Nervous system

1. INTRODUCTION TO
CENTRAL NERVOUS
SYSTEM –
STRUCTURE, FUNCTION &
DISEASES
Pavithra Narasimhan

3. ◦ The central nervous system is the part of the nervous system consisting of the brain and spinal
cord.
◦ The brain is encased in the skull, and protected by the cranium.
◦ The spinal cord is continuous with the brain and lies caudally to the brain, and is protected by the
vertebrae
◦ The spinal cord reaches from the base of the skull, continues through or starting below the
foramen magnum, and terminates roughly level with the first or second lumbar vertebra,
occupying the upper sections of the vertebral canal.
◦ The CNS is contained within the dorsal body cavity, with the brain housed in the cranial cavity
and the spinal cord in the spinal canal. In vertebrates, the brain is protected by the skull, while the
spinal cord is protected by the vertebrae.
◦ The brain and spinal cord are both enclosed in the meninges. In central nervous systems, the
inter-neuronal space is filled with a large amount of supporting non-nervous cells
called neuroglial cells.

4. The brain
 The brain is the most complex organ in the human body; the cerebral cortex (the outermost part of the brain
and the largest part by volume) contains an estimated 15–33 billion neurons, each of which is connected to
thousands of other neurons.
 In total, around 100 billion neurons and 1,000 billion glial (support) cells make up the human brain. Our
brain uses around 20 percent of our body’s total energy.
 The brain is the central control module of the body and coordinates activity. From physical motion to the
secretion of hormones, the creation of memories, and the sensation of emotion.
 To carry out these functions, some sections of the brain have dedicated roles. However, many higher
functions — reasoning, problem-solving, creativity — involve different areas working together in networks

6. Cerebrum
 The cerebrum (front of brain) comprises gray matter (the cerebral cortex) and white matter at its center.
 The largest part of the brain, the cerebrum initiates and coordinates movement and regulates
temperature.
 Other areas of the cerebrum enable speech, judgment, thinking and reasoning, problem-solving,
emotions and learning. Other functions relate to vision, hearing, touch and other senses.

7. Cerebral Cortex
 Cortex is Latin for “bark,” and describes the outer gray matter covering of the cerebrum. The cortex has a
large surface area due to its folds, and comprises about half of the brain’s weight.
 The cerebral cortex is divided into two halves, or hemispheres.
 It is covered with ridges (gyri) and folds (sulci).
 The two halves join at a large, deep sulcus (the interhemispheric fissure, AKA the medial longitudinal
fissure) that runs from the front of the head to the back.
 The right hemisphere controls the left side of the body, and the left half controls the right side of the body.
 The two halves communicate with one another through a large, C-shaped structure of white matter and
nerve pathways called the corpus callosum. The corpus callosum is in the center of the cerebrum.

8. Brainstem
The brainstem (middle of brain) connects the cerebrum with the spinal cord. The brainstem includes the
midbrain, the pons and the medulla.
 Midbrain. The midbrain (or mesencephalon) is a very complex structure with a range of different neuron
clusters (nuclei and colliculi), neural pathways and other structures. These features facilitate various
functions, from hearing and movement to calculating responses and environmental changes. The midbrain
also contains the substantia nigra, an area affected by Parkinson’s disease that is rich in dopamine neurons
and part of the basal ganglia, which enables movement and coordination.
 Pons. The pons is the origin for four of the 12 cranial nerves, which enable a range of activities such as tear
production, chewing, blinking, focusing vision, balance, hearing and facial expression. Named for the Latin
word for “bridge,” the pons is the connection between the midbrain and the medulla.
 Medulla. At the bottom of the brainstem, the medulla is where the brain meets the spinal cord. The medulla
is essential to survival. Functions of the medulla regulate many bodily activities, including heart rhythm,
breathing, blood flow, and oxygen and carbon dioxide levels. The medulla produces reflexive activities such
as sneezing, vomiting, coughing and swallowing.
 The spinal cord extends from the bottom of the medulla and through a large opening in the bottom of the
skull. Supported by the vertebrae, the spinal cord carries messages to and from the brain and the rest of the
body.

9. Cerebellum
 The cerebellum (“little brain”) is a fist-sized portion of the brain located at the back of the head, below the
temporal and occipital lobes and above the brainstem.
 Like the cerebral cortex, it has two hemispheres.
 The outer portion contains neurons, and the inner area communicates with the cerebral cortex.
 Its function is to coordinate voluntary muscle movements and to maintain posture, balance and
equilibrium.
 New studies are exploring the cerebellum’s roles in thought, emotions and social behavior, as well as its
possible involvement in addiction, autism and schizophrenia.

11. The brain is roughly split into four lobes:
Temporal lobe : important for processing sensory input and assigning it emotional meaning.
It is also involved in laying down long-term memories. Some aspects of language perception are also housed
here.
Occipital lobe : visual processing region of the brain, housing the visual cortex.
Parietal lobe : the parietal lobe integrates sensory information including touch, spatial awareness, and
navigation.
Touch stimulation from the skin is ultimately sent to the parietal lobe. It also plays a part in language
processing.
Frontal lobe : positioned at the front of the brain, the frontal lobe contains the majority of dopamine-sensitive
neurons and is involved in attention, reward, short-term memory, motivation, and planning.

