Art & science life sciences: 22
Nervous system: part 3 Farley A et al (2014) Nervous system: part 3 Nursing Standard. 28, 33, 46-50. Date of submission: August 6 2013; date of acceptance: November 25 2013.
Abstract This article, which forms part of the life sciences series and is the last of three articles on the nervous system, explores the major divisions of the peripheral nervous system. Motor and sensory nerves will be described before a more detailed examination of the cranial and spinal nerves is provided. The autonomic nervous system will be explored, including the diverse roles of the parasympathetic and sympathetic divisions. The role of spinal reflexes in maintaining homeostasis is identified. Motor neurone disease will be discussed briefly as an example of a disorder of the peripheral nervous system.
Authors Alistair Farley Retired, was lecturer in nursing, School of Nursing and Midwifery, University of Dundee, Dundee, Scotland. 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. Charles Hendry Retired, was senior lecturer, School of Nursing and Midwifery, University of Dundee. Correspondence to: [email protected]
Keywords Cranial nerves, motor neurone disease, peripheral nervous system, spinal nerves, spinal reflexes
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.
46 april 16 :: vol 28 no 33 :: 2014
THE CENTRAL NERVOUS system (CNS) is composed of the brain and spinal cord, while the remaining nervous tissue forms the peripheral nervous system. The peripheral nervous system is composed of cranial nerves, spinal nerves and the autonomic nervous system (Waugh and Grant 2010). Spinal nerves connect the body to the spinal cord and cranial nerves connect the body directly to the brain. Most of the nerves of the peripheral nervous system consist of sensory (afferent) nerve fibres that transmit impulses (information) from sensory organs to the brain. Efferent nerves transmit impulses from the brain to effector organs or tissues such as skeletal muscle (Waugh and Grant 2010). Efferent nerves can be divided into somatic nerves, which are involved in voluntary movement of skeletal muscle, and autonomic nerves, which are involved in cardiac and smooth muscle contraction (Tortora and Derrickson 2012).
Cranial nerves There are 12 pairs of cranial nerves, each of which is named according to its distribution or function. Cranial nerves arise from the inferior surface of the brain and are numbered in the order in which they connect with the brain from anterior to posterior (Patton and Thibodeau 2010). They may be sensory, motor or mixed nerves (Table 1).
Spinal cord An adult’s spinal cord is approximately 42-45cm long and 2cm wide in the mid-thoracic region (Jenkins and Tortora 2013). It extends from the foramen magnum in the base of the skull, to the first or second lumbar vertebra (L1-L2). The spinal cord ends at around L1 in a tapered cone known as the conus medullaris, from which many nerve roots (initial segment of a nerve leaving the CNS) extend to form the cauda equina, which resembles a horse’s tail (Patton and Thibodeau 2010). The diameter of the spinal cord decreases from the cervical region to the sacral region, with larger regions found where the limbs are innervated (Seeley et al 2008). The spinal cord is composed of 31 segments, each having a pair of dorsal and ventral roots that form a spinal nerve. Pairs of nerves are present throughout the length of the spinal cord (Watson 2011).
© NURSING STANDARD / RCN PUBLISHING
Downloaded from RCNi.com by ${individualUser.displayName} on Nov 19, 2015. For personal use only. No other uses without permission. Copyright © 2015 RCNi Ltd. All rights reserved.
In the spinal cord, grey matter (cell bodies) is found in columns and arranged in an H shape, with white matter (nerve pathways) in tracts surrounding it. A central canal runs down the middle of the spinal cord containing cerebrospinal fluid. The white matter is divided into columns running the length of the spinal cord, each of which contains a number of tracts or fasciculi. The ascending tracts carry sensory information from the body to the brain, while the descending tracts carry motor information to motor neurones in the spinal cord. Figure 1 shows a cross section of the spinal cord. The posterior (dorsal) horns of the H shape of the spinal cord are stimulated by sensory impulses travelling to the CNS from the body. The cell body of the sensory neurone lies outside the spinal cord in the posterior root ganglion. A ganglion is a group of nerve cell bodies that lie outside the CNS (Jenkins and Tortora 2013). The axon of a sensory neurone enters the posterior horn of the spinal cord where it synapses with another sensory neurone, the axon of which passes up the sensory pathways in the spinal cord to the brain.
The anterior (ventral) horn of the H shape of the spinal cord deals with motor information leaving the CNS and travelling to the body. Similarly, the axon of a motor neurone passes down the motor pathway in the spinal cord. At the appropriate level, it will synapse with another motor neurone in the anterior horn, and the axon of this neurone forms the motor nerve. The upper motor neurone has its cell body in the motor area of the cerebrum and carries motor information down to the lower motor neurones. The lower motor neurones are located in the anterior horn of the grey matter in the spinal cord.
