A Physiological Approach to Clinical Neurology

James W. Lance, James G. McLeod

• Publisher: Butterworth-Heinemann
• Year: 2013
• Print ISBN: 9780407001961
• eBook ISBN: 9781483163314
• Edition: 3
• Topic: Medicine
• Subtopic: Clinical Medicine

A Physiological Approach to Clinical Neurology, Third Edition is a 13-chapter book that first describes pain and other sensations, weakness, neuromuscular disorders, spinal reflexes, as well as muscle tone and movement. This text also explores the disordered muscle tone, a term used to describe the sensation of resistance felt by the clinician as he manipulates a joint through a range of movement with the subject attempting to relax. Other chapters discuss the basal ganglia and their disorders and the cerebellum and its disorders. The anatomy and physiology of the special senses, cranial nerves, and autonomic nervous system are also explained. The last three chapters elucidate consciousness, unconsciousness, epilepsy, and the relationship between brain and mind. This book will be useful to those in the field of clinical neurology.

1

Pain and other sensations

Publisher Summary

The nervous system of a normal individual is constantly active in conveying information to the brain about the state of the body and of the world outside it. The perception of any sensation, therefore, depends not only on the appropriate receptor organ in skin, muscle, joint, or viscus and the integrity of the peripheral nerve and spinal cord pathways but also on complex connections within the cerebral cortex that may be influenced by the thoughts and emotions of the subject. Pain is the most consistently unpleasant symptom that the nervous system can provide and may signal a disorder in any part of the body through irritation or distortion of sensory end organs or may arise from the disease of the sensory pathways at any level from end organ to cortex. Pain is often associated with an emotional change so that it may be hard to determine as to which is primary and which is secondary. There are two main types of nociceptor in the skin, mechanical and thermal. Mechanical nociceptors respond to pricking, pinching, or squeezing of the skin; thermal nociceptors respond to high and low temperatures. The viscera are insensitive to touching, cutting, or pinching but give rise to the sensation of pain, given an adequate stimulus such as distention, excessive contraction, or irritation by toxins and chemicals. Visceral pain is transmitted by the afferent fibres in the sympathetic nervous system from the thoracic and abdominal cavities or the sacral parasympathetic nerves from the pelvis.

The nervous system of a normal individual is constantly active in conveying information to the brain about the state of the body and of the world outside it. If all these neuronal messages were received in equal measure, consciousness would become a nightmare of confused and largely irrelevant stimuli, so that a selective response would become impossible. Fortunately, there are various physiological processes which speed the passage of pertinent stimuli and retard awareness of the background activity. We thus become oblivious to the touch of clothes, the pressure of a hard seat and the functioning of contented viscera. The processes involved in this selectivity of sensations are as follows.

(1) Adaptation of sensory end organs, which cease to respond after variable periods of stimulation.

(2) Presynaptic inhibition of adjacent nerve cells by collaterals from an active nerve cell, thus assuring priority for ‘the stimulus of the moment’¹¹. This process probably takes place at all levels of the nervous system, thus repeatedly ‘refining’ the impulses representing a particular sensation, or, in electronic jargon, ensuring ‘a high signal-to-noise ratio’.

(3) Regulation of synaptic transmission in sensory nuclei from the motor cortex by pyramidal tract fibres which send collaterals to the cuneate, gracile and trigeminal nuclei, and to the ventrobasal thalamus⁴³. This provides a mechanism for the voluntary suppression of sensory information or for involuntary suppression during movement.

(4) Alteration in the state of awareness at a cortical or subcortical level. A subject, while fully conscious, may so concentrate his attention on a particular sensation, thought or response as to preclude perception of other sensations.

The perception of any sensation therefore depends not only on the appropriate receptor organ in skin, muscle, joint or viscus, and the integrity of the peripheral nerve and spinal cord pathways, but also on complex connections within the cerebral cortex which may be influenced by the thoughts and emotions of the subject. Thus sensation is subjective and each individual has his own ‘perceptual world’ which is unique to him and can be known to others solely by his description of it. A certain stimulus may be registered by some as pleasant, by others as unpleasant but tolerable, and by others as so uncomfortable that they use the term ‘pain’ to describe it. Each person may therefore be regarded as having a ‘pain threshold’, and if the level of sensory stimulation exceeds this, pain is experienced.

