Art & science life sciences: 20
Nervous system: part 1 Farley A et al (2014) Nervous system: part 1. Nursing Standard. 28, 31, 46-51. Date of submission: March 30 2012: date of acceptance: November 5 2012.
Abstract This article, which forms part of the life sciences series and is the first of three articles on the nervous system, explores the major divisions of the nervous system and their functions. The basic structure of a nerve cell is described, and generation and conduction of nerve impulses is discussed. Blood supply to the brain is also covered. The second article will examine the central nervous system (CNS) in greater detail, including protection of the CNS, and the structure and function of the cerebral cortex and cerebellum. The third article will examine the peripheral nervous system and the autonomic nervous system, and provides an overview of some of the disorders of the nervous system.
Authors Alistair Farley, Retired, was lecturer in nursing, School of Nursing and Midwifery, University of Dundee, Dundee, Scotland. Carolyn Johnstone, Lecturer in nursing, School of Nursing and Midwifery, University of Dundee. Charles Hendry, Retired, was senior lecturer, School of Nursing and Midwifery, University of Dundee. Ella McLafferty, Retired, was senior lecturer, School of Nursing and Midwifery, University of Dundee. Correspondence to: [email protected]
Keywords Autonomic nervous system, central nervous system, nervous system, neurone, peripheral nervous system, stroke
Review All articles are subject to external double-blind peer review and checked for plagiarism using automated software.
Online Guidelines on writing for publication are available at www.nursing-standard.co.uk. For related articles visit the archive and search using the keywords above.
HOMEOSTASIS IS VITAL to human wellbeing and health, and is maintained through the combined action of the nervous and endocrine systems. The nervous system monitors and responds to changes in the internal and external environment. It is also responsible for perception, behaviour and memory, and initiates all voluntary movements. The nervous system 46 april 2 :: vol 28 no 31 :: 2014
includes all of the neural tissue in the body. Neural tissue carries information from one region of the body to another. Integration and co-ordination occurs within the brain and spinal cord, which form the central nervous system (CNS). The peripheral nervous system includes all of the neural tissue outside the CNS (Tortora and Derrickson 2013).
Brain The adult brain contains almost 100 billion nerve cells or neurones (Tortora and Derrickson 2013). Neuroglia (glial cells) make up the remaining 90% of nervous tissue located in the brain (Thibodeau and Patton 2010). There are six major divisions in the adult brain: the medulla oblongata, pons, midbrain, cerebellum, diencephalon and cerebrum. The medulla, pons and midbrain are often referred to as the brain stem (Thibodeau and Patton 2010). The cerebrum is the structure most frequently mentioned when referring to the brain. The diencephalon is situated between the cerebrum and the midbrain and consists of the thalamus, hypothalamus, optic chiasma, pineal gland and other small structures.
Spinal cord The spinal cord is continuous with the brain stem and exits the cranium at the foramen magnum of the occipital bone. It extends to the lower border of the first lumbar vertebra. It aids homeostasis by providing rapid reflexive responses to many stimuli (Tortora and Derrickson 2013). The spinal cord carries sensory information in the form of nerve impulses to the brain and motor responses away from the brain.
Peripheral nervous system The brain and spinal cord form the CNS. Cranial nerves communicate directly with the brain and spinal nerves communicate with the spinal cord. The peripheral nervous system consists of all nervous tissue outside the CNS. The nervous system has a number of subdivisions as shown in Figure 1.
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Somatic nervous system
Sensory and motor nerves, also known as afferent and efferent nerves, communicate with the CNS. Motor nerves communicate with skeletal muscle only and are under voluntary control. Sensory nerves carry information to the brain and spinal cord from somatic receptors found throughout the body.
Autonomic nervous system
The activity of the autonomic nervous system (ANS) is not under conscious or voluntary control. The ANS is divided into two parts: the sympathetic division and parasympathetic division. The ANS regulates temperature, heart rate, blood pressure and blood glucose.
protein synthesis and maintaining the health of the cell. Cell bodies are found mainly in the CNS packed together forming tissue known as grey matter. They are responsible for the analysis, integration and storage of information. The soma does not contain a centrosome and, therefore, neurones cannot undergo mitosis (Thibodeau and Patton 2010). It is important to appreciate that this means damaged brain tissue, for example which occurs following stroke, cannot be replaced.
FIGURE 2 Structure of a myelinated neurone
Enteric nervous system
Nucleus
A further subdivision of the peripheral nervous system has been identified and is known as the enteric nervous system (ENS). The ENS is the intrinsic nervous system of the gastrointestinal tract. It has been referred to as the ‘brain of the gut’ (Tortora and Derrickson 2013) and exercises local control over gastrointestinal function.
