4. nervous system

Human Nervous system
Basic Anatomy
Saira Aftab

Nervous system and its divisions
All mammals have developed nervous system despite the differences in the
complexity of nervous system
Main functions of nervous systems include
1. Gathering information from both inside and outside the body - Sensory Function
2. Transmitting information to the processing areas of the brain and spine--Integration
Function
3. Sending information to the muscles, glands, and organs so they can respond
appropriately – Motor Function
It controls and coordinates all essential functions of the body including all other
body systems allowing the body to maintain homeostasis or its delicate balance.

• The human nervous system is a complex communications network,
consisting of two integrated systems:
1- Central nervous system (CNS)
• The CNS is made up of the brain and spinal cord.
• It acts as a command center, interpreting incoming sensory information
from your eyes and ears, for example, and controlling outgoing
commands to muscles.
• Communication occurs via nerve impulses – electrical signals that travel
along nerves.
• The brain also performs complex functions like learning, memory,
emotion and abstract thought.

• 2-Peripheral nervous system (PNS)
• The PNS consists of sensory and motor nerves:
• The peripheral nervous system (PNS) is the division of the nervous
system containing all the nerves that lie outside of the central
nervous system (CNS).
• The primary role of the PNS is to connect the CNS to the organs,
limbs, and skin.
• Unlike the CNS, the PNS is not protected by the vertebral column and
skull, or by the blood–brain barrier, which leaves it exposed to toxins
and mechanical injuries.
• Consists of 12 pairs of cranial nerves attached to brain
• 31 pairs of spinal nerves attached to spinal cord

Main Components of Central Nervous system
1. Brain
2. Neurons
3. Spinal cord
4. Sensory organs

1-Brain
• The human brain is the central organ of the human nervous system, and with the
spinal cord makes up the central nervous system.
• The adult male human brain weighs on average about 1.2–1.4 kg and for women 1.0–
1.4 kg.
• Neurological differences between the genders have not been shown to correlate in
any simple way with IQ or other measures of cognitive performance
• The brain is protected by the skull, suspended in cerebrospinal fluid (CSF) (filled
cavities called ventricles), and isolated from the bloodstream by the blood–brain
barrier.
• However, the brain is still susceptible to damage, disease, and infection. Damage can
be caused by trauma, or a loss of blood supply known as a stroke.
• The brain is susceptible to degenerative disorders, such as Parkinson's disease,
dementias including Alzheimer's disease, and multiple sclerosis.

Layers of Brain
• Meninges are the capsule-like coverings that provide protection to the
brain and spinal cord. They include three membranes;
1. Dura mater,
2. Arachnoid mater
3. Pia mater.

• The dura mater is the most external and toughest layer among the meninges.
• It has two more layers;
• Endosteal layer/periosteal layer, that covers the inner surface of the skull bones
• Meningeal layer, that covers the brain and forms folds.
• The four important folds of the meningeal layer are
1. Falx cerebri, that lies between two cerebral hemispheres
2. Tentorium cerebelli, that forms the roof of posterior cranial fossa
3. Falx cerebelli, that lies between the two cerebellar hemispheres
4. Diaphragma sella, that bridges the sella turcica
• Dura mater is sensitive to pain and is supplied by various nerves.
• It receives nutrition from the blood supplied by various dural blood vessels.
• Dural venous sinuses are present in the folds of the endosteal layer.

• The arachnoid mater is the middle layer of the meninges.
• It is separated from the dura mater by subdural space, and from pia mater by
subarachnoid space.

• Pia mater is the innermost layer, that directly invests the brain and
spinal cord. It forms tela choroidea and choroid plexus in the brain. In
the spinal cord, it forms modifications such as ligamentum flavum and
ligamentum denticulatum.

Grey Matter Vs White Matter
• The CNS has two kinds of tissue: grey matter and
white matter,
• Grey matter, which has a pinkish-grey color in
the living brain, contains the cell bodies,
dendrites and axon terminals of neurons, so it is
where all synapses are.
• White matter is made of axons connecting
different parts of grey matter to each other

Regions of the Brain
• Cerebellum – coordination of
movement and aspects of motor
learning
• Cerebrum – conscious activity
including perception, emotion,
thought, and planning
• Medulla – vital reflexes as heart
beat and respiration

• Di-encephalon= Thalamus and
Hypothalamus
 Thalamus – Brain’s switchboard – filters
and then relays information to various
brain regions
 Hypothalamus– involved in regulating
activities internal organs, monitoring
information from the autonomic
nervous system, controlling the
pituitary gland and its hormones, and
regulating sleep and appetite
• Brain stem – medulla, pons, and
midbrain (involuntary responses) and
relays information from spine to
upper brain

Cerebellum
• The cerebellum is divided into an anterior lobe, a posterior lobe, and the
flocculonodular lobe
• The anterior and posterior lobes are connected in the middle by the vermis.
• Compared to the cerebral cortex, the cerebellum has a much thinner outer cortex that is
narrowly furrowed into numerous curved transverse fissures.

