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Chapter 12: The Central Nervous System (CNS)
Objectives
The Brain
1. Describe how space constraints affect brain development.
2. Name the major regions of the adult brain.
3. Name and locate the ventricles of the brain.
4. List the major lobes, fissures, and functional areas of the cerebral cortex.
5. Explain lateralization of hemisphere function.
6. Differentiate between commissures, association fibers, and projection fibers.
7. Describe the general function of the basal nuclei (basal ganglia).
8. Describe the location of the diencephalon, and name its subdivisions and functions.
9. Identify the three major regions of the brain stem, and note the functions of each area.
10. Describe the structure and function of the cerebellum.
11. Locate the limbic system and the reticular formation, and explain the role of each functional system.
Higher Mental Functions
12. Define EEG and distinguish between alpha, beta, theta, and delta brain waves.
13. Describe consciousness clinically.
14. Compare and contrast the events and importance of slow-wave and REM sleep, and indicate how their patterns change through life.
15. Compare and contrast the stages and categories of memory.
16. Describe the roles of the major brain structures believed to be involved in declarative and procedural memories.
Protection of the Brain
17. Describe how meninges, cerebrospinal fluid, and the blood brain barrier protect the CNS.
18. Explain how cerebrospinal fluid is formed and describe its circulatory pathway.
Homeostatic Imbalances of the Brain
19. Describe the cause (if known) and major signs and symptoms of cerebrovascular accidents, Alzheimer’s disease, Parkinson’s disease, and Huntington’s disease.
The Spinal Cord
20. Describe the gross and microscopic structure of the spinal cord.
21. List the major spinal cord tracts, and classify each as a motor or sensory tract.
22. Distinguish between flaccid and spastic paralysis, and between paralysis and paresthesia.
Diagnostic Procedures for Assessing CNS Dysfunction
23. List and explain several techniques used to diagnose brain disorders.
Developmental Aspects of the Central Nervous System
24. Describe the development of the brain and spinal cord.
25. Indicate several maternal factors that can impair development of the nervous system in an embryo.
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26. Explain the effects of aging on the brain.
Suggested Lecture Outline
I. The Brain (pp. 429–452; Figs. 12.1–12.17; Table 12.1)
A. Embryonic Development (pp. 429–430; Figs. 12.1–12.2)
1. The brain and spinal cord begin as the neural tube, which rapidly differentiates into the CNS.
2. The neural tube develops constrictions that divide the three primary brain vesicles: the prosencephalon
(forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain).
Secondary brain vesicles:
A. Telencephalon – develops into cerebral hemispheres, including basal nuclei, and lateral ventricles.
B. Diencephalon – develops into thalamus, hypothalamus, & epithalmus.
C. Mesencephalon – midbrain % aqueduct.
D. Metencephalon – pons, cerebellum & 4th ventricle.
E. Myelencephalon – medulla & 4th ventricle.
3. Since the brain grows more rapidly than the developing skull, the brain forms folds, allowing it to fit inside
the space of the skull.
B. Regions and Organization (p. 430)
1. The medical scheme of brain anatomy divides the adult brain into four parts: the cerebral hemispheres, the
diencephalon, the brain stem (consisting of the midbrain, pons, and medulla oblongata), and the cerebellum.
2. The basic pattern of the CNS consists of a central cavity surrounded by a gray matter core, external to which
is white matter.
3. In the brain, the cerebrum and cerebellum have an outer gray matter layer, which is reduced to scattered
gray matter nuclei in the spinal cord.
C. Ventricles (p. 430; Fig. 12.3)
1. The ventricles of the brain are continuous with one another and with the central canal of the spinal cord.
a. The ventricles are lined with ependymal cells and are filled with cerebrospinal fluid.
b. The paired lateral ventricles lie deep within each cerebral hemisphere and are separated by the septum
pellucidum.
c. The third ventricle lies within the diencephalon and communicates with the lateral ventricles via two interventricular foramina.
d. The fourth ventricle lies in the hindbrain and communicates with the third ventricle via the cerebral aqueduct.
e. The lateral and median apertures within the fourth ventricle connect the ventricles with the subarachnoid
space surrounding the brain.
D. Cerebral Hemispheres (pp. 430–441; Figs. 12.4–12.10, 12.12; Table 12.1)
1. The cerebral hemispheres form the superior part of the brain and are characterized by ridges and grooves
called gyri and sulci.