Cell body Axon
Neurilemma
Neurones
Dendrites
Neurones are the functional units of the nervous system. They are composed of three parts: the cell body, dendrites and axon (Thibodeau and Patton 2010). The dendrites and axon may be referred to as nerve fibres (Figure 2). A neurone consists of a cell body, also known as a soma or perikaryon, containing a nucleus surrounded by cytoplasm. It is responsible for
Nucleus of Schwann cell
PETER LAMB
Cell body
Nodes of Ranvier Synaptic knob
Myelin sheath Axon terminal
FIGURE 1 Subdivisions of the nervous system Central nervous system
Peripheral nervous system
Sensory (afferent) nerves
Motor (efferent) nerves
Somatic nerves
Brain and spinal cord
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Autonomic nerves
Sympathetic nerves
Parasympathetic nerves
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Art & science life sciences: 20 Dendrites
Dendrites are branching cytoplasmic projections of the cell body. They receive information and direct it into the cell body. They are often highly branched and may account for most of the total surface area of the neurone.
Axon
The axon is a single cytoplasmic projection of the cell body and directs information away from the cell body. It can vary in length from less than one millimetre to more than one metre. The axon may or may not have collateral branches. At its distal tip, the axon gives rise to several finer terminal branches, the axon terminal or telodendria. When gathered together into
TABLE 1 Structural classification of neurones Structural classification
Characteristics
Multipolar
Contains several dendrites communicating directly with the cell body, and a single axon. Most neurones in the central nervous system are multipolar.
Bipolar
Contains one main dendrite and one axon. These neurones are found in the retina of the eye, inner ear and olfactory area of the brain.
Unipolar
Contains dendrites and one axon fused together forming a single process extending from the cell body. These are sensory neurones.
Anaxonic
Small neurones in which the dendrites and axon are indistinguishable.
(Thibodeau and Patton 2010, Tortora and Derrickson 2013)
TABLE 2 Functional classification of neurones Functional classification
Characteristics
Sensory or afferent neurones
Transmit nerve impulses to the central nervous system (CNS) via cranial or spinal nerves. Monitor the internal and external environment and convey this information to the CNS.
Motor or efferent neurones
Transmit nerve impulses away from the CNS to muscles and glands, bringing about an action, for example muscular contraction or glandular secretion.
Interneurons or association neurones
Found mainly in the CNS and are responsible for linking sensory and motor neurones.
(Thibodeau and Patton 2010, Tortora and Derrickson 2013)
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bundles, dendrites and axons form white matter and are responsible for the transmission of nerve impulses. The white appearance is the result of the presence of myelin surrounding the axon (Tortora and Derrickson 2013).
Structural classification of neurones
Structurally, neurones are classified according to the number of processes extending from the cell body (Tortora and Derrickson 2013). General categories include multipolar, bipolar, unipolar and anaxonic neurones (Table 1). Neurones may also be classified according to function (Table 2).
Nerve impulse transmission To be effective, nerves must communicate with one another and with target tissues. Information must be conveyed to the CNS via sensory or afferent neurones, and the CNS must process this information before initiating a response via motor or efferent neurones. Information can be conveyed via electrical and/or chemical means. Nerve cells are unique within the body in that they generate and conduct signals called nerve impulses (Thibodeau and Patton 2010). These nerve impulses are electrical in nature and are often referred to as action potentials. Transmission of the impulse or action potential occurs as a result of the movement of ions across the nerve cell membrane (Waugh and Grant 2010). Differences in electrical charge exist on either side of the cell membrane. This is called the potential difference and is the result of unequal distribution of potassium ions and sodium ions on either side of the membrane. In the resting state, the charge on the inside of the cell membrane is negative, while the charge on the outside of the cell is positive (Waugh and Grant 2010). Within the cell, the potassium level is 148mmol/L and the sodium level is 10mmol/L, whereas outside the cell the potassium level is 5mmol/L and the sodium level is 142mmol/L. It is important to note that substances tend to move down a concentration gradient by diffusion. At rest, the cell membrane is impermeable to sodium ions, however potassium ions diffuse slowly out of the cell through open potassium channels. These channels are protein structures located within the phospholipid bilayer of the cell membrane, with a central pore or channel through which ions can pass. As the positively charged potassium leaves the cell, the inside of the cell carries a greater negative charge (Seeley et al 2008). There is an overall difference in charge on either side of the membrane – positive outside and negative inside. This difference is known as
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Synapse Impulses are conducted from one neurone to another cell across a type of junction known as a synapse. This other cell can be either another neurone or an effector cell or organ such as a muscle or gland. At a synapse, the activity of one neurone affects the membrane characteristics of another cell. This is dependent on the presence of chemicals known as neurotransmitters. Communication usually occurs from the presynaptic neurone to the postsynaptic neurone or effector cell (Figure 3). A synapse where a neurone communicates with another cell type represents a neuroeffector junction. The presynaptic membrane and the postsynaptic membrane are separated by a narrow gap known as the synaptic cleft (Figure 3). The diffusion of a neurotransmitter across this cleft accounts for the observed synaptic delay in the transmission of the impulse (Jenkins and Tortora 2013). When the nerve impulse reaches the end of the axon, it divides and enters the small branches terminating in synaptic knobs (Waugh and Grant 2010). This impulse causes the release of a neurotransmitter from the presynaptic neurone into the synaptic gap. The neurotransmitter diffuses across the gap to the postsynaptic
