The Neurologic System

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CHAPTER 4

The Neurologic System

SYSTEMWIDE ELEMENTS

Physiologic Anatomy

1. Brain

a. Coverings

i. Scalp

ii. Cranium

(a) Part of the skull that houses and protects the brain (Figure 4-1)

(b) Bones: Frontal, sphenoid, ethmoid, occipital, two temporal and two parietal bones

(c) Basilar skull: Base of the skull has three depressions—the anterior, middle, and posterior fossae (Figure 4-2)

iii. Meninges (Figure 4-3)

(a) Dura mater

(b) Arachnoid mater

(c) Pia mater

b. Divisions of the brain

i. Cerebrum

(a) Telencephalon: Two cerebral hemispheres separated by a longitudinal fissure; joined by the corpus callosum

(1) Functional localization in the cerebral cortex, including cerebral dominance (Table 4-1)

(2) Corpus callosum: Commissural fibers that transfer learned discriminations, sensory experiences, and memory from one cerebral hemisphere to corresponding parts of the other

(3) Basal ganglia (basal nuclei) (Figure 4-4)

(b) Diencephalon (Figure 4-5)

(1) Thalamus: Two egg-shaped masses of gray matter that abut the lateral walls of the third ventricle; subdivided into several nuclei

(2) Hypothalamus: Below the thalamus; regulates

a) Body temperature

b) Food and water intake

c) Behavior: Part of the limbic system; concerned with aggressive and sexual behavior; elicits physical expressions associated with emotions; may be involved with sleep-wake cycles and circadian rhythm control

d) Autonomic responses: Control center for the autonomic nervous system (ANS); controls numerous visceral and somatic activities (e.g., heart rate, pupil constriction and dilation)

e) Hormonal secretion of the pituitary gland (see Chapter 6)

(3) Subthalamus: Functionally related to the basal ganglia

(4) Epithalamus: Dorsal part of the diencephalon

(c) Limbic system (Figure 4-6)

ii. Brainstem (Figure 4-7; also see Figure 4-5)

(a) Midbrain (mesencephalon): Located between the diencephalon and pons

(b) Pons (metencephalon): Between the midbrain and medulla

(c) Medulla (myelencephalon): Between the pons and spinal cord

(d) Reticular formation (RF) (Figure 4-8): Diffuse cellular network in the brainstem, with axons projecting to the thalamus and into the cortex; receives input from the cerebrum, spinal cord, other brainstem nuclei, and the cerebellum; has a role in the control of autonomic and endocrine functions, skeletal muscle activity, and visceral and somatic sensation. The reticular activating system is part of the RF.

iii. Cerebellum: Lies in the posterior fossa behind the brainstem; separated from the cerebrum by the tentorium cerebelli

c. Cerebral circulation (Figure 4-9)

i. Arterial system: Supplied by the internal carotid and vertebral arteries

(a) Circle of Willis: Anastomosis of arteries at the base of the brain formed by a short segment of the internal carotid and anterior and posterior cerebral arteries, which are connected by an anterior communicating artery and two posterior communicating arteries. This anastomosis may permit collateral circulation if a carotid or vertebral artery becomes occluded.

(b) Internal carotid system: Internal carotid arteries arise from the common carotid arteries. Table 4-2 shows the branches of this system and the areas they supply.

(c) Vertebral system: Vertebral arteries arise from the subclavian arteries and join at the lower pontine border to form the basilar artery. Branches of this system and the areas they supply are summarized in Table 4-2.

(d) Branches of the internal carotid, external carotid, and vertebral arteries (e.g., anterior, middle, posterior meningeal arteries) provide blood supply to the meninges

ii. Cerebral blood flow (CBF)

(a) Normal CBF averages 50 ml/100 g of brain tissue per minute

(b) Cerebral perfusion pressure (CPP) and intrinsic regulatory mechanisms affect CBF

(1) CPP: Pressure gradient that drives blood into the brain; calculated as the difference between the mean arterial pressure (MAP) and the intracranial pressure (ICP): CPP = MAP − ICP

(2) Regulatory mechanisms influence the diameter of the cerebrovasculature

(3) Inadequate CBF results in brain tissue ischemia (CBF <18 to 20 ml/100 g/min) and death (CBF <8 to 10 ml/100 g/min)

(4) CBF higher than metabolic demand is called hyperemia

iii. Venous system: Brain surface drains into the superficial veins; the central interior cerebrum drains into the internal veins beneath the corpus callosum (Figure 4-10). Veins have no valves.

(a) Veins empty into venous sinuses between dural layers (Table 4-3)

TABLE 4-3

Major Venous Drainage Structures, Their Locations, and Areas Drained

Venous Structure Location and Area Drained
Superior sagittal sinus Courses along the midline at the superior border of the falx cerebri; superior cerebral veins empty into it
Straight sinus Lies in the midline attachment of the falx cerebri and the tentorium; drains the system of internal cerebral veins (including the inferior sagittal sinus and great cerebral vein of Galen)
Transverse sinuses Lie in the bony groove along the fixed edge of the tentorium cerebelli; drain the straight sinus and the superior sagittal sinus
Sigmoid sinuses Lie on the mastoid process of the temporal bone and jugular process of the occipital bone; receive blood from the transverse sinuses and empty into the internal jugular veins
Inferior sagittal sinus Lies along the free inferior border of the falx cerebri just above the corpus callosum; receives blood from the medial aspects of the hemispheres
Emissary veins Connect the dural sinuses with veins outside the cranial cavity

(b) Internal jugular veins collect blood from the large dural venous sinuses and return blood to the heart

iv. Blood-brain barrier: Specialized permeability of the brain capillaries that limits transfer of certain substances from blood into brain tissue. Barrier formed by tight junctions between brain capillary endothelial cells, reduced transport mechanisms of these cells, and footlike projections from the astrocytes that encase the capillaries.

