Art & science life sciences: 21
Nervous system: part 2 Hendry C et al (2014) Nervous system: part 2. Nursing Standard. 28, 32, 45-49. Date of submission: May 29 2013; date of acceptance: September 2 2013.
Cerebrum
Abstract
Charles Hendry, retired, was senior lecturer, School of Nursing and Midwifery, University of Dundee, Dundee, Scotland. Alistair Farley, retired, was lecturer in nursing, School of Nursing and Midwifery, University of Dundee. Ella McLafferty, retired, was senior lecturer, School of Nursing and Midwifery, University of Dundee. Carolyn Johnstone, lecturer in nursing, School of Nursing and Midwifery, University of Dundee. Correspondence to: [email protected]
The cerebrum forms the bulk of the brain. It is composed of grey matter made up of neuronal cell bodies, and white matter composed of axons. The surface, which is called the cerebral cortex, is divided into two hemispheres by a longitudinal fissure. The cerebral cortex has many folds or furrows, which increase its surface area. These folds are called gyri, the deep grooves between gyri are referred to as fissures and the shallow grooves are called sulci (Tortora and Derrickson 2012). The cerebral hemispheres are connected by a band of tissue called the corpus callosum, which contains white matter (Tortora and Derrickson 2012) through which information can pass from one side of the brain to the other and each hemisphere can communicate with the other. Each hemisphere is further divided into lobes named after the cranial bones under which they lie (frontal, parietal, temporal and occipital), with each lobe having specific functions.
Keywords
Frontal lobe
This article, which forms part of the life sciences series and is the second of three articles on the nervous system, explores the major divisions of the brain and their functions. The various ways in which the delicate structures of the brain and spinal cord are protected and supported by the cranial and vertebral bones, the meninges and the cerebrospinal fluid are examined. Neuroglia (glial cells), which form a major component of the total volume of brain tissue, are described. Dementia is discussed briefly to illustrate the effects of a common central nervous system disorder.
Authors
Brain, central nervous system, cerebrospinal fluid, dementia, meninges, nervous system, neurone, spinal cord
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.
THE CENTRAL NERVOUS system (CNS) is composed of the brain and spinal cord. The brain is a complex and delicate structure, which can be subdivided into several elements. This article examines these elements and explores their functions. There are six major divisions in the adult brain: the cerebrum, diencephalon, cerebellum, midbrain, pons and medulla oblongata (Figure 1).
The frontal lobe contains centres that deal with voluntary motor function, motivation, aggression and mood. Aspects of the sensation of smell are perceived here (Seeley et al 2008), although the primary area for the perception of smell is found in the temporal lobe (Jenkins and Tortora 2013). The frontal lobe also contains the anterior speech centre (Broca’s area), which is concerned with constructing sentences. Damage to this area may occur in stroke, where an individual knows what he or she wants to say but is unable to construct the sentences.
Parietal lobe
The parietal lobe is responsible for perceiving most sensory information. It receives and interprets sensations from receptors in the skin, muscle and viscera (abdomen and thorax) (Seeley et al 2008).
Temporal lobe
The temporal lobe contains the olfactory centre, which is responsible for the perception of smell (Waugh and Grant 2010). It also contains the posterior speech centre (Wernicke’s area), which is concerned with vocabulary and comprehension (Waugh and Grant 2010). Damage to this area
©Downloaded NURSINGfrom STANDARD RCN PUBLISHING aprilNo 9 other :: vol uses 28 no 32 :: permission. 2014 45 RCNi.com/ by ${individualUser.displayName} on Nov 13, 2015. For personal use only. without Copyright © 2015 RCNi Ltd. All rights reserved. 045-049w32NEW.indd 45
04/04/2014 15:20
Art & science life sciences: 21 will produce incomprehensible speech with inappropriate words. The auditory centre is also located in the temporal lobe.
Occipital lobe
The occipital lobe contains the visual centre (Jenkins and Tortora 2012).
Midbrain, pons and medulla oblongata
Diencephalon The diencephalon lies deep within the brain and contains the thalamus and hypothalamus. The thalamus, which comprises about 80% of the diencephalon, is an area of grey matter and is a major relay station for most sensory impulses (with the exception of olfaction) which are sent to the appropriate area of the cerebral cortex (Seeley et al 2008). The hypothalamus is situated below and in front of the thalamus, immediately above the pituitary gland (Waugh and Grant 2010). The nervous and endocrine systems work together to achieve homeostasis. The hypothalamus receives information from sensory nerves about the current state of the body. This information initiates a response in the autonomic nervous and endocrine systems. The hypothalamus influences or regulates control of body temperature, eating and drinking, strong emotions and circadian rhythms (Waugh and Grant 2010).
Cerebellum The cerebellum is similar in structure to the cerebrum. It occupies the posterior and inferior
FIGURE 1 Major divisions of the brain Diencephalon: Thalamus
Cerebrum
aspects of the cranial cavity, situated posterior to the pons and immediately below the cerebrum (Waugh and Grant 2010). It is responsible for the maintenance of posture and balance, muscle tone and co-ordination and refinement of movement (Jenkins and Tortora 2013).