Spinal nerves Spinal nerves connect the CNS with sensory receptors, muscles and glands throughout the body. There are 31 pairs of spinal nerves: eight cervical, 12 thoracic, five lumbar, five sacral and one coccygeal (Jenkins and Tortora 2013). Spinal nerves are surrounded by a series of protective connective tissue layers: the endoneurium (innermost layer), the perineurium, which surrounds each bundle of fibres, and the epineurium (outermost layer) (Waugh
TABLE 1 Classification and function of the cranial nerves Cranial nerve (number)
Classification
Function
Olfactory nerves (I)
Sensory
Provide the sense of smell. Impulses are conveyed to the primary olfactory area in the temporal lobe of the cerebral cortex.
Optic nerves (II)
Sensory
Send information from the eye to the diencephalon and onward to the visual area of the occipital lobe providing the sense of sight.
Oculomotor nerves (III)
Motor
Supply the superior, medial and inferior recti and inferior oblique muscles; the ciliary muscles; the circular muscles of the iris; and muscles that raise the upper eyelids, the levator palpebrae muscles.
Trochlear nerves (IV)
Motor
Innervate the superior oblique muscles of the eye.
Trigeminal nerves (V)
Mixed
The ophthalmic branch innervates the scalp and skin of the forehead; the maxillary branch innervates the upper lip and gums, teeth, cheeks, nose and lower eyelids; and the mandibular branch innervates the muscles of mastication.
Abducent nerves (VI)
Motor
Innervate the lateral rectus muscle of the eye.
Facial nerves (VII)
Mixed
Innervate the facial muscles (facial expression) and control secretion of saliva and tears, as well as sensory information from the taste buds along the anterior two thirds of the tongue.
Vestibulocochlear nerves (VIII)
Sensory
The cochlear branch provides the sensation of hearing and the vestibular branch provides the sensation of equilibrium.
Glossopharyngeal nerves (IX)
Mixed
Stimulate the posterior third of the tongue and the pharynx. Monitor blood pressure and respiratory gas levels in the blood.
Vagus nerves (X)
Mixed
Supply visceral structures such as the pharynx, larynx, trachea and heart and have diverse sensory functions such as taste, touch, pain and temperature.
Accessory nerves (XI)
Motor
Innervate muscles that control movement of the head and shoulders.
Hypoglossal nerves (XII)
Motor
Innervate the muscles of the tongue.
(Tortora and Derrickson 2012)
© NURSING STANDARD / RCN PUBLISHING
april 16 :: vol 28 no 33 :: 2014 47
Downloaded from RCNi.com by ${individualUser.displayName} on Nov 19, 2015. For personal use only. No other uses without permission. Copyright © 2015 RCNi Ltd. All rights reserved.
Art & science life sciences: 22 and Grant 2010). A typical spinal nerve has two connections with the spinal cord – an anterior and a posterior nerve root. The anterior nerve root contains motor nerve fibres and the posterior root contains sensory nerve fibres that combine at the intervertebral foramen to form a spinal nerve, therefore a spinal nerve can be considered a mixed nerve (Jenkins and Tortora 2013). On emerging from the intervertebral foramen, spinal nerves divide into branches or rami. A typical spinal nerve has anterior, posterior and communicating rami. The anterior rami innervate the anterior and lateral aspects of the neck, trunk and upper and lower limbs, while the posterior rami innervate the skin and muscles of the posterior of the trunk. The communicating rami are part of the autonomic nervous system (ANS).
Plexuses Plexuses are large masses of nerves. Several segments of the spinal cord are involved in the innervations of complex peripheral musculature, with nerves arising at different points along the spinal cord. Anterior roots intermingle to form a plexus. This intermingling means that damage to one spinal nerve does not cause total loss of function to a given region. There are five large plexuses of mixed nerves on the left and right sides of the vertebral column: the cervical plexus, brachial plexus, lumbar plexus, sacral plexus and coccygeal plexus (Waugh and Grant 2010) (Table 2). The thoracic nerves do not form plexuses (Waugh and Grant 2010).
Spinal reflexes The spinal cord has a role to play in maintaining homeostasis by acting as an integrating centre for some reflexes (Tortora and Derrickson 2012). A reflex is an immediate, involuntary motor response to a particular stimulus (Waugh and Grant 2010). In a spinal reflex, the processing events take place in the spinal cord rather than involving the brain, however individual reflexes may vary in their complexity. The simplest reflex exists where sensory neurones synapse directly with motor neurones; these are referred to as monosynaptic reflexes. An example of a simple reflex is the response that occurs when a person touches a very hot surface – the withdrawal reflex. Even before the person is conscious of the heat, the spinal reflex causes the hand to be removed from the heat source, thus minimising the risk of harm. Polysynaptic spinal reflexes have more complex pathways and consist of additional elements such as integrating centres and interneurones. Interneurones are relay neurones found between the sensory and motor neurones, and often have an inhibitory function (Seeley et al 2008). In an intersegmental reflex, more than one segment of the spinal cord will be involved.
