P. VNormal Atrioventricular septum defect
16.3.4 The role of subcortical structures in
The voluntary control of muscle action is still not fully understood.
Electrical stimulation of M1 produces much simpler movements than normal voluntary movements under conscious control, indicating that many more areas of the cortex are involved in motor activity. Closed motor loops between the motor areas, basal ganglia, cerebellum, and thalamus play an important role in initiation and coordination of complex movement sequences although their precise functions are still being clarifi ed.
One closed loop originates in the SMA which projects to the basal ganglia and these in turn project back via the thalamus to the SMA.
Another loop originates in the motor areas and travels with the corticos- pinal and corticonuclear pathways. These pathways give off collateral axons that synapse in the pontine nuclei; the pontine nuclei project to
the cerebellum that in turn sends neuronal processes back to M1 and PM via the thalamus.
The cerebellum
The cerebellum plays an important part in the maintenance of equilibrium and posture and the control of muscle tone and coordination of muscle activity. Through its connections with the motor cortical areas, the cerebel- lum ensures that voluntary movements are carried out smoothly and pre- cisely in the correct sequence, using the necessary rate, range, and force of muscle contractions to execute a given movement. It also plays a vital part in learning motor skills and probably has some role in cognitive activities.
The cerebellum receives inputs from:
• The vestibular, visual, and auditory system via the midbrain colliculi;
• The proprioceptive endings in muscles, tendons, and joints that supply
information about the state of muscle contraction and joint position
Box 16.4 Upper and lower motor neuron injuries
Disorders of movement take many forms. The simplest example is that produced by a lesion aff ecting lower motor neurons . These neurons are the fi nal common pathway through which information from all levels of the CNS is transmitted to the muscles. A lower motor neuron lesion eff ectively denervates the associated muscles, producing fl accid paralysis characterized by muscle fl accidity and absent tendon refl exes. If lower motor neurons are damaged beyond repair and do not regenerate, after about 6 months the muscles begin to atrophy, producing muscle wasting . Muscular wasting may produce cosmetic defects as well as function defi cits if the aff ected muscle is superfi cial. In the longer term, after about a year, denervated muscles degenerate and are replaced by fi brous tissue. The fi brous tissue shrinks by about 20% of its length as it forms. This has the equivalent eff ect of the muscle contracting by the same length and produces a fi xed contracture of the muscle.
Contracture is not only functionally debilitating, but can cause seri- ous cosmetic problems, especially in areas such as the face where contracture is obvious.
Upper motor neuron lesions produce absence of voluntary move- ment with increased muscle tone ( spastic paralysis ) and enhanced tendon refl exes ( hyperrefl exia ) or weak voluntary movements of the muscles ( paresis ); wasting of the muscles is not marked. Den- tists are most likely to encounter patients with upper motor neuron lesions due to the eff ects of a CVA (a ‘stroke’) disrupting the blood supply to the lateral motor pathways; CVAs occurring in the poste- rior limb of the internal capsule often aff ect these pathways. After the initial eff ects wear off , the victim is left with some degree of spastic paralysis and hyperrefl exia on the opposite side of the body to which the lesion occurred due to the crossing of corticospinal pathways. The fl exor muscles are stronger than the extensors in the upper limb while the reverse is true in the lower limb. If spastic paral- ysis aff ects the lower limb, a characteristic gait is produced in which the leg is swung forwards with the knee and ankle joint extended; in
other words, the leg is kept straight. If the arm is aff ected, it is carried with the elbow fl exed and the forearm held across the chest and the hand tightly bunched into a fi st.
The reason that spastic paralysis is observed in upper motor neuron lesions is because proprioceptive information from muscle spindles in the aff ected muscles can still reach the spinal cord through the intact peripheral spinal nerve and form refl ex connections with the lower motor neurons. However, this information cannot be modi- fi ed by inputs from higher centres in the brain; the unmodifi ed prop- rioceptive information produces the characteristic spastic paralysis and hyperrefl exia.
It was, at one time, thought that upper motor neuron lesions were caused specifi cally by interruption of the lateral motor pathways.
However, experiments in animals have indicated that damage to these pathways only produces reduced tone in the contralateral muscles with little or no eff ect upon the tendon refl exes. The prin- cipal defi cit appears to be clumsiness in the use of the contralat- eral limbs. Lesions limited to the corticospinal pathways are rare in humans; clinical observations suggest that they produce defi cits similar to those in experimental animals when they do occur. This has led to the view that the principal function of the corticospinal and corticobulbar pathways is in the control of highly skilled move- ments. It is now known that upper motor neuron lesions also involve corticoreticular pathways which, although functionally part of the medial motor pathways, travel from the motor cortex to the brain- stem reticular nuclei alongside the lateral motor pathways. Corti- coreticular pathways are, therefore, also involved in lesions such as internal capsule CVA. One function of reticulospinal pathways is to act to inhibit spinal refl exes. When this action is lost, the result is hyperrefl exia and spasticity.