The midbrain, pons and medulla oblongata, or the medulla, are referred to collectively as the brainstem. It is within the structures of the brainstem that most of the cranial nerves originate (Tortora and Derrickson 2012). The midbrain is located between the diencephalon and the pons (Tortora and Derrickson 2012). It acts as a relay station for receptors for hearing passing from the ear to the thalamus. The midbrain contains several reflex centres, including the centres associated with controlling movement of the eyes and head in response to visual stimuli (Tortora and Derrickson 2012). The pons also acts as a relay station passing information, such as signals pertaining to the coordination of voluntary movement, between the cerebrum and cerebellum. It contains the pneumotaxic and apneustic areas, which form part of the respiratory centre and control the rhythm of breathing (Waugh and Grant 2010). The pons is situated directly superior to the medulla and anterior to the cerebellum (Tortora and Derrickson 2012). The medulla oblongata is situated between the foramen magnum and the pons. The medulla contains the vital centres, the remainder of the respiratory centre, the cardiac centre and the vasomotor centre. It is in the lower end of the medulla that most motor nerve pathways cross to the opposite side so that the left cerebral hemisphere controls movement on the right side of the body and vice versa – this is known as the decussation of the pyramids (Jenkins and Tortora 2013).
Hypothalamus
Protection of the central nervous system
Brainstem: Midbrain Pons
PETER LAMB
Cerebellum
Posterior
Medulla oblongata Spinal cord
Anterior
There are many layers that protect the CNS. These include hair, skin, subcutaneous tissue, adipose tissue, bone, the meninges, and cerebrospinal fluid (CSF). However, in practice it is usual to consider only the bone, meninges and CSF as offering significant protection to the brain and spinal cord. A special class of cells known as neuroglia (glial cells) also protects the delicate structures of the brain. This will be discussed in further detail later.
Bones
Eight bones of the cranium protect the brain: frontal, parietal (two), temporal (two), occipital,
46 april 9 :: vol from 28 noRCNi.com 32 :: 2014 NURSING STANDARD / RCN Downloaded by ${individualUser.displayName} on Nov 13, 2015.©For personal use only. No other usesPUBLISHING without permission. Copyright © 2015 RCNi Ltd. All rights reserved. 045-049w32NEW.indd 46
04/04/2014 15:20
sphenoid and ethmoid. Together they form a rigid bony box that encloses the brain (McCance and Huether 2006). This rigidity is achieved as a result of the fusion of the joints between the bones. The bones of the cranium are united by sutures, which are immovable fibrous joints. The interlocking nature of these cranial sutures adds strength to the cranium and reduces the likelihood of fracturing at the joints (Tortora and Derrickson 2012). There are four prominent cranial sutures (Tortora and Derrickson 2012): The coronal suture unites the frontal bone and parietal bones. The sagittal suture unites the two parietal bones. The lambdoid suture unites the parietal bones and occipital bone. The squamous sutures unite the parietal bones and temporal bones. At birth the bones of the skull are not fused, which allows for some compression of the skull during labour and birth, enabling the cranium to pass through the birth canal. Fusion of the sutures of a child’s skull is not complete until approximately 12 years of age (McCance and Huether 2006). The vertebral column is also protective, with the vertebrae enclosing and protecting the spinal cord (Kneale and Davis 2005, Tortora and Derrickson 2012). The space between the vertebral arch and the body of the vertebra – known as the vertebral foramen – contains the spinal cord (Waugh and Grant 2010). The spinal cord contained within the vertebral column extends from the foramen magnum of the skull to the region of the second lumbar vertebra (Seeley et al 2008). Lying between the vertebrae from the second cervical vertebra to the sacrum are intervertebral discs, which permit movement of the vertebral column and act as shock absorbers (Tortora and Derrickson 2012). Samples of CSF can be obtained by inserting a cannula into the space below the spinal cord through L3-L4 or L4-L5. In this way there should be no risk of damaging the spinal cord. Following this procedure, patients should be kept supine for one hour to minimise leakage of CSF (Lim et al 2007). There is a true space between the dura mater of the spinal cord and the wall of the vertebrae known as the epidural space. Epidural anaesthesia of the spinal nerves can be achieved by injecting anaesthetic agents into the epidural space. An epidural can be given at different locations along the spinal canal, depending on where anaesthesia is required, although it is most frequently used in childbirth (Seeley et al 2008, NHS Choices 2013).
Meninges
The meninges provide the next layer of protection – they surround the brain and spinal cord. There
are three distinct layers that together form the meninges: the dura mater (outer layer), the arachnoid mater (middle layer) and the pia mater (inner layer). The cerebral dura mater is a double layer of dense fibrous tissue with the outer layer lining the inner surface of the skull (replacing the periosteum) and the inner layer forming the meningeal dura, which is continuous with the dura of the spinal cord (Seeley et al 2008). In addition, dense dural folds, formed from the meningeal dura, project between the two cerebral hemispheres (falx cerebri) and between the cerebral hemispheres and the cerebellum (tentorium cerebelli). These folds help to support and stabilise the brain, preventing excessive movement. The cerebral dura has a central region known as the superior sagittal sinus, which contains blood vessels. This structure acts as a venous reservoir and collects blood from brain tissue before returning it to the heart via the internal jugular vein. There are a number of other smaller sinuses within the dural folds, which also act as venous reservoirs (Patton and Thibodeau 2010). Lying beneath the dura mater is a potential space – which is an anatomical space that can be realised between two structures that in normal circumstances remain closed – known as the subdural space, which is filled with serous fluid (Waugh and Grant 2010, Tortora and Derrickson 2012). The arachnoid mater (so called because of its web-like structure) is a layer of thin wispy connective tissue below which lies the subarachnoid space. It merges with the dura mater within the vertebral column at the level of the second sacral vertebra (Waugh and Grant 2010). The subarachnoid space is filled with CSF. The pia mater, the innermost layer of connective tissue, is delicate and thin, contains numerous minute blood vessels and is adherent to the surface of the brain and spinal cord (Seeley et al 2008). Capillaries leaving the blood vessels of the pia mater penetrate the neural tissue of the brain.
