CHAPTER 14: The Brain and Cranial Nerves
Brain
Principal Parts
1. The four principal parts of the brain are: (1) the cerebrum; (2) the diencephalon; (3) the
brainstem; and (4) the cerebellum.
2. The diencephalon consists of the thalamus and hypothalamus; the brainstem consists of the
midbrain, pons, and medulla oblongata.
Protection and Coverings
1. The brain is protected by neurocranial skull bones (8 of these – can you name them?), meninges,
and cerebrospinal fluid (CSF).
2. The cranial meninges are continuous with the spinal meninges and (from superficial to deep) are: the dura
mater, arachnoid mater, and pia mater.
Cerebrospinal Fluid (CSF)
1. CSF is formed by the choroid plexus via filtration from the blood and circulates continually
through
the ventricles, subarachnoid space, and central canal of the cord. It is reabsorbed back into
venous
blood via arachnoid villi.
2. CSF protects, serving as a shock absorber; delivers nutritive substances from the blood and removes
wastes.
Blood Supply to the Brain
1. Any interruption of the oxygen supply to the brain can weaken, permanently damage, or kill brain
cells.
2. Glucose deficiency may produce dizziness, convulsions, and unconsciousness.
3. The blood-brain barrier (BBB) explains the differential rates of passage of certain material from
the
blood into the brain.
4. Circle of Willis (aka: Cerebral Arterial Circle) is fed from two sources: (1) the R + L Vertebral
Arteries (off the Subclavian Arteries) and (2) the R + L Internal Carotid Arteries (off the
Common Carotid Arteries). (See pp. 724 – 726)
Cerebrum
1.
The Cerebrum (“big brain”) is the largest part of the brain. Its cortex contains convolutions (gyri)
alternating with sulci and fissures.
2. The cerebral lobes are named for the neurocranial bones that overly them: frontal, parietal,
temporal, and occipital.
3. The left hemisphere is more important for right-handed control, spoken and written language,
numerical and scientific skills, and reasoning. The right hemisphere is more important for lefthanded control, musical and artistic awareness, spatial +pattern perceptions, insight, imagination,
and generating mental images of sight, sound, touch, taste, and smell.
4. The median fissure separates the two hemispheres; the central sulcus separates the frontal / parietal
lobes; the lateral sulcus separates the temporal/parietal lobes; the parieto-occipital sulcus separates
the parietal/occipital lobes.
5. Cerebral cortex is grey matter; white matter is under the cortex and consists of myelinated axons
running in three principal directions: (1) Associate tracts: conduct impulses between gyri in the
same hemisphere; (2) Commissural tracts: conduct impulses from gyri in one hemisphere to
corresponding gyri in the other (corpus callosum, anterior commissure, and posterior commissure)
and (3) Projection tracts: conduct nerve impulses from the cerebrum to lower parts of the
CNS (thalamus, brainstem, or spinal cord) or from these lower parts of CNS to the cerebrum.
(Example: the Internal Capsule)
6. The basal ganglia are paird masses of gray matter in the cerebral hemispheres that help control
muscular movements. (These should be called the “basal nuclei” – Why? But, they were
originally named “basal ganglia” and the name “stuck”.)
7. The limbic system, found in the cerebral hemispheres and diencephalon, functions in emotional
aspects of behavior and memory.
8. The sensory areas of the cerebral cortex interpret sensory impulses. The motor areas govern
muscular movement. (The primary somatosensory area is the post-central gyrus in each temporal
lobe; the primary motor area is the precentral gyrus in each frontal lobe. (There are additional
sensory and motor areas). The association areas are concerned with emotional and intellectual processes.
9. Memory is the ability to recall thoughts. It is classified into two kinds:
short-term and long-term memory.
10. Brain waves generated by the cerebral cortex are recorded as an electroencephalogram (EEG).
These “records” may be used to diagnose epilepsy, infections, and tumors.
Diencephalon
1.
The diencephalon consists of the superior-lying thalamus and the inferior-lying hypothalamus
(There is also a small epithalamus, consisting of the pineal gland and habenular nuclei just superior
and posterior to the thalamus which is considered part of the diencephalon.)
