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Neurological Assessment and Monitoring 423

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the medulla. The pain system crosses low, in the spinal nicotinic acetylcholine receptors). White matter at the
cord. The proprioceptive sensory system that guards anterior of the midbrain conducts impulses between the
balance and position goes to the cerebellum, which works higher centres of the cerebrum and the lower centres of
ipsilaterally and therefore doesn’t cross. Almost every the pons, medulla, cerebellum and spinal cord. The mid-
region of the body is represented by a corresponding brain contains the autonomic reflex centres for pupillary
region in both the primary motor cortex and the somatic accommodations to light, which constrict the pupil and
sensory cortex. 14 accommodate the lens. The fibres travel in cranial nerve
The homunculus (see Figure 16.6) visualises the connec- III, so damage to that nerve will also produce a dilated
tion between different areas of the body and areas in pupil. It also contains the ventral tegmental area, packed
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brain hemispheres. The body on the right side is the with dopamine-releasing neurons that synapse deep
motor homunculus and on the left the sensory homun- within the forebrain and seem to be involved in pleasure:
culus. Representations of parts of the body that exhibit amphetamines and cocaine bind to the same receptors
fine motor control and sensory capabilities occupy a that it activates, and this may account at least in part for
greater amount of space than those that exhibit less their addictive qualities.
precise motor or sensory functions. The medulla oblongata lies between the pons and the
spinal cord and looks like a swollen tip to the spinal cord.
Basal ganglia and cerebellum Running down the ventral aspect of the medulla are the
The basal ganglia, consisting of the caudate, putamen, pyramids, which contain corticospinal fibres. The func-
globus pallidus, substantia nigra, subthalamic nucleus, tion of the medulla oblongata is to control automatic
and related nuclei in the brainstem, play an important functions (e.g. breathing and heart rate) and to relay
role in movement, as evidenced by the hypokinetic/rigid nerve messages from the brain to the spinal cord. Process-
and hyperkinetic disorders seen with lesions of various ing of interaural time differences for sound localisation
components. However, their role in the initiation and occurs in the olivary nuclei. The neurons controlling
control of movement cannot be isolated from the motor breathing have mu (µ) receptors, the receptors to which
activities of the cortex and brainstem centres discussed opiates bind. This accounts for the suppressive effect of
previously. Procedural memories for motor and other opiates on breathing. Impairment of any of the vital func-
unconscious skills depend on the integrity of the premo- tions or reflexes involving these cranial nerves suggests
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tor cortex, basal ganglia and cerebellum. The cerebel- medullary damage. 19
lum plays a more obvious role in coordinating movements
by giving feedback to the motor cortex, as well as by The pons varolii is the part of the brainstem that lies
providing important influences on eye movements between the medulla oblongata and the mesencephalon.
through brainstem connections, and on postural activity It contains pneumotaxic and apneustic respiratory centres
through projections down the spinal cord. and fibre tracts connecting higher and lower centres,
including the cerebellum. The pons seems to serve as a
Brainstem relay station, carrying signals from various parts of the
The brainstem is composed of the midbrain, the pons cerebral cortex to the cerebellum. Nerve impulses coming
from the eyes, ears and touch receptors are sent on to the
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and the medulla oblongata. These structures connect the cerebellum. The pons also participates in the reflexes that
cerebrum and diencephalon with the spinal cord. Brain- regulate breathing. Table 16.4 contains a description of
stem centres are organised into medial, lateral and amin- the cranial nerves including their type of tract, their func-
ergic systems. Collectively, these integrate vestibular, tion and location of origin.
visual and somatosensory inputs for the control of eye
movements and, through projections to the spinal cord, Hypothalamus and limbic system
provide for postural adjustments. For example, these The hypothalamus, the cingulate gyrus of the cortex,
centres keep the images on matching regions of the the amygdala and hippocampus in the temporal lobes,
retinas when the head moves by causing conjugate eye and the septum and interconnecting nerve fibre tracts
movements in the opposite direction to which the head among these areas comprise the limbic system. The
is turned. This is the basis for the ‘doll’s eyes’ test in neu- hypothalamus and limbic systems, which are closely
rological assessment, in which the head is rapidly turned linked to homeostasis, act to regulate endocrine secre-
and the eyes move conjugately in the opposite direction, tion and the autonomic nervous system, and to influence
demonstrating the integrity of much of the brainstem. behaviour through emotions and drives. The hypothala-
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The sequence of sleep states is governed by a group of mus integrates information from the forebrain, brain-
brainstem nuclei that project widely throughout the brain stem, spinal cord and various endocrine systems. This
and spinal cord. 17 area of the brain also contains some of the centres for
The midbrain, inferior to the centre of the cerebrum, coordinated parasympathetic and sympathetic stimula-
forms the superior part of the brainstem. It contains the tion, as well as those for temperature regulation, appetite
reticular formation (which collects input from higher regulation, regulation of water balance by antidiuretic
brain centres and passes it on to motor neurons), the hormone (ADH), and regulation of certain rhythmic
substantia nigra (which regulates body movements; psychobiological activities (e.g. sleep). The release of
damage to the substantia nigra causes Parkinson’s stored serotonin from axon terminals in the diencepha-
disease) and the ventral tegmental area (which contains lon, medulla, thalamus, and a small forebrain area
dopamine-releasing neurons that are activated by (DMTF), results in inactivation of the RAS and