FIGURE 3 A synapse Presynaptic neurone
Dendrites Nucleus
Axon Cell body Synaptic knobs
Axon
Postsynaptic neurone Presynaptic neurone Vesicles containing neurotransmitter Synaptic knob Synaptic cleft
Dendrite
Postsynaptic neurone PETER LAMB
the resting potential or membrane potential. In this state, the membrane is said to be polarised. The membrane potential is typically -70mV, the minus sign indicates that the inside of the cell has a negative charge relative to the outside (Tortora and Derrickson 2013). If a stimulus of adequate strength is applied to a polarised membrane, the membrane depolarises – the electrical charge across the membrane moves towards a positive value of >0mV – and once the events of depolarisation have occurred, an action potential (nerve impulse) is initiated. The threshold at which depolarisation occurs completely is -55mV. If this value is not reached then depolarisation cannot proceed and an action potential is not initiated. However, if it is reached then depolarisation is an all or nothing event and the membrane potential is +30mV (Tortora and Derrickson 2013). The action potential is conducted along the length of the axon in a segmental wave-like fashion. By the time the impulse has travelled from one part of the axon membrane to the adjacent part, the previous part becomes repolarised – its resting potential is restored. Until repolarisation occurs, the neurone cannot conduct another impulse; this is known as the refractory period (Waugh and Grant 2010). During repolarisation, potassium leaves the cell returning the membrane potential to its resting state. It should be noted that at this point the distribution of potassium and sodium across the cell membrane is not in balance, such that there is an excess of sodium inside the cell and an excess of potassium outside the cell. To restore the appropriate ionic balance on either side of the cell membrane, the nerve cell is actively transporting ions across its membrane using a mechanism known as the sodium-potassium pump. Sodium ions are transported out of the cell, while potassium ions are transported into the cell (Waugh and Grant 2010). Large axons and those of the peripheral nerves in the body are surrounded by a myelin sheath (Waugh and Grant 2010). The myelin sheath prevents leakage of electrical charge from the axon and conducts the impulse more efficiently. Between the segments of the myelin sheath, unmyelinated gaps called nodes of Ranvier can be found. At these nodes, depolarisation can occur. When an impulse is conducted along a myelinated sheath, it moves rapidly from one node to another through surrounding extracellular fluid. This speeds up the rate of nerve impulse conduction, compared to unmyelinated neurones, and is referred to as saltatory conduction (Waugh and Grant 2010).
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Art & science life sciences: 20 neurone or effector muscle, where it binds to receptor molecules on the postsynaptic membrane (Seeley et al 2008). The effect of this may be excitatory or inhibitory depending on the type of neurotransmitter and the type of postsynaptic membrane (Seeley et al 2008). Box 1 provides some examples of neurotransmitters. Subsequently, neurotransmitters can be broken down by the action of enzymes, diffuse away from the synapse or be transported back into the presynaptic neurone (reuptake). The phenomenon of reuptake has led to the development of a class of drugs known as selective serotonin re-uptake inhibitors.
BOX 1 Examples of neurotransmitters
FIGURE 4 Circle of Willis
Anterior communicating artery
Internal carotid artery Basilar artery
Posterior communicating artery
Vertebral artery
Spinal cord
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Neurones have a high demand for adenosine triphosphate to support synthetic and active transport activities. They obtain energy through aerobic breakdown of glucose and do not maintain glycogen reserves. As a result, these cells are completely dependent on the oxygen and glucose delivered by the circulation and thus any interruption in the circulatory supply may damage or destroy neurones (Jenkins and Tortora 2013).
The brain demands a constant supply of oxygen and glucose. It receives about 15% of cardiac output (750mL/minute) (Waugh and Grant 2010). Blood reaches the brain via two internal carotid arteries and two vertebral arteries. Within the brain, these arteries form the circle of Willis (Figure 4). This circle helps to ensure that no part of the brain receives an inadequate supply of blood because the circulation can reach any part of the brain from this arrangement of blood vessels. Posteriorly, the right and left vertebral arteries originate from the subclavian arteries. Once in the skull, on the underside of the brain, they unite to form the basilar artery. Anteriorly, the right and left internal carotids arise from the common carotid arteries. As they enter the skull they become the anterior cerebral arteries. These arteries are linked by the anterior communicating artery. The anterior cerebral arteries unite with the basilar artery via the posterior communicating artery forming the circle of Willis. Arteries arise from this circle and serve the brain (Waugh and Grant 2010). The main veins returning blood from the brain are the internal jugular veins draining into the subclavian veins (Waugh and Grant 2010).
