Anatomically, the human brain shares many basic structures and brain areas with the
brains of other animals. For example, in the much smaller sheep brain (see Image
below), one observes a cerebrum, a brain stem, cerebellum, medulla oblongata and
glands, such as the pineal gland and the pituitary gland.
Like the human brain it's surface is also thrown into many convolusions, called gyri and
fissures. It possesses a rather large olfactory bulb, a pronounced optic nerve (CN II) and
an optic chiasma. Cerebral peduncles are found on the ventral aspect of the midbrain,
posterior to the single mammilary body. The same number of 12 cranial nerves as found
in the human brain, emerge from the base of the brain. Internally, the sheep brain also
reveals a thalamus, a corpus callosum, ventricles, hypothalamus, the corpora
quadrigemina of the midbrain, the pineal gland, and the pons.
The inner anatomy of a sheep brain
Know the names and location of the different functional (sensory, motor &
association) areas of the cerebral cortex, which are: (see also Figure 10-3 below).
1. Sensory areas
- the areas are located posterior to the central sulcus
- the individual areas are:
1. Primary somatosensory area
2. Primary visual area
3. Primary auditory area
4. Primary olfactory area
2. Motor areas
- the areas, such as the Primary motor area, Premotor area, are located anterior to
the central sulcus.
3. Association areas
Figure 10-3: Lobes and functional areas of the human cerebrum
Know that within the sensory cortex and the motor cortex the neurons are
communicating with different regions of the human body. The number of neurons
and neuron density is different for the different regions of the human body.
- See and study the maps of the human sensory and motor cortex below
- The hands, feet and face are over-proportionally represented in the sensory cortex
due to their high importance in perception and transmission of sensory inputs, e.g.
touch, warmth, cold, etc., from the periphery towards the brain.
- The hands and face (especially the lips and tongue) are over-proportionally
represented in the motor cortex.
Know the difference between the somatic sensory and somatic motor pathway of the
cerebrum.
All efferent somatic motor information descends mainly from the primary motor area
of the cortex via the brain stem to the skeletal muscles.
- they control all voluntary and involuntary movements of the body
Route: axons of motor neurons begin in primary motor area → internal capsule →
upper motor neurons → pons → medulla → left lateral corticospinal tract or right
anterior corticospinal tract → lower motor neurons (brain stem or in anterior grey
horn of spinal cord) → spinal cord → muscle
- axons converge at the lower motor neurons located in the brain stem or in anterior
grey horn of spinal cord
- lower motor neurons receive modulatory signals from other neurons such as:
1. "Local interneurons"
- involved in rhythmic muscle activity, e.g. walking
2. Upper motor neurons
- control of voluntary movements
3. Basal ganglia
- help to initiate and terminate movements, control muscle tone, suppression of
unwanted movements
4. Cerebellum neurons
- help to coordinate body movements
Pathology
"Damage or disease (e.g. viruses) of LOWER MOTOR NEURONS leads to flaccid
paralysis of muscles on the SAME SIDE of the body. Under these conditions, muscles
lack voluntary control and reflexes; the muscles remain flaccid (limp) due to decreased
or lost muscle tone. Injury or disease of UPPER MOTOR NEURONS causes spastic
paralysis of muscles located on the opposite side of the body; in this condition the
muscle tone is increased, reflexes are exaggerated, and pathological reflexes appear."
All afferent) somatic sensory information from the body surface ascends to the
primary somatosensory area of the cerebral cortex via 2 main somatic sensory
pathways, which are:
1. The posterior column - medial lemniscus pathway
Carries nerve signals or fine touch, proprioception and vibration from somatic sensory
receptors in peripheral body parts via tracts located in the posterior column of the
spinal cord and via the medial lemniscus area of the midbrain towards the primary
somatosensory area of cortex. Nerve impulses conducted along the posterior columnmedial lemniscus pathway give rise to three main types of sensations:
I. Fine touch
- ability to recognize what point on the body is touched
- recognition of shape, size and texture
II. Proprioception
- awareness of the precise position of body parts
- signals the awareness of the directions of body movements = kinesthesia
III. Vibratory sensations
- signals of rapidly fluctuating touch stimuli
2. The anterior spinothalamic pathway
Carries nerve signals from peripheral receptors for pain, hot and cold temperature,
tickle, and itch sensations via two spinal cord tracts towards the primary
somatosensory area of the cerebral cortex. The two, ascending nerve signals conducting
spinal cord tracts are:
I. Anterior spinothalamic tract
II. Lateral spinothalamic tract
Know that (despite the brain symmetry) there exist functional differences between
the two brain hemispheres, which is termed Hemispheric laterization:
1. Left hemisphere
- receives sensory signals from right body side
- controls motor functions of right body side
- important for spoken and written language, numerical and scientific skills
- important for reasoning in most people
2. Right hemispere
- receives sensory signals from left body side
- controls motor functions of left body side
- important for musical and artistic awareness
- important for spatial and pattern perception
- recognition of faces and emotional content of language
- generates mental images of sight, sound, touch, taste and smell
Know that memory is the capacity and great achievement of the brain to store and
retrieve information once acquired through learning or experience. Know that the parts
of the brain important for memory include:
1. Association cortex of the frontal, parietal, occipital and temporal lobes
2. Parts of the limbic system
3. Parts of the diencephalon
4. Basal ganglia & cerebellum (→ memory of motor skills)
Know that brain functions, e.g. consciousness, can be altered by a series of drugs and
pharmaceuticals. Drugs (legal of illegal) as well as pharmaceuticals can interfere with
nervous transmission and in many cases can cause the long-term development of habit
or a drug addiction. Some can induce immediate mental alterations and hallucinations,
e.g. LSD, cocaine, and show measurable changes in brain functions (--> see PET Figures
below). All of them cause tolerance or addiction.