2. The thalamus contains many nuclei that serve as relay stations for sensory impulses to the cerebral
cortex: ALL SENSATIONS except for SMELL are relayed through here! It also registers
conscious recognition of pain, temperature, and some awareness of light touch and pressure.
3. The thalamus constitutes 4/5ths of the diencephalon and forms most of the side walls of the
midlying Third Ventricle.
4. The hypothalamus is inferior to the thalamus. It controls and integrates the ANS. (It is the main
regulator of visceral activities since it acts as a liason between the cerebral cortex and the
autonomic nerves.) It also (1) processes feelings of hunger, fullness, and thirst so that you can
respond accordingly; (2) functions in rage and aggression behaviors; (3) controls normal body
temperature; (4) coordinates mind-over-body-phenomena; (5) maintains conciousness (waking
state) / sleep patterns and acts as a “pace-maker” of biological rhythm; and (6) makes chemicals
called “regulating factors”(aka “releasing hormones”) that, in turn, control the functioning of the
Pituitary Gland, as well as two hormones: Oxytocin and Antidiuretic Hormone (ADH).
5. The hypothalamus constitutes 1/5th of the diencephalon, and form the floor of the Third Ventricle.
Brain Stem
1. The Midbrain connects the pons and the diencephalon. It conveys (1) motor impulses from the
cerebrum to the cerebellum and cord and (2) sensory impulses from the cord to the thalamus;
(3) Cranial Nerves III and IV arise here; (4) location of nucleus for / origin of the spinal tracts
concerned with muscle tone and posture; (5) Auditory and Visual reflexes regulated here.
2. The Pons is inferior to the Midbrain (and superior to the Medulla). It connects the spinal cord with
the brain and (1) links parts of the brain with one another via tracts; (2) acts as the relay “station”
for impulses to (voluntary) skeletal movements from cerebral cortex to cerebellum; (3) Cranial
Nerves V – VIII1(Vestibular N. ) arise here; (4) contains two important respiration control areas:
Pneumotaxic Area – inhibits inspiration and facilitates expiration, and the Apneustic Area –
prolongs inspiration and inhibits expiration.
3. The Medulla Oblongata extends from the inferior border of the Pons and is continuous with the
upper part of the spinal cord. It consists of all sensory (ascending) and motor (descending) tracts
extending between the cord and other parts of the brain. It contains (1) regions for regulating
vital reflexes including heart rate, blood pressure and respiration; (2) nuclei for non-vital reflexes
having to do with diameter of blood vessels (vasoconstriction/vasodilation), swallowing,
coughing, vomiting, sneezing, and hiccupping; (3) Cranial Nerves VIII2(Cochlear) – XII arise
here; and (4) the Reticular Formation which functions in consciousness and arousal from sleep.
Cerebellum
The cerebellum (“Little Brain”) occupies the inferior and posterior aspects of the cranial cavity. It
consists of two hemispheres with a cortex of gray matter (thrown into tiny folds known a “folia”) and
an interior of white matter (in a configuration known as the “arbor vitae”).
2. It constitutes only about one-tenth of the mass of the brain, but contains nearly half of all of the
brain’s neurons.
1.
3. The cerebellum coordinates skeletal muscles and maintains normal muscle tone and body
equilibrium.
Neurotransmitters
1. Numerous substances are either known or suspected neurotransmitters that function to facilitate
and excite or inhibit postsynaptic neurons.
2. Examples of neurotransmitters include acetylcholine (ACh), dopamine, epinephrine, norepinephrine, serotonin, gamma aminobutyric acid (GABA), substance P, endorphins, and enkephalins.
Cranial Nerves – PNS
1. Twelve pairs of Cranial Nerves originate from the brain. (Actually, only 10 come off the brain;
Cr. N. I (Olfactory) originates from the nasal mucosa; and Cr. N. II (Optic) originates from
the retina of the oculus.
2. Like Spinal Nerves, they are part of the PNS. See Table 14.3 for a summary of Cranial Nerves
Also, refer to Pages 477 – 484 of the Textbook.
3. You need to know the Names, Roman Numeral designations, and Functions of the
12 (pairs of) Cranial Nerves (and whether they are motor, sensory, or BOTH)