Stroke
PETER LAMB
Posterior cerebral artery
Neurones and metabolic processes
Blood supply to the brain
Acetylcholine. Dopamine. Adrenaline (epinephrine). Gamma aminobutyric acid. Glycine. Histamine. Noradrenaline (norepinephrine). Serotonin.
Anterior cerebral artery
These drugs are used in the treatment of depression and include citalopram, fluoxetine and paroxetine (White and Clare 2009).
A common neurological condition resulting from an abnormality of cerebral circulation is stroke. Lim et al (2007) defined stroke as ‘a sudden onset of a focal neurological deficit that persists for more than 24 hours.’ Stroke results from an interruption in blood supply to a part of the brain. This may be caused by occlusion or rupture of a blood vessel within the brain. This results in damage and/or death to an area of the brain and if extensive, can result in death of the
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individual. As a consequence, the person may be left with motor, sensory or language deficits and higher brain dysfunction (Lim et al 2007). Each year in the UK, about 150,000 people have a stroke, with about 25% of these being under the age of 65 (NHS Choices 2012). It is the third most common cause of death in the Western world, following heart disease and dementia (Lim et al 2007, Office for National Statistics 2013). Damage to and/or death of areas of the brain can be reduced by recognising the features of stroke and responding promptly. Healthcare workers have a role in increasing public awareness of stroke and helping to reduce risk factors, such as those listed in Box 2. The acronym FAST is currently recommended in the UK for first responders to stroke (Stroke Association 2014): Facial weakness: can the person smile? Has his or her mouth or eye drooped? Arm weakness: can the person raise both arms? Speech problems: can the person speak clearly and understand what you say? Time to call 999.
POINTS FOR PRACTICE Identify risk factors for stroke that might be modifiable. Outline ways in which the risk of stroke may be reduced. Referring to Box 1, identify the actions of the named neurotransmitters. Using resources of your choice, identify tools that may be used when conducting a neurological examination.
GLOSSARY Axon terminal The end structure of the axon; axon terminals are separated from neighbouring neurones by the synapse. Neuroglia Also known as glial cells; non-neuronal cells that provide support, protection and insulation for neurones. Homeostasis The mechanisms by which a stable internal environment is maintained within the body. Neurotransmitter Chemicals that transmit signals from a neurone to another neurone or target cell across a synapse. Sodium-potassium pump An active transport mechanism that maintains the correct balance of sodium and potassium on either side of the cell membrane.
BOX 2 Risk factors for stroke
Conclusion
Age. Gender. Race. Heredity. Hypertension. Smoking. Diabetes. Hyperlipidaemia. Atrial fibrillation. Obesity. High alcohol consumption.
Nurses need to have an understanding of the structure and function of the nervous system to provide appropriate care for patients who may be experiencing neurological deficits. While many patients with such deficits will be cared for in specialised units, some may receive care in general medical and surgical wards. Similarly, community nursing staff are increasingly caring for patients with long-term neurological deficits. The second article in this series examines the CNS in greater detail NS
References Jenkins G, Tortora GJ (2013) Anatomy and Physiology from Science to Life. International Student Version. Third edition. John Wiley and Sons, Singapore. Lim E, Loke YK, Thompson A (Eds) (2007) Medicine and Surgery. An Integrated Textbook. Churchill Livingstone Elsevier, Edinburgh.
Office for National Statistics (2013) What are the Top Causes of Death by Age and Gender? www.ons.gov.uk/ ons/rel/vsob1/mortalitystatistics--deaths-registeredin-england-and-wales--seriesdr-/2012/sty-causes-of-death. html (Last accessed: March 11 2014.)
NHS Choices (2012) Stroke. www. nhs.uk/conditions/stroke/pages/ introduction.aspx (Last accessed: March 11 2014.)
Seeley RR, Stephens TD, Tate P (2008) Anatomy and Physiology. Eighth edition. McGraw Hill, Boston MA.
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Stroke Association (2014) Recognise the Symptoms: You Can Recognise A Stroke Using the FAST Test. www.stroke.org. uk/information/about_stroke/ recognising_symptoms/index.html (Last accessed: March 11 2014.) Thibodeau GA, Patton KT (2010) Anatomy and Physiology. Seventh edition. Mosby Elsevier, Missouri. Tortora GJ, Derrickson B (2013) Essentials of Anatomy and Physiology. International Student
Version. Ninth edition. John Wiley and Sons, Singapore. Waugh A, Grant A (2010) Ross and Wilson Anatomy and Physiology in Health and Illness. 11th edition. Churchill Livingstone Elsevier, Edinburgh. White PD, Clare AW (2009) Psychological medicine. In Kumar P, Clark M (Eds) Kumar and Clark’s Clinical Medicine. Seventh edition. Saunders Elsevier, Edinburgh, 1185-1223.
