Overview of the Brain
Major Portions of the Brain
The brain is composed of about 100 billion (1011)
multipolar neurons and innumerable nerve fibers by
which these neurons communicate with one another
and with neurons in other parts of the nervous system.
The brain can be divided into three major portions the cerebrum, the cerebellum, and the brain stem. The
cerebrum, the largest part, contains nerve centers
associated with sensory and motor functions and
provides higher mental functions, including memory
and reasoning. The cerebellum includes centers that
coordinate voluntary muscular movements. Nerve
pathways in the brain stem connect various parts of
the nervous system and regulate visceral activities.
Development
(a)
Formation of the Neural Tube
21-day-old human embryo. (b) Cross sections
through the embryo. The level of each
section is indicated by a line on (a).
Development of the Brain Segments and Ventricles
By the fourth week, the neural tube exhibits three primary vesicles, and by
the fifth week the neural tube subdivides further into five secondary
vesicles
Meninges
Meninges
Bones, membranes, and fluid surround the organs of the CNS. The brain lies
within the cranial cavity of the skull, and the spinal cord occupies the
vertebral canal within the vertebral column. Beneath these bony coverings,
membranes called meninges (singular, meninx), located between the bone and
soft tissues of the nervous system, protect the brain and spinal cord.
The meninges have three layers - dura mater, arachnoid mater, and pia mater.
The dura mater is the outermost layer of the meninges. It is composed of
tough, white fibrous connective tissue and contains many blood vessels and
nerves. The dura mater continues into the vertebral canal as a strong, tubular
sheath that surrounds the spinal cord. The arachnoid mater is a thin, weblike
membrane that lacks blood vessels and is located between the dura and pia
maters. It spreads over the brain and spinal cord but generally does not dip
into the grooves and depressions on their surfaces.
Between the arachnoid and pia maters is subarachnoid space which contains the clear, watery cerebrospinal fluid (CSF). The pia mater is
very thin and contains many nerves and blood vessels that nourish underlying cells of the brain and spinal cord. This layer hugs the
surfaces of these organs and follows their irregular contours, passing over gyri and dipping into sulci.
Meningeal Coverings of the Spinal Cord
Meningeal Coverings of the Brain
Spinal Cord Structure
Spinal Cord Structure
The spinal cord is extremely important to the overall function of the nervous
system. It is the communication link between the brain and the peripheral
nervous system (PNS) inferior to the head, integrating incoming information
and producing responses through reflex mechanisms.
The spinal cord extends from the foramen magnum to the level of the second
lumbar vertebra, (L2) where it tapers to a point called the conus medullaris.
Arising from the conus medullaris is a bundle of nerve roots called the cauda
equina.
It is considerably shorter than the vertebral column because it does not grow
as rapidly as does the vertebral column during development.
Spinal Cord Functions
Functions of the Spinal Cord
The spinal cord has three major functions: maintaining repetitive, coordinated skeletal muscle contractions; serving as a center for spinal
reflexes; and conducting information to and from the brain. The nerve tracts of the spinal cord provide a two-way communication system
between the brain and body parts outside the nervous system. The tracts that carry sensory information to the brain are called ascending
tracts, and those that conduct motor impulses from the brain to the muscles and glands are called descending tracts.
All nerve fibers in a given tract have a similar origin, destination, and function.
Many of the fibers exhibit decussation - they cross from one side of the body to the other within the spinal cord or the medulla oblongata. Not
all spinal nerve fibers decussate. Those that do not cross remain on the ipsilateral side of the body.
Sensory impulses originating in skin receptors cross over in the spinal cord and ascend to the brain. As
a result, each side of the brain receives sensory information from the contralateral (opposite) side of
the body.
The names of most ascending tracts consist of the prefix spino-followed by a root denoting the
destination of its fibers.
Motor fibers of the corticospinal tract begin in the cerebral cortex, cross over in the medulla oblongata. As a result each side of the
brain issues motor impulses to the opposite side of the brain. From the medulla the fibers descend in the spinal cord. There, they
synapse with neurons whose fibers lead to the spinal nerves that supply skeletal muscles.
The names of most descending tracts consist of a word root denoting the point of origin in the brain, followed by the suffix -spinal.
Brain Stem
The medulla oblongata, pons, midbrain and diencephalon constitute the brain stem. The brain stem connects the spinal cord to the remainder of
the brain and is responsible for many essential functions. Damage to even small brain stem areas often causes death because reflexes essential
for survival are integrated in the brain stem, whereas relatively large areas of the cerebrum or cerebellum may be damaged without being lifethreatening. All but two of the 12 cranial nerves enter or exit the brain through the brain stem
Diencephalon
The diencephalon is considered, by some, to be part of the brain stem. It is between the midbrain and the corpus callosum. Its main components
are the thalamus, hypothalamus, and epithalamus.
The thalamus comprises the majority (4/5) of the diencephalon. Most sensory information synapses in the thalamus before going to the
cerebrum. The thalamus integrates sensory information and directs it to the appropriate location in the cerebrum.