Cerebrospinal fluid
CSF is composed of 99% water and appears clear and colourless; it contains glucose, proteins, lactic acid, urea and white blood cells (Waugh and Grant 2010, Tortora and Derrickson 2012). Its functions are to (Waugh and Grant 2010): Support the weight of the brain (the brain effectively ‘floats’ within the cranium). Act as a shock absorber. Ensure a uniform pressure around the delicate structures of the brain and spinal cord. Provide some nutrition to the brain and spinal cord and remove waste.
©Downloaded NURSINGfrom STANDARD RCN PUBLISHING aprilNo 9 other :: vol uses 28 no 32 :: permission. 2014 47 RCNi.com/ by ${individualUser.displayName} on Nov 13, 2015. For personal use only. without Copyright © 2015 RCNi Ltd. All rights reserved. 045-049w32NEW.indd 47
04/04/2014 15:20
Art & science life sciences: 21 Lying within the brain are four CSF-filled cavities known as ventricles: the right and left lateral ventricles; third ventricle; and fourth ventricle (Figure 2). The ventricles communicate with each other, with the central space of the spinal cord and with the subarachnoid space via small connecting ducts and/or small openings in their surfaces. The lateral ventricles are two main ventricles and they are located in the cerebral hemisphere under the corpus callosum. The third ventricle is a slit between and below the lateral ventricles and is connected to the lateral ventricles by a narrow opening, the intraventricular foramen (Waugh and Grant 2010). The fourth ventricle is a diamond-shaped cavity that lies between the pons and the cerebellum. It communicates with the third ventricle via the cerebral aqueduct, a small connecting duct. The fourth ventricle has three openings through which CSF flows out into the central canal of the spinal cord and the subarachnoid space (Patton and Thibodeau 2010). CSF is formed at the choroid plexuses of the ventricles at a rate of 20mL/hour or approximately 500mL/day (Jenkins and Tortora 2013). Choroid plexuses are networks of blood vessels located in the walls of each of the four ventricles, and are covered by cells known as ependymal cells. Ependymal cells in the lateral ventricles produce about 80-90% of the CSF, with the remainder being formed in the third and fourth ventricles (Seeley et al 2008). The ventricles, subarachnoid space and central canal of
FIGURE 2 Brain and spinal cord showing the ventricles and the central canal Posterior
Anterior
Subarachnoid space
Choroid plexus of lateral ventricle
Arachnoid villus
Cerebrum
Corpus callosum
Choroid plexus of third ventricle
Lateral ventricle Third ventricle
Choroid plexus of fourth ventricle
Cerebral aqueduct Fourth ventricle Spinal cord
PETER LAMB
Central canal Subarachnoid space
spinal cord contain approximately 140mL of CSF, with about 23mL in the ventricles and 117mL in the subarachnoid space (Seeley et al 2008, Patton and Thibodeau 2010). The rate of production of CSF equals the rate of reabsorption such that the volume of CSF remains constant (Tortora and Derrickson 2012). CSF reaches the subarachnoid space through the lateral and median apertures in the roof of the fourth ventricle. The CSF returns to the vascular circulation through the arachnoid villi that project into the superior sagittal sinus and other smaller sinuses.
Neuroglia Neuroglia support and protect the cells of the nervous system (Tortora and Derrickson 2012). They can be described according to whether they are present in the CNS or the peripheral nervous system (PNS). Neuroglia outnumber neurones; the greatest variety of neuroglia is found within the CNS, where they account for roughly half of the total volume of neural tissue (Tortora and Derrickson 2012). Types of neuroglia present in the CNS are: astrocytes, oligodendrocytes, microglia and ependymal cells. Astrocytes are star-shaped with fine branching processes and are the main supporting structure of the CNS (Waugh and Grant 2010). They are found in large numbers adjacent to blood vessels where the terminal ends of the branching processes surround blood capillaries, creating a sleeve that enables CNS blood capillaries to pass various substances selectively between the blood and brain tissue, thereby protecting the brain from harmful substances – this is known as the blood-brain barrier (Jenkins and Tortora 2013). Oligodendrocytes are smaller than astrocytes and have fewer branching processes. They form and maintain the myelin sheath around axons in the CNS (Jenkins and Tortora 2013). Microglia are phagocytic cells derived from monocytes that have migrated into neural tissues before birth (Waugh and Grant 2010). Microglia help protect the brain and spinal cord by removing microbes and damaged tissue (Waugh and Grant 2010). Ependymal cells line the central canal of the spinal cord and the ventricles of the brain (Waugh and Grant 2010). The apical surfaces of these cells have microvilli that may assist in the circulation of the enclosed CSF (Jenkins and Tortora 2013).
