Anatomy
Abstract
Knowledge of the anatomy of the nervous system is crucial for nurses caring for patients with neurologic diseases or disorders because they must thoroughly understand the normal neuroanatomy to make meaningful correlations between anatomical and clinical findings. The nervous system consists of the central nervous system and the peripheral nervous system. This overview of the normal neuroanatomy includes basic surface structure, the ventricular system, and the vasculature of both the brain and the spine.
Keywords: autonomic nervous system, brain, cranial nerves, peripheral nervous system, spinal cord, vertebrae
1.1 Nervous System
The nervous system, widely thought to be the most complicated system in the human body, is divided into two parts: the central nervous system (CNS) and the peripheral nervous system (PNS) (Box 1.1 Components of the Nervous System). Nurses caring for patients with neurologic diseases or disorders must possess a thorough knowledge of normal neuroanatomy, including the cranial nerves (CN), so that they can recognize anatomical abnormalities. This knowledge will allow nurses to make meaningful correlations between anatomical and clinical findings.
Box 1.1 Components of the Nervous System
Central nervous system
Brain
Spinal cord
Peripheral nervous system
Spinal nerves
Cranial nerves
Autonomic nervous system
Sympathetic nervous system
Parasympathetic nervous system
1.1.1 Cellular Anatomy
The nervous system comprises two fundamental cell types: neurons (▶ Fig. 1.1) and neuroglial cells (▶ Fig. 1.2).
Fig. 1.1 Neuron.
Fig. 1.2 Neurons and neuroglial cells.
Neurons
Fundamental building blocks
Highly specialized
Conduct and receive nerve impulses, as well as release chemical transmitters
Soma: Cell body of the neuron
Dendrites: Receive impulses
Axons: Carry impulses away from the cell
Myelin sheath (Box 1.2 Clinical Correlation: Demyelinating Disease)
Formed by Schwann’s cells
Composed of a lipid substance
Provides insulation for nerve impulses
Synapse
The junction between neurons, in which impulses are transmitted
Neurotransmitters
Chemical substances that promote, inhibit, or alter cellular response
Over 100 neurotransmitters have been identified, including
Amines (e.g., acetylcholine and serotonin)
Catecholamines (e.g., dopamine and norepinephrine)
Box 1.2 Clinical Correlation: Demyelinating Disease
Multiple sclerosis is an example of a demyelinating disease
In demyelination, the protective Schwann’s cells lose their myelin sheaths, making them less effective in shielding the nerves
Neuroglial Cells
Specialized support cells. They include
Astrocytes: Supply nutrients to the brain
Ependymal cells: Line the ventricles and help produce cerebrospinal fluid (CSF)
Oligodendrocytes: Form protective myelin sheath around the axons
Microglial cells: Scavengers; associated with immune response
Schwann’s cells: Form the myelin sheath around the peripheral nerves
1.2 Central Nervous System
The CNS includes the brain and spinal cord (Video 1.1). Of all the systems in the body, the CNS has the largest number of axons and synapses. The brain alone consists of approximately 100 billion cells. The brain is encased and protected by the skull. In turn, the skull is covered and cushioned by the scalp, which consists of multiple layers. Both the scalp and the skull protect the brain.
1.2.1 Scalp
The scalp consists of several layers that cover the skull.
Skin (dermal) layer
Protects the skull
Contains hair
Subcutaneous layer
Vascular; may bleed profusely
Galea
Tough innermost layer
Subgaleal space: Potential location for blood to collect, commonly called a goose egg
Periosteum
Thin layer of connective tissue that covers the skull
1.2.2 Skull
The skull (▶ Fig. 1.3) is composed of eight fused bone plates.
Fig. 1.3 Skull.
Cranial Bones (n = 8)
Frontal
Temporal (n = 2); thinnest portion of bone
Parietal (n = 2)
Occipital
Sphenoid
Ethmoid
Fourteen facial bones form the anterior portion of the skull.
Other Important Bone Segments
Clivus: The thin bone that rests against the brainstem
Sella turcica: Houses the pituitary gland, which is not considered part of the brain
1.2.3 Brain
The brain (Box 1.3 Just the Facts: The Brain), largely recognized as the most complex organ in the human body, has a complicated anatomy. Sulci (singular, sulcus) are small separations between brain tissue. Gyri (singular, gyrus) are folds (i.e., wrinkles) on the surface of the brain. The purpose of the sulci and gyri is to increase the surface area of the brain.
Fissures are deep separations between the cerebral hemispheres (or lobes) of the brain, commonly used as geographical markers. The best-known fissures are as follows
Great longitudinal fissure
Lateral fissure of Sylvius
Central fissure of Rolando
Parieto-occipital fissure
Box 1.3 Just the Facts: The Brain
The adult brain weighs about 3 lb
Brain tissue is also called parenchyma
The brain is composed of 78% water, 10% fat, 8% protein, and 4% organic and inorganic substances
The brain makes up only about 2% of the body’s total weight, but it uses about 20% of its oxygen supply and 20% of its blood flow
The brain consists of 40% gray matter and 60% white matter
The adult brain has the consistency of thickened JELL-O
Meninges
The meninges (▶ Fig. 1.4) are the three layers of thick connective tissue that cover the entire brain and spinal cord.
