The brain is the control center of the nervous system and, along with the spinal [619460]

BRAIN
The brain is the control center of the nervous system and, along with the spinal
cord, forms the
central nervous system
.
It occupies the
cranial cavity and can be divided into four main parts: the
brainstem
,
cerebellum
,
diencephalon
, and
cerebrum
.
It is covered by layers of fascia known as
meninges
and
contains cavities filled with cerebrospinal fluid.
The gross appearance of the brain shows gray and white matter.
Gray matter
contains the neuronal cell bodies and is
found in
the surface of the cerebral and cerebellar hemispheres, as well as in several deep
nuclei (ganglia).
White matter
is formed by myelinated
neuronal axons and forms most of the brain, connecting it to the spinal cord and
cranial nerves.
BRAINSTEM
The brainstem consists of the
medulla oblongata
,
pons
, and
midbrain
. It lies medially
and inferiorly and is continuous
inferiorly with the cervical spinal cord at the
foramen magnum. Its fibers connect the peripheral and central nervous
systems. It
contains the nuclei from which most
cranial nerves
originate,
as well as the
vital centers
that regulate
breathing,
digestion, heart rate, blood pressure, and consciousness.
Medulla oblongata
The medulla oblongata is 3 cm in length and is the most inferior portion of the
brainstem. It is continuous with the
spinal cord and extends superiorly from the
foramen magnum to the pons where the border is marked by a groove.
It displays the following surface features:
Pyramids
The pyramids are two club-like enlargements on the anterior surface of the length
of the medulla. They
taper towards the spinal cord and contain the
corticospinal
tracts
, of which 90% of their fibers cross
over to the opposite
side to form the
pyramidal decussation
.
Olive
The olives are two bulges located on the anterolateral side of the medulla, just
lateral to the pyramids.
Each contains the
inferior olivary nucleus

which relays sensory information to the cerebellum via the
inferior cerebellar peduncles.
Gracile fasciculus
The gracile fasciculus lies on the posterior aspect of the medulla, on either side
of the posterior median
septum. It is formed by the gracile nucleus which relays
sensory information from the lower body to the
thalamus
,
via the
medial lemniscus
.
Cuneate fasciculus
The cuneate fasciculus lies on the posterior aspect of the medulla, lateral to the
gracile fasciculus. It is
formed by the caudate nucleus which relays sensory information
from the upper body to the
thalamus
,
via the
medial
lemniscus
.
Anterior median fissure
The anterior median fissure is a groove that runs along the midline of the anterior
surface of the
brainstem.
© Primal Pictures Ltd. 2014

Internally, the medulla oblongata contains the following nuclei:
Cardiac center
Regulates the heart rate and force of contraction.
Respiratory center
Regulates respiratory movements.
Vasomotor center
Regulates blood vessel diameter.
Special senses nuclei
The nuclei of the following cranial nerves are located in the medulla oblongata:
Gustatory nucleus (IX)
Cochlear nuclei (VIII)
Vestibular nuclei (VIII)
Cranial nerve nuclei
The nuclei of the following cranial nerves are located in the medulla oblongata:
Glossopharyngeal (IX)
Vagus (X)
Accessory (XI)
Hypoglossal (XII)
Pons
The pons is a bulge located on the anterior surface of the brainstem, in front of
the cerebellum. It is 2.5 cm in length
and forms the origin of the middle cerebellar
peduncles.
It consists of fibers descending from the cerebrum to the cerebellum and spinal
cord, ascending fibers to the
thalamus, and fibers that connect the two lobes
of the cerebellum.
Internally, the pons contains the following nuclei:
Pontine nuclei
Located anteriorly in the pons, they connect the cerebrum to the cerebellum and
co-ordinate voluntary movement.
Cranial nerve nuclei
The nuclei of the following cranial nerves are located in the posterior part of
the pons:
T
rigeminal (V)
A
bducens (VI)
F
acial (VII)
Vestibulocochlear (VIII)
Midbrain
The midbrain is the smallest part of the brainstem measuring 1.5 cm. It is responsible
for the visual and gustatory
response, as well as the co-ordination of movement.
It displays the following features:
Tectum
The tectum forms the dorsal surface of the midbrain and the roof of the cerebral
aqueduct.
It contains four nuclei which form four mounds, collectively known as the
quadrigeminal bodies
:

