Neurone Structure and Function

Neurones are the cells in the nervous system which are responsible for sending messages.

They have three major purposes

Neurones are divided into different regions each having a different function and are characterised as having:

Figure 10. Diagram of a typical neurone

Note that the function of the nervous system as a whole depends very much upon the complexity of the network of connections between the various neurones rather than the specific features of any single neurone.

Neurones display an extremely high level of metabolic activity. All neurones have a massive surface area compared to other cells in the body which in itself requires huge energy input. In addition they need to produce electrochemical gradients which are in effect the nerve impulses which also requires a high level of metabolic activity.

This high level of activity is reflected in the cells cytological appearance. The nucleus is typically large, rounded and with a prominent nucleolus which represents a high degree of cellular activity.

There is abundant rough endoplasmic reticulum which secretes the proteins required and is visible in the cytoplasm. This is often called the Nissl substance. There is a well developed Golgi apparatus to provide secretory products especially neurotransmitters. In addition there are large numbers of mitochondria needed to produce the large amount of energy required by the neurone.

Figure 11. A photomicrograph of the cell body of a neurone showing [1] nucleus with a prominent nucleolus [2] Nissl substance [3] axon

Neuroglial cells are associated with the axons of neurones. In the central nervous system these are the oligodendrocytes whereas in the peripheral nervous system these are the Schwann cells. Both of these types of cell can associate with axons in two different ways. This will produce either a myelinated or a non-myelinated neurone.

In the first case the Schwann cell (or oligodendrocyte) wraps it's cell membrane around the axon a number of times.

This forms a segmented sheath around the axon known as the myelin sheath. Myelin is a lipid (fatty) substance which is pale in colour and it is this colour which leads to the terms white matter and grey matter within the nervous system. The white matter contains myelinated axons predominantly whereas the grey matter contains mainly neurone cell bodies which are non-myelinated.

Figure 12. The above diagram shows the various stages in the myelinisation of an axon by a Schwann cell

The main functions of myelin are to increase the electrical capacitance of neurones and also to insulate against any leakage of the bio-electrical nerve impulse.

The higher the capacitance and the better the insulation the faster the nerve impulse will travel along the neurone.

The main difference between the Schwann cell and the oligodendrocyte is that the former will myelinate only one neurone whereas the latter may associate with a number of axons.

The gaps between the segments of the myelin sheath represent the junction between adjacent neuroglial cells. They are approximately 1mm. apart along the length of the axon. These tiny gaps are known as the nodes of Ranvier.

Figure 13. Diagram of node of Ranvier

These nodes play an important role in the physiology of the neurone. At these points there are gated sodium ion channels of which there are none in the myelinated areas of the axon. How this affects nerve transmission is discussed later in these pages.

In the second type of relationship between the neuroglial cells and the neurones the axon is still enveloped in the Schwann cell (or oligodendrocyte) but it does not continue to wrap itself around to complete the process of myelinisation. Myelinated neurones will transmit nerve impulses more quickly than non-myelinated neurone for the reasons mentioned above.

It is important that there is a range of speeds at which nerve impulses are transmitted as what determines the body's response depends upon the number of impulses being sent and their speed of transmission.

Neurones can be classified by either structural or functional terms.

Structural classification.

Figure 14. Types of neurone.

 

Bipolar neurones (Fig.14-1) are generally small simple cells with short processes that provide local connections within the central nervous system. They have two main processes similar in length, one being a dendrite the other an axon.

These types of neurones are also known as association neurones or interneurones.

Unipolar neurones (Fig.14-2) are characterised by the possession of one major process which subdivides into two branches. One runs to the central nervous system and is axonic in function. The other runs to a part of the body and is dendritic in function.

Unipolar neurones are usually sensory neurones by function. This means that they send messages from parts of the body to the brain which convey information about touch, heat, smell etc.

Multipolar neurones tend to have a large cell body with on long axon carrying an impulse away from the cell body. They also have several shorter dendritic processes carrying impulses towards the cell body. They tend to be motor neurones by function.