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Nervous system: part 1 Farley A et al (2014) Nervous system: part 1. Nursing Standard. 28, 31, 46-51. Date of submission: March 30 2012: date of acceptance: November 5 2012.
Abstract This article, which forms part of the life sciences series and is the first of three articles on the nervous system, explores the major divisions of the nervous system and their functions. The basic structure of a nerve cell is described, and generation and conduction of nerve impulses is discussed. Blood supply to the brain is also covered. The second article will examine the central nervous system (CNS) in greater detail, including protection of the CNS, and the structure and function of the cerebral cortex and cerebellum. The third article will examine the peripheral nervous system and the autonomic nervous system, and provides an overview of some of the disorders of the nervous system.
Authors Alistair Farley, Retired, was lecturer in nursing, School of Nursing and Midwifery, University of Dundee, Dundee, Scotland. Carolyn Johnstone, Lecturer in nursing, School of Nursing and Midwifery, University of Dundee. Charles Hendry, Retired, was senior lecturer, School of Nursing and Midwifery, University of Dundee. Ella McLafferty, Retired, was senior lecturer, School of Nursing and Midwifery, University of Dundee. Correspondence to: [email protected]
Keywords Autonomic nervous system, central nervous system, nervous system, neurone, peripheral nervous system, stroke
Review All articles are subject to external double-blind peer review and checked for plagiarism using automated software.
Online Guidelines on writing for publication are available at www.nursing-standard.co.uk. For related articles visit the archive and search using the keywords above.
HOMEOSTASIS IS VITAL to human wellbeing and health, and is maintained through the combined action of the nervous and endocrine systems. The nervous system monitors and responds to changes in the internal and external environment. It is also responsible for perception, behaviour and memory, and initiates all voluntary movements. The nervous system 46 april 2 :: vol 28 no 31 :: 2014
includes all of the neural tissue in the body. Neural tissue carries information from one region of the body to another. Integration and co-ordination occurs within the brain and spinal cord, which form the central nervous system (CNS). The peripheral nervous system includes all of the neural tissue outside the CNS (Tortora and Derrickson 2013).
Brain The adult brain contains almost 100 billion nerve cells or neurones (Tortora and Derrickson 2013). Neuroglia (glial cells) make up the remaining 90% of nervous tissue located in the brain (Thibodeau and Patton 2010). There are six major divisions in the adult brain: the medulla oblongata, pons, midbrain, cerebellum, diencephalon and cerebrum. The medulla, pons and midbrain are often referred to as the brain stem (Thibodeau and Patton 2010). The cerebrum is the structure most frequently mentioned when referring to the brain. The diencephalon is situated between the cerebrum and the midbrain and consists of the thalamus, hypothalamus, optic chiasma, pineal gland and other small structures.
Spinal cord The spinal cord is continuous with the brain stem and exits the cranium at the foramen magnum of the occipital bone. It extends to the lower border of the first lumbar vertebra. It aids homeostasis by providing rapid reflexive responses to many stimuli (Tortora and Derrickson 2013). The spinal cord carries sensory information in the form of nerve impulses to the brain and motor responses away from the brain.
Peripheral nervous system The brain and spinal cord form the CNS. Cranial nerves communicate directly with the brain and spinal nerves communicate with the spinal cord. The peripheral nervous system consists of all nervous tissue outside the CNS. The nervous system has a number of subdivisions as shown in Figure 1.
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Somatic nervous system
Sensory and motor nerves, also known as afferent and efferent nerves, communicate with the CNS. Motor nerves communicate with skeletal muscle only and are under voluntary control. Sensory nerves carry information to the brain and spinal cord from somatic receptors found throughout the body.
Autonomic nervous system
The activity of the autonomic nervous system (ANS) is not under conscious or voluntary control. The ANS is divided into two parts: the sympathetic division and parasympathetic division. The ANS regulates temperature, heart rate, blood pressure and blood glucose.
protein synthesis and maintaining the health of the cell. Cell bodies are found mainly in the CNS packed together forming tissue known as grey matter. They are responsible for the analysis, integration and storage of information. The soma does not contain a centrosome and, therefore, neurones cannot undergo mitosis (Thibodeau and Patton 2010). It is important to appreciate that this means damaged brain tissue, for example which occurs following stroke, cannot be replaced.
FIGURE 2 Structure of a myelinated neurone
Enteric nervous system
Nucleus
A further subdivision of the peripheral nervous system has been identified and is known as the enteric nervous system (ENS). The ENS is the intrinsic nervous system of the gastrointestinal tract. It has been referred to as the ‘brain of the gut’ (Tortora and Derrickson 2013) and exercises local control over gastrointestinal function.
