The neural system is arguably the most important system in the body, as it controls every function that occurs, within the body. This is achieved through the intake of information by afferent nerves, followed by the appropriate response by efferent nerves.
The functions of the nervous system can be summed up as follows:
1. Sensory intake:
Sensory receptors around the body collect information, and transmit this information to the CNS for processing.
The brain and spinal cord assimilate and process received information. Here, impulses are evaluated, and appropriate responses are determined. They will be compared to previously received information, and reacted to. Where the information is new (there is no previous encounter) the information will be stored in the memory, to be used for comparison in the event of a repeat encounter with the stimulus.
Once a response is decided upon, the CNS send out nerve stimuli to affect the chosen response. This will mean more than one of the body systems may be brought into action, to respond to the received stimulus.
The Divisions of the Nervous System
The neural system can be broken down into two major divisions, in order to understand how it goes about performing these functions, and controlling the rest of the body.
The Central Nervous System (CNS), which consists of the brain and spinal cord, is responsible for all integration of afferent information, and the effecting of responses. It is responsible for the control of all other parts of the nervous system, and therefore also responsible for all other systems in the body.
The Peripheral Nervous System (PNS) consists of all nervous tissue outside of the brain and spinal cord. This is essentially all afferent and efferent nerves responsible for the conveying of information to and from the processing unit, to obtain information, and cause responses.
On closer inspection, there are separate systems contained within these two larger systems:
- Autonomic Nervous System (ANS), comprised of the sympathetic and parasympathetic nervous systems, and is responsible for all functioning that we don’t have to think about (hence autonomic).
- The Somatic Nervous System, responsible for all functions we consciously choose to do.
For the purposes of this section, it is important to understand the different functions of these systems, as they will each respond to exercise in a different way; and they will also play different roles in causing exercise/movement to take place.
Nerves can be looked at as having two distinct parts. These are the dendrites, and the axons. The dendrites are essentially the brains of the cells, where most of the cellular organs for function are found. The axons then serve as the long arms of the cell, which serve to transmit nerve impulses, and thus carry information from one place to another. Typically, along any neural pathway there will be a sequence of nerve cells, lined up in series. Between each nerve in the series lies a microscopic gap, known as a synapse. This is where nerves end at the terminal end of the axon, and the dendrite of the next nerve is found. These will pass information between the CNS, and every other part of the body.
There are three basic classes of neurons: afferent neurons, efferent neurons, and interneurons.
- Afferent neurons. Also known as sensory neurons, afferent neurons transmit sensory signals to the central nervous system from receptors in the body.
- Efferent neurons. Also known as motor neurons, efferent neurons transmit signals from the central nervous system to effectors in the body such as muscles and glands.
- Interneurons. Interneurons form complex networks within the central nervous system to integrate the information received from afferent neurons and to direct the function of the body through efferent neurons.
- Neuroglia: Neuroglia, also known as glial cells, act as the “helper” cells of the nervous system. Each neuron in the body is surrounded by anywhere from 6 – 60 neuroglia that protect, feed, and insulate the neuron.
OTHER NEURAL STRUCTURES
Extending from the inferior side of the brain are 12 pairs of cranial nerves. Each cranial nerve pair is identified by a Roman numeral 1 to 12 based upon its location along the anterior-posterior axis of the brain. Each nerve also has a descriptive name (e.g. olfactory, optic, etc.) that identifies its function or location. The cranial nerves provide a direct connection to the brain for the special sense organs, muscles of the head, neck, and shoulders, the heart, and the GI tract.
Extending from the left and right sides of the spinal cord are 31 pairs of spinal nerves. The spinal nerves are mixed nerves that carry both sensory and motor signals between the spinal cord and specific regions of the body. The 31 spinal nerves are split into five groups named for the five regions of the vertebral column:
- 8 pairs of cervical nerves
- 12 pairs of thoracic nerves
- 5 pairs of lumbar nerves
- 5 pairs of sacral nerves
- 1 pair of coccygeal nerves
Each spinal nerve exits from the spinal cord through the intervertebral foramen between a pair of vertebrae or between the C1 vertebra and the occipital bone of the skull.
The meninges are the protective coverings of the central nervous system (CNS). They consist of three layers: the dura mater, arachnoid mater, and pia mater.
- Dura mater.
The dura mater, which means “tough mother,” is the thickest, toughest, and most superficial layer of meninges. Made of dense irregular connective tissue, it contains many tough collagen fibres and blood vessels. Dura mater protects the CNS from external damage, contains the cerebrospinal fluid that surrounds the CNS, and provides blood to the nervous tissue of the CNS.
- Arachnoid mater.
The arachnoid mater, which means “spider-like mother,” is much thinner and more delicate than the dura mater. It lines the inside of the dura mater and contains many thin fibres that connect it to the underlying pia mater. These fibres cross a fluid-filled space called the subarachnoid space between the arachnoid mater and the pia mater.