The hypothalamus is a major control center of the autonomic nervous system and endocrine system. Nuclei of the hypothalamus also regulate
body temperature, food and water intake, sleep and circadian rhythms, memory and emotional behavior. The epithalamus consists mainly of the
pineal gland.
Brain Stem
Nerve fibers connect the hypothalamus to the cerebral cortex,
thalamus, and other parts of the brain stem. The hypothalamus
maintains homeostasis by regulating a variety of visceral activities
and by linking the nervous and endocrine systems.
The hypothalamus is involved in the regulation of:
1.
2.
3.
4.
5.
6.
7.
Heart rate and arterial blood pressure
Body temperature
Water and electrolyte balance
Control of hunger and body weight
Control of movements and glandular secretions of the
stomach and intestines
Production of neurosecretory substances that stimulate
the pituitary gland to secrete hormones
Sleep and wakefulness
Development of the Brain Segments and Ventricles - Adult
Cerebrum
The cerebrum is the part of the brain that most people think of when the term "brain" is mentioned. It is the largest portion of the
brain, weighing about 1200 g in females and 1400 g in males. Brain size is related to body size; larger brains are associated with
larger bodies, not necessarily with greater intelligence.
The cerebrum is divided into left and right hemispheres by a longitudinal fissure. A thin layer of gray matter called the
cerebral cortex is the outermost portion of the cerebrum. It contains nearly 75% of all the neuron cell bodies in the
nervous system. The most conspicuous features on the surface of each hemisphere are numerous folds called gyri,
(convolutions) which greatly increase the surface area of the cortex. The intervening grooves between the gyri are called
sulci. A central sulcus, which runs along the lateral surface of the cerebrum from superior to inferior, is located about
midway along the length of the brain. The central sulcus is located between the precentral gyrus anteriorly and a
postcentral gyrus posteriorly.
Certain fissues and deep sulci divide each cerebral hemisphere into five anatomically and functionally distinct lobes:
frontal lobe, parietal lobe, occipital lobe, temporal lobe, and the insula
Functional Regions of the Cerebral Cortex
Sensory areas are located in several lobes of the cerebrum. These interpret impulses that arrive from sensory receptors, producing feelings or
sensations. For example, sensations from all parts of the skin (cutaneous senses) arise in the anterior portions of the parietal lobes along the
central sulcus. The posterior parts of the occipital lobes interpret visual information and the temporal lobes contain the centers for hearing
(auditory area). The sensory areas for taste are located within the parietal lobe and sense of smell arises from the temporal lobe.
Like motor fibers, sensory fibers cross over in either the spinal cord or the brain stem. Thus, the centers in the right cerebral hemisphere
interpret impulses originating from the left side of the body, and vice versa.
Association areas are neither primarily sensory nor motor.
Association areas connect with other association areas and with
other brain structures. These areas analyze and interpret sensory
experiences and oversee memory, reasoning, verbalizing, judgment,
and emotion. Association areas occupy the anterior portions of the
frontal lobes and are widespread in the lateral portions of the
parietal, temporal, and occipital lobes.
A stroke may occur when there is an insufficient supply of blood to a brain region, or when damage to the vasculature in a specific brain
region causes bleeding into the nervous tissue. In general, there are three types of strokes that occur. They are classified based on the
type of vascular defect that causes them. Cerebral hemorrhaging occurs when a vessel bursts, causing bleeding in a region of the cerebral
cortex. An embolic stroke occurs when small clots that formed on other parts of the circulatory system occlude cerebral arteries and cut
off the blood supply to a part of the cerebral cortex. An ischemic stroke occurs when a narrowing or blockage of cerebral vasculature
results in oxygen deprivation in a specific part of the cerebral cortex. The specific functional deficits observed in someone who has
suffered a stroke can be correlated with the functions of the damaged cortical region. In a healthy brain, the nerve impulses from the
cerebral cortex travel through the brain and cross over to the opposite (contralateral) side of the body. When a stroke occurs, damage to
the cortex is usually unilateral since information to and from the cortex crosses over the midline, the effects of a stroke on the left side
of the brain occur on the right side of the body. A person who has suffered a stroke may experience paralysis that may involve the face,
weakness or paralysis of limbs, often limited to one side of the body, loss of sensation that parallels paralysis, difficulty with speech,
double vision, loss of vision, dizziness, or vomiting, severe headache, and loss of consciousness.
MRI of Stroke
These are magnetic resonance images (MRI) of a normal brain
(left) and the brain of a person who has suffered a stroke
(right). The arrow indicates the region of stroke damage. The
plane of the image is indicated in the lower left of each film.
PNS
The peripheral nervous system (PNS) consists of nerves that branch out from the CNS and connect it to other body parts. The PNS includes
the cranial nerves, which emerge from the brain, and the spinal nerves, which emerge from the spinal cord.