Cell body Axon
Neurilemma
Neurones
Dendrites
Neurones are the functional units of the nervous system. They are composed of three parts: the cell body, dendrites and axon (Thibodeau and Patton 2010). The dendrites and axon may be referred to as nerve fibres (Figure 2). A neurone consists of a cell body, also known as a soma or perikaryon, containing a nucleus surrounded by cytoplasm. It is responsible for
Nucleus of Schwann cell
PETER LAMB
Cell body
Nodes of Ranvier Synaptic knob
Myelin sheath Axon terminal
FIGURE 1 Subdivisions of the nervous system Central nervous system
Peripheral nervous system
Sensory (afferent) nerves
Motor (efferent) nerves
Somatic nerves
Brain and spinal cord
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Autonomic nerves
Sympathetic nerves
Parasympathetic nerves
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Art & science life sciences: 20 Dendrites
Dendrites are branching cytoplasmic projections of the cell body. They receive information and direct it into the cell body. They are often highly branched and may account for most of the total surface area of the neurone.
Axon
The axon is a single cytoplasmic projection of the cell body and directs information away from the cell body. It can vary in length from less than one millimetre to more than one metre. The axon may or may not have collateral branches. At its distal tip, the axon gives rise to several finer terminal branches, the axon terminal or telodendria. When gathered together into
TABLE 1 Structural classification of neurones Structural classification
Characteristics
Multipolar
Contains several dendrites communicating directly with the cell body, and a single axon. Most neurones in the central nervous system are multipolar.
Bipolar
Contains one main dendrite and one axon. These neurones are found in the retina of the eye, inner ear and olfactory area of the brain.
Unipolar
Contains dendrites and one axon fused together forming a single process extending from the cell body. These are sensory neurones.
Anaxonic
Small neurones in which the dendrites and axon are indistinguishable.
(Thibodeau and Patton 2010, Tortora and Derrickson 2013)
TABLE 2 Functional classification of neurones Functional classification
Characteristics
Sensory or afferent neurones
Transmit nerve impulses to the central nervous system (CNS) via cranial or spinal nerves. Monitor the internal and external environment and convey this information to the CNS.
Motor or efferent neurones
Transmit nerve impulses away from the CNS to muscles and glands, bringing about an action, for example muscular contraction or glandular secretion.
Interneurons or association neurones
Found mainly in the CNS and are responsible for linking sensory and motor neurones.
(Thibodeau and Patton 2010, Tortora and Derrickson 2013)
48 april 2 :: vol 28 no 31 :: 2014
bundles, dendrites and axons form white matter and are responsible for the transmission of nerve impulses. The white appearance is the result of the presence of myelin surrounding the axon (Tortora and Derrickson 2013).
Structural classification of neurones
Structurally, neurones are classified according to the number of processes extending from the cell body (Tortora and Derrickson 2013). General categories include multipolar, bipolar, unipolar and anaxonic neurones (Table 1). Neurones may also be classified according to function (Table 2).
Nerve impulse transmission To be effective, nerves must communicate with one another and with target tissues. Information must be conveyed to the CNS via sensory or afferent neurones, and the CNS must process this information before initiating a response via motor or efferent neurones. Information can be conveyed via electrical and/or chemical means. Nerve cells are unique within the body in that they generate and conduct signals called nerve impulses (Thibodeau and Patton 2010). These nerve impulses are electrical in nature and are often referred to as action potentials. Transmission of the impulse or action potential occurs as a result of the movement of ions across the nerve cell membrane (Waugh and Grant 2010). Differences in electrical charge exist on either side of the cell membrane. This is called the potential difference and is the result of unequal distribution of potassium ions and sodium ions on either side of the membrane. In the resting state, the charge on the inside of the cell membrane is negative, while the charge on the outside of the cell is positive (Waugh and Grant 2010). Within the cell, the potassium level is 148mmol/L and the sodium level is 10mmol/L, whereas outside the cell the potassium level is 5mmol/L and the sodium level is 142mmol/L. It is important to note that substances tend to move down a concentration gradient by diffusion. At rest, the cell membrane is impermeable to sodium ions, however potassium ions diffuse slowly out of the cell through open potassium channels. These channels are protein structures located within the phospholipid bilayer of the cell membrane, with a central pore or channel through which ions can pass. As the positively charged potassium leaves the cell, the inside of the cell carries a greater negative charge (Seeley et al 2008). There is an overall difference in charge on either side of the membrane – positive outside and negative inside. This difference is known as
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Synapse Impulses are conducted from one neurone to another cell across a type of junction known as a synapse. This other cell can be either another neurone or an effector cell or organ such as a muscle or gland. At a synapse, the activity of one neurone affects the membrane characteristics of another cell. This is dependent on the presence of chemicals known as neurotransmitters. Communication usually occurs from the presynaptic neurone to the postsynaptic neurone or effector cell (Figure 3). A synapse where a neurone communicates with another cell type represents a neuroeffector junction. The presynaptic membrane and the postsynaptic membrane are separated by a narrow gap known as the synaptic cleft (Figure 3). The diffusion of a neurotransmitter across this cleft accounts for the observed synaptic delay in the transmission of the impulse (Jenkins and Tortora 2013). When the nerve impulse reaches the end of the axon, it divides and enters the small branches terminating in synaptic knobs (Waugh and Grant 2010). This impulse causes the release of a neurotransmitter from the presynaptic neurone into the synaptic gap. The neurotransmitter diffuses across the gap to the postsynaptic