From the above it can be seen that axons always carry impulses away from the cell body whereas dendrites always carry impulses towards the cell body.

All neurones, whatever their structure or function can only carry a nerve impulse in one direction.

Dendrite ® Cell Body ® Axon

Whether the nerve impulse goes into the central nervous system or comes away from the central nervous system depends upon the alignment of the cells. Sensory neurones always have their dendrites at their ending in a sensory receptor in the body and their axon ends within the central nervous system (either the spinal cord or the brain depending on the part of the body in which the sensory receptor is located.

Motor neurones are arranged relative to the central nervous system in the opposite direction.

Transmission of nerve impulses

The transmission of nerve impulses around the body depend on two properties which are unique to neurones.

Excitability refers to the fact that nerve cells are able to respond to a stimulus.

This stimulus may be internal or external. Muscle fibres are also able to demonstrate excitability giving them the ability to contract if stimulated.

Conductivity refers to the property that neurones alone have of transferring their excitability along their length and then on to other neurones or even muscle tissue.

It is these two properties together which allow neurones to deliver appropriate messages to appropriate parts of the body as and when required.

The way in which neurones transmit signals along their length is controlled by an electrical (ionic) gradient across their cell membranes. Along with muscle fibres, neurones are said to be excitable tissues.

At rest the neurone cell membrane has a negative membrane potential of -70 millivolts (mv). Firing is associated with depolarisation which creates a positive potential of around + 40 mv.

This is possible because, unlike other cells in the body, nerve cells and muscle cells are able to alter their transmembrane potential reversibly. The significant feature which makes this possible is the difference in the ionic concentration inside the cell to that outside it.

This difference is partly as a result of the imbalance between Sodium (Na+) and Potassium (K+) ions on either side of the neurone cell membrane which is selectively permeable.

In a resting neurone the concentration of K+ inside the cell is around 30 times greater than it is outside whereas the concentration of Na+ is around 14 times less inside the cell than it is outside.

The maintenance of the neurone at rest seems to be due to the action of the sodium-potassium pump. The precise action of this mechanism is not as yet fully understood but appears to utilise Adenosine Tri-Phosphate (ATP) as it's energy source.

Even when neuronal cells are not conducting nerve impulses they are continually sending ions across the membrane via the action of this pump. Na+ ions are actively transported out and K+ ions are actively transported in.

In addition to this a large number of negatively charged ions (mostly protein anions) are trapped within the cell membrane because of their size (i.e. they are too big to diffuse through the cell membrane). Although potassium ions are positively charged there are insufficient quantities of them to balance out the protein anions in the cell leading to an overall negative charge within the cell. Outside the cell the higher quantity of sodium ions (which are positively charged) plus the lack of protein anions (which cannot get out of the cell) leads to an overall positive charge in the extracellular fluid. This state is known as the resting potential.

As mentioned above nerve cells and muscle fibres are both able to alter the potential across their membranes and this allows them to have the property of excitability.

When this occurs the polarity of the membrane is reversed and the inside of the cell momentarily becomes positive with respect to the outside of the cell. This shift is referred to as the action potential and within nerve cells this has an amplitude of approximately +110mv. and lasts for approximately 1 millisecond (ms) [or one thousandth of a second].

In order for this excitability response to occur there must be a stimulus. This represents the delivery of energy to the nerve cell membrane in some way. There are numerous ways which this can be achieved. For example the energy from sound waves excite neurones in the inner ear, the energy from heat excites neurones in the skin, the energy from light excites neurones in the eye etc.

Action potentials are said to follow the all-or-nothing principle which means that they are not graded responses but are either full sized or absent depending upon the strength of the stimulus. (i.e. if the stimulus is strong enough the action potential is present at its full strength of 110mv if the stimulus is not strong enough there is no action potential at all.)