d. Ventricular system and CSF (Figure 4-11)

i. Ventricles: Four cavities containing CSF

ii. CSF functions

iii. CSF properties: See Table 4-4

iv. CSF formation

v. Circulation and absorption of CSF (see Figure 4-11)

vi. Blood-CSF barrier: Choroid plexus epithelium imposes a barrier analogous to the blood-brain barrier; permits selective transport of substances from the blood into the CSF

e. Brain metabolism

i. Brain has high metabolic energy requirements; energy primarily used for neuronal conductive and metabolic activities

ii. At rest, the brain consumes 25% of body glucose and 20% of body oxygen; cerebral oxygen consumption averages 49 ml/min

iii. Brain utilizes glucose as its principal energy source

iv. Minimal storage of oxygen and glucose in the brain necessitates a constant supply for normal neuronal function

v. Anaerobic glucose metabolism (glycolysis) yields insufficient adenosine triphosphate (ATP) to meet cerebral energy demands. Rate of glycolysis increases markedly during hypoxia in an attempt to maintain functional neuronal activity.

vi. Within seconds to minutes of anoxia, the energy-dependent sodium-potassium pump fails; cytotoxic cerebral edema results

vii. Hypoglycemia causes neuronal dysfunction and may lead to convulsions, coma, and death

f. Cells of the nervous system

i. Neuron: Basic functional unit of the nervous system; transmits nerve impulses

(a) Components of each cell (Figure 4-12)

(1) Cell body: Carries out the metabolic functions of the cell; contains a nucleus, cytoplasm, and organelles surrounded by a lipoprotein cell membrane

(2) Dendrites: Short branching extensions of the cell body; conduct impulses toward the cell body

(3) Axon hillock: Thickened area of the cell body from which the axon originates

(4) Axon: Long extension of the cell body; conducts impulses away from the cell body; usually myelinated. Outside the brain, axons are also covered with neurilemma. Branch into several processes at the terminal end.

(5) Myelin sheath: White protein-lipid complex that surrounds some axons; laid down by oligodendrocytes in the CNS and by Schwann cells in the PNS

(6) Nodes of Ranvier: Periodic interruptions in the myelin covering along the axon. Impulses are conducted from node to node (saltatory conduction), which makes conduction more rapid and efficient.

(7) Synaptic knobs: At the terminal ends of the axon; contain vesicles that store neurotransmitter substances

(b) Functions

ii. Neuroglial cells: Support, nourish, and protect the neurons; about 5 to 10 times as numerous as neurons. Four types:

g. Synaptic transmission of impulses: Unidirectional conduction of an impulse from a presynaptic neuron across a junction or synapse to a postsynaptic neuron

i. Resting membrane potential (RMP): Voltage difference across the cell membrane when the neuron is resting. Determined by the difference in ion concentrations on either side of the membrane. At rest, cells are positively charged outside and negatively charged inside.

ii. Depolarization: Stimulus causes sodium channels to open, which results in an intracellular influx of sodium ions (Na+)

iii. Action potential: If a transient voltage change that occurs with depolarization is of sufficient magnitude (threshold level), an action potential is produced and transmitted (conducted as an impulse) along the nerve fibers in an active, self-propagating process

iv. Summation: Simultaneous excitation of numerous excitatory presynaptic terminals (or rapidly successive discharges from the same presynaptic terminal) can add together to cause a progressive increase in the postsynaptic potential that may eventually reach threshold to generate an action potential

v. Neurotransmitters (Table 4-5): Chemicals secreted by presynaptic knobs or vesicles (usually located at the axon terminal) that excite, inhibit, or modify the response of a postsynaptic neuron. When an action potential reaches the synaptic knob, calcium channels are opened, allowing Ca++ influx into the knob, which triggers neurotransmitter release.

vi. Refractory period

vii. Repolarization

viii. Inhibition

2. Spine and spinal cord

a. Vertebral column (Figure 4-13)

i. Composed of 33 vertebrae

ii. Anatomic features of a typical vertebra

(a) Body: Flat round, solid portion; lies anteriorly

(b) Arch: Posterior part of the vertebra. Consists of:

(c) Intervertebral foramina: Openings between the vertebrae through which spinal nerves pass

(d) Spinal foramina: Opening between the arch and the body through which the spinal cord passes

iii. Intervertebral discs

iv. Spinal ligaments: Hold the vertebrae and discs in alignment; prevent excessive spinal flexion or extension

b. Spinal cord

i. Location: Extends from the superior border of the atlas to the first or second lumbar vertebra

ii. Meninges: Continuous with the layers covering the brain

iii. Gray matter (Figure 4-14)

iv. White matter (see Figure 4-14)

(a) Composed of three longitudinal columns (funiculi): Anterior, lateral, and posterior

(b) Contains mostly myelinated axons

(c) Funiculi contain tracts (fasciculi): Composed of axons with similar origin, course, and termination that perform specific functions; clinically significant tracts are summarized in Table 4-6; classified as follows (Figure 4-15):

(d) Most tracts are named to indicate the column in which the tract travels, the location of its cells of origin, and the location of axon termination

v. Upper and lower motor neurons

c. Reflexes

i. Reflex arc: Requires a receptor, sensory neuron, motor neuron, and effector (e.g., muscle or gland) (Figure 4-16)

ii. Monosynaptic reflex arc: Direct synapse between the afferent and efferent neurons

iii. Polysynaptic reflex arc

iv. Reciprocal innervation: Impulses that excite motor neurons supplying a particular muscle also inhibit motor neurons of antagonistic muscles

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