The PNS can also be subdivided into the somatic and autonomic nervous systems. Generally, the somatic nervous system consists of the cranial
and spinal nerve fibers that connect the CNS to the skin and skeletal muscles; it oversees conscious activities. The autonomic nervous system
includes fibers that connect the CNS to viscera, such as the heart, stomach, intestines, and glands; it controls unconscious activities.
A nerve fiber is the axon of a single neuron. A nerve is an organ composed of multiple nerve fibers bound together by sheaths of connective
tissue. Its structure is similar to that of skeletal muscle.
Nerve fibers from the PNS are ensheathed in Schwann cells, which form a neurilemma around the axon. External to the neurilemma, each
fiber is surrounded by a sparse layer of connective tissue, the endoneurium.
Nerve fibers are gathered together in bundles called nerve fascicles, surrounded by a fibrous sheath called the perineurium. The entire
nerve is surrounded by a fibrous epineurium.
Cranial Nerves
Inferior Surface of the Brain Showing the Origin of the Cranial Nerves
By convention, the 12 cranial nerves are indicated by Roman numerals (I - XII) from anterior to posterior. The three general
categories of cranial nerve function are (1) sensory, (2) somatic motor, and (3) parasympathetic. Sensory functions include the special
senses such as vision and the more general senses such as touch and pain. Somatic motor functions refer to the control of skeletal muscles
through motor neurons. Parasympathetic functions include regulation of glands and smooth muscles, such as salivary and lacrimal glands
Olfactory Nerves (I) - Sensory fibers that transmit impulses associated with the sense of smell
Optic Nerves (II) - Sensory fibers that transmit impulses associated with the sense of vision
Oculomotor Nerves (III) - Motor fibers that transmit impulses to muscles that raise eyelids, move eyes, adjust the amount of light entering
the eyes and focus lenses.
Trochlear Nerves (IV) - Motor fibers that transmit impulses to muscles that move the eyes. Some sensory fibers transmit impulses associated
with the condition of muscles.
Trigeminal Nerves (V) - Sensory fibers that transmit impulses from the surface of the eyes, tear glands, scalp, forehead and upper eyelids.
Opthalmic Division- Sensory fibers that transmit impulses from the upper teeth, upper gum, upper lip, lining of the palate, and skin of the face.
Maxillary Division- Sensory fibers that transmit impulses from the skin of the jaw, lower teeth, lower gum and lower lip
Mandibular Division- Motor fibers that transmit impulses to muscles of mastication and to muscles in the floor of the mouth.
Abducens (VI) - Motor fibers that transmit impulses to muscles that move the eyes. Some sensory fibers transmit impulses associated with
the condition of muscles. Sensory fibers transmit impulses associated with taste receptors of the anterior tongue.
Facial Nerves (VII) - Motor fibers that transmit impulses to muscles of facial expression, tear glands and salivary glands.
Vestibulocochlear Nerve (VIII)
Vestibular Branch- Sensory fibers that transmit impulses associated with the sense of equilibrium.
Cochlear Branch- Sensory fibers that transmit impulses associated with the sense of hearing.
Glossopharyngeal Nerve (IX) - Sensory fibers that transmit impulses from the pharynx, tonsils, posterior tongue and carotid arteries.
Motor fibers that transmit impulses to muscles of the pharynx used in swallowing and to salivary glands.
Vagus Nerves (X) - Somatic motor fibers that transmit impulses to muscles associated with speech and swallowing; autonomic motor fibers
transmit impulses to the heart, smooth muscles and glands in the thorax and abdomen. Sensory fibers that
transmit impulses from the pharynx, larynx, esophagus and viscera of the thorax and abdomen.
Accessory Nerves (XI) Cranial Branch- Motor fibers that transmit impulses to the muscles of the soft palate, pharynx and larynx.
Spinal Branch- Motor fibers that transmit impulses to muscles of the neck and back.
Hypoglossal Nerves (XII) - Motor fibers that transmit impulses that move the tongue
Spinal Nerves
The spinal nerves arise through numerous rootlets along the dorsal and ventral surfaces of the spinal cord. About six to eight of these
rootlets combine to form a ventral root on the ventral (anterior) side of the spinal cord, and another six to eight form a dorsal root on the
dorsal (posterior) side of the cord at each segment. The ventral root contains efferent (motor) fibers, and the dorsal root contains
afferent (sensory) fibers. The dorsal and ventral roots join one another just lateral to the spinal cord to form the spinal nerve. The dorsal
root contains a ganglion, called the dorsal root, or spinal ganglion, near where it joins the ventral root.
All of the 31 pairs of spinal nerves, except the first pair and those in the sacrum, exit the vertebral column through an intervertebral
foramen located between adjacent vertebrae. The first pair of spinal nerves exits between the skull and the first cervical vertebra. The
nerves of the sacrum exit from the single bone of the sacrum through the sacral foramina. Eight spinal nerve pairs exit the vertebral
column in the cervical region, 12 in the thoracic region, 5 in the lumbar region, 5 in the sacral region,