Superior colliculi
The superior colliculi are the two superior quadrigeminal bodies and they control
the visual response.
Inferior colliculi
The inferior colliculi are the two inferior quadrigeminal bodies and they control
the auditory response.
Cerebral crus
Located inferior to the tegmentum, the cerebral crus consists of descending tracts
from the cerebrum to
the spinal cord and cerebellum.
Internally, the midbrain contains the following nuclei:
Tegmentum
The tegmentum forms the inner mass of the midbrain and lies between the substantia
nigra and the reticular
formation. It contains ascending tracts from the spinal
cord to the brain and the red nucleus. It controls fine motor
functions.
Substantia nigra
The substantia nigra is a pigmented lamina located between the tegmentum and cerebral
crus that helps to co-
ordinate movement.
Red nuclei
The red nuclei make up a highly vascular area that contains cell bodies of fibers
traveling from the cerebrum and
cerebellum to control subconscious movement.
Medial lemniscus
The medial lemniscus is a continuation of the gracile and cuneate tracts of the
brainstem and spinal cord.
Cranial nerve nuclei
The nuclei of the following cranial nerves are located in the central part of the
midbrain:
Oculomotor (III)
Trochlear (IV)
Central gray substance
The central gray substance surrounds the cerebral aqueduct and controls our perception
of pain.
Reticular
formation
The reticular formation is a series of important nuclei that are scattered throughout
the brainstem and upper spinal
cord. They receive sensory information from the body,
and motor signals from the cerebrum. They are important in
the arousal and maintenance
of consciousness and the sleep/wake cycle.
CEREBELLUM
The cerebellum is located in the
posterior part of the cranium and consists of two hemispheres. It controls
muscle co-
ordination, maintains balance and equilibrium, and fine tunes movements
at the conscious and subconscious levels.
It displays the following characteristics:
Vermis
The vermis is a worm-like band
running down the midline, connecting the two cerebellar hemispheres.

Folia
The surface area of the cortex of the cerebellum is greatly increased by folds known
as folia. This increases
the number of neurons that can be contained within the
cortical layers.
Deep nuclei
Within the white matter of each hemisphere are four deep nuclei, through which
all information leaving the
cerebellum passes.
The cerebellum is connected to the brainstem by three tracts:
Inferior cerebellar peduncle
The thin inferior peduncles connect the cortex of the cerebellum to the
medulla oblongata
.
They consist of both motor and sensory fibers; ascending and descending tracts from
the spinal cord.
Middle cerebellar peduncle
The middle peduncles are the largest and connect the cerebellar hemispheres to the
pons
.
They consist mainly of motor and sensory tracts connecting to the pons.
Superior cerebellar peduncle
The superior peduncles connect the deep nuclei of the cerebellum to the midbrain,
diencephalon, and
cerebrum.
They consist mainly of motor fibers leaving the cerebellum to reach the brain.
The cerebellum is separated from the pons and medulla oblongata anteriorly by the
fourth ventricle
. The roof and floor of
the ventricle
are formed by the superior and inferior medullary velum.
The cerebellum receives information from the cerebral cortex, eye, ear, and the
muscles of the body. It monitors the
intentions for movement and the actual movements
that occur, combining this information to evaluate how the body is
performing. It
then sends feedback to the cortex to initiate any necessary adjustments via the
thalamus
. This process
helps to smooth
and co-ordinate complex movements, and regulates balance. It also stores this data
which allows for the
learning of skilled activities.

HISTOLOGY IMAGES
The thumbnail below shows a photomicrograph of the cerebellar cortex. Note the large
neurons known as Purkinje
cells with their complex dendritic arborizations, and
the densely packed neuronal cell bodies.
DIENCEPHALON
The diencephalon lies between the brainstem and the cerebrum.
It surrounds the third ventricle and is formed by the
thalamus, hypothalamus, and
epithalamus.