Dementia Dementia is a common disorder that affects the functioning of the brain. Dementia describes the
48 april 9 :: vol from 28 noRCNi.com 32 :: 2014 NURSING STANDARD / RCN Downloaded by ${individualUser.displayName} on Nov 13, 2015.©For personal use only. No other usesPUBLISHING without permission. Copyright © 2015 RCNi Ltd. All rights reserved. 045-049w32NEW.indd 48
04/04/2014 15:20
progressive loss of cortical tissue from the cerebral cortex (Alexander et al 2006, McCance and Huether 2006). This leads to a progressive decline in mental functioning including universal deterioration in memory, cognition (including decision making), motor function, emotional responsiveness and social behaviour (Alexander et al 2006). Dementia can therefore affect all aspects of a person’s life (Jenkins and McKay 2013). Dementia will affect approximately one in three people over the age of 65 in the UK (Alzheimer’s Society 2013). It is therefore likely that all nurses will encounter a person with dementia in their clinical practice. There are many types of dementia, however Alzheimer’s disease is the most common and familiar type. A total of 800,000 people are affected by dementia in the UK, of which 62% are caused by Alzheimer’s disease (Alzheimer’s Society 2013). The greatest risk factor for developing Alzheimer’s disease is increasing age. The Mini-Mental State Examination is commonly used to screen for cognitive function (Alzheimer’s Society 2014). Acetylcholinesterase inhibiting drugs donepezil, galantamine or rivastigmine may be a treatment option for patients with mild to moderate Alzheimer’s disease. These drugs are thought to slow intellectual deterioration. In patients with moderate to severe dementia in Alzheimer’s disease, memantine, a glutamate receptor antagonist, may be beneficial (British National Formulary 2011, National Institute for Health and Care Excellence 2011). People with dementia benefit from being in familiar surroundings with familiar routines and with people known to them (Jenkins and McKay 2013).
USEFUL RESOURCES The Age UK website provides a good general introduction to dementia. www.ageuk.org.uk/health-wellbeing/conditions-illnesses/dementia/ The Alzheimer’s Society website is an excellent resource on all issues relating to a person with Alzheimer’s disease. www.alzheimers.org.uk/
POINTS FOR PRACTICE What is the role of the nurse in caring for patients before, during and after lumbar puncture? Consider the clinical environment in which you work. How might you ensure the safety and wellbeing of patients who may have dementia?
GLOSSARY Acetylcholinesterase inhibitors Chemicals that inhibit the acetylcholinesterase enzyme from breaking down acetylcholine, thereby increasing both the level and duration of action of the neurotransmitter acetylcholine. Meninges Three layers of protective tissue that surround the brain and spinal cord. Neuroglia (glial cells) Non-neuronal cells of the central nervous system, which support and protect neurones.
Conclusion Having a clear understanding of the structures of the central nervous system and how they are protected enables the nurse to provide improved care and support to people with nervous system disorders. Since one in three adults over the age of 65 will develop dementia, it is crucial that nurses understand the challenges associated with this condition. A third article will conclude the examination of the nervous system with a detailed look at the PNS, including the autonomic nervous system NS
References Alexander MF, Fawcett JN, Runciman PJ (Eds) (2006) Nursing Practice. Hospital and Home: The Adult. Third edition. Churchill Livingstone Elsevier, Edinburgh.
Jenkins C, McKay A (2013) Collaborative health promotion in middle and later stages of dementia. Nursing Standard. 27, 37, 49-56.
Alzheimer’s Society (2013) Dementia 2013: The Hidden Voice of Loneliness. www.alzheimers.org.uk/infographic (Last accessed: March 24 2014.)
Jenkins GW, Tortora GJ (2013) Anatomy and Physiology: from Science to Life. International Student Version. Third edition. John Wiley and Sons, Hoboken NJ.
Alzheimer’s Society (2014) Diagnosing Dementia. tiny.cc/ diagnosing_dementia (Last accessed: March 25 2014.)
Kneale J, Davis P (Eds) (2005) Orthopaedic and Trauma Nursing. Second edition. Churchill Livingstone, Edinburgh.
British National Formulary (2011) British National Formulary No. 61. BMJ Group and the Royal Pharmaceutical Society of Great Britain, London.
Lim E, Loke YK, Thompson A (Eds) (2007) Medicine and Surgery. An Integrated Textbook. Churchill Livingstone Elsevier, Edinburgh.
McCance KL, Huether SE (2006) Pathophysiology: The Biologic Basis for Disease in Adults and Children. Fifth edition. Elsevier Mosby, St Louis MO.
Patton KT, Thibodeau GA (2010) Anatomy & Physiology. Seventh edition. Mosby Elsevier, St Louis MO. Seeley RR, Stephens TD, Tate P (2008) Anatomy and Physiology. Eighth edition. McGraw Hill, Boston MA.
National Institute for Health and Care Excellence (2011) Donepezil, Galantamine, Rivastigmine and Memantine for the Treatment of Alzheimer’s Disease. Review of NICE technology appraisal guidance 111. NICE technology appraisal guidance 217. NICE, London.
Tortora GJ, Derrickson B (2012) Principles of Anatomy and Physiology. Thirteenth edition. John Wiley and Sons, Hoboken NJ.
NHS Choices (2013) Epidural Anaesthesia. www.nhs.uk/ Conditions/Epidural-anaesthesia/ Pages/Introduction.aspx (Last accessed: March 17 2014.)
Waugh A, Grant A (2010) Ross and Wilson Anatomy and Physiology in Health and Illness. Eleventh edition. Churchill Livingstone Elsevier, Edinburgh.
©Downloaded NURSINGfrom STANDARD RCN PUBLISHING aprilNo 9 other :: vol uses 28 no 32 :: permission. 2014 49 RCNi.com/ by ${individualUser.displayName} on Nov 13, 2015. For personal use only. without Copyright © 2015 RCNi Ltd. All rights reserved. 045-049w32NEW.indd 49
04/04/2014 15:20
Nervous system: part 2 Hendry C et al (2014) Nervous system: part 2. Nursing Standard. 28, 32, 45-49. Date of submission: May 29 2013; date of acceptance: September 2 2013.