The outcome from upper motor neuron lesions aff ecting the cor- ticonuclear pathways is somewhat diff erent and is described in Box 18.4 .
Motor pathways 151
via the thalamus and to the red nuclei; both routes aff ect motor output through the lateral motor pathways.
The lateral parts of each cerebellar hemisphere form the cerebrocer- ebellum (or neocerebellum). Once again, the name of this area indi- cates where it receives its major inputs from; they are from the cerebral cortex via the pontine nuclei. Note that inputs via this route are not restricted to motor information; there is a considerable input from cer- ebral areas concerned with cognitive activity. The output is to the motor areas of the cerebral cortex via the thalamus and to lateral motor path- ways via the red nuclei. The output also goes to cortical areas involved in cognitive functions.
Essentially, the cerebellum computes information from many sensory sources and compares it with the intentions of the motor systems. The actions of the motor system at all levels from spinal activity to motor cortex can be modifi ed to correct errors of specifi c movements. Over a longer period, these mechanisms operate during learning of new motor tasks.
Box 16.5 describes the eff ects of disease and trauma on cerebellar function.
The basal nuclei (basal ganglia)
The precise functions of the basal nuclei and their connections are still being discovered, but clinical fi ndings show that they play essential roles in the control of motor activity. As indicated above, the basal nuclei are one of a group of structures connected to the cortical motor areas through closed motor loops. In this case, inputs from the motor cortical areas enter the basal nuclei and then pass back via the thalamus to the motor areas.
The basal nuclei comprise the caudate and lentiform nuclei in the deep parts of each cerebral hemisphere (see Figure 15.20 and 15.21), the subthalamic nuclei in the diencephalon, and the substantia nigra in the midbrain. There are several complex interconnections between the cerebral cortex and diff erent components of the basal nuclei, creating several separate loops. Two of these loops are directly involved with through spinocerebellar pathways and corresponding pathways from
cranial nerves; these pathways also carry other sensory information, including discriminative touch;
• The cerebral motor cortex via the pontine nuclei ;
• A relatively small, but nevertheless important, input is from the infe-
rior olivary nuclei which receives inputs from the red nuclei, motor cortex, and spinal cord.
By computing the information from all these sources, the cerebel- lum feeds back to motor areas of the brain and into both the lateral and medial motor pathways to ensure smooth coordinated muscular activity.
Three functional parts of the cerebellum can be distinguished by their connections, activities, and their order of appearance in the evo- lutionary scale.
The evolutionary oldest part is the vestibulocerebellum (or arche- ocerebellum) which occupies a small area known as the fl occulonodular lobe. As its name suggests, it receives direct inputs from the vestibular apparatus in the inner ear and sends outputs to the vestibular nuclei in the brainstem from which the vestibulospinal tracts arise; these are part of the medial pathways as described on p. 147 . It also connects with reticulospinal components of the medial pathways.
The next oldest part is the spinocerebellum (or paleocerebellum) which occupies the midline area of the cerebellar cortex. As might be anticipated from its name, this area receives inputs from dorsal spinoc- erebellar tracts carrying proprioception and touch sensations and ven- tral spinocerebellar tracts conveying information from spinal motor circuits. It also receives inputs from several other sources, including the vestibular apparatus, visual and auditory pathways, and a massive input from the pontine nuclei carrying information from the motor and sen- sory cortices. This is also the area that receives inputs from the inferior olivary nuclei. The output from the spinocerebellum operates via two routes. One goes to the vestibular nuclei and hence to the medial motor pathways. The other output projects back to the motor cortical areas
Box 16.5 Cerebellar dysfunction
There are many causes of cerebellar dysfunction, ranging from tumours or other space-occupying lesions, traumatic head injuries, degenerative diseases such as multiple sclerosis or Friedrich’s ataxia ( Box 16.3 ) that aff ects spinocerebellar tracts and the cerebellum specifi cally. Alcoholic intoxication interferes with cerebellar func- tion and we have all probably observed or may be experienced these short-term eff ects at some time. An inebriated person has dif- fi culty standing, walks with a wide-based staggering gait, has slurred speech, and often ‘misses’ when trying to perform actions such as putting a key into a lock. These signs and symptoms sum up the major eff ects of cerebellar disease or damage.
Most of the causes of cerebellar damage listed above aff ect the whole cerebellum, but localized tumours give insight into the eff ects of disease and damage on the subdivisions of the cerebel- lum. Vestibulocerebellar damage is responsible for the staggering
gait ( cerebellar ataxia ) and a rapid side-to-side eye movement ( nystagmus ). A patient with spinocerebellar damage has extreme diffi culty with walking and standing, thus keeps falling. They also cannot coordinate the complex motor activities required for speech which becomes slurred as a result ( dysarthria ). Another manifes- tation of spinocerebellar dysfunction is intention tremor, large amplitude shaking when a movement is attempted, resulting in
‘misses’ during motor activity. Damage to the cerebrocerebellum causes delay and overshoot of movement that is most noticeable with complex movement involving several joints.
