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NE.RVOUS SYSTE.M

271

Supenor saglnal Sinus

Choroid plexus

Cerebral aqueduct

Fourth veolflcle

Foramen olluschka

Foramen 01 Magendle

Figure 4-3. The velltricular system of the brai". Arrows indicate the ciratlaliol1 of cerebrospinal fluid from the site of formatioll in the choroid plexus to the site of absorptioll ill the villi of the sagittal 5;11115. (\Vith permission from J

Bogouss/avsky, M Fisher /edsJ. Textbook of Neurology. Bostol1: Blitterworth

Heillemallll, 1998;656.)

\Vhen ventricular pressure is greater than venous pressure, CSF is

absorbed into the venous system via the arachnoid villi, capillary

walls of the pia mater, and lymphatics of the subarachnoid space near

the optic nerve.2

Blood-Brain Barrier

The blood-brain barrier is the physiologic mechanism responsible for

keeping toxins, such as amino acids, hormones, ions, and urea, from

altering neuronal firing of the brain. It readily allows water, oxygen,

carbon dioxide, glucose, some amino acids, and substances thar are

highly soluble in fat (e.g., alcohol, nicotine, and anesthetic agents) to

pass across the barrier,s,· The barrier consists of fused endothelial

cells on a basement membrane that is surrounded by astrocytic foot

extensions.· Substances must therefore pass through, rather than

272 ACUTE CARE HANDBOOK FOR PHYSICAL THERAPISTS

around, these cells. The blood-brain barrier is absent ncar the hypothalamus, pineal region, anterior third ventricle, and floor of the fourth ventricle.3

Central Brain Systems

The central brain systems are the reticular activating systcm and the

limbic system. The rcticular activating system is responsible for

human consciousness level and integrates the functions of the brain

stem with conical, cerebellar, thalamic, hypothalamic, and sensory

receptor functions.s

The limbic system is a complex interactive system, with primary

connections berween the cortex, hypothalamus, and sensory receptors.5 The limbic system is the emotional system, mediating cortical autonomic function of internal and external stimuli.

Circulation

The brain reccives blood from the internal carotid and vertebral arteries, which are linked together by the circle of Willis, as shown in Figure 4-4. Each vessel supplies blood to a certain part of the brain (Table 4-5). The circulation of the brain is discussed in tcrms of a sin-Ant communicating att.

AnI. cerebral art

Middle cerebral att

POSI.

carotid art

'".

POSI cerebral art

Greal cerebfal vein

(Galen)

CIrcle 01 WilNs

& associated veins

Figure 4-4. Schematic representation of the arterial circle of Willis and

accompanying veins. (Ant. anterior; art. artery; Post. posterior.) (\Vith

=

=

=

permission from £G Gonzalez, S) Meyers reds/. Downey and Darling's Physiological Basis of Rehabilitation Medicine (Jrd ed/. Boston: Butterworth

Heinemann, 2001 ;22.)

NERVOUS SYSTEM

273

Table 4-5. Blood Supply of the Major Areas of the Brain

Artery

Area of Perfusion

Anterior circulation

Internal carotid artery

The dura, optic tract, basal ganglia, midbrain,

(ICA)

uncus, lateral geniculate body, and tympanic

cavity. Ophthalmic branch supplies the eyes

and orbits.

External carotid artery

All structures external to the skull, the larynx,

(ECA)

and the thyroid.

Anterior cerebral artery

Medial and superior surface of frontal and

(ACA)

parietal lobes. Medial striate branch supplies anterior portion of the internal capsule,

optic chiasm and nerve, portions of the

hypothalamus, and basal ganglia.

Middle cerebral artery

Lateral surface of the frontal, parietal, and

(MCA)

occipital lobes, including the superior and

lareral surfaces of temporal lobes.

Posterior circulation

Vertebral artery

Medulla, dura of the posterior fossa, including

the falx cerebri and tentorium cerebelli.

Basilar artery

Pons and midbrain.

Posterior inferior

Posrerior and inferior surface of cerebellum.

cerebellar artery (PICA)

Anterior inferior

Anterior surface of the cerebellum, flocculus,

cerebellar artery (AICA)

and inferior vermis.

Superior cerebellar artery

Superior surface of cerebellum and vermis.

(SCA)

I)osterior cerebellar artery

Occipital lobe and medial and lateral surfaces

(PCA)

of the remporal lobes, thalamus, lareral geniculate bodies, hippocampus, and choroid

plexus of the third and lareral ventricles.

Sources: Data (rom CL Rumbaugh, A Wang, FY Tsai (cds). Cerebrovascular Disease

Imaging and Interventional Tre:nment Oprions. New York: Igaku-Shoin Medical Publishers, 1995; and KL Moore (cd). Clinically Oriented Anaromy (2nd cd). Baltimore: Williams & Wilkins, 1985.

gle vessel or by region (usually as the anterior or posterior circulation). There are several anastomotic systems of the cerebral vasculature that provide essential blood flow to the brain. Blood is drained from the brain through a series of venous sinuses. The superior sagittal sinus, with its associated lacunae and villi, is the primary drainage

274 ACUTE CARE HANDBOOK FOR PHYSICAL THERAI)ISTS

site. The superior sagittal sinus and sinuses located in the dura and

scalp then drain blood into the internal jugular vein for return to the

heart.

Spinal Cord

The spinal cord lies within the spinal column and extends from the

foramen magnum to the first lumbar vertebra, where ir forms the

conus medullaris and the cauda equina and attaches to the coccyx

via the filum terminale. Divided into the cervical, thoracic, and

lumbar portions, it is prorected by mechanisms similar ro those

supporting the brain. The spinal cord is composed of gray and

white matter and provides the pathway for the ascending and

descending tracts, as shown in cross section in Figure 4-5 and outlined in Table 4-6.

Peripheral Nervous System

The peripheral nervous sysrem consists of rhe cranial and spinal

nerves and the reflex system. The primary structures include

peripheral nerves, associated ganglia, and sensory receptors. There

are 12 pairs of CNs, each with a unique parhway and function

(sensory, motor, mixed, or autonomic). Thirty-one pairs of spinal

nerves (all mixed) exit the spinal cord to form distinct plexuses

(excepr T2 rhrough T12). The peripheral nerves of rhe upper and

lower extremities and thorax are listed in Tables 4-7 through 4-9,

and the dermatomal system is shown in Figure 4-6. The reflex system includes spinal, deep tendon, stretch, and superficial reflexes and protective responses.

Autonomic Nervous System

The portion of the peripheral nervous system thar innervates glands

and cardiac and smooth muscle is the autonomic nervous system.

The parasympathetic division is activated in time of rest, whereas

the sympathetic division is activated in times of work or "fight or

flight" situations. The two divisions work closely togerher, wirh dual

innervation of most organs, to ensure homeostasis.