Cerebrum
Abstract
Charles Hendry, retired, was senior lecturer, School of Nursing and Midwifery, University of Dundee, Dundee, Scotland. Alistair Farley, retired, was lecturer in nursing, School of Nursing and Midwifery, University of Dundee. Ella McLafferty, retired, was senior lecturer, School of Nursing and Midwifery, University of Dundee. Carolyn Johnstone, lecturer in nursing, School of Nursing and Midwifery, University of Dundee. Correspondence to: [email protected]
The cerebrum forms the bulk of the brain. It is composed of grey matter made up of neuronal cell bodies, and white matter composed of axons. The surface, which is called the cerebral cortex, is divided into two hemispheres by a longitudinal fissure. The cerebral cortex has many folds or furrows, which increase its surface area. These folds are called gyri, the deep grooves between gyri are referred to as fissures and the shallow grooves are called sulci (Tortora and Derrickson 2012). The cerebral hemispheres are connected by a band of tissue called the corpus callosum, which contains white matter (Tortora and Derrickson 2012) through which information can pass from one side of the brain to the other and each hemisphere can communicate with the other. Each hemisphere is further divided into lobes named after the cranial bones under which they lie (frontal, parietal, temporal and occipital), with each lobe having specific functions.
Keywords
Frontal lobe
This article, which forms part of the life sciences series and is the second of three articles on the nervous system, explores the major divisions of the brain and their functions. The various ways in which the delicate structures of the brain and spinal cord are protected and supported by the cranial and vertebral bones, the meninges and the cerebrospinal fluid are examined. Neuroglia (glial cells), which form a major component of the total volume of brain tissue, are described. Dementia is discussed briefly to illustrate the effects of a common central nervous system disorder.
Authors
Brain, central nervous system, cerebrospinal fluid, dementia, meninges, nervous system, neurone, spinal cord
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.
THE CENTRAL NERVOUS system (CNS) is composed of the brain and spinal cord. The brain is a complex and delicate structure, which can be subdivided into several elements. This article examines these elements and explores their functions. There are six major divisions in the adult brain: the cerebrum, diencephalon, cerebellum, midbrain, pons and medulla oblongata (Figure 1).
The frontal lobe contains centres that deal with voluntary motor function, motivation, aggression and mood. Aspects of the sensation of smell are perceived here (Seeley et al 2008), although the primary area for the perception of smell is found in the temporal lobe (Jenkins and Tortora 2013). The frontal lobe also contains the anterior speech centre (Broca’s area), which is concerned with constructing sentences. Damage to this area may occur in stroke, where an individual knows what he or she wants to say but is unable to construct the sentences.
Parietal lobe
The parietal lobe is responsible for perceiving most sensory information. It receives and interprets sensations from receptors in the skin, muscle and viscera (abdomen and thorax) (Seeley et al 2008).
Temporal lobe
The temporal lobe contains the olfactory centre, which is responsible for the perception of smell (Waugh and Grant 2010). It also contains the posterior speech centre (Wernicke’s area), which is concerned with vocabulary and comprehension (Waugh and Grant 2010). Damage to this area
©Downloaded NURSINGfrom STANDARD RCN PUBLISHING aprilNo 9 other :: vol uses 28 no 32 :: permission. 2014 45 RCNi.com/ by ${individualUser.displayName} on Nov 13, 2015. For personal use only. without Copyright © 2015 RCNi Ltd. All rights reserved. 045-049w32NEW.indd 45
04/04/2014 15:20
Art & science life sciences: 21 will produce incomprehensible speech with inappropriate words. The auditory centre is also located in the temporal lobe.
Occipital lobe
The occipital lobe contains the visual centre (Jenkins and Tortora 2012).
Midbrain, pons and medulla oblongata
Diencephalon The diencephalon lies deep within the brain and contains the thalamus and hypothalamus. The thalamus, which comprises about 80% of the diencephalon, is an area of grey matter and is a major relay station for most sensory impulses (with the exception of olfaction) which are sent to the appropriate area of the cerebral cortex (Seeley et al 2008). The hypothalamus is situated below and in front of the thalamus, immediately above the pituitary gland (Waugh and Grant 2010). The nervous and endocrine systems work together to achieve homeostasis. The hypothalamus receives information from sensory nerves about the current state of the body. This information initiates a response in the autonomic nervous and endocrine systems. The hypothalamus influences or regulates control of body temperature, eating and drinking, strong emotions and circadian rhythms (Waugh and Grant 2010).
Cerebellum The cerebellum is similar in structure to the cerebrum. It occupies the posterior and inferior
FIGURE 1 Major divisions of the brain Diencephalon: Thalamus
Cerebrum
aspects of the cranial cavity, situated posterior to the pons and immediately below the cerebrum (Waugh and Grant 2010). It is responsible for the maintenance of posture and balance, muscle tone and co-ordination and refinement of movement (Jenkins and Tortora 2013).