FIGURE 3 A synapse Presynaptic neurone
Dendrites Nucleus
Axon Cell body Synaptic knobs
Axon
Postsynaptic neurone Presynaptic neurone Vesicles containing neurotransmitter Synaptic knob Synaptic cleft
Dendrite
Postsynaptic neurone PETER LAMB
the resting potential or membrane potential. In this state, the membrane is said to be polarised. The membrane potential is typically -70mV, the minus sign indicates that the inside of the cell has a negative charge relative to the outside (Tortora and Derrickson 2013). If a stimulus of adequate strength is applied to a polarised membrane, the membrane depolarises – the electrical charge across the membrane moves towards a positive value of >0mV – and once the events of depolarisation have occurred, an action potential (nerve impulse) is initiated. The threshold at which depolarisation occurs completely is -55mV. If this value is not reached then depolarisation cannot proceed and an action potential is not initiated. However, if it is reached then depolarisation is an all or nothing event and the membrane potential is +30mV (Tortora and Derrickson 2013). The action potential is conducted along the length of the axon in a segmental wave-like fashion. By the time the impulse has travelled from one part of the axon membrane to the adjacent part, the previous part becomes repolarised – its resting potential is restored. Until repolarisation occurs, the neurone cannot conduct another impulse; this is known as the refractory period (Waugh and Grant 2010). During repolarisation, potassium leaves the cell returning the membrane potential to its resting state. It should be noted that at this point the distribution of potassium and sodium across the cell membrane is not in balance, such that there is an excess of sodium inside the cell and an excess of potassium outside the cell. To restore the appropriate ionic balance on either side of the cell membrane, the nerve cell is actively transporting ions across its membrane using a mechanism known as the sodium-potassium pump. Sodium ions are transported out of the cell, while potassium ions are transported into the cell (Waugh and Grant 2010). Large axons and those of the peripheral nerves in the body are surrounded by a myelin sheath (Waugh and Grant 2010). The myelin sheath prevents leakage of electrical charge from the axon and conducts the impulse more efficiently. Between the segments of the myelin sheath, unmyelinated gaps called nodes of Ranvier can be found. At these nodes, depolarisation can occur. When an impulse is conducted along a myelinated sheath, it moves rapidly from one node to another through surrounding extracellular fluid. This speeds up the rate of nerve impulse conduction, compared to unmyelinated neurones, and is referred to as saltatory conduction (Waugh and Grant 2010).
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Art & science life sciences: 20 neurone or effector muscle, where it binds to receptor molecules on the postsynaptic membrane (Seeley et al 2008). The effect of this may be excitatory or inhibitory depending on the type of neurotransmitter and the type of postsynaptic membrane (Seeley et al 2008). Box 1 provides some examples of neurotransmitters. Subsequently, neurotransmitters can be broken down by the action of enzymes, diffuse away from the synapse or be transported back into the presynaptic neurone (reuptake). The phenomenon of reuptake has led to the development of a class of drugs known as selective serotonin re-uptake inhibitors.
BOX 1 Examples of neurotransmitters
FIGURE 4 Circle of Willis
Anterior communicating artery
Internal carotid artery Basilar artery
Posterior communicating artery
Vertebral artery
Spinal cord
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Neurones have a high demand for adenosine triphosphate to support synthetic and active transport activities. They obtain energy through aerobic breakdown of glucose and do not maintain glycogen reserves. As a result, these cells are completely dependent on the oxygen and glucose delivered by the circulation and thus any interruption in the circulatory supply may damage or destroy neurones (Jenkins and Tortora 2013).
The brain demands a constant supply of oxygen and glucose. It receives about 15% of cardiac output (750mL/minute) (Waugh and Grant 2010). Blood reaches the brain via two internal carotid arteries and two vertebral arteries. Within the brain, these arteries form the circle of Willis (Figure 4). This circle helps to ensure that no part of the brain receives an inadequate supply of blood because the circulation can reach any part of the brain from this arrangement of blood vessels. Posteriorly, the right and left vertebral arteries originate from the subclavian arteries. Once in the skull, on the underside of the brain, they unite to form the basilar artery. Anteriorly, the right and left internal carotids arise from the common carotid arteries. As they enter the skull they become the anterior cerebral arteries. These arteries are linked by the anterior communicating artery. The anterior cerebral arteries unite with the basilar artery via the posterior communicating artery forming the circle of Willis. Arteries arise from this circle and serve the brain (Waugh and Grant 2010). The main veins returning blood from the brain are the internal jugular veins draining into the subclavian veins (Waugh and Grant 2010).