2. The cerebral hemispheres are separated along the midline by the longitudinal fissure and are separated
from the cerebellum along the transverse cerebral fissure.
3. The five lobes of the cerebral hemispheres separated by specific sulci are: frontal, parietal, temporal, occipital, and insula.
a. The central sulcus separates the frontal and parietal lobes.
b. The precentral gyrus lies anterior to the central sulcus, while the postcentral gyrus lies posterior to the
central sulcus.
c. The parieto-occipital sulcus separates the parietal lobe from the occipital lobe.
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4.
5.
6.
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9.
d. The lateral sulcus separates the temporal lobe from the parietal and frontal lobes.
e. The insula forms part of the floor of the lateral sulcus and is covered by the temporal, parietal, and
frontal lobes.
f. Each cerebral hemisphere has three regions: the superficial cortex of gray matter, internal white matter,
and areas of gray matter deep within the white matter, the basal nuclei.
The cerebral cortex is the location of the conscious mind, allowing us to communicate, remember, and understand, and comprises about 40% of the total brain mass.
The cerebral cortex has three kinds of functional areas: motor areas, sensory areas, and association areas.
Each hemisphere has contralateral control over sensory and motor functions, meaning that each hemisphere controls the opposite side of the body.
The hemispheres exhibit lateralization of function, meaning that there is specialization of one side of the
brain for certain functions.
Areas in the posterior part of the frontal lobes control voluntary movement. Precentral gyrus; contralateral;
beginning of descending tracts.
a. The primary motor cortex allows conscious control of skilled voluntary movement of skeletal muscles.
b. The premotor cortex is the region controlling learned motor skills.
c. Broca’s area is a motor speech area that controls muscles involved in speech production. Left hemisphere
in majority; coordinates speech & breathing muscles. Lesion l/t nonfluent aphasia.
d. The frontal eye field controls eye movement. Voluntary scanning of eyes.
There are several sensory areas of the cerebral cortex that occur in the parietal,
temporal, and occipital lobes.
a. The primary somatosensory cortex allows spatial discrimination and the ability to detect the
location of stimulation. Postcentral gyrus; sensory homunculus – size of cortical area depends on number of receptors present. Termination of ascending tracts.
b. The somatosensory association cortex integrates sensory information and produces an understanding of
the stimulus being felt.
c. The primary visual cortex (visual perception) and visual association (interpretation) of visual stimuli. Occipital lobe.
d. The primary auditory cortex allows detection of the properties of sound and auditory association area
the contextual recognition of sound.
e. The vestibular cortex is responsible for conscious awareness of balance.
f. The olfactory cortex allows detection of odors.
g. The gustatory cortex allows perception of taste stimuli.
h. The visceral sensory areas are involved in conscious awareness of visceral (organ) sensation.
10. Multimodal association areas are not connected to any specific sensory cortex, but are highly
interconnected areas throughout the cerebral cortex.
a. The anterior association area, or prefrontal cortex, is involved with intellect, cognition, recall, and personality and is closely linked to the limbic system. As well as learning, mood, planning, & judgment.
b. The posterior association area aids in recognition of patterns and faces, as well as understanding of written and spoken language, and includes Wernicke’s area – lesion l/t “word salad”
c. The limbic association area deals with emotion surrounding situations and includes the cingulate gyrus,
parahippocampal gyrus, and hippocampus.
11. There is lateralization of cortical functioning, in which each cerebral hemisphere has unique control over
abilities not shared by the other half.
a. Often, the left hemisphere dominates language abilities, math, and logic, and the right hemisphere dominates visual-spatial skills, intuition, emotion, and artistic and musical skills.
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b. The term “cerebral dominance” refers to which side of the cerebrum is dominant for language, not
which side of the brain is dominant overall.
12. Cerebral white matter is responsible for communication between cerebral areas and the cerebral cortex and lower CNS centers. Aka: “seat of intelligence”. Gray matter = “bark”; split into 2 hemispheres. Define gyri, fissure, sulci. Lobes of Cerebrum – frontal, parietal, temporal, occipital & insula.