The midbrain, pons and medulla oblongata, or the medulla, are referred to collectively as the brainstem. It is within the structures of the brainstem that most of the cranial nerves originate (Tortora and Derrickson 2012). The midbrain is located between the diencephalon and the pons (Tortora and Derrickson 2012). It acts as a relay station for receptors for hearing passing from the ear to the thalamus. The midbrain contains several reflex centres, including the centres associated with controlling movement of the eyes and head in response to visual stimuli (Tortora and Derrickson 2012). The pons also acts as a relay station passing information, such as signals pertaining to the coordination of voluntary movement, between the cerebrum and cerebellum. It contains the pneumotaxic and apneustic areas, which form part of the respiratory centre and control the rhythm of breathing (Waugh and Grant 2010). The pons is situated directly superior to the medulla and anterior to the cerebellum (Tortora and Derrickson 2012). The medulla oblongata is situated between the foramen magnum and the pons. The medulla contains the vital centres, the remainder of the respiratory centre, the cardiac centre and the vasomotor centre. It is in the lower end of the medulla that most motor nerve pathways cross to the opposite side so that the left cerebral hemisphere controls movement on the right side of the body and vice versa – this is known as the decussation of the pyramids (Jenkins and Tortora 2013).
Hypothalamus
Protection of the central nervous system
Brainstem: Midbrain Pons
PETER LAMB
Cerebellum
Posterior
Medulla oblongata Spinal cord
Anterior
There are many layers that protect the CNS. These include hair, skin, subcutaneous tissue, adipose tissue, bone, the meninges, and cerebrospinal fluid (CSF). However, in practice it is usual to consider only the bone, meninges and CSF as offering significant protection to the brain and spinal cord. A special class of cells known as neuroglia (glial cells) also protects the delicate structures of the brain. This will be discussed in further detail later.
Bones
Eight bones of the cranium protect the brain: frontal, parietal (two), temporal (two), occipital,
46 april 9 :: vol from 28 noRCNi.com 32 :: 2014 NURSING STANDARD / RCN Downloaded by ${individualUser.displayName} on Nov 13, 2015.©For personal use only. No other usesPUBLISHING without permission. Copyright © 2015 RCNi Ltd. All rights reserved. 045-049w32NEW.indd 46
04/04/2014 15:20
sphenoid and ethmoid. Together they form a rigid bony box that encloses the brain (McCance and Huether 2006). This rigidity is achieved as a result of the fusion of the joints between the bones. The bones of the cranium are united by sutures, which are immovable fibrous joints. The interlocking nature of these cranial sutures adds strength to the cranium and reduces the likelihood of fracturing at the joints (Tortora and Derrickson 2012). There are four prominent cranial sutures (Tortora and Derrickson 2012): The coronal suture unites the frontal bone and parietal bones. The sagittal suture unites the two parietal bones. The lambdoid suture unites the parietal bones and occipital bone. The squamous sutures unite the parietal bones and temporal bones. At birth the bones of the skull are not fused, which allows for some compression of the skull during labour and birth, enabling the cranium to pass through the birth canal. Fusion of the sutures of a child’s skull is not complete until approximately 12 years of age (McCance and Huether 2006). The vertebral column is also protective, with the vertebrae enclosing and protecting the spinal cord (Kneale and Davis 2005, Tortora and Derrickson 2012). The space between the vertebral arch and the body of the vertebra – known as the vertebral foramen – contains the spinal cord (Waugh and Grant 2010). The spinal cord contained within the vertebral column extends from the foramen magnum of the skull to the region of the second lumbar vertebra (Seeley et al 2008). Lying between the vertebrae from the second cervical vertebra to the sacrum are intervertebral discs, which permit movement of the vertebral column and act as shock absorbers (Tortora and Derrickson 2012). Samples of CSF can be obtained by inserting a cannula into the space below the spinal cord through L3-L4 or L4-L5. In this way there should be no risk of damaging the spinal cord. Following this procedure, patients should be kept supine for one hour to minimise leakage of CSF (Lim et al 2007). There is a true space between the dura mater of the spinal cord and the wall of the vertebrae known as the epidural space. Epidural anaesthesia of the spinal nerves can be achieved by injecting anaesthetic agents into the epidural space. An epidural can be given at different locations along the spinal canal, depending on where anaesthesia is required, although it is most frequently used in childbirth (Seeley et al 2008, NHS Choices 2013).
Meninges
The meninges provide the next layer of protection – they surround the brain and spinal cord. There
are three distinct layers that together form the meninges: the dura mater (outer layer), the arachnoid mater (middle layer) and the pia mater (inner layer). The cerebral dura mater is a double layer of dense fibrous tissue with the outer layer lining the inner surface of the skull (replacing the periosteum) and the inner layer forming the meningeal dura, which is continuous with the dura of the spinal cord (Seeley et al 2008). In addition, dense dural folds, formed from the meningeal dura, project between the two cerebral hemispheres (falx cerebri) and between the cerebral hemispheres and the cerebellum (tentorium cerebelli). These folds help to support and stabilise the brain, preventing excessive movement. The cerebral dura has a central region known as the superior sagittal sinus, which contains blood vessels. This structure acts as a venous reservoir and collects blood from brain tissue before returning it to the heart via the internal jugular vein. There are a number of other smaller sinuses within the dural folds, which also act as venous reservoirs (Patton and Thibodeau 2010). Lying beneath the dura mater is a potential space – which is an anatomical space that can be realised between two structures that in normal circumstances remain closed – known as the subdural space, which is filled with serous fluid (Waugh and Grant 2010, Tortora and Derrickson 2012). The arachnoid mater (so called because of its web-like structure) is a layer of thin wispy connective tissue below which lies the subarachnoid space. It merges with the dura mater within the vertebral column at the level of the second sacral vertebra (Waugh and Grant 2010). The subarachnoid space is filled with CSF. The pia mater, the innermost layer of connective tissue, is delicate and thin, contains numerous minute blood vessels and is adherent to the surface of the brain and spinal cord (Seeley et al 2008). Capillaries leaving the blood vessels of the pia mater penetrate the neural tissue of the brain.