Stroke
PETER LAMB
Posterior cerebral artery
Neurones and metabolic processes
Blood supply to the brain
Acetylcholine. Dopamine. Adrenaline (epinephrine). Gamma aminobutyric acid. Glycine. Histamine. Noradrenaline (norepinephrine). Serotonin.
Anterior cerebral artery
These drugs are used in the treatment of depression and include citalopram, fluoxetine and paroxetine (White and Clare 2009).
A common neurological condition resulting from an abnormality of cerebral circulation is stroke. Lim et al (2007) defined stroke as ‘a sudden onset of a focal neurological deficit that persists for more than 24 hours.’ Stroke results from an interruption in blood supply to a part of the brain. This may be caused by occlusion or rupture of a blood vessel within the brain. This results in damage and/or death to an area of the brain and if extensive, can result in death of the
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individual. As a consequence, the person may be left with motor, sensory or language deficits and higher brain dysfunction (Lim et al 2007). Each year in the UK, about 150,000 people have a stroke, with about 25% of these being under the age of 65 (NHS Choices 2012). It is the third most common cause of death in the Western world, following heart disease and dementia (Lim et al 2007, Office for National Statistics 2013). Damage to and/or death of areas of the brain can be reduced by recognising the features of stroke and responding promptly. Healthcare workers have a role in increasing public awareness of stroke and helping to reduce risk factors, such as those listed in Box 2. The acronym FAST is currently recommended in the UK for first responders to stroke (Stroke Association 2014): Facial weakness: can the person smile? Has his or her mouth or eye drooped? Arm weakness: can the person raise both arms? Speech problems: can the person speak clearly and understand what you say? Time to call 999.
POINTS FOR PRACTICE Identify risk factors for stroke that might be modifiable. Outline ways in which the risk of stroke may be reduced. Referring to Box 1, identify the actions of the named neurotransmitters. Using resources of your choice, identify tools that may be used when conducting a neurological examination.
GLOSSARY Axon terminal The end structure of the axon; axon terminals are separated from neighbouring neurones by the synapse. Neuroglia Also known as glial cells; non-neuronal cells that provide support, protection and insulation for neurones. Homeostasis The mechanisms by which a stable internal environment is maintained within the body. Neurotransmitter Chemicals that transmit signals from a neurone to another neurone or target cell across a synapse. Sodium-potassium pump An active transport mechanism that maintains the correct balance of sodium and potassium on either side of the cell membrane.
BOX 2 Risk factors for stroke
Conclusion
Age. Gender. Race. Heredity. Hypertension. Smoking. Diabetes. Hyperlipidaemia. Atrial fibrillation. Obesity. High alcohol consumption.
Nurses need to have an understanding of the structure and function of the nervous system to provide appropriate care for patients who may be experiencing neurological deficits. While many patients with such deficits will be cared for in specialised units, some may receive care in general medical and surgical wards. Similarly, community nursing staff are increasingly caring for patients with long-term neurological deficits. The second article in this series examines the CNS in greater detail NS
References Jenkins G, Tortora GJ (2013) Anatomy and Physiology from Science to Life. International Student Version. Third edition. John Wiley and Sons, Singapore. Lim E, Loke YK, Thompson A (Eds) (2007) Medicine and Surgery. An Integrated Textbook. Churchill Livingstone Elsevier, Edinburgh.
Office for National Statistics (2013) What are the Top Causes of Death by Age and Gender? www.ons.gov.uk/ ons/rel/vsob1/mortalitystatistics--deaths-registeredin-england-and-wales--seriesdr-/2012/sty-causes-of-death. html (Last accessed: March 11 2014.)
NHS Choices (2012) Stroke. www. nhs.uk/conditions/stroke/pages/ introduction.aspx (Last accessed: March 11 2014.)
Seeley RR, Stephens TD, Tate P (2008) Anatomy and Physiology. Eighth edition. McGraw Hill, Boston MA.
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Stroke Association (2014) Recognise the Symptoms: You Can Recognise A Stroke Using the FAST Test. www.stroke.org. uk/information/about_stroke/ recognising_symptoms/index.html (Last accessed: March 11 2014.) Thibodeau GA, Patton KT (2010) Anatomy and Physiology. Seventh edition. Mosby Elsevier, Missouri. Tortora GJ, Derrickson B (2013) Essentials of Anatomy and Physiology. International Student
Version. Ninth edition. John Wiley and Sons, Singapore. Waugh A, Grant A (2010) Ross and Wilson Anatomy and Physiology in Health and Illness. 11th edition. Churchill Livingstone Elsevier, Edinburgh. White PD, Clare AW (2009) Psychological medicine. In Kumar P, Clark M (Eds) Kumar and Clark’s Clinical Medicine. Seventh edition. Saunders Elsevier, Edinburgh, 1185-1223.
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