Central sulcus ÷ frontal & parietal. Precentral Gyrus – primary motor area. Postcentral Gyrus – primary
somatosensory area.
a. Association fibers are tracts of cerebral white matter that run horizontally, connecting different parts of
the same hemisphere.
b. Commissural fibers run horizontally and connect corresponding areas of gray matter in the two hemispheres, allowing the hemispheres to function together as a whole (includes the corpus callosum).
c. Projection fibers run vertically, and connect the cerebral cortex to the lower brain or cord centers, tying
together the rest of the nervous system to the body’s receptors and effectors. Connects cerebrum to
thalamus, brain stem, and spinal cord.
13. Basal nuclei consist of a group of subcortical nuclei that have overlapping motor control with the cerebellum that regulate cognition and emotion. Helps regulate initiation & termination of movements. Controls
subconscious contractions of skeletal muscle.
a. Putamen – continues or anticipates body movements.
b. Globus Pallidus – regulates muscle tone.
c. Limbic System – “emotional brain”. Plays primary role in pain, pleasure, docility, affection, & anger.
 Hippocampus – relation memory
 Amygdala – tameness
E. The diencephalon - central core, superior to midbrain. Is a set of gray matter areas and consists of the
thalamus, hypothalamus, and epithalamus (pp. 441–443; Figs. 12.11, 12.13; Table 12.1). CVO & optic
tracts.
1. The thalamus plays a key role in mediating sensation, motor activities, cortical arousal, learning, and
memory. Major relay station for most sensory impulses, transmits motor info from cerebellum and plays
role in maintenance of consciousness. 7 nuclei.
2. The hypothalamus - 4 major regions; one of the major regulators of homeostasis. Connected with pituitary
& produces many hormones.
a. Control of ANS – regulates glandular secretion, contraction of smooth & cardiac muscle.
b. Production of Hormones – releasing & inhibiting; linked to anterior & posterior pituitary.
c. Emotional & Behavioral Pattern
d. Eating & Drinking – feeding, satiety & thirst centers.
e. Body’s Thermostat – Thermoregulation
f. Circadian Rhythm = biological clock; sleep-wake cycle. Associated with eyes, RAS & pineal gland.
3. The epithalamus includes the pineal gland & habenular nulei.
a. Pineal Gland – secretes melatonin = sleepiness.
b. Habenular Nuclei – involved in olfaction & emotional response; positive & negative response to familiar
scents.
4. Circumventricular Organs (CVOs) – lack BBB. HIV entry?
F. The brain stem, consisting of the midbrain, pons, and medulla oblongata, produces rigidly programmed, automatic behaviors necessary for survival (pp. 443–447; Figs. 12.12–12.14; Table 12.1). Between spinal cord & diencephalon with Reticular Formation throughout.
1. The midbrain is comprised of the cerebral peduncles, corpora quadrigemina, and substantia nigra; mesencephalon; contains nuclei & tracts.
a. Descending Tracts – corticospinal, corticobulbar, & corticopontine.
b. Tectum
c. Superior Colliculi – elicit eye tracking, scanning, & reflex.
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d. Inferior Colliculi – auditory & startle reflex.
e. Substantial Nigra – release dopamine t basal nuclei; helps control subconscious muscle activity, emotion
(Parkinson)
f. Red Nuclei – rich blood supply; helps control muscular movements
g. CN – III, IV.
2. The pons (“bridge”) contains fiber tracts that complete conduction pathways between the brain and spinal
cord. Consists of nuclei; ascending & descending tracts.
Nuclei:
a. Pontine essential in efficiency of voluntary motor output.
b. Pneumotacic & apneustic – help control breathing.
c. CN – V, VI, VII, VIII.
3. The medulla oblongata is the location of several visceral motor nuclei controlling vital functions such as
cardiac and respiratory rate. Alcohol overdose = suppresses MO rhythmicity l/t death.
a. White mater – Contains ascending & descending tracts.
b. Pyramids – corticospinal tracts controlling voluntary movement; 90% decussation.
c. Nuclei – (cell bodies CNS) several areas include CVC, RMR, vomiting, deglutition, sneezing, coughing, hiccups.
d. Olive – inferior olivary nucleus to cerebellum which provides adjustments to muscle activity – new motor
skills.
e. Gracile & cuneate – sensation for touch, pressure, vibration, & conscious proprioception. (PCML)
f. Others: gustatory, cochlear, & vestibular.
g. CN – VIII, IX, X, XI, XII
G. Cerebellum (pp. 447–449; Fig. 12.15; Table 12.1) inferior & posterior to cerebrum; 2 hemispheres.
1. The cerebellum processes inputs from several structures and coordinates skeletal muscle contraction to
produce smooth movement.