THALAMUS
The thalamus is a pair of oval masses of gray matter that lie beneath the cerebrum
and form most of the
diencephalon. The masses are connected to one another by the
intermediate mass. Each is made up of four groups of
nuclei which are separated
by a Y-shaped sheet of white matter.
Anterior group of nuclei
The anterior group of nuclei forms the anterior portion of the thalamus and functions
as part of the
limbic
system
, helping
to control mood.
Lateral group of nuclei
The lateral group of nuclei forms the lateral portion of the thalamus. The lateral
group are linked to the
association areas

and the
limbic system
.
Medial
group of nuclei
The medial group of nuclei forms the medial portion of the thalamus. The medial
group is involved with
emotions
and is
connected to the prefrontal cortex.
Ventral group of nuclei
The ventral group of nuclei forms the ventral portion of the thalamus. It is involved
in motor functions and
connects the basal nuclei and the motor cortex. There are
five nuclei in the ventral group:
Ventral anterior nucleus
Connects the basal ganglia and the motor cortex and controls
movement.
Ventral lateral nucleus
Connects the cerebellum and basal ganglia with the motor cortex and controls movement.
Ventral posterior nucleus
Relays somatic sensation to the cortex.
Lateral geniculate nucleus
An eminence formed by the lateral geniculate nucleus on the posterior surface of
the
thalamus. The lateral geniculate nucleus receives
visual information
from the optic tract,
which it relays to the visual
cortex.
Medial geniculate nucleus
An eminence formed by the medial geniculate nucleus, located on the posterior surface
of the
thalamus. The medial geniculate nucleus relays
auditory information
from the lateral
lemniscus to the auditory cortex.
The thalamus is a major relay center and receives fibers from the following three
pathways:
Sensory fibers
Receives sensory input from the spinal cord and brainstem. These fibers relay and
continue to the cerebral cortex.
Motor fibers
Receives motor input from the cerebellum. These fibers relay and continue to the
cerebral cortex.
Intercerebellar fibers
Relays fibers from one area of the cerebral cortex to another.
HYPOTHALAMUS
The hypothalamus is a small area of the brain that lies inferior and lateral to
the anterior aspect of the third ventricle. It
is constricted anteriorly by the
optic chiasma and posteriorly by the mammillary bodies. Its inferior portion is
stretched
into a hollow stalk that attaches the
pituitary gland
.

It constitutes a large number of neurosecretory cell bodies, divided into a number
of small nuclei, all with varying
functions.
Infundibulum
The infundibulum is a narrow, hollow stalk that connects the hypothalamus to the
pituitary gland. It
extends from between the mammillary bodies and the optic chiasma
to the posterior lobe of the gland.
Mammillary bodies
The mammillary bodies are a pair of pea-sized white lumps protruding from the posterior
surface of the
hypothalamus. They are continuous superiorly with the fornix. They
function in recognition memory
especially with regards to smell memory.
Through connections to the limbic system, hippocampus, striatum, and brainstem it
regulates emotions, autonomic
control, hunger, satiety, immunity, memory input,
and anger control.
Autonomic control
Regulates and controls the output of the autonomic nervous system. Thus, it is involved
with the regulation of
visceral activities.
Emotional response
Functions as part of the limbic system and is involved in both negative and positive
emotions.
Thermoregulation
Functions as the body's
thermostat, stimulating the autonomic
nervous system to promote or reduce
heat loss.
Control of the endocrine system
Secretes releasing and inhibiting hormones which either stimulate or inhibit the
release of hormones from the
anterior pituitary gland. Also, neurons from the hypothalamus
produce hormones that travel along their axons to the
posterior pituitary gland,
stimulating it to release hormones.
Hunger and satiety
Controls the feelings of hunger and fullness.
Water balance and thirst
Responds to increase in concentration of extracellular fluid, stimulating the feeling
of thirst.
Sleep/wake cycle
When darkness is detected by the retina, the suprachiasmatic nucleus of the hypothalamus
stimulates the pineal
gland to release
melatonin
.
Melatonin
regulates the body clock by promoting
sleepiness.
EPITHALAMUS
The epithalamus is a small area of tissue that lies posterior to the third ventricle
and contains the habenular nuclei.
The pineal gland protrudes from its posterior
aspect.
Habenular nuclei
Habenular nuclei are involved in the emotional response to olfaction.
Pineal gland
A small, round endocrine gland that lies above the tectum of the midbrain, posterior
to the third ventricle
and between the superior colliculi. When darkness is detected
by the retina, the suprachiasmatic nucleus
of the hypothalamus stimulates the pineal
gland to release
melatonin
.
Melatonin
regulates the body clock
by promoting sleepiness.