Autonomic nervous system The nervous system receives information from sensory nerves. This information travels to the hypothalamus and is the basis for the response of the ANS. While somatic nerves are motor nerves that serve structures under voluntary or conscious
FIGURE 1 Cross section of the spinal cord Posterior horn
Posterior root of spinal nerve
Posterior root ganglion
Central canal
Spinal nerve
Axon of sensory neurone
Sensory information
Anterior root of spinal nerve Anterior horn
Nerve impulses Axon of motor neurone
48 april 16 :: vol 28 no 33 :: 2014
PETER LAMB
White matter
© NURSING STANDARD / RCN PUBLISHING
Downloaded from RCNi.com by ${individualUser.displayName} on Nov 19, 2015. For personal use only. No other uses without permission. Copyright © 2015 RCNi Ltd. All rights reserved.
control, the ANS contains motor nerves that supply structures under involuntary control. The ANS exerts its effects on smooth muscle, cardiac muscle and glands (Waugh and Grant 2010). It is subdivided into the sympathetic nervous system and the parasympathetic nervous system. These two divisions tend to exert opposing effects, but overall work together to achieve homeostasis. A summary of the characteristics of the ANS is outlined in Table 3. Sympathetic activity tends to predominate in stressful situations, while parasympathetic activity is most obvious during rest. Examples of sympathetic activity include (Waugh and Grant 2010): Bronchodilation. Increased heart rate and force of contraction. Dilation of pupils. Examples of parasympathetic activity include (Waugh and Grant 2010): Bronchoconstriction. Decreased heart rate and force of contraction. Constriction of pupils. Increased gastrointestinal activity. The autonomic pathway, the nerve pathway associated with the ANS, consists of two motor neurones. The first has its cell body in the CNS and is known as the preganglionic neurone. It synapses with a ganglion outside the spinal cord. The second motor neurone, the postganglionic neurone, leaves the ganglion and synapses with the target organ. The cell bodies for the sympathetic division are located in the spinal cord at T1-L2/L3. The axons for these neurones lead to a chain of ganglia situated just outside the vertebral column, where they synapse with a second neurone leading to the target organ. The cell bodies for the parasympathetic division are found in the spinal cord at S2-S4 (Seeley et al 2008). The long axons of these neurones lead to ganglia in the walls of the target organ where they synapse with the second neurone leading into the organ.
Neurotransmitters
There are a number of neurotransmitters acting in the ANS. A neurotransmitter is a chemical that transmits signals from one neurone to another or to a target cell, across a synapse (Farley et al 2014). In the sympathetic nervous system, the neurotransmitter acetylcholine is released between preganglionic and postganglionic neurones, while the neurotransmitter noradrenaline (norepinephrine) is released between postganglionic neurones and target organs. In the parasympathetic nervous system, the neurotransmitter acetylcholine is released between preganglionic and postganglionic neurones and
© NURSING STANDARD / RCN PUBLISHING
between postganglionic neurones and target organs (Jenkins and Tortora 2013).
Regulation
The ANS is regulated by the hypothalamus, which receives input from sensory nerves and from many other parts of the brain, for example the higher centres in the frontal lobe (Jenkins and Tortora 2012).
Motor neurone disease Motor neurone disease involves progressive degeneration of upper and lower motor neurones in the brain and spinal cord, affecting muscle activity such as speaking, swallowing, walking and general movement of the body. It is rare, with about two in 100,000 people in the UK developing the disease each year and with a prevalence of between seven and ten cases in 100,000 people (McKevitt et al 2007, Jarman 2012, Motor Neurone Disease Association
TABLE 2 Functions of plexuses Plexus
Function
Cervical
Sensory nerves supply the scalp, ears, chest, neck and shoulders; motor nerves supply the neck muscles and diaphragm.
Brachial
Sensory and motor nerves innervate the shoulders and upper limbs.
Lumbar
Sensory and motor nerves innervate the lower abdomen, external genitals and part of the lower limbs.
Sacral
Sensory and motor nerves innervate the buttocks, perineum and lower limbs.
Coccygeal
Sensory and motor nerves innervate the skin around the coccyx and anal area.
TABLE 3 Characteristics of the autonomic nervous system Characteristic
Sympathetic division
Parasympathetic division
Point of origin
Thoracolumbar
Craniosacral
General effects
Functions adapted for stressful situations
Functions adapted for resting situations
Neurotransmitters
Acetylcholine Noradrenaline (norepinephrine)
Acetylcholine
Position of ganglia
Outside but close to the central nervous system
Inside or close to the effector structure
Length of fibres
Short preganglionic Long postganglionic
Long preganglionic Short postganglionic
april 16 :: vol 28 no 33 :: 2014 49
Downloaded from RCNi.com by ${individualUser.displayName} on Nov 19, 2015. For personal use only. No other uses without permission. Copyright © 2015 RCNi Ltd. All rights reserved.