• The cerebellum rests at the back of the cranial
cavity, lying beneath the occipital lobes, and is
separated from these by the cerebellar tentorium
(tentorium= tent or extension) which is a sheet of
fiber.
• The cerebellum consists of an inner medulla of
white matter and an outer cortex of richly folded
grey matter.
• The cerebellum's anterior and posterior lobes
appear to play a role in the coordination and
smoothing of complex motor movements, and the
flocculonodular lobe in the maintenance of and in
cognitive motor functions

Cerebrum
• The cerebrum, consisting of the cerebral
hemispheres, forms the largest part of the brain
and overlies the other brain structures.
• divided into nearly symmetrical left and right
hemispheres by a deep groove, the longitudinal
fissure
• The outer region of the hemispheres, the
cerebral cortex, is grey matter, consisting of
cortical layers of neurons.
• The hemispheres are connected by five bridges
that span the longitudinal fissure, the largest of
these is the corpus callosum
• Each hemisphere is divided into four main lobes
 Frontal lobe
 Parietal lobe
 Temporal lobe
 Occipital lobe
1
2
3
4
Corpus callosum

Three other lobes are also
considered to be distinctive
lobes
 a central lobe
 a limbic lobe
 An insular lobe.
• The central lobe comprises
the precentral gyrus and
the postcentral gyrus.
Sulcus= a groove/ furrow
Gyrus= a fold between two clefts

The insular cortex (also insula and insular lobe) is a portion of
the cerebral cortex folded deep within the lateral sulcus (the
fissure separating the temporal lobe from the parietal and
frontal lobes) within each hemisphere of the mammalian brain.

Medulla Oblongata
• It is anterior and partially inferior to the cerebellum. The bulb is a medical term
for the medulla oblongata
• It is a cone-shaped neuronal mass that controls autonomic functions and
connects the higher levels of the brain to the spinal cord.
• It is also responsible for regulating several basic functions of the autonomic
nervous system, including:
Respiration
Cardiac center
Vasomotor center
Reflex centers of vomiting, coughing, sneezing, and swallowing

• The medulla oblongata is a fairly long structure
comprised of many parts. Anatomical features of
the medulla oblongata include:
Median fissures: Shallow groves located along the
anterior and posterior portions of the medulla.
Olivary bodies: Paired oval structures on the medulla's
surface that contain nerve fibers connecting the
medulla to the pons and cerebellum. Olivary bodies are
sometimes called olives.
Pyramids: Two rounded masses of white matter located
on opposite sides of the anterior median fissure. These
nerve fibers connect the medulla to the spinal cord,
pons, and cerebral cortex.
Fasciculus gracilis: A continuation of the bundle of
nerve fiber tracts that extend from the spinal cord to
the medulla.
Fasciculus gracilis

Thalamus
• The thalamus (from Greek, "chamber") is a large mass of gray matter
in the dorsal part of the di-encephalon of the brain with several
functions such as relaying of sensory signals, including motor signals
to the cerebral cortex, and the regulation of consciousness, sleep, and
alertness.
• It is a midline symmetrical structure of two halves, within the
vertebrate brain, situated between the cerebral cortex and the
midbrain.

DiencephalonJust for Understanding

• The thalamus is made up of two symmetrical structures formed from the
diencephalon.
• Each half of the thalamus is elongated along the anteroposterior axis giving
it an ovoid appearance. It is narrowest at the anterior end and widest at
the posterior part.
• The thalami are made up of grey matter that is partitioned by a “Y” shaped
white matter structure known as the internal medullary lamina.
• As a result of the location of the internal medullary lamina, each thalamus
is divided into roughly three main parts:
 the anterior,
 medial
 and lateral thalamus.
• The anterior part lies between the short limbs of the internal medullary
lamina, while the medial and lateral parts lie on the respective side of the
main stem of the “Y”. The left thalamus communicates with the right
thalamus by way of the inter-thalamic adhesion.

Hypothalamus
• The hypothalamus is a small but
important area of the brain formed
by various nucleus and nervous
fibers.
• Through its neuronal connections in
Limbic system, it is involved in many
complex functions of the organism
such as vegetative system control,
homeostasis of the organism,
thermoregulation, and also in
adjusting the emotional behavior.
Just for Understanding

• The hypothalamus is divided
• by the anterior horns of the fornix in
a lateral, medial, and periventricular (median) region
• and by a coronal plane passing through the infundibulum
 in an anterior and posterior region.
o The anterior region is also referred to as
the pre-chiasmatic region, due to its location above the chiasma optic,
o while the posterior region is called the mammillary region.
o The infundibular region is situated between the previous two regions.

Brain Stem
• The brainstem lies beneath the cerebrum and consists
of the midbrain, pons and medulla.
• Pons: Contains nuclei that relay signals from the
forebrain to the cerebellum, along with nuclei deal
primarily with sleep, respiration, swallowing, bladder
control, hearing, equilibrium, taste, eye movement,
facial expressions, facial sensation, and posture.
• Midbrain: Associated with vision, hearing, motor
control, sleep and wake cycles, alertness, and
temperature regulation.
• Medulla: The lower half of the brainstem that contains
the cