moves a machine. They pass nerve impulses from the brain or spinal cord to a muscle or gland. They send impulses to the muscles and make them contract. All life is movement - from the heart beating, the diaphragm moving up and down to create breathing in the lungs, to conscious movement of the muscles in your legs when you walk to the kitchen in the morning.

The brain constantly sends nerve impulses to the body via the motor neurones. These neurones have some of the longest axons in the human body, with some of them stretching from the spine to the foot. Specialised myelin sheaths, called Schwann cells, surround the axons. Schwann cells allow the electrical impulse to travel faster, so they are ideally suited to transmit signals over long distances (from the brain to the big toe) without loss of charge. They can also repair damaged cells, which their counterparts in the central nervous system, the oligodendrocytes, can't.

Damaged motor neurones

When motor neurones are damaged, individuals have problems moving or controlling vital functions such as breathing, chewing and swallowing. Muscle movement and coordination can be impaired, and individuals might have twitching limbs or become paralysed. This is the case with ALS and other motor neurone diseases such as multiple sclerosis.

What is the difference between the endocrine and nervous systems?

The nervous and endocrine systems are responsible for passing information, responding to stimuli, and creating homeostasis (biological balance) in the body.

Table 1. Examples for comparison between nervous and endocrine systems.