In tolerance higher drug doses are required to achieve the same effect, while in
addiction lack of drug access can cause severe physiological and psychological
problems.
Drugs can be divided into 3 major groups which are:
1. Popular, legal drugs
- e.g. caffeine, nicotine (tobacco), alcohol
2. Pharmaceutics
- sold in pharmacies with a medical prescription
- e.g. stimulants, sleeping pills, sedatives, pain killers
3. Prohibited, illegal substances/drugs
- e.g. heroin, cocaine, amphetamines, cannabis (ashis/marijuana), hallucinogens
(e.g. LSD, mescal, ecstasy)
Be able to identify the 12 pairs of cranial nerves (by name and numbers) and know
their biological functions.
The 12 pairs of cranial nerves (CN) which are designated with roman numerals (CN I
→ CN XII), are part of the PNS. The names, components and biological function of the
cranial nerves are summarized in the Table and Figure below.
The 12 pairs of cranial nerves primarily serve the head and neck and only the Vagus
nerve (CN X), which is part of the ANS, extends into the thoracic and abdominal
cavities. With the exception of CN I & CN II, all cranial nerves are mixed nerves, which
means, that they are containing motor AND sensory nerve fibers.
Techniques for testing cranial nerve function/condition is an important part of routine
neurological examination of patients for traumatic injury of the brain. An easy way to
remember the sequence of cranial nerves within the human brain is the following
catchy saying:
"On occasion our trusty truck acts funny - very good Vehicle anyhow"
Number
Name
Components (Receptors/Axons)
Function
I
Olfactory nerve
Purely sensory (Nose)
Transmission of
sense of smell
via olfactory
bulb
II
Optic nerve
Purely sensory (Retina)
Vision
Sensory part (Eye balls)
III
Oculomotor nerve
Motor part (muscles of eye balls)
IV
Trochlear nerve
Sensory part (Superior oblique
muscle)
Proprioception
(= muscle sense)
of eye balls
Movement of
skeletal muscles
controlling
eyelid and eye
balls;
Parasympathetic
control of ciliary
muscle of eye
ball
and of smooth
muscles
sphincter
muscle of iris
Proprioception
of superior
oblique muscles
Motor part (Superior oblique
muscle)
V
Trigeminal nerve
Touch, pain
and
temperature
sensations from
skin of face and
anterior scalp,
mucosae of
Sensory part
mouth and nose;
1. Ophthalmic nerve
+
(scalp/forehead)
proprioception
2. Maxillary nerve
(Test: Patient's
facial sensation
(lower eyelid, nose,
upper teeth, upper lip & pharynx of pain, touch,
and temperature
3. Mandibular nerve
are tested with
(tongue, lower teeth
the help of
& lower side of face)
safety pins, hot
and cold
objects)
Motor part (motor control of
chewing muscles)
VI
VII
Abducens nerve
Facial nerve
Movement of
the eye balls
Activation of
chewing
muscles
(Test: Patient is
asked to clench
teeth, open
mouth against
resistance, and
to move jaw
side to side)
Proprioception
of lateral rectus
Sensory part (lateral rectus
muscle
muscles, eye ball positioning)
Control of
Motor part (lateral rectus muscles) movement of
eye balls
Sensory part (Taste buds on
Taste &
tongue, proprioceptors of muscles Proprioception
of face and scalp)
(Test: Anterior
two thirds of
tongue is tested
for
Motor part (facial, scalp and neck ability to taste
sweet, salty,
muscles
Control of lacrimal and salivary
glands)
Sensory part (Vestibule)
Motor part (Vestibule)
VIII
Vestibulocochlear nerve
Sensory part (Cochlea)
Motor part (Cochlea)
IX
Glossopharyngeal
nerve
Sensory part (Taste buds on
posterior third, of tongue,
swallowing proprioception,
carotid sinus, carotid body)
sour and bitter
substances)
1. Control of
facial
expressions via
skeletal muscles
(Test: Patient is
asked to close
eyes, smile,
whistle, etc.)