CEREBRUM
The cerebrum is the largest part of the brain and is divided into
left

and
right hemispheres
by a longitudinal fissure that
runs
along the median sagittal plane.
The outer layer of the cerebrum is composed of gray matter and is called the
cerebral cortex
. It is responsible for the
analysis of sensory
input, memory, learning, and cognitive thought.
Each cerebral hemisphere can be divided into lobes, the names of which correlate
with the bones that protect them as
follows:
Frontal lobe
The frontal lobe is the largest lobe and is found at the front of the brain, protected
by the
frontal bone
. It is
separated from the parietal
lobe posteriorly by the central sulcus and from the temporal lobe inferiorly by
the
lateral sulcus.
Areas
Primary motor area (movement).
Motor association area (movement).
Primary olfactory cortex (smell).
Broca's area (motor speech production).
Function
Cognitive thought and memory.
Control of voluntary movements.
Temporal lobe
The temporal lobe is located at the side of the brain and is protected by the
temporal bone
. It is separated
above from the frontal lobe
by the lateral sulcus.
Areas
Primary auditory area (hearing).
Auditory association area (hearing).
Wernicke's area (speech comprehension).
Function
Special senses (hearing, smell).
Learning and memory (retrieval).
Emotions.
Parietal lobe
The parietal lobe is located at the top of the brain and is protected by the
parietal bone
. Anteriorly, it
contains the postcentral gyrus
and is separated from the frontal lobe by the central sulcus. Posteriorly, it is
separated from the occipital lobe by the parietal-occipital sulcus.
Areas
Primary
somatosensory area (cortex).
Sensory association area (general senses).
Function
Body orientation.
Primary gustatory cortex (taste).

Occipital lobe
Located at the back of the brain protected by the
occipital bone
.
Areas
Primary visual area (cortex).
Visual association area (vision).
Function
Visual interpretation.
Insula
The insula is the smallest lobe of the brain and located deep in the cerebrum, deep
to the parietal and
temporal lobe.
Function
Special senses (taste, hearing).
Visceral sensation.
Each hemisphere is greatly folded, forming folds and creases known as
gyri
and
sulci
that increase the surface area of
the cerebral cortex.
Although the exact location of the sulci and gyri varies between different individuals,
there are a
number of large gyri and deep sulci which can be identified as constant
landmarks.
The main features have been listed below:

Longitudinal fissure
A large fissure running from back to front along the sagittal plane. It divides
the
cerebrum into left and right cerebral hemispheres.
Central sulcus
Descending downwards and forwards from the top of the hemisphere. It divides the
frontal and parietal lobes.
Parietal-occipital
sulcus
Descending downwards and forwards mainly inside the longitudinal fissure. It divides
the parietal and occipital lobes.
Precentral sulcus
Forms the anterior boundary of the precentral gyrus.
Precentral gyrus
This is located at the posterior border of the frontal lobe, in front of the central
sulcus. It descends downwards and forwards from the top of the hemisphere. It forms
the
primary motor area
(cortex).
Postcentral gyrus
This is found at the anterior border of the parietal lobe, behind the central sulcus.
It
descends downwards and forwards from the top of the hemisphere. It forms the
primary somatosensory area
(cortex).
Postcentral sulcus
Forms the posterior boundary of the postcentral sulcus.
Lateral sulcus
Found on the lateral side of the brain, it is almost horizontal and ascends gradually
from the front of the brain to the angular gyrus. It separates the temporal lobe
from
the frontal lobe above.
The majority of the cerebral mass is formed by
white
matter.
It is composed of myelinated axons that form tracts, which
connect
the cerebrum to the other parts of the central nervous system.
There are three types of tracts defined by the areas they connect,
association fibers
,
commissural fibers
,
and
projection fibers
:
Association
tracts
Association tracts connect gyri within the same hemisphere.
Commissural
tracts
Commissural tracts connect gyri in different hemispheres. Examples include:
Anterior commissure
The anterior commissure is a tract of myelinated axons that connects the vestibular
cortex of the two cerebral hemispheres. It passes across the midline in front of
the
fornix and grooves the inferior surface of the putamen.

Posterior commissure
The posterior commissure is a thin tract of myelinated axons that lie above the
superior colliculi and connect the midbrain and diencephalon.
Corpus callosum
The corpus callosum is a C-shaped bundle of myelinated axons that arches over the
thalamus and ventricles, connecting the left and right cerebral hemispheres; it
forms
the base of the longitudinal fissure.
Projection
tracts
Projection tracts connect gyri with different areas of the CNS. Examples include:
Internal capsule
The internal capsule is a large sheet of myelinated axons connecting the cerebral
cortex to the brainstem and cerebrospinal tracts of the spinal cord. In the cerebrum
it
separates the thalamus and caudate nucleus from the globus pallidus and putamen;
superiorly it diverges to form the
corona radiata
.
Inferiorly the fibers converge to form
the
cerebral
crus
in the midbrain, continuing to the
pyramids
of the medulla
oblongata and passing to the opposite side in the pyramidal
decussation.
During the first three or four months after conception, the fetal brain has a smooth
surface which is similar in appearance
to that of an adult bird or reptile brain.
As fetal development continues, the surface of the brain begins to fold until it
takes
on the walnut-like appearance of the adult human brain.
MOTOR AND SENSORY HOMUNCULI
The motor and sensory homunculi are disproportionate maps of the body used to demonstrate
the relative portion of
cerebral cortex dedicated to each area of the body.
The primary motor and somatosensory cortices in the cerebrum deal with motor and
sensory information for the whole
body. Each strip of cortex is arranged
topographically
i.e., different areas of the cortex deal with
different pieces of
information.
The more control that is needed over a body part, the more area of the cerebral
cortex is dedicated to that part. For
example, even though the thigh has more muscles
than the hand, the hand requires more control because it is involved in
more intricate
skills, like writing, while the thigh is involved in less intricate skills, like
walking. Therefore, more of the
cerebral cortex will be involved in controlling
the hand than in controlling the thigh.
The homunculus picture shows the relative
amount of the cortex dedicated to the specific body area. If the area has a lot
of control, then that part will be disproportionately larger; if there is less control
it will be smaller. The feature works both
for motor stimulation and for sensory
feedback.

Sensory homunculus
The tongue, lips, fingers, toes, and sex organs are shown as relatively large
as they have high sensitivity
and therefore have a relatively large portion of the
somatosensory cortex attributed to them.
Motor homunculus
The tongue, lips, hands, fingers, and toes are shown relatively large as they
all have precise motor
control and therefore have a relatively large portion of
the motor cortex attributed to them.
BASAL GANGLIA
The basal nuclei are three masses of cerebral gray matter embedded in the white
matter surrounding the thalamus.
The basal nuclei include the caudate, putamen, and globus pallidus.
Caudate nucleus
The caudate nucleus is a C-shaped nucleus that lies under the lateral ventricles.
It has a large head that
tapers posteriorly and is connected to the putamen by thin
striations.
It is involved in the sub-control of voluntary movement.
Putamen
The putamen is an oval nucleus located lateral to the internal capsule.
It is
connected to the caudate
nucleus by thin striations.

It is involved in the reinforcement and co-ordination of learned motor skills.
Globus pallidus
The globus pallidus is the most medial of the basal ganglia. It is a small nucleus
that lies medial to the
putamen and lateral to the internal capsule.
It is involved in inhibiting muscular activity and reducing muscle tone.
The basal ganglia are important in co-ordinating muscle movement and posture; they
suppress unwanted movements and
control muscle tone.
LIMBIC SYSTEM
The limbic system is the main area of the brain involved with
emotion
and
learning
. It influences
the formation of
memory by integrating emotional states with stored memories of
physical sensations. It is also involved in linking smell
and memory.
It exerts influence on the endocrine and autonomic nervous systems producing both
negative and positive emotional
responses.
It is formed by a ring of structures which surround the diencephalon and include
the following:

Cerebral gyri
The c
ingulate gyrus of the frontal and parietal lobes runs above the corpus callosum
within the longitudinal
fissure. The dentate and parahippocampal gyri surround the
hippocampus.
Hippocampus
A gyrus that runs medially along the temporal lobe, beneath the diencephalon. It
is connected to the inferior
limbs of the fornix. The hippocampus is critical to
forming new memories.
Amygdala
The amygdala is a
large swelling located at the anterior aspect of the hippocampus.
It is involved with the
emotions of fear and aggression.
Septal nuclei
The septal nuclei are clusters of cell bodies found within the septum pellucidum,
a thin sheet of gray and
white matter that lies vertically in the midline between
the two cerebral hemispheres. It joins the corpus
callosum superiorly to the fornix
inferiorly, and separates the left and right lateral ventricles.
Mammillary bodies
Mammillary bodies protrude from the posterior of the hypothalamus. They are continuous
superiorly with the
fornix.
Anterior thalamic nuclei
The anterior thalamic nuclei form the anterior portion of the thalamus.
Olfactory bulbs
The olfactory bulbs receive information regarding olfaction.
Fornix
The fornix is a fibrous band of myelinated axons that arches over the thalamus,
connecting the hippocampus
and the mammillary bodies.
BLOOD SUPPLY TO THE BRAIN
The right and left
common carotid
arteries supply a large proportion
of the head and neck with blood. These arteries
ascend at the side of the neck and
divide to form the
internal
and
external
carotids.
For more information, see
'Cardiovascular system: Vessels of the head and neck'.
The brain is supplied by two internal carotid arteries, and two vertebral arteries:
Internal carotid artery
The internal carotid artery is a
deep artery of the neck that enters the skull
to supply
the brain, eyes, nose, and forehead.
Branches
include
:
Ophthalmic.
Anterior cerebral.
Middle cerebral.

Vertebral artery
The vertebral artery arises from the subclavian artery, and ascends the neck through
the transverse foramen of the cervical vertebrae and enters the skull via the foramen
magnum, where it unites with its opposite to form the basilar artery.
The vertebral arteries are important as they supply the cervical vertebrae, brainstem,
cerebellum, and the spinal cord with blood.
Branches
include:
Spinal.
Posterior inferior
cerebellar.
Basilar.
The branches of the internal carotid and vertebral arteries
form an arterial circle around the pituitary gland and optic
chiasm at the base
of the brain, called the '
circle of Willis'
.

This anastomosis means that if an artery supplying the brain
becomes damaged, blood flow from the other vessels can
often replace it.

