The Central Nervous System
Cerebrovascular Accidents (CVAs)
Cerebrovascular accidents (strokes) result when blood circulation
to brain neurons is impaired and brain tissue dies. Te result may
be hemiplegia, sensory deﬁcits, or speech impairment.
Degenerative Brain Disorders
Alzheimer’s disease is a degenerative brain disease in which
beta-amyloid peptide deposits and neuroﬁbrillary tangles appear.
Marked by a deﬁcit of ACh, it results in slow, progressive loss of
memory and motor control and increasing dementia.
Parkinson’s disease and Huntington’s disease are
neurodegenerative disorders of the basal nuclei. Both involve
abnormalities of the neurotransmitter dopamine (too little or too
much secreted) and are characterized by abnormal movements.
The Spinal Cord
Gross Anatomy and Protection
Te spinal cord, a two-way impulse conduction pathway and a
reﬂex center, resides within the vertebral column and is protected
by meninges and cerebrospinal ﬂuid. It extends from the foramen
magnum to the end of the ﬁrst lumbar vertebra.
Tirty-one pairs of spinal nerves issue from the cord. Te cord is
enlarged in the cervical and lumbar regions, where spinal nerves
serving the limbs arise.
Spinal Cord Cross-Sectional Anatomy
Te central gray matter of the cord is H shaped. Ventral horns mainly
contain somatic motor neurons. Lateral horns contain visceral
(autonomic) motor neurons. Dorsal horns contain interneurons.
Axons of neurons of the lateral and ventral horns emerge in
common from the cord via the ventral roots. Axons of sensory
neurons (with cell bodies located in the dorsal root ganglion)
form the dorsal roots and enter the dorsal aspect of the cord. Te
ventral and dorsal roots combine to form the spinal nerves.
Each side of the white matter of the cord has dorsal, lateral, and
ventral columns (funiculi), and each funiculus contains a number
of ascending and descending tracts. All tracts are paired and most
Ascending (sensory) tracts include the fasciculi gracilis and
cuneatus, spinothalamic tracts, and spinocerebellar tracts.
Te dorsal column–medial lemniscal pathway consists of the
dorsal white column (fasciculus cuneatus, fasciculus gracilis)
and the medial lemniscus, which are concerned with straight-
through, precise transmission of one or a few related types
of sensory input. Te spinothalamic pathways transmit pain,
temperature, and coarse touch, and permit brain stem processing
of ascending impulses. Te spinocerebellar tracts, which
terminate in the cerebellum, serve muscle sense, not conscious
Descending tracts include the pyramidal tracts (ventral and
lateral corticospinal tracts) and a number of motor tracts
originating from subcortical motor nuclei. Tese descending
ﬁbers issue from the brain stem motor areas [indirect system] and
cortical motor areas [direct (pyramidal) system].
Spinal Cord Trauma and Disorders
Injury to the ventral horn neurons or the ventral roots results in
ﬂaccid paralysis. (Injury to the upper motor neurons in the brain
results in spastic paralysis.) If the dorsal roots or sensory tracts
are damaged, paresthesias occur.
In most people the leF hemisphere controls language. Te
language implementation system, which includes Broca’s and
Wernicke’s areas and the basal nuclei, analyzes incoming and
produces outgoing language. Te opposite hemisphere deals with
the emotional content of language.
Memory is the storage and retrieval of information. It is essential
for learning and is part of consciousness.
Memory storage has two stages: short-term memory (S±M) and
long-term memory (L±M). ±ransfer of information from S±M to
L±M takes minutes to hours, but more time is required for L±M
Declarative memory is the ability to learn and consciously
remember information. Procedural memory is the learning of
motor skills, which are then performed without conscious thought.
Declarative memory appears to involve the medial temporal lobe
(hippocampus and surrounding temporal cortical areas), thalamus,
basal forebrain, and prefrontal cortex. Procedural memory (a type
of nondeclarative memory) relies on the basal nuclei.
Te nature of memory formation at the molecular level is not
fully known, but NMDA receptors (essentially calcium channels),
activated by depolarization and glutamate binding, play a major
role in long-term potentiation (L±P). Te calcium inﬂux that
follows NMDA receptor activation mobilizes enzymes that
mediate events necessary for memory formation.
Protection of the Brain
Te delicate brain is protected by bone, meninges, cerebrospinal
ﬂuid, and the blood brain barrier.
Te meninges from superﬁcial to deep are the dura mater,
arachnoid mater, and pia mater. Tey enclose the brain and spinal
cord and their blood vessels. Inward folds of the inner layer of the
dura mater secure the brain to the skull.
Cerebrospinal ﬂuid (CS²), formed by the choroid plexuses from
blood plasma, circulates through the ventricles and into the
subarachnoid space. It returns to the dural venous sinuses via the
arachnoid villi. CS² supports and cushions the brain and cord
and helps to nourish them.
Blood Brain Barrier
Te blood brain barrier reﬂects the relative impermeability of the
epithelium of capillaries of the brain. It allows water, respiratory
gases, essential nutrients, and fat-soluble molecules to enter the
neural tissue, but blocks other, water-soluble, potentially harmful
Homeostatic Imbalances of the Brain
Traumatic Brain Injuries
Head trauma may cause brain injuries called concussions or, in
severe cases, contusions (bruising). When the brain stem is aﬀected,
unconsciousness (temporary or permanent) occurs. ±rauma-
induced brain injuries may be aggravated by intracranial hemorrhage
or cerebral edema, both of which compress brain tissue.