2. There are two cerebellar hemispheres consisting of three lobes each:
a. Anterior and posterior lobes coordinate body movements (governs subconscious aspect of skeletal muscle) and the flocculonodular lobes adjust posture to maintain balance via proprioception.
b. Three paired fiber tracts, the cerebellar peduncles, communicate between the cerebellum and the brain
stem.
3. Cerebellar processing follows a functional scheme in which the frontal cortex communicates the intent to initiate voluntary movement to the cerebellum, the cerebellum collects input concerning balance and tension
in muscles and ligaments (via proprioceptors), and the best way to coordinate muscle activity is relayed back
to the cerebral cortex.
4. The cerebellum may also play a role in thinking, language, and emotion. Ataxia – a-“without”, Taxia-“order”loss of coordinated movements; ie. Alcohol – sobriety tests (touching nose)
H. Functional brain systems consist of neurons that are distributed throughout the brain but work together (pp.
449–452; Figs. 12.16–12.17; Table 12.1).
1. The limbic system is involved with emotions and is extensively connected throughout the brain, allowing it to
integrate and respond to a wide variety of environmental stimuli.
2. The reticular formation extends through the brain stem, keeping the cortex alert via the reticular
activating system and dampening familiar, repetitive, or weak sensory inputs. Ascending & descending
functions
1. Ascending – RAS = consciousness, arousal, attention, prevents sensory overload & muscle tone; inactivation results in sleep or coma.
2. Receives sensory from eyes, ears, others but not nose. Significance during fire?
II. Higher Mental Functions (pp. 452–458; Figs. 12.18–12.20)
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A. Brain Wave Patterns and the EEG (pp. 452–453; Fig. 12.18)
1. Normal brain function results from continuous electrical activity of neurons and can be recorded with an
electroencephalogram, or EEG.
2. Patterns of electrical activity are called brain waves and fall into four types:
a. Alpha waves are regular, rhythmic, low-amplitude, synchronous waves that indicate calm wakefulness.
Ex. person awake with eyes closed; absent when asleep
b. Beta waves have a higher frequency than alpha waves and are less regular, usually occurring when the
brain is mentally focused. NS is active (sensory input & mental activity)
c. Theta waves are irregular waves that are not common when awake, but may occur when concentrating
or emotional stress.
d. Delta waves are high amplitude waves seen during deep sleep, but indicate brain damage if observed in
awake adults.
3. Brain waves change with age, sensory stimuli, brain disease, and chemical state of the body.
4. An absence of brain waves defines brain death.
HI: epileptic seizures
B. Consciousness can be clinically defined on a continuum that measures behavior in response to stimuli and
ranges through several stages: alertness, drowsiness or lethargy, stupor, and coma (pp. 453–454).
1. Consciousness involves simultaneous activity of large areas of the cerebral cortex, is superimposed on other
types of neural activity, and is totally interconnected throughout the brain.
HI: fainting (syncope), coma
C. Sleep and Sleep-Wake Cycles (pp. 454–455; Fig. 12.19)
1. Sleep is a state of partial unconsciousness from which a person can be aroused and has two major types
that alternate through the sleep cycle: non–rapid eye movement (NREM) and rapid eye movement (REM).
a. Non–rapid eye movement (NREM) sleep has four stages, which are characterized by progressively slower
frequency but higher amplitude EEG waves.
b. After reaching NREM stage 4, an abrupt change in brain waves occurs, indicating the onset of rapid eye
movement (REM) sleep. Allows daily analysis and rids unnecessary information; Decreased BP & HR; night
terrors vs. nightmares; alcohol and sleep aids suppress REM.
c. Eyes move rapidly under the eyelids during REM sleep, skeletal muscles are inhibited (prevent acting out
dreams), and most dreaming occurs during this stage.
2. Sleep patterns change throughout life and are regulated by the hypothalamus (circadian rhythm). Suprachiasmatic nucleus (biological clock) regulates the proptic nucleus (sleep-inducing center); orexins act as “sleep
waking chemicals”
3. The hypothalamus inhibits the reticular activating system (RAS), which mediates some sleep stages, especially dreaming.