When the normal functioning of the body is disturbed, sensory impulses of unusual quantity, quality or pattern are received by the brain, and the resulting ‘sense data’ are expressed by the subject as ‘symptoms’.

SENSORY SYMPTOMS

Symptoms bring the patient to the doctor. It is part of the art of medicine to record the patient’s symptoms accurately and to interpret them in the light of the patient’s intellectual and educational endowment, his personality and his emotional state.

Symptoms may be negative in that the patient complains of numbness or inability to feel touch, pain, temperature or position of the limbs. Symptoms may also be positive, providing curious abnormal sensory experiences (paraesthesiae).

Ischaemia or irritation of peripheral nerves or the central projection of touch pathways gives rise to pain or to prickling sensation described as ‘pins and needles’ or the arm or leg ‘going to sleep’. For example, compression of the lateral cutaneous nerve of the thigh in the inguinal ligament produces a curious creeping feeling in the outer aspect of the lower thigh which has been likened to the sensation of ants crawling under the skin (formication).

A disturbance within the posterior root entry zone or posterior columns of the spinal cord, or pressure upon them, may be responsible for a girdle sensation around the trunk, described as a tight band, or a feeling of pressure in the limbs as though they were being wrapped by a bandage. Sudden flexion of the neck may induce an electric shock sensation which shoots down the back when there is a cervical lesion irritating the posterior columns. This phenomenon (Lhermitte’s sign) is found most commonly in cervical spondylosis and multiple sclerosis. A lesion in the spinothalamic tracts or thalamus produces an unpleasant burning sensation or pain which spreads diffusely down the opposite side of the body.

Irritation or ischaemia of the sensory cortex evokes paraesthesiae, which may spread rapidly over the contralateral side in epilepsy and transient ischaemic attacks, or advance more slowly when caused by migrainous vasospasm. Disturbance of the sensory association areas in the parietal lobe may give rise to weird illusions of the body image so that parts of the body appear larger or smaller than normal.

Pain is the most consistently unpleasant symptom which the nervous system can provide and may signal a disorder in any part of the body through irritation or distortion of sensory endorgans, or may arise from disease of the sensory pathways at any level from endorgan to cortex.

Pain is often associated with an emotional change so that it may be hard to determine which is primary and which secondary. In spite of all the complexities of the individual reaction to pain, it is usually possible to analyse the description of the pain so as to determine its site of origin and often its cause.

THE PERCEPTION OF DIFFERENT KINDS OF SENSATION

Cutaneous nerves and sensory receptors

Human cutaneous nerves contain myelinated fibres which range in diameter from about 1 to 16 μm, and unmyelinated fibres which are less than 2 μm in diameter. The myelinated fibres are designated A fibres and are subdivided into Aα,β (6–16 μm) and Aγ,δ (2–6 μm) groups. The unmyelinated fibres are C fibres. All the fibres have their cell bodies in the dorsal root ganglia, and they terminate peripherally in skin and subcutaneous structures. The sensory receptors in the skin may be encapsulated endings of nerves such as Pacinian corpuscles and Meissner’s corpuscles; specialized free endings, such as Merkel’s discs; and simple free endings. The encapsulated endings are concentrated in areas of the body which are particularly sensitive-the tips of the fingers, the lips, the areola of the breast, and the genitalia. There now seems little doubt that many cutaneous receptors display stimulus specificity and include...

Table of contents

• Cover image
• Title page
• Table of Contents
• Inside Front Cover
• Copyright
• Preface to the Third Edition
• Preface to the First Edition
• Acknowledgements
• Chapter 1: Pain and other sensations
• Chapter 2: Weakness
• Chapter 3: Neuromuscular disorders
• Chapter 4: Spinal reflexes
• Chapter 5: Muscle tone and movement
• Chapter 6: Disordered muscle tone
• Chapter 7: The basal ganglia and their disorders
• Chapter 8: The cerebellum and its disorders
• Chapter 9: The special senses and cranial nerves
• Chapter 10: Autonomic nervous system
• Chapter 11: Consciousness and unconsciousness
• Chapter 12: Epilepsy
• Chapter 13: The relationship between brain and mind
• Index