Depolarization of one part of the membrane sends an electrical current to neighbouring unstimulated membrane, the outer surface of which is still positively charged. This local current stimulates the adjacent portion of the neuron’s membrane to depolarize in a similar fashion. This event repeats itself along the membrane of the cell, thereby conveying the nerve impulse along the neuron.

The size of the action potential is self-limiting, because the membrane becomes less permeable to Na+ again. The membrane then becomes more permeable to K+, which rapidly leaves the cell. The neuron is then said to be repolarized. The whole process takes less than one thousandth of a second. After a very brief period, called the refractory period, the neuron can repeat this process. By then, the Na+/K+ pump has re-established the necessary concentration differences across the membrane.

This is the way that impulses pass down unmyelinated nerves and is termed continuous conduction. In myelinated nerves, however, the myelin sheath around the nerve does not conduct an electric current and forms an insulatory layer around the axon. However, depolarization can occur at the short sections of non-myelination along myelinated nerves, the nodes of Ranvier. In such nerves, the impulse is conducted by jumping sequentially from one node to another along the nerve. This form of impulse conduction is usually quicker than continuous conduction and is known as saltatory conduction.

It is important for us as nurses to remember that the correct functioning of nerve cells (and as a result, most of the correct functioning of body organs) depends upon the correct levels of electrolytes (especially sodium and potassium) and water being present.

Many neurological problems can occur if the fluid and electrolyte balance is severely disturbed in any way.

Synapses

A synapse occurs where the axon of one neurone (the pre-synaptic neurone) meets either the dendrite or the cell body of another (the post-synaptic neurone). At the tip of the pre-synaptic axon is a button shaped swelling called a synaptic knob, inside which are numerous mitochondria and vesicles packed with a substance called a neurotransmitter. The membrane enclosing the synaptic knob is separated from the membrane of the post-synaptic cell by a gap of about 20 nanometres. This space is called the synaptic cleft.

Synapses are very similar in structure to neuromuscular junctions, which occur where nerve cells meet muscle cells.

Transmission across the synapse

When a nerve impulse reaches the membrane of a synaptic knob, it opens ion channels in the membrane that allow calcium ions to enter the cell. The presence of these ions prompts movement of the vesicles inside the knob towards the membrane at the synaptic cleft. The vesicles fuse with this membrane (exocytosis), discharging neurotransmitter molecules into the space between the two neurones. The neurotransmitters then diffuse across the space and bind to receptors in the membrane of the post-synaptic cell. This opens ion channels in the post-synaptic membrane resulting in a change in the post-synaptic cell's membrane potential and (if the cell is sufficiently excited) in the consequent generation of a nerve impulse.

The neurotransmitter molecules left in the synaptic cleft are broken down by enzymes, reabsorbed by the pre-synaptic cell, where they are resynthesized (using energy from ATP generated by the nearby mitochondria), and packaged once again into vesicles.

 

Figure 15. A synaptic knob or bouton showing vesicles

 

 

 

Figure 16. Release of neurotransmitter

Figure 17. Reuptake of neurotransmitter chemical

Excitation and inhibition

Sometimes an impulse can cross a synapse unimpeded, but more often, the effect of just one synapse is not enough to make the post-synaptic cell fire. In such cases the neurone must be stimulated by several synapses at once before a nerve impulse is generated. Each synapse produces a small rise in an electrical property of the post-synaptic cell called the excitatory post-synaptic potential (EPSP). The small rises add together, raising the EPSP to a level high enough to trigger a nerve impulse. This is called spatial summation. Some neurones fire after receiving several impulses from the same synapse in quick succession, an effect called temporal summation.

Certain synapses have the opposite effect, increasing an electrical property called the inhibitory post-synaptic potential. This causes the inside of the post-synaptic membrane to become more negative, and makes it more difficult for excitatory impulses to get across.

Synapses that are continually bombarded with signals eventually stop working and are described as fatigued. This is a temporary effect. It is caused by depletion of the supply of neurotransmitter in the synaptic knob.