Bear in mind that in the majority of cases involving cerebellar dys- function, all components of the cerebellum are aff ected so the patient is likely to show many of the signs and symptoms described above. Charcot’s triad of nystagmus, dysarthria, and intention tremor is strongly indicative of disease aff ecting cerebellar function.
152 Major sensory and motor systems
motor function; one is involved in the overall control of motor activ- ity and the other is specifi cally concerned with eye movements. The other loops are involved in emotion, memory, and cognitive functions.
The numerous interconnections of the individual basal nuclei with each other and with other areas of the brain indicate that much integrative activity occurs in these structures.
The closed motor loop concerned specifi cally with movement has two parallel circuits that involve neurons originating from the
substantia nigra that use dopamine as their neurotransmitter.
These act on two populations of neurons that look identical, but carry two diff erent dopamine receptors; one receptor excites the cells whereas the other inhibits them. It appears that the excita- tory loop amplifi es movement intentions signalled from the motor cortex whereas the other circuit inhibits extraneous unintended movement. The eff ects of damage or disease on the basal nuclei are described in Box 16.6 .
Box 16.6 Basal nuclei dysfunctions
The functions of the basal nuclei are not fully understood, but they appear to amplify the intended movement while suppressing extra- neous movement. Involuntary movements ( dyskinesia ), abnormal motor activity, and alterations in muscle tone characterize lesions of the basal nuclei and related nuclei, usually due to degenerative diseases. Dyskinesia includes intermittent, purposeless twitching of individual muscles ( choreiform movements ), slow sinuous writhing most obvious in the limbs ( athetoid movements ), or rapid short amplitude movements of the limb extremities at rest ( tremors ). These involuntary movements are believed to arise as a result of the inhibitory pathways in the basal ganglia failing to oper- ate effi ciently to suppress unwanted movement. Abnormalities of
movement usually show as slowness ( bradykinesia ) or weak and incomplete actions known as poverty of movement or hypoki- nesia because the intended movement is not amplifi ed.
Parkinson’s disease is due to progressive degeneration in the sub- stantia nigra. It produces tremor in the extremities, most noticeably the hands, accompanied by poverty of movement and muscular rigidity. The patient has a shuffl ing gait, stooped posture, lack of facial expression, and often fails to complete verbal statements because of the hypokinesia. Huntington’s disease is progressive degeneration of the basal ganglia, usually the caudate nucleus and putamen, characterized by choreiform movement and progressive dementia.
17
The autonomic nervous system
Chapter contents
17.1 Introduction 154
17.2 Visceral motor neurons 154
17.3 Visceral sensory neurons 157
154 The autonomic nervous system
Visceral motor neurons innervate smooth muscle and secretory cells of the gastrointestinal and respiratory systems, the smooth and cardiac muscle of the cardiovascular system, the sweat glands and arrector pili muscles of the skin, and the muscles of the ciliary body and iris of the eyeball. In many cases, there is a dual supply from the sympathetic and parasympathetic divisions of the autonomic nervous system.
In both divisions of the autonomic nervous system, there is a sequence of two neurons between the CNS and the eff ector organ which synapse in peripheral autonomic ganglia. The neurons from the CNS to the synapse in the ganglion are the preganglionic neu- rons and those from the ganglia to the eff ector organs are the post- ganglionic neurons . The enteric plexus is a third set of neurons interposed between the post-ganglionic neurons and the eff ector cells in the gastrointestinal tract.
Figure 17.1 compares the general arrangement of the sympathetic and parasympathetic nervous system. The cell bodies of sympathetic visceral preganglionic motor neurons are located in the intermedi- olateral horns of the thoracic and upper lumbar segments of the spinal cord while those of the parasympathetic visceral preganglionic (secre- tomotor) neurons are in the nuclei of four of the cranial nerves and the sacral segments of the spinal cord.
The functional eff ects of the two divisions of the autonomic system on a target tissue or organ are antagonistic; under normal circumstances, there is a balance between them, which maintains an appropriate level of visceral activity to maintain bodily homeostasis . In states of alarm or anger, there is a massive stimulation of the sympathetic outfl ow which completely overrides the parasympathetic eff ects and results in the body being placed in a state of activity suitable for violent physical activity, the so-called ‘fi ght or fl ight reaction’ . This eff ect is backed up and prolonged by increased secretion of adrenalin from the adrenal medulla. Excess levels of adrenalin may cause fainting as outlined in Box 17.2.
As indicated in Figure 17.1 , there are also pharmacological diff erences between the two components of the autonomic nervous system. Acety- lcholine is the neurotransmitter in the synapses between pre- and post- ganglionic neurons in both divisions. The same neurotransmitter is used at the synapses between post- ganglionic parasympathetic neurons and the target organs. The neurotransmitter between post-ganglionic sympathetic neurons and their targets is noradrenalin; those to sweat glands are, however, cholinergic. Many pharmacological agents can interfere with autonomic neurotransmission as described in Box 17.1.