Pia mater
Delicate innermost layer
Adheres to the brain
Arachnoid mater
Situated above the pia mater
Space below the arachnoid is called the subarachnoid space (Box 1.4 Clinical Correlation: Subarachnoid Hemorrhage)
Contains CSF
Dura mater
Latin for “tough mother”
Tough, fibrous layer between the arachnoid mater and the skull bone
Spaces above (epidural) and below (subdural) the dura are typical locations for dural hematomas, a common type of brain injury (Box 1.5 Clinical Correlation: Dural Hematomas)
The tentorium, a tent-like fold of dura that separates the cerebrum and cerebellum, serves as an important anatomical marker
Fig. 1.4 Layers of the meninges.
Box 1.4 Clinical Correlation: Subarachnoid Hemorrhage
Defined as bleeding into the space below the arachnoid layer
Common causes of subarachnoid hemorrhage are trauma and aneurysmal rupture
Box 1.5 Clinical Correlation: Epidural and Subdural Hematomas
The main cause of cerebral and spinal hematomas is trauma
This common traumatic brain injury involves bleeding into the space above or below the dura
Epidural hematoma: Bleeding above the dura
Subdural hematoma: Bleeding below the dura
May result from surgery or rapid decompression of ventricles
Ventricular System
The ventricular system produces and circulates CSF (Box 1.6 Cerebrospinal Fluid). It includes four cavities (i.e., ventricles) that contain CSF and the transport system that circulates CSF throughout the brain and spinal cord (Box 1.7 Clinical Correlation: Disorders of the Ventricular System). The components of the ventricular system are
Ventricles
Situated in the center of the brain
The four ventricles include the right and left lateral ventricles, the third ventricle, and the fourth ventricle (▶ Fig. 1.5. and ▶ Fig. 1.6)
Communicate with other ventricles
Composed of ependymal cells
Choroid plexus
Refers to the group of blood vessels in each ventricle
Produces CSF (approximately 22 mL/h)
Cisterns
CSF reservoirs
Foramina of Monro
Connect the lateral ventricles with the third ventricle
Also called the interventricular foramina
Cerebral aqueduct of Sylvius
Passageway from the third to fourth ventricle
Also called the sylvian aqueduct
Foramen of Magendie and foramen of Luschka
Openings from the fourth ventricle into the subarachnoid space
Arachnoid villi
Reabsorb CSF into blood
Transport CSF to dural sinus
Box 1.6 Cerebrospinal Fluid
Produced by the choroid plexus
Circulates through the ventricular system and in the subarachnoid space
Cushions the brain and spinal cord
Produced at a rate of 500 mL/day
The body contains about 150 mL of CSF at any one time
Absorbed by the arachnoid granules
Intracranial pressure is commonly measured in the ventricular CSF
Box 1.7 Clinical Correlation: Disorders of the Ventricular System
Hydrocephalus
Ventriculitis
Fig. 1.5 Ventricles, lateral view.
Fig. 1.6 Ventricles, posterior view.
Cerebrum
The cerebrum consists of two cerebral hemispheres, right and left.
Separated by the great longitudinal fissure
Surface of the brain is covered by gray matter, which is composed of millions of neurons
Deeper brain tissue contains white matter, which is composed of millions of highly specialized neuroglial cells
Pathways carrying information to the cerebral hemispheres cross over from one hemisphere to the other (Box 1.8 Clinical Concern)
The hemispheres are further divided into two parts.
Supratentorial (above the tentorium)
Infratentorial (below the tentorium)
Box 1.8 Clinical Concern
CNS pathways carrying sensory or motor information to the cerebral hemispheres cross to the opposite hemisphere
Each hemisphere therefore controls the movement of, and perceives sensation from, the contralateral (opposite) side
Supratentorial Region
The supratentorial area of the brain consists of the frontal, parietal, temporal, and occipital lobes (▶ Fig. 1.7) and the corpus callosum.
The frontal lobe controls
Emotions and behavior
Attention
Motivation
Judgment
Broca’s area: Motor aspect of speech (i.e., expressive speech)
Initiation of motor integration
Problem-solving
Bowel and bladder function
The parietal lobe controls
Interpretation of characteristics of sensory input (e.g., pain, temperature, and touch)
Processing of visual–spatial information (nondominant hemisphere)
Praxis (dominant hemisphere)
The temporal lobe controls
Hearing
Wernicke’s area: Interpretation of language (i.e., receptive speech)
Memory
Musical awareness
Sequencing
The occipital lobe controls
Visual perception
Interpretation of the written word
The corpus callosum is a thick band of nerve fibers running longitudinally that connects the hemispheres
Fig. 1.7 Lobes of the brain.
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