There are many neurotransmitters present in the human nervous system. Some are simple chemical ions such as calcium, others are more complex chemicals such as Dopamine, Serotonin (5HT), gamma-amino-butyric acid (GABA), Acetylcholine, Adrenaline, Noradrenaline etc.

The interference of many drugs with neurotransmitter processes has resulted in their use either as legitimate drugs for treatment of mental health problems or as illegal or recreational drugs such as cannabis, LSD, cocaine etc.

Again as nurses we should be aware that certain drugs we give as medication may have an effect upon the patient’s neurotransmitters and cause resultant anxiety states, hypermanic states, drowsiness etc. all as a result of interfering with the normal process of synaptic transmission.

 

 

Focus on Psychoactive Drugs, chemical substances that alter mood, behaviour, perception, or mental functioning. Throughout history, many cultures have found ways to alter consciousness through the ingestion of substances. In current professional practice, psychoactive substances known as psychotropic drugs have been developed to treat patients with severe mental illness.

Psychoactive substances exert their effects by modifying biochemical or physiological processes in the brain. The message system of nerve cells, or neurons, relies on both electrical and chemical transmission. Neurons rarely touch each other; the microscopic gap between one neuron and the next, called the synapse, is bridged by chemicals called neuroregulators, or neurotransmitters. Psychoactive drugs act by altering neurotransmitter function. The drugs can be divided into six major pharmacological classes based on their desired behavioural or psychological effect: alcohol, sedative-hypnotics, opiate analgesics, stimulant-euphoriants, hallucinogens, and psychotropic agents.

Alcohol has always been the most widely used psychoactive substance. In most countries it is the only psychoactive drug legally available without prescription. Pleasant relaxation is commonly the desired effect, but intoxication impairs judgement and motor performance. When used chronically, alcohol can be toxic to liver and brain cells and can be physiologically addicting (giving rise to alcoholism), producing dangerous withdrawal syndromes.

Sedative-hypnotics, such as the barbiturates and diazepam (widely known under the brand name Valium), include brain depressants, which are used medically to help people sleep (sleeping pills), and antianxiety agents, which are used to calm people without inducing sleep. Sedative-hypnotics are used illegally to produce relaxation, tranquillity, and euphoria. Overdoses of sedative-hypnotics can be fatal; all can be physiologically addicting, and some can cause a life-threatening withdrawal syndrome.

Opiate analgesics—opiates such as opium, morphine, and heroin—are prescribed to produce analgesia. Because the relief of pain is one of the primary tasks of medical treatment, opiates have been among the most important and valuable drugs in medicine. Illegal use of opiate analgesics involves injecting these substances, particularly heroin, into the veins to produce euphoria. Opiates are physiologically addicting and can produce a quite unpleasant withdrawal syndrome.

Stimulant-euphoriants, such as amphetamines, are prescribed by doctors to suppress the appetite and to treat children often diagnosed as hyperactive. Although amphetamines stimulate adults, they have a paradoxically calming effect on certain children who have short attention spans and are hyperactive. Cocaine is used medically as a local anaesthetic. Amphetamines and cocaine are used illegally to produce alertness and euphoria, to prevent drowsiness, and to improve performance in physical and mental tasks such as athletic events and college examinations.

Hallucinogens—psychedelic drugs such as LSD (lysergic acid diethylamide), mescaline, and PCP (Phencyclidine)—thus far have little medical use. They are taken illegally to alter perception and thinking patterns. Marijuana is a weak hallucinogen that may be medically useful in suppressing the nausea caused by cancer treatments and possibly in reducing eye pressure in certain severe glaucomas.

Psychotropic drugs have been in use since the early 1950s. Antipsychotic drugs decrease the symptoms of schizophrenia, allowing many schizophrenic patients to leave the hospital and rejoin community life. Antidepressant drugs help the majority of patients with severe depression recover from their disorder. Lithium salts eliminate or diminish the episodes of mania and depression experienced by manic-depressive patients.