Cerebrospinal fluid
CSF is composed of 99% water and appears clear and colourless; it contains glucose, proteins, lactic acid, urea and white blood cells (Waugh and Grant 2010, Tortora and Derrickson 2012). Its functions are to (Waugh and Grant 2010): Support the weight of the brain (the brain effectively ‘floats’ within the cranium). Act as a shock absorber. Ensure a uniform pressure around the delicate structures of the brain and spinal cord. Provide some nutrition to the brain and spinal cord and remove waste.
©Downloaded NURSINGfrom STANDARD RCN PUBLISHING aprilNo 9 other :: vol uses 28 no 32 :: permission. 2014 47 RCNi.com/ by ${individualUser.displayName} on Nov 13, 2015. For personal use only. without Copyright © 2015 RCNi Ltd. All rights reserved. 045-049w32NEW.indd 47
04/04/2014 15:20
Art & science life sciences: 21 Lying within the brain are four CSF-filled cavities known as ventricles: the right and left lateral ventricles; third ventricle; and fourth ventricle (Figure 2). The ventricles communicate with each other, with the central space of the spinal cord and with the subarachnoid space via small connecting ducts and/or small openings in their surfaces. The lateral ventricles are two main ventricles and they are located in the cerebral hemisphere under the corpus callosum. The third ventricle is a slit between and below the lateral ventricles and is connected to the lateral ventricles by a narrow opening, the intraventricular foramen (Waugh and Grant 2010). The fourth ventricle is a diamond-shaped cavity that lies between the pons and the cerebellum. It communicates with the third ventricle via the cerebral aqueduct, a small connecting duct. The fourth ventricle has three openings through which CSF flows out into the central canal of the spinal cord and the subarachnoid space (Patton and Thibodeau 2010). CSF is formed at the choroid plexuses of the ventricles at a rate of 20mL/hour or approximately 500mL/day (Jenkins and Tortora 2013). Choroid plexuses are networks of blood vessels located in the walls of each of the four ventricles, and are covered by cells known as ependymal cells. Ependymal cells in the lateral ventricles produce about 80-90% of the CSF, with the remainder being formed in the third and fourth ventricles (Seeley et al 2008). The ventricles, subarachnoid space and central canal of
FIGURE 2 Brain and spinal cord showing the ventricles and the central canal Posterior
Anterior
Subarachnoid space
Choroid plexus of lateral ventricle
Arachnoid villus
Cerebrum
Corpus callosum
Choroid plexus of third ventricle
Lateral ventricle Third ventricle
Choroid plexus of fourth ventricle
Cerebral aqueduct Fourth ventricle Spinal cord
PETER LAMB
Central canal Subarachnoid space
spinal cord contain approximately 140mL of CSF, with about 23mL in the ventricles and 117mL in the subarachnoid space (Seeley et al 2008, Patton and Thibodeau 2010). The rate of production of CSF equals the rate of reabsorption such that the volume of CSF remains constant (Tortora and Derrickson 2012). CSF reaches the subarachnoid space through the lateral and median apertures in the roof of the fourth ventricle. The CSF returns to the vascular circulation through the arachnoid villi that project into the superior sagittal sinus and other smaller sinuses.
Neuroglia Neuroglia support and protect the cells of the nervous system (Tortora and Derrickson 2012). They can be described according to whether they are present in the CNS or the peripheral nervous system (PNS). Neuroglia outnumber neurones; the greatest variety of neuroglia is found within the CNS, where they account for roughly half of the total volume of neural tissue (Tortora and Derrickson 2012). Types of neuroglia present in the CNS are: astrocytes, oligodendrocytes, microglia and ependymal cells. Astrocytes are star-shaped with fine branching processes and are the main supporting structure of the CNS (Waugh and Grant 2010). They are found in large numbers adjacent to blood vessels where the terminal ends of the branching processes surround blood capillaries, creating a sleeve that enables CNS blood capillaries to pass various substances selectively between the blood and brain tissue, thereby protecting the brain from harmful substances – this is known as the blood-brain barrier (Jenkins and Tortora 2013). Oligodendrocytes are smaller than astrocytes and have fewer branching processes. They form and maintain the myelin sheath around axons in the CNS (Jenkins and Tortora 2013). Microglia are phagocytic cells derived from monocytes that have migrated into neural tissues before birth (Waugh and Grant 2010). Microglia help protect the brain and spinal cord by removing microbes and damaged tissue (Waugh and Grant 2010). Ependymal cells line the central canal of the spinal cord and the ventricles of the brain (Waugh and Grant 2010). The apical surfaces of these cells have microvilli that may assist in the circulation of the enclosed CSF (Jenkins and Tortora 2013).