Art & science life sciences: 22 2014). However, the effect of the disease is significant and is usually fatal, with death occurring within three to five years, although some patients may live up to ten years or even longer from diagnosis (Ballinger 2012, NHS Choices 2013). It typically presents in middle age and is more common in men (Jarman 2012). In motor neurone disease, the exact pattern of the nerves affected and the rate of degeneration is variable and clinical features will differ between individuals. Because degeneration is progressive, symptoms will worsen over time. There is no cure for motor neurone disease, and patients will need to be treated symptomatically. Riluzole may be given, but it only extends survival for people with motor neurone disease by on average two or three months and does not prevent the disease progressing (NHS Choices 2013). Four broad clinical patterns can be seen, as identified in Table 4, although the features may merge as the disease progresses. None of the clinical types include involvement of the sensory system, nor of the sensory or motor nerves of the eyes (Ballinger 2012). Diagnosis is largely by history and examination since there are no specific tests for motor neurone disease, although the
TABLE 4 Clinical patterns of motor neurone disease Type
Features
Amyotrophic lateral sclerosis
Muscle wasting, weakness, fasciculations, speech and swallowing problems, and muscle spasms.
Progressive muscular atrophy
Weakness, fasciculations, and wasting in the hands and arms.
Progressive bulbar palsy
Dysarthria, dysphagia, nasal regurgitation of fluids and choking.
Primary lateral sclerosis
Progressive tetraparesis.
clinician may order various tests to rule out other causes of presenting symptoms such as cervical spine lesion or bulbar myasthenia gravis.
Conclusion In this concluding article on the nervous system, the focus has been on the peripheral nervous system. Understanding the structure and function of the nervous system will enable the nurse to better care for patients who may be experiencing neurological deficits. While many patients with such deficits may be cared for in specialised units, many others will be cared for in general medical and surgical wards. Similarly, community nursing staff are increasingly caring for patients with long-term neurological deficits such as motor neurone disease NS
POINTS FOR PRACTICE In motor neurone disease, swallowing may be affected. If so, how would you assess and assist the patient in the maintenance of adequate fluid and nutrient intake? Disorders such as motor neurone disease lead to life-limiting and life-changing consequences for patients and significant others. In your area of practice, identify suitable resources that you may access to assist you in caring for a patient with motor neurone disease.
GLOSSARY Dorsal Pertaining to the posterior surface or portion of a named tissue or structure. Plexus A large mass of nerves. Ventral Pertaining to the anterior surface or portion of a named tissue or structure.
References Ballinger A (2012) Essentials of Kumar & Clark’s Clinical Medicine. Fifth edition. Saunders, Edinburgh.
Jenkins G, Tortora GJ (2013) Anatomy and Physiology: From Science to Life. Third edition. John Wiley and Sons, Hoboken NJ.
Farley A, Johnstone C, Hendry C, McLafferty E (2014) Nervous system: part 1. Nursing Standard. 28, 31, 46-51.
McKevitt F, Rowe J, Hadjivassiliou M (2007) Diseases of the nervous system and voluntary muscle. In Lim E, Loke YK, Thompson A (Eds) Medicine and Surgery. An Integrated Textbook. Churchill Livingstone Elsevier, Edinburgh, 571-634.
Jarman P (2012) Neurological disease. In Kumar P, Clark M (Eds) Kumar & Clark’s Clinical Medicine. Eighth edition. Saunders Elsevier, Edinburgh, 1067-1154.
Motor Neurone Disease Association (2014) Brief Guide to MND. tinyurl.
50 april 16 :: vol 28 no 33 :: 2014
com/oqran77 (Last accessed: March 27 2014.) NHS Choices (2013) Motor Neurone Disease. tinyurl.com/ptmc38p (Last accessed: March 27 2014.) 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.
Tortora GJ, Derrickson B (2012) Principles of Anatomy and Physiology. 13th edition. John Wiley and Sons, Hoboken NJ. Watson R (2011) Anatomy and Physiology for Nurses. 13th edition. Baillière Tindall Elsevier, Edinburgh. Waugh A, Grant A (2010) Ross and Wilson Anatomy and Physiology in Health and Illness. 11th edition. Churchill Livingstone Elsevier, Edinburgh.
© NURSING STANDARD / RCN PUBLISHING
Downloaded from RCNi.com by ${individualUser.displayName} on Nov 19, 2015. For personal use only. No other uses without permission. Copyright © 2015 RCNi Ltd. All rights reserved.