- Pia mater.
The pia mater, which means “tender mother,” is a thin and delicate layer of tissue that rests on the outside of the brain and spinal cord. Containing many blood vessels that feed the nervous tissue of the CNS, the pia mater penetrates into the valleys of the sulci and fissures of the brain as it covers the entire surface of the CNS.
The space surrounding the organs of the CNS is filled with a clear fluid known as cerebrospinal fluid (CSF). CSF is formed from blood plasma by special structures called choroid plexuses. The choroid plexuses contain many capillaries lined with epithelial tissue that filters blood plasma and allows the filtered fluid to enter the space around the brain.
Cerebrospinal fluid provides several vital functions to the central nervous system:
1. CSF absorbs shocks between the brain and skull and between the spinal cord and vertebrae. This shock absorption protects the CNS from blows or sudden changes in velocity, such as during a car accident.
2. The brain and spinal cord float within the CSF, reducing their apparent weight through buoyancy. The brain is a very large but soft organ that requires a high volume of blood to function effectively. The reduced weight in cerebrospinal fluid allows the blood vessels of the brain to remain open and helps protect the nervous tissue from becoming crushed under its own weight.
3. CSF helps to maintain chemical homeostasis within the central nervous system. It contains ions, nutrients, oxygen, and albumins that support the chemical and osmotic balance of nervous tissue. CSF also removes waste products that form as by-products of cellular metabolism within nervous tissue.
When any information is passed along a nerve, it is carried as an electrical pulse along the length of the nerve. Each nerve will carry specific information, to its designated destination. As such, information does not need to be coded. Rather, the coding for the information is held in which nerves are carrying the information.
If we take an efferent nerve as our example; the impulse is generated by the brain, and then sent along the nerve to the point where action is required. The brain will generate an electric pulse. This is sent down the nerve through a chemical exchange along the cell wall of the axon. As the impulse moves down the axon, it causes sodium ions (Na+) to move into the axon, resulting in a negative charge outside of the cell. This is quickly rectified, though, through what is known as the sodium-potassium pump, where potassium ions (K+) are drawn into the cell, in order to pump the Na+ back out. The K+ then move back through osmosis. This demonstrates one of the functions of these minerals, and why it is so important to maintain a good salt balance.
If you consider the length of some neural pathways, it becomes apparent that the nerve impulse must travel at incredible speed, in order to effectively carry information both to, and from, the CNS. To overcome this, the axons are covered in a fatty layer, called a myelin sheath. This sheath leaves intermittent points along the axon exposed. As a result, the nerve impulse is forced to “jump” from one exposed area, to the next. This vastly increases the speed at which the impulse is carried along the nerve.
For the purpose of this section, the more important nerve ending to focus on is that where the nerves meet the muscles, or the neuro-muscular junction. This is where the nerve impulse is passed to the muscle, to cause a muscular contraction. Like all synapses, the impulse is passed through the release of a chemical, or neurotransmitter. These chemicals are stored in small pockets, called vesicles. And, the neurotransmitter at the neuro-muscular junction is called acetylcholine. This chemical is released in response to the nerve impulse reaching the end of the axon of the motor nerve, and is taken up by the muscle cell, resulting in the sequence of chemical reactions which stimulate the muscle to contract. This sequence will be discussed later.
The two main organelles involved in proprioception are the muscle spindle, and the golgi tendon.
This organelle is found within the muscle fibres. An easy way to think about it is as a coiled spring. As the muscle is stretched, due to being lengthened, the coil is also pulled straighter. This then causes a protective reflex, at the spinal level. The muscle fibres contract, to prevent being stretched to the point of tearing, without control. This is what is known as the Stretch Reflex, and is responsible for protecting the muscle, as well as for assistance in force generation in muscular contraction.
This organelle is found in the tendons of all of the skeletal muscles. And, it too has a protective reflex as a function. When tension in a muscle becomes too great, to prevent damage to the muscle, tendon, and attachment to the bone, the golgi tendon causes inhibition of the muscle contraction. In other words, it stops the contraction. This does not always mean it stops it completely. Just that it inhibits it enough, so that the chance of injury is removed.
Everyone has felt some form of physical pain, at some point. Take, for example, the feeling in your toe immediately after it becomes quickly and closely acquainted with the leg of a table. Even if you don’t see that you have stubbed your toe, you suddenly become acutely aware of it. All throughout the body are tiny receptors called Nociceptors. These little organelles are responsible for our ability to perceive pain. When aggravated, they release neurotransmitters which tell the body that something unpleasant is happening, and needs to be stopped. We perceive this as pain.
An example of how this works could be when you touch a hot plate. Your hand quickly realizes that the heat is too much for it, and you feel pain. The body, in response, pulls the hand away from the plate. This is not the only time we feel pain. But, in all instances, it is a reaction to the body wanting something to end. This is why pain is so uncomfortable, and far from a desirable state to be in.