4. The sleep cycle alternates between NREM and REM sleep, with each REM episode lasting longer than the
previous REM stage.
5. NREM sleep is considered restorative, and REM sleep allows the brain to analyze events or eliminate meaningless information.
6. A person’s sleep requirement declines throughout life; REM sleep declines and then stabilizes at around age
ten, with stage 4 sleep declining steadily from birth.
HI: narcolepsy, insomnia, sleep apnea
D. The ability to both speak and understand language is produced through coordination of several brain areas,
notably Broca’s area and Wernicke’s area (pp. 455–456). Afluent aphasia vs. fluent aphasia
E. Memory is the storage and retrieval of information (pp. 456–458; Figs. 12.20–12.21).
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1. Short-term memory (STM), or working memory, allows the memorization of a few units of information for a
short period of time.
2. Long-term memory (LTM) allows the memorization of potentially limitless amounts of information for very
long periods.
3. Transfer of information from short-term to long-term memory can be affected by a high emotional state,
rehearsal, association of new information with old, or the automatic formation of memory while concentrating on something else.
4. Declarative memory entails learning explicit information “facts” learning; is often stored with the learning
context, and is related to the ability to manipulate symbols and language. NT: ACh – decrease is correlated
with Alzheimer’s
5. Nondeclarative memory “skilled” learning; is less conscious and is categorized into procedural memory, motor memory, and emotional memory. NT: Dopamine – decrease is correlated with Parkinson’s
6. Learning causes changes in neuronal RNA, dendritic branching, deposition of unique proteins at LTM synapses, increase of presynaptic terminals, increase of neurotransmitter, and development of new neurons in the
hippocampus.
HI: Amnesia – anterograde vs retrograde
III. Protection of the Brain (pp. 458–462; Figs. 12.22–12.25)
A. Meninges (developed from ectoderm neural tube) are three connective tissue membranes that cover and protect the CNS, protect blood vessels and enclose venous sinuses, contain cerebrospinal fluid, and partition the
brain (pp. 459–460; Figs. 12.22–12.23). Protective coverings similar to the spinal cord. Exception: dura mater
has 2 layers – which enclose the venous sinuses drains blood into internal jugular.
1. The dura mater is the most durable, outermost covering that extends inward in certain areas to limit
movement of the brain within the cranium.
a. Falx cerebri – separates cerebral hemispheres
b. Falx cerebelli – separates the cerebellar hemispheres
c. Tentorium cerebelli – separates the cerebrum & cerebellum
2. The arachnoid mater is the middle meninx that forms a loose brain covering.
3. The pia mater is the innermost layer that clings tightly to the brain.
HI: meningitis, encephalitis, lumbar tap
B. Cerebrospinal Fluid (pp. 460–462; Figs. 12.24–12.25)
1. Cerebrospinal fluid (CSF) is the fluid found within the ventricles of the brain and surrounding the
brain and spinal cord. Clear liquid 2%; consumes 20% body’s O2 & glucose = ATP. No glycogen; supply of glucose must be continuous. Continuously circulates in subarachnoid space pathway through 4 primary ventricles
and associated structures. Also contains small amounts proteins, lactic acid, ions (more Na+, Cl-, H+ than blood
but less Ca2+ & K+), urea, WBCs.
HI: hydrocephalus
2. CSF Functions
a. Mechanical shock absorber. Creates buoy; prevents crushing of brain from own weight.
b. Homeostatic control of pH affecting pulmonary ventilation & cerebral blood flow. Transport system for
polypeptide hormones secreted by hypothalamus (ex. Prolactin, ACTH, ADH, insulin, etc.)
c. Circulation – Carries small amounts of O2, glucose, etc. to neurons & neuroglia.
3. Formation from choroid plexuses covered by ependymal cells with tight junctions (BCSFB). Bidirectional &
continuous.
4. Circulation: Reabsorbed into blood through arachnoid villi. Rate of formation = rate of absorption. D/O =
hydrocephalus.
C. The blood brain barrier is a mechanism that helps maintain a protective environment for the brain (p. 462).
Formed by astrocytes; drugs enter concentrated sugar solution causing high osmotic pressure = endothelial
cells & capillaries to shrink resulting in gaps in between tight junctions  drug leakage.
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1. Nutrients, essential amino acids, and some electrolytes are allowed to pass into cerebrospinal fluid, but
metabolic wastes, proteins, toxins, and drugs are excluded.