Nervous system: part 3 Farley A et al (2014) Nervous system: part 3 Nursing Standard. 28, 33, 46-50. Date of submission: August 6 2013; date of acceptance: November 25 2013.
Abstract This article, which forms part of the life sciences series and is the last of three articles on the nervous system, explores the major divisions of the peripheral nervous system. Motor and sensory nerves will be described before a more detailed examination of the cranial and spinal nerves is provided. The autonomic nervous system will be explored, including the diverse roles of the parasympathetic and sympathetic divisions. The role of spinal reflexes in maintaining homeostasis is identified. Motor neurone disease will be discussed briefly as an example of a disorder of the peripheral nervous system.
Authors Alistair Farley Retired, was lecturer in nursing, School of Nursing and Midwifery, University of Dundee, Dundee, Scotland. 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. Charles Hendry Retired, was senior lecturer, School of Nursing and Midwifery, University of Dundee. Correspondence to: [email protected]
Keywords Cranial nerves, motor neurone disease, peripheral nervous system, spinal nerves, spinal reflexes
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.
46 april 16 :: vol 28 no 33 :: 2014
THE CENTRAL NERVOUS system (CNS) is composed of the brain and spinal cord, while the remaining nervous tissue forms the peripheral nervous system. The peripheral nervous system is composed of cranial nerves, spinal nerves and the autonomic nervous system (Waugh and Grant 2010). Spinal nerves connect the body to the spinal cord and cranial nerves connect the body directly to the brain. Most of the nerves of the peripheral nervous system consist of sensory (afferent) nerve fibres that transmit impulses (information) from sensory organs to the brain. Efferent nerves transmit impulses from the brain to effector organs or tissues such as skeletal muscle (Waugh and Grant 2010). Efferent nerves can be divided into somatic nerves, which are involved in voluntary movement of skeletal muscle, and autonomic nerves, which are involved in cardiac and smooth muscle contraction (Tortora and Derrickson 2012).
Cranial nerves There are 12 pairs of cranial nerves, each of which is named according to its distribution or function. Cranial nerves arise from the inferior surface of the brain and are numbered in the order in which they connect with the brain from anterior to posterior (Patton and Thibodeau 2010). They may be sensory, motor or mixed nerves (Table 1).
Spinal cord An adult’s spinal cord is approximately 42-45cm long and 2cm wide in the mid-thoracic region (Jenkins and Tortora 2013). It extends from the foramen magnum in the base of the skull, to the first or second lumbar vertebra (L1-L2). The spinal cord ends at around L1 in a tapered cone known as the conus medullaris, from which many nerve roots (initial segment of a nerve leaving the CNS) extend to form the cauda equina, which resembles a horse’s tail (Patton and Thibodeau 2010). The diameter of the spinal cord decreases from the cervical region to the sacral region, with larger regions found where the limbs are innervated (Seeley et al 2008). The spinal cord is composed of 31 segments, each having a pair of dorsal and ventral roots that form a spinal nerve. Pairs of nerves are present throughout the length of the spinal cord (Watson 2011).
© NURSING STANDARD / RCN PUBLISHING
Downloaded from RCNi.com by ${individualUser.displayName} on Nov 19, 2015. For personal use only. No other uses without permission. Copyright © 2015 RCNi Ltd. All rights reserved.
In the spinal cord, grey matter (cell bodies) is found in columns and arranged in an H shape, with white matter (nerve pathways) in tracts surrounding it. A central canal runs down the middle of the spinal cord containing cerebrospinal fluid. The white matter is divided into columns running the length of the spinal cord, each of which contains a number of tracts or fasciculi. The ascending tracts carry sensory information from the body to the brain, while the descending tracts carry motor information to motor neurones in the spinal cord. Figure 1 shows a cross section of the spinal cord. The posterior (dorsal) horns of the H shape of the spinal cord are stimulated by sensory impulses travelling to the CNS from the body. The cell body of the sensory neurone lies outside the spinal cord in the posterior root ganglion. A ganglion is a group of nerve cell bodies that lie outside the CNS (Jenkins and Tortora 2013). The axon of a sensory neurone enters the posterior horn of the spinal cord where it synapses with another sensory neurone, the axon of which passes up the sensory pathways in the spinal cord to the brain.
The anterior (ventral) horn of the H shape of the spinal cord deals with motor information leaving the CNS and travelling to the body. Similarly, the axon of a motor neurone passes down the motor pathway in the spinal cord. At the appropriate level, it will synapse with another motor neurone in the anterior horn, and the axon of this neurone forms the motor nerve. The upper motor neurone has its cell body in the motor area of the cerebrum and carries motor information down to the lower motor neurones. The lower motor neurones are located in the anterior horn of the grey matter in the spinal cord.