13. Brain regions
Basal ganglia: involved in the control of voluntary motor movements, procedural learning, and decisions
about which motor activities to carry out. Diseases that affect this area include Parkinson’s
disease and Huntington’s disease.
Cerebellum: mostly involved in precise motor control, but also in language and attention. If the cerebellum is
damaged, the primary symptom is disrupted motor control, known as ataxia.
Broca’s area: this small area on the left side of the brain (sometimes on the right in left-handed individuals) is
important in language processing. When damaged, an individual finds it difficult to speak but can still
understand speech

15. Brain Regions:
Corpus callosum: a broad band of nerve fibers that join the left and right hemispheres. It is the largest white
matter structure in the brain and allows the two hemispheres to communicate. Dyslexic children have
smaller corpus callosums; left-handed people, ambidextrous people, and musicians typically have larger
ones.
Medulla oblongata: extending below the skull, it is involved in involuntary functions, such as vomiting,
breathing, sneezing, and maintaining the correct blood pressure.
Hypothalamus: sitting just above the brain stem and roughly the size of an almond, the hypothalamus
secretes a number of neurohormones and influences body temperature control, thirst, and hunger.
Thalamus: positioned in the center of the brain, the thalamus receives sensory and motor input and relays it
to the rest of the cerebral cortex. It is involved in the regulation of consciousness, sleep, awareness, and
alertness.
Amygdala: two almond-shaped nuclei deep within the temporal lobe. They are involved in decision-making,
memory, and emotional responses; particularly negative emotions.

16.  The spinal cord, running almost the full length of the back, carries information between the brain and body,
but also carries out other tasks.
 From the brainstem, where the spinal cord meets the brain, 31 spinal nerves enter the cord.
 Along its length, it connects with the nerves of the peripheral nervous system (PNS) that run in from the skin,
muscles, and joints.
 Motor commands from the brain travel from the spine to the muscles and sensory information travels from
the sensory tissues — such as the skin — toward the spinal cord and finally up to the brain.
 The spinal cord contains circuits that control certain reflexive responses, such as the involuntary movement
your arm might make if your finger was to touch a flame.
 The circuits within the spine can also generate more complex movements such as walking. Even without
input from the brain, the spinal nerves can coordinate all of the muscles necessary to walk.

17. White and gray matter
The CNS can be roughly divided into white and gray matter. As a very general rule, the brain consists of an
outer cortex of gray matter and an inner area housing tracts of white matter.
Both types of tissue contain glial cells, which protect and support neurons. White matter mostly consists of
axons (nerve projections) and oligodendrocytes — a type of glial cell — whereas gray matter consists
predominantly of neurons.

18. Central glial cells
Also called neuroglia, glial cells are often called support cells for neurons. In the brain, they outnumber nerve
cells 10 to 1.
Without glial cells, developing nerves often lose their way and struggle to form functioning synapses.
Glial cells are found in both the CNS and PNS but each system has different types.
The following are brief descriptions of the CNS glial cell types:
Astrocytes: these cells have numerous projections and anchor neurons to their blood supply. They also
regulate the local environment by removing excess ions and recycling neurotransmitters.
Oligodendrocytes: responsible for creating the myelin sheath — this thin layer coats nerve cells, allowing
them to send signals quickly and efficiently.
Ependymal cells: lining the spinal cord and the brain’s ventricles (fluid-filled spaces), these create and secrete
cerebrospinal fluid (CSF) and keep it circulating using their whip-like cilia.
Radial glia: act as scaffolding for new nerve cells during the creation of the embryo’s nervous system.

19. Cranial nerves
The cranial nerves are 12 pairs of nerves that arise directly from the brain and pass through holes in the skull
rather than traveling along the spinal cord. These nerves collect and send information between the brain and
parts of the body – mostly the neck and head.
Of these 12 pairs, the olfactory and optic nerves arise from the forebrain and are considered part of the central
nervous system:
Olfactory nerves (cranial nerve I): transmit information about odors from the upper section of the nasal cavity
to the olfactory bulbs on the base of the brain.
Optic nerves (cranial nerve II): carry visual information from the retina to the primary visual nuclei of the
brain. Each optic nerve consists of around 1.7 million nerve fibers.

20. Below are the major causes of disorders that affect the CNS:
Trauma: depending on the site of the injury, symptoms can vary widely from paralysis to mood disorders.
Infections: some micro-organisms and viruses can invade the CNS; these include fungi, such as cryptococcal
meningitis; protozoa, including malaria; bacteria, as is the case with leprosy, or viruses.
Degeneration: in some cases, the spinal cord or brain can degenerate. One example is Parkinson’s disease
which involves the gradual degeneration of dopamine-producing cells in the basal ganglia.
Structural defects: the most common examples are birth defects; including anencephaly, where parts of the
skull, brain, and scalp are missing at birth.
Tumors: both cancerous and noncancerous tumors can impact parts of the central nervous system. Both types
can cause damage and yield an array of symptoms depending on where they develop.
Autoimmune disorders: in some cases, an individual’s immune system can mount an attack on healthy cells.
For instance, acute disseminated encephalomyelitis is characterized by an immune response against the brain
and spinal cord, attacking myelin (the nerves’ insulation) and, therefore, destroying white matter.
Stroke: is an interruption of blood supply to the brain; the resulting lack of oxygen causes tissue to die in the
affected area.

21. References
◦ https://www.medicalnewstoday.com/articles/307076#cns-diseases
◦ https://www.hopkinsmedicine.org/health/conditions-and-diseases/anatomy-of-the-brain