Dementia Dementia is a common disorder that affects the functioning of the brain. Dementia describes the
48 april 9 :: vol from 28 noRCNi.com 32 :: 2014 NURSING STANDARD / RCN Downloaded by ${individualUser.displayName} on Nov 13, 2015.©For personal use only. No other usesPUBLISHING without permission. Copyright © 2015 RCNi Ltd. All rights reserved. 045-049w32NEW.indd 48
04/04/2014 15:20
progressive loss of cortical tissue from the cerebral cortex (Alexander et al 2006, McCance and Huether 2006). This leads to a progressive decline in mental functioning including universal deterioration in memory, cognition (including decision making), motor function, emotional responsiveness and social behaviour (Alexander et al 2006). Dementia can therefore affect all aspects of a person’s life (Jenkins and McKay 2013). Dementia will affect approximately one in three people over the age of 65 in the UK (Alzheimer’s Society 2013). It is therefore likely that all nurses will encounter a person with dementia in their clinical practice. There are many types of dementia, however Alzheimer’s disease is the most common and familiar type. A total of 800,000 people are affected by dementia in the UK, of which 62% are caused by Alzheimer’s disease (Alzheimer’s Society 2013). The greatest risk factor for developing Alzheimer’s disease is increasing age. The Mini-Mental State Examination is commonly used to screen for cognitive function (Alzheimer’s Society 2014). Acetylcholinesterase inhibiting drugs donepezil, galantamine or rivastigmine may be a treatment option for patients with mild to moderate Alzheimer’s disease. These drugs are thought to slow intellectual deterioration. In patients with moderate to severe dementia in Alzheimer’s disease, memantine, a glutamate receptor antagonist, may be beneficial (British National Formulary 2011, National Institute for Health and Care Excellence 2011). People with dementia benefit from being in familiar surroundings with familiar routines and with people known to them (Jenkins and McKay 2013).
USEFUL RESOURCES The Age UK website provides a good general introduction to dementia. www.ageuk.org.uk/health-wellbeing/conditions-illnesses/dementia/ The Alzheimer’s Society website is an excellent resource on all issues relating to a person with Alzheimer’s disease. www.alzheimers.org.uk/
POINTS FOR PRACTICE What is the role of the nurse in caring for patients before, during and after lumbar puncture? Consider the clinical environment in which you work. How might you ensure the safety and wellbeing of patients who may have dementia?
GLOSSARY Acetylcholinesterase inhibitors Chemicals that inhibit the acetylcholinesterase enzyme from breaking down acetylcholine, thereby increasing both the level and duration of action of the neurotransmitter acetylcholine. Meninges Three layers of protective tissue that surround the brain and spinal cord. Neuroglia (glial cells) Non-neuronal cells of the central nervous system, which support and protect neurones.
Conclusion Having a clear understanding of the structures of the central nervous system and how they are protected enables the nurse to provide improved care and support to people with nervous system disorders. Since one in three adults over the age of 65 will develop dementia, it is crucial that nurses understand the challenges associated with this condition. A third article will conclude the examination of the nervous system with a detailed look at the PNS, including the autonomic nervous system NS
References Alexander MF, Fawcett JN, Runciman PJ (Eds) (2006) Nursing Practice. Hospital and Home: The Adult. Third edition. Churchill Livingstone Elsevier, Edinburgh.
Jenkins C, McKay A (2013) Collaborative health promotion in middle and later stages of dementia. Nursing Standard. 27, 37, 49-56.
Alzheimer’s Society (2013) Dementia 2013: The Hidden Voice of Loneliness. www.alzheimers.org.uk/infographic (Last accessed: March 24 2014.)
Jenkins GW, Tortora GJ (2013) Anatomy and Physiology: from Science to Life. International Student Version. Third edition. John Wiley and Sons, Hoboken NJ.
Alzheimer’s Society (2014) Diagnosing Dementia. tiny.cc/ diagnosing_dementia (Last accessed: March 25 2014.)
Kneale J, Davis P (Eds) (2005) Orthopaedic and Trauma Nursing. Second edition. Churchill Livingstone, Edinburgh.
British National Formulary (2011) British National Formulary No. 61. BMJ Group and the Royal Pharmaceutical Society of Great Britain, London.
Lim E, Loke YK, Thompson A (Eds) (2007) Medicine and Surgery. An Integrated Textbook. Churchill Livingstone Elsevier, Edinburgh.
McCance KL, Huether SE (2006) Pathophysiology: The Biologic Basis for Disease in Adults and Children. Fifth edition. Elsevier Mosby, St Louis MO.
Patton KT, Thibodeau GA (2010) Anatomy & Physiology. Seventh edition. Mosby Elsevier, St Louis MO. Seeley RR, Stephens TD, Tate P (2008) Anatomy and Physiology. Eighth edition. McGraw Hill, Boston MA.
National Institute for Health and Care Excellence (2011) Donepezil, Galantamine, Rivastigmine and Memantine for the Treatment of Alzheimer’s Disease. Review of NICE technology appraisal guidance 111. NICE technology appraisal guidance 217. NICE, London.
Tortora GJ, Derrickson B (2012) Principles of Anatomy and Physiology. Thirteenth edition. John Wiley and Sons, Hoboken NJ.
NHS Choices (2013) Epidural Anaesthesia. www.nhs.uk/ Conditions/Epidural-anaesthesia/ Pages/Introduction.aspx (Last accessed: March 17 2014.)
Waugh A, Grant A (2010) Ross and Wilson Anatomy and Physiology in Health and Illness. Eleventh edition. Churchill Livingstone Elsevier, Edinburgh.
©Downloaded NURSINGfrom STANDARD RCN PUBLISHING aprilNo 9 other :: vol uses 28 no 32 :: permission. 2014 49 RCNi.com/ by ${individualUser.displayName} on Nov 13, 2015. For personal use only. without Copyright © 2015 RCNi Ltd. All rights reserved. 045-049w32NEW.indd 49
04/04/2014 15:20