Spinal nerves Spinal nerves connect the CNS with sensory receptors, muscles and glands throughout the body. There are 31 pairs of spinal nerves: eight cervical, 12 thoracic, five lumbar, five sacral and one coccygeal (Jenkins and Tortora 2013). Spinal nerves are surrounded by a series of protective connective tissue layers: the endoneurium (innermost layer), the perineurium, which surrounds each bundle of fibres, and the epineurium (outermost layer) (Waugh
TABLE 1 Classification and function of the cranial nerves Cranial nerve (number)
Classification
Function
Olfactory nerves (I)
Sensory
Provide the sense of smell. Impulses are conveyed to the primary olfactory area in the temporal lobe of the cerebral cortex.
Optic nerves (II)
Sensory
Send information from the eye to the diencephalon and onward to the visual area of the occipital lobe providing the sense of sight.
Oculomotor nerves (III)
Motor
Supply the superior, medial and inferior recti and inferior oblique muscles; the ciliary muscles; the circular muscles of the iris; and muscles that raise the upper eyelids, the levator palpebrae muscles.
Trochlear nerves (IV)
Motor
Innervate the superior oblique muscles of the eye.
Trigeminal nerves (V)
Mixed
The ophthalmic branch innervates the scalp and skin of the forehead; the maxillary branch innervates the upper lip and gums, teeth, cheeks, nose and lower eyelids; and the mandibular branch innervates the muscles of mastication.
Abducent nerves (VI)
Motor
Innervate the lateral rectus muscle of the eye.
Facial nerves (VII)
Mixed
Innervate the facial muscles (facial expression) and control secretion of saliva and tears, as well as sensory information from the taste buds along the anterior two thirds of the tongue.
Vestibulocochlear nerves (VIII)
Sensory
The cochlear branch provides the sensation of hearing and the vestibular branch provides the sensation of equilibrium.
Glossopharyngeal nerves (IX)
Mixed
Stimulate the posterior third of the tongue and the pharynx. Monitor blood pressure and respiratory gas levels in the blood.
Vagus nerves (X)
Mixed
Supply visceral structures such as the pharynx, larynx, trachea and heart and have diverse sensory functions such as taste, touch, pain and temperature.
Accessory nerves (XI)
Motor
Innervate muscles that control movement of the head and shoulders.
Hypoglossal nerves (XII)
Motor
Innervate the muscles of the tongue.
(Tortora and Derrickson 2012)
© NURSING STANDARD / RCN PUBLISHING
april 16 :: vol 28 no 33 :: 2014 47
Downloaded from RCNi.com by ${individualUser.displayName} on Nov 19, 2015. For personal use only. No other uses without permission. Copyright © 2015 RCNi Ltd. All rights reserved.
Art & science life sciences: 22 and Grant 2010). A typical spinal nerve has two connections with the spinal cord – an anterior and a posterior nerve root. The anterior nerve root contains motor nerve fibres and the posterior root contains sensory nerve fibres that combine at the intervertebral foramen to form a spinal nerve, therefore a spinal nerve can be considered a mixed nerve (Jenkins and Tortora 2013). On emerging from the intervertebral foramen, spinal nerves divide into branches or rami. A typical spinal nerve has anterior, posterior and communicating rami. The anterior rami innervate the anterior and lateral aspects of the neck, trunk and upper and lower limbs, while the posterior rami innervate the skin and muscles of the posterior of the trunk. The communicating rami are part of the autonomic nervous system (ANS).
Plexuses Plexuses are large masses of nerves. Several segments of the spinal cord are involved in the innervations of complex peripheral musculature, with nerves arising at different points along the spinal cord. Anterior roots intermingle to form a plexus. This intermingling means that damage to one spinal nerve does not cause total loss of function to a given region. There are five large plexuses of mixed nerves on the left and right sides of the vertebral column: the cervical plexus, brachial plexus, lumbar plexus, sacral plexus and coccygeal plexus (Waugh and Grant 2010) (Table 2). The thoracic nerves do not form plexuses (Waugh and Grant 2010).
Spinal reflexes The spinal cord has a role to play in maintaining homeostasis by acting as an integrating centre for some reflexes (Tortora and Derrickson 2012). A reflex is an immediate, involuntary motor response to a particular stimulus (Waugh and Grant 2010). In a spinal reflex, the processing events take place in the spinal cord rather than involving the brain, however individual reflexes may vary in their complexity. The simplest reflex exists where sensory neurones synapse directly with motor neurones; these are referred to as monosynaptic reflexes. An example of a simple reflex is the response that occurs when a person touches a very hot surface – the withdrawal reflex. Even before the person is conscious of the heat, the spinal reflex causes the hand to be removed from the heat source, thus minimising the risk of harm. Polysynaptic spinal reflexes have more complex pathways and consist of additional elements such as integrating centres and interneurones. Interneurones are relay neurones found between the sensory and motor neurones, and often have an inhibitory function (Seeley et al 2008). In an intersegmental reflex, more than one segment of the spinal cord will be involved.