2.
Parasympathetic
control of
secretion of
tears and saliva
(Test: Patient's
ability to
produce tears is
tested with the
help of
ammonia
fumes)
Sensation of
Equilibrium
Sensitivity of
hair cells
adjustment
Sensation of
hearing
(Test: Patient's
hearing ability
is checked by air
and bone
conduction
using tuning
forks)
Modulation of
cochlear hair
cell responses
Taste, touch,
pain,
temperature
sensation from
tongue;
blood pressure
monitoring,
oxygen and CO2
monitoring,
Motor part (swallowing muscles, Actions: Somatic
motor neurons
throat, salivary gland)
activate
swallowing
muscles;
Parasympathetic
axons stimulate
secretion of
saliva
(Test: Patient is
checked for gag
and swallowing
reflexes;
patient is asked
to speak and
cough;
X
Vagus nerve
(also major part of
the parasympathetic
division of the ANS)
Taste, touch &
temperature
sensation from
throat and
Sensory portion (Proprioception
pharynx;
of muscles of neck and throat,
monitoring of
carotid sinus;
blood pressure;
Stretch and chemoreceptors in
Monitoring of
carotid body and carotid sinus;
blood oxygen
Visceral sensory receptors in
and CO2 level;
organs of thoracic and abdominal sensations from
cavities
visceral organs
Swallowing,
coughing and
voice
production;
Motor portion (Muscles of throat
smooth muscle
and neck, smooth muscles in the
contraction &
airways, esophagus, stomach,
relaxation of GI
small intestines, most of large
tract organs;
intestine, gall bladder, cardiac
slowing of heart
muscle, glands of GI tract)
rate;
secretion of
digestive fluids
Sensory part (Proprioceptors in
muscles of throat and voice box)
XI
Accessory nerve
Proprioception
Actions:
Swallowing;
rotation of head
and movement
of shoulders
(Test: patient is
Motor part (Muscles of throat and asked to rotate
neck; provides motor fibers to
head and
sternocleidomastoid and trapezius elevate shoulder
muscle)
against
resistance)
Proprioception
Sensory part ( Proprioceptors in
tongue muscles)
XII
Hypoglossal nerve
Motor part ( Motor fibers serve
muscles
of tongue)
Actions:
Movement of
tongue during
speech and
swallowing
(Test: Patient is
asked to
protrude and
retract tongue)
Figure 10-3: Brain stem and the cranial nerves of the human brain
Medicine & Pathology:
Make yourself familiar with the meaning of the following conditions, diseases and
disorders connected to the functions of the tissues of the central nervous system
(CNS):
Key term
Neurology
The scientific study of nerve tissue & brain function
Neurogenesis
The biological process through which new neurons are created; very limited in the adult
human brain.
Neuralgia
Severe pain that occurs along a nerve with unknown cause; may occur as repeated
stabs of pain in the teeth, sinuses, eyes, tongue, face, or throat; occurs most frequently in
two cranial nerves:
1. Trigeminal nerve (= CN V)
- pain in eyes, face, sinuses, and teeth
2. Glossopharyngeal nerve (= CN IX)
- pain in the back of tongue and throat
Neuritis
A painful inflammation of a nerve usually caused by disease or traumatic injury.
Infection by bacteria (tuberculosis, syphilis), viruses (Herpes zoster), bad diet habits,
vitamin deficiency and certain diseases (diabetes) can cause neuritis. If a neuritis
remains untreated and continues for a long period, the affected nerve(s) can become
damaged beyond repair.
Primary Amebic Meningoencephalitis (PAM)
A human disease caused by the waterborne protozoan Naegleria fowleri. It is
characterized by an infection of the meninges and other parts of the central nervous
system. The disease usually occurs in children or young adults after swimming in lakes
or pools, bathing in natural hot springs, or after water skiing in waters carrying the
pathogenic microbe. After nasal infection the protozoan reaches the brain where it
initiates the - often fatal - inflammatory reaction.
Electroencephalogram (EEG)
Technical recording of the nerve signals of the brain.
Positron emission tomography (PET)
A very sensitive medical technology that uses radioactive tracers to visualize brain
function. The brain scans of measured subjects' reveal the overall brain activity while
they perform certain tasks, e.g. working memory, or during exposure to certain drugs,
such as cocaine or amphetamine (see Figure below).