2. Lipid-soluble molecules easily cross the blood brain barrier. As well as O2, CO2, alcohol, nicotine, anesthetics.
D. Homeostatic Imbalances of the Brain (pp. 462–464) refer to In-class Warm Ups
1. Traumatic head injuries can lead to brain injuries of varying severity: concussion, contusion, and subdural or
subarachnoid hemorrhage.
2. Cerebrovascular accidents (CVAs), or strokes, occur when blood supply to the brain is blocked, resulting in
tissue death. Compare to TIAs (transient Ischemia Attacks) – temporary cerebral dysfunction d/t impaired
blood flow; 5-10 mins; S/SX: dizziness, weakness, numbness, and paralysis in a limb; Bell’s palsy, slurred or
difficulty understanding speech; loss of vision or double vision.
3. Alzheimer’s disease is a progressive degenerative disease that ultimately leads to dementia.
4. Parkinson’s disease results from deterioration of dopamine-secreting neurons of the substantia nigra and
leads to a loss in coordination of movement and a persistent tremor.
5. Huntington’s disease is a fatal hereditary disorder that results from deterioration of the basal nuclei and
cerebral cortex.
IV. The Spinal Cord (pp. 464–474; Figs. 12.26–12.32; Tables 12.2–12.3)
A. Gross Anatomy and Protection (pp. 464–466; Figs. 12.26–12.27)
1. The spinal cord extends from the foramen magnum of the skull to the level of the first or second lumbar
vertebra and provides a two-way conduction pathway to and from the brain and serves as a major reflex
center. Define & locate the following structures:
1. Conus Medullaris –tapering of spinal cord at L1/L2
2. Filium Terminale – extension of pia mater & anchors the spinal cord to coccyx.
3. Dorsal Root Ganglia vs. Ventral Root Ganglia
4. Cauda Equina – aka: horse’s tail; inferior to conus medullaris
2. Fibrous extensions of the pia mater anchor the spinal cord to the vertebral column and coccyx, preventing
excessive movement of the cord.
1. Dura Mater – first meninx; tough, strong DICT.
a. Subdural Space – interstitial fluid; serous fluid
2. Arachnoid Mater – middle layer. Avascular, loose collage & elastic fibers.
a. Subarchnoid Space – contains CSF.
3. Pia Mater – innermost layer; contains b.v. Adhered to brain & spinal cord. Denticulate ligaments fuse
other layers to protect the spinal cord from sudden displacement that could result in shock.
 Administer – anesthesia, antibiotic, chemo, media (myelography)
 Withdraw – CSF biopsy.
 Pressure regulation
3. The spinal cord has 31 pairs of spinal nerves along its length that define the segments of the cord.
4. There are cervical and lumbar enlargements for the nerves that serve the limbs and a collection of nerve
roots (cauda equina) that travel through the vertebral column to their intervertebral foramina.
B. Spinal Cord Cross-Sectional Anatomy (pp. 466–468; Figs. 12.28–12.30)
1. Two grooves partially divide the spinal cord into two halves: the anterior and posterior median fissures.
2. Two arms that extend posteriorly are dorsal horns (sensory), and the two arms that extend anteriorly are
ventral horns (motor).
3. In the thoracic and superior lumbar regions, there are also paired lateral horns that extend laterally between the dorsal and ventral horns (ANS – visceral motor).
4. Afferent fibers from peripheral receptors form the dorsal roots of the spinal cord.
5. The white matter of the spinal cord allows communication between the cord and brain.
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C. Neuronal Pathways (pp. 468–473; Figs. 12.31–12.32; Tables 12.2–12.3) Recall that tracts are the similar to
nerves but in the CNS. Usually named by beginning and end of pathway.
1. All major spinal tracts are part of paired multineuron pathways that mostly cross from one side to the other,
consist of a chain of two or three neurons, and exhibit somatotopy.
2. Ascending (Ascending) pathways conduct sensory impulses upward through a chain of three neurons. Input
from PNS to the CNS. Know: DCML, Lateral spinothalamic, & Spinocerebellar pathways
a. Nonspecific ascending pathways receive input from many different types of sensory receptors, and make
multiple synapses in the brain.
b. Specific ascending pathways mediate precise input from a single type of sensory receptor.
c. Spinocerebellar tracts convey information about muscle and tendon stretch to the cerebellum.
