Page 375 - Clinical Anatomy

ECA6 7/18/06 6:54 PM Page 360

360 The central nervous system

particularly the basal ganglia, actually control movement by altering activ-
ity in the premotor cortex and, thus, the pyramidal motor projections. This
clearly emphasizes the blur between the two systems.
Components of the extrapyramidal system include the red nuclei,
vestibular nuclei, superior colliculus and reticular formation in the brain
stem, all of which project via discrete pathways to influence spinal cord
motor neurons. Cerebellar projections (see page 344) are also included
since they influence not only these brainstem motor pathways, but also
the motor cortex itself via the dentatothalamic projection.
Perhaps the most important structures to retain an extrapyramidal defi-
nition are the basal ganglia (see pages 353 and 354). The neostriatum
(caudate and putamen) receives widespread cortical afferents, including
those from high order sensory association and motor areas, and projects
mainly to the globus pallidus. The latter nucleus is the major outflow for the
basal ganglia and, via the ventral anterior thalamus, exerts its major influ-
ence on premotor and hence the motor cortices. This pattern of connections
suggests that the basal ganglia are involved in complex aspects of motor
control, including motor planning and the initiation of movement.
A variety of motor disorders are associated with basal ganglia pathol-
ogy and, in some instances, neuroanatomically discrete deficits in specific
neurotransmitters. For example, Parkinson’s disease involves the degener-
ation of dopaminergic neurons in the substantia nigra in the midbrain. This
pigmented nucleus provides the neostriatum with a dense dopaminergic
innervation which may be completely lost in severe cases of Parkinsonism.
Knowledge of this selective chemical neuropathology has resulted in the
development of a treatment of the disease which involves the oral adminis-
tration of the dopamine precursor l-dopa.

The membranes of the brain (the meninges)
The three membranes surrounding the spinal cord, the dura mater, arach-
noid mater and pia mater, are continued upwards as coverings to the brain.
The dura is a dense membrane which, within the cranium, is made up of
two layers. The outer layer is intimately adherent to the skull; the inner
layer is united to the outer layer except where separated by the great dural
venous sinuses and where it projects to form four sheets (Fig. 214):
•◊◊the falx cerebri;
•◊◊the falx cerebelli;
•◊◊the tentorium cerebelli;
•◊◊the diaphragma sellae.
The arachnoid is a delicate membrane separated from the dura by the
potential subdural space. It projects only into the longitudinal fissure and the
stem of the lateral fissure.
The pia is closely moulded to the outline of the brain; it dips down into
the cerebral sulci leaving the subarachnoid space between it and the arach-
noid. This space is broken up by trabeculae of fine fibrous strands and con-
tains the cerebrospinal fluid.