Autonomic nervous system The nervous system receives information from sensory nerves. This information travels to the hypothalamus and is the basis for the response of the ANS. While somatic nerves are motor nerves that serve structures under voluntary or conscious
FIGURE 1 Cross section of the spinal cord Posterior horn
Posterior root of spinal nerve
Posterior root ganglion
Central canal
Spinal nerve
Axon of sensory neurone
Sensory information
Anterior root of spinal nerve Anterior horn
Nerve impulses Axon of motor neurone
48 april 16 :: vol 28 no 33 :: 2014
PETER LAMB
White matter
© NURSING STANDARD / RCN PUBLISHING
Downloaded from RCNi.com by ${individualUser.displayName} on Nov 19, 2015. For personal use only. No other uses without permission. Copyright © 2015 RCNi Ltd. All rights reserved.
control, the ANS contains motor nerves that supply structures under involuntary control. The ANS exerts its effects on smooth muscle, cardiac muscle and glands (Waugh and Grant 2010). It is subdivided into the sympathetic nervous system and the parasympathetic nervous system. These two divisions tend to exert opposing effects, but overall work together to achieve homeostasis. A summary of the characteristics of the ANS is outlined in Table 3. Sympathetic activity tends to predominate in stressful situations, while parasympathetic activity is most obvious during rest. Examples of sympathetic activity include (Waugh and Grant 2010): Bronchodilation. Increased heart rate and force of contraction. Dilation of pupils. Examples of parasympathetic activity include (Waugh and Grant 2010): Bronchoconstriction. Decreased heart rate and force of contraction. Constriction of pupils. Increased gastrointestinal activity. The autonomic pathway, the nerve pathway associated with the ANS, consists of two motor neurones. The first has its cell body in the CNS and is known as the preganglionic neurone. It synapses with a ganglion outside the spinal cord. The second motor neurone, the postganglionic neurone, leaves the ganglion and synapses with the target organ. The cell bodies for the sympathetic division are located in the spinal cord at T1-L2/L3. The axons for these neurones lead to a chain of ganglia situated just outside the vertebral column, where they synapse with a second neurone leading to the target organ. The cell bodies for the parasympathetic division are found in the spinal cord at S2-S4 (Seeley et al 2008). The long axons of these neurones lead to ganglia in the walls of the target organ where they synapse with the second neurone leading into the organ.
Neurotransmitters
There are a number of neurotransmitters acting in the ANS. A neurotransmitter is a chemical that transmits signals from one neurone to another or to a target cell, across a synapse (Farley et al 2014). In the sympathetic nervous system, the neurotransmitter acetylcholine is released between preganglionic and postganglionic neurones, while the neurotransmitter noradrenaline (norepinephrine) is released between postganglionic neurones and target organs. In the parasympathetic nervous system, the neurotransmitter acetylcholine is released between preganglionic and postganglionic neurones and
© NURSING STANDARD / RCN PUBLISHING
between postganglionic neurones and target organs (Jenkins and Tortora 2013).
Regulation
The ANS is regulated by the hypothalamus, which receives input from sensory nerves and from many other parts of the brain, for example the higher centres in the frontal lobe (Jenkins and Tortora 2012).
Motor neurone disease Motor neurone disease involves progressive degeneration of upper and lower motor neurones in the brain and spinal cord, affecting muscle activity such as speaking, swallowing, walking and general movement of the body. It is rare, with about two in 100,000 people in the UK developing the disease each year and with a prevalence of between seven and ten cases in 100,000 people (McKevitt et al 2007, Jarman 2012, Motor Neurone Disease Association
TABLE 2 Functions of plexuses Plexus
Function
Cervical
Sensory nerves supply the scalp, ears, chest, neck and shoulders; motor nerves supply the neck muscles and diaphragm.
Brachial
Sensory and motor nerves innervate the shoulders and upper limbs.
Lumbar
Sensory and motor nerves innervate the lower abdomen, external genitals and part of the lower limbs.
Sacral
Sensory and motor nerves innervate the buttocks, perineum and lower limbs.
Coccygeal
Sensory and motor nerves innervate the skin around the coccyx and anal area.
TABLE 3 Characteristics of the autonomic nervous system Characteristic
Sympathetic division
Parasympathetic division
Point of origin
Thoracolumbar
Craniosacral
General effects
Functions adapted for stressful situations
Functions adapted for resting situations
Neurotransmitters
Acetylcholine Noradrenaline (norepinephrine)
Acetylcholine
Position of ganglia
Outside but close to the central nervous system
Inside or close to the effector structure
Length of fibres
Short preganglionic Long postganglionic
Long preganglionic Short postganglionic
april 16 :: vol 28 no 33 :: 2014 49
Downloaded from RCNi.com by ${individualUser.displayName} on Nov 19, 2015. For personal use only. No other uses without permission. Copyright © 2015 RCNi Ltd. All rights reserved.
Art & science life sciences: 22 2014). However, the effect of the disease is significant and is usually fatal, with death occurring within three to five years, although some patients may live up to ten years or even longer from diagnosis (Ballinger 2012, NHS Choices 2013). It typically presents in middle age and is more common in men (Jarman 2012). In motor neurone disease, the exact pattern of the nerves affected and the rate of degeneration is variable and clinical features will differ between individuals. Because degeneration is progressive, symptoms will worsen over time. There is no cure for motor neurone disease, and patients will need to be treated symptomatically. Riluzole may be given, but it only extends survival for people with motor neurone disease by on average two or three months and does not prevent the disease progressing (NHS Choices 2013). Four broad clinical patterns can be seen, as identified in Table 4, although the features may merge as the disease progresses. None of the clinical types include involvement of the sensory system, nor of the sensory or motor nerves of the eyes (Ballinger 2012). Diagnosis is largely by history and examination since there are no specific tests for motor neurone disease, although the
TABLE 4 Clinical patterns of motor neurone disease Type
Features
Amyotrophic lateral sclerosis
Muscle wasting, weakness, fasciculations, speech and swallowing problems, and muscle spasms.
Progressive muscular atrophy
Weakness, fasciculations, and wasting in the hands and arms.
Progressive bulbar palsy
Dysarthria, dysphagia, nasal regurgitation of fluids and choking.
Primary lateral sclerosis
Progressive tetraparesis.
clinician may order various tests to rule out other causes of presenting symptoms such as cervical spine lesion or bulbar myasthenia gravis.
Conclusion In this concluding article on the nervous system, the focus has been on the peripheral nervous system. Understanding the structure and function of the nervous system will enable the nurse to better care for patients who may be experiencing neurological deficits. While many patients with such deficits may be cared for in specialised units, many others will be cared for in general medical and surgical wards. Similarly, community nursing staff are increasingly caring for patients with long-term neurological deficits such as motor neurone disease NS
POINTS FOR PRACTICE In motor neurone disease, swallowing may be affected. If so, how would you assess and assist the patient in the maintenance of adequate fluid and nutrient intake? Disorders such as motor neurone disease lead to life-limiting and life-changing consequences for patients and significant others. In your area of practice, identify suitable resources that you may access to assist you in caring for a patient with motor neurone disease.
GLOSSARY Dorsal Pertaining to the posterior surface or portion of a named tissue or structure. Plexus A large mass of nerves. Ventral Pertaining to the anterior surface or portion of a named tissue or structure.
References Ballinger A (2012) Essentials of Kumar & Clark’s Clinical Medicine. Fifth edition. Saunders, Edinburgh.
Jenkins G, Tortora GJ (2013) Anatomy and Physiology: From Science to Life. Third edition. John Wiley and Sons, Hoboken NJ.
Farley A, Johnstone C, Hendry C, McLafferty E (2014) Nervous system: part 1. Nursing Standard. 28, 31, 46-51.
McKevitt F, Rowe J, Hadjivassiliou M (2007) Diseases of the nervous system and voluntary muscle. In Lim E, Loke YK, Thompson A (Eds) Medicine and Surgery. An Integrated Textbook. Churchill Livingstone Elsevier, Edinburgh, 571-634.
Jarman P (2012) Neurological disease. In Kumar P, Clark M (Eds) Kumar & Clark’s Clinical Medicine. Eighth edition. Saunders Elsevier, Edinburgh, 1067-1154.
Motor Neurone Disease Association (2014) Brief Guide to MND. tinyurl.
50 april 16 :: vol 28 no 33 :: 2014
com/oqran77 (Last accessed: March 27 2014.) NHS Choices (2013) Motor Neurone Disease. tinyurl.com/ptmc38p (Last accessed: March 27 2014.) 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.
Tortora GJ, Derrickson B (2012) Principles of Anatomy and Physiology. 13th edition. John Wiley and Sons, Hoboken NJ. Watson R (2011) Anatomy and Physiology for Nurses. 13th edition. Baillière Tindall Elsevier, Edinburgh. Waugh A, Grant A (2010) Ross and Wilson Anatomy and Physiology in Health and Illness. 11th edition. Churchill Livingstone Elsevier, Edinburgh.
© NURSING STANDARD / RCN PUBLISHING
Downloaded from RCNi.com by ${individualUser.displayName} on Nov 19, 2015. For personal use only. No other uses without permission. Copyright © 2015 RCNi Ltd. All rights reserved.