3. Descending (Descending) pathways involve two neurons: upper motor neurons and lower motor neurons.
Output from CNS to PNS Know: pyramidal (corticospinal), Tectospinal & Rubrospinal Pathways
a. The direct, or pyramidal, system regulates fast, finely controlled or skilled movements. (EX: needle-work
& writing)
b. The indirect, or extrapyramidal, system regulates muscles that maintain posture and balance and control
coarse limb movements (Rubrospinal) and head, neck, and eye movements involved in tracking visual objects (Tectospinal).
D. Spinal Cord Trauma and Disorders (p. 474)
1. Any localized damage to the spinal cord or its roots leads to paralysis (loss of motor function) or paresthesias (loss of sensory function).
a. Severe damage to the ventral root or ventral horn results in flaccid paralysis, since nerve impulses are not
transmitted to the skeletal muscles.
b. When upper motor neurons of the primary motor cortex are damaged, spastic paralysis occurs, in which
voluntary control over skeletal muscle is lost.
c. If damage to the spinal cord occurs between T1 and L1, lower limbs are affected, resulting in paraplesia,
but if the damage occurs in the cervical region, all four limbs are affected, resulting in quadriplegia.
2. Poliomyelitis results from destruction of ventral horn motor neurons by the poliovirus.
3. Amyotrophic lateral sclerosis (ALS), or Lou Gehrig’s disease, is a neuromuscular condition that involves progressive destruction of ventral horn motor neurons and fibers of the pyramidal tracts.
V. Diagnostic Procedures for Assessing CNS Dysfunction (pp. 474–475) Refer to pp. 18-19
A. CT scans and MRI scanning techniques allow visualization of most tumors, intracranial lesions, multiple sclerosis plaques, and areas of dead brain tissue (p. 474).
B. PET scans can localize brain lesions that generate seizures and diagnose Alzheimer’s disease (p. 474).
C. Ultrasound of the carotid arteries can detect narrowing of the artery, or a reduction in blood flow (p. 475).
VI. Developmental Aspects of the Central Nervous System (pp. 475–477;
Figs. 12.33–12.35)
A. In a three-week-old embryo, the ectoderm thickens, forming a neural plate that in turn folds inward to form a
neural groove between neural folds (p. 475; Fig. 12.33). Begins in 3rd week with thickening of ectoderm called
neural plate  neural tube.
B. Neural fold cells migrate laterally which, after the superior edges fuse, form the neural crest (p. 475; Fig.
12.33).
C. Following the formation of the neural tube, the brain forms on the rostral end, while the spinal cord forms on
the caudal portion (p. 475).
D. By six weeks into development, each side of the neural tube has formed two clusters (p. 475; Fig. 12.33):
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1. The dorsal alar plate becomes interneurons that, along with some basal plate neurons, give rise to the white
matter of the spinal cord that expands to form the H-shaped central mass of gray matter.
2. The ventral basal plate develops into motor neurons that grow axons outward toward effector organs.
E. Neural crest cells alongside the cord form the dorsal root ganglia and send axons into the dorsal side of the
cord (p. 475; Fig. 12.34).
F. Gender-specific areas of the brain and spinal cord develop depending on the presence or absence of testosterone (p. 475).
G. Several homeostatic imbalances exist that involve improper development of the nervous system (p. 476; Fig.
12.35). *Folic Acid deficiency, in 1st few weeks, can lead to spina bifida & anencephaly
1. Lack of oxygen to the developing fetus may result in cerebral palsy, a neuromuscular disability in which voluntary muscles are poorly controlled or paralyzed as a result of brain damage.
2. Anencephaly results when the cerebrum and part of the brain stem do not develop.
3. Spina bifida occurs when the vertebral arches form incompletely and can be mild to severe.
H. Growth and maturation of the nervous system continue throughout childhood and largely reflect development
of myelination, which progresses in a superior-to-inferior direction (p. 477).
I. Age brings some cognitive decline, but losses are not significant until the seventh decade of life (p. 477). Aging
NS = number of neurons is same but mass & synaptic contacts decline; velocity decreases, processing diminishes, voluntary motor control slows & reflex time increases.
***Don’t forget the At-The-Clinic Terms p. 482***
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INSTRUCTOR GUIDE FOR HUMAN ANATOMY & PHYSIOLOGY, 9e
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