Branches of the
INTERNAL CAROTID
and
VERTEBRAL ARTERIES
:

BRAIN WAVES
Brain waves are an amalgamation of the many action potentials generated by the neurons
in the brain.
They represent
the total electrical activity of neurons in the brain, as recorded
by
electrodes
placed on the forehead
and scalp. A
recording of brain waves is called an
electroencephalogram
or EEG. Different patterns of neuronal firing in
the brain
produce four different brain waves:
alpha
waves
,
beta waves
,
theta waves
, and
delta waves
.
The frequency of a wave is
measured in
hertz
(
Hz), which represents the number of cycles per second.
Wave
Frequency
Incidence
Alpha
9-14 Hz
Appear when awake and resting. Absent during sleep.
Beta
15-30 Hz
Appear when the nervous system is active, during sensory reception, and during mental
activity.
Theta
4-8 Hz
Seen in children and adults under emotional stress. Also seen in individuals with
brain disorders.
Delta
1-3 Hz
Seen in adults during deep sleep and in infants when they are awake. An indication
of brain
damage is seen in the EEG of an adult in a state of wakefulness.

SLEEP
Humans follow a 24 hour cycle of alternation between sleep and wakefulness known
as a
'circadian rhythm'
.
The
suprachiasmatic nucleus
of the hypothalamus
is key to the generation and control of the circadian rhythm.
Wakefulness is a state
of consciousness, where one possesses the ability to react to various stimuli. Sleep
is a state of
partial unconsciousness, where one is less able to respond to stimuli.
Two different types of sleep have been
established:
non-rapid eye movement sleep
(NREM) and
rapid
eye-movement sleep
(REM). An individual will
alternate between NREM
and REM sleep during the night.
Non-rapid eye movement
sleep (NREM)
NREM sleep is controlled by neurons in the preoptic nucleus
of the hypothalamus, basal nuclei, and medulla
oblongata.
During NREM sleep, an individual will pass through the following three stages in
under an hour.
Transition
stage
The period between wakefulness and sleep lasting for up to
7 minutes. A state of relaxation, with eyes
closed and a wandering mind. An individual
in this stage is easily awakened.
Light
sleep
Eyes are closed and may slowly roll in their sockets. Incomplete
dreams may be experienced. An
individual in this stage is a little less easily awakened.
Deep
sleep
The final and deepest stage of sleep, during which sleepwalking
may occur. A further drop in body
temperature, a slight decrease in muscle tone,
and a slowing of brain metabolism may be experienced.
Rapid eye movement sleep
(REM)
REM sleep occurs in episodes, breaking up NREM sleep.
An individual will typically have five episodes of REM sleep, at 90 minute intervals,
during an 8 hour sleep period; they
will get progressively longer, with the first
lasting about 10-20 minutes and the last about 50 minutes.
During REM sleep, eyes move around rapidly and continuously under closed lids. Neuronal
activity in the brain is high.
REM sleep is triggered by neurons in the pons and
midbrain.
The majority of
dreaming
occurs during
REM sleep, producing EEG readings comparable to those of someone in a
state of wakefulness.
Sleep paralysis may be experienced when an individual is awoken from REM sleep which
is due
to the inhibition of motor neurons controlling skeletal muscle.
LEARNING AND MEMORY
Learning
is acquiring new information
or skills through experience.
Memory
is the ability to store and retain this
information within the brain for future
retrieval. Plasticity is the term used to describe the structural and functional
changes in the brain, through which learning and memory are attained. Plasticity
occurs in response to stimuli from both
internal and external environments, and
involves changes in synaptic connections such as an increase in the number of
and
enlargement of presynaptic axon terminals, growth of new dendrites, and synthesis
of new proteins in particular
brain regions.
Immediate memory
Immediate memory is the capacity for recalling snippets of information over a few
seconds, for example, dialing a
phone number that was just told to you.
Short-term memory
Short-term memory, also referred to as
active
or
primary memory
, is the capacity
for holding small amounts of
information over a short period of time.
Long-term memory
Long-term memory is the capacity for storing information for much longer periods
of time. This information can usually
be retrieved at any point.
Memory consolidation
Memory consolidation is the process by which memories become more established through
frequent retrieval.

Long term potentiation (LTP)
Long term potentiation (LTP) is the lasting amplification of synaptic transmission
between two neurons, resulting from
high-frequency stimulation. LTP is essentially
the ability of a synapse to change its strength and therefore, it is
thought to
be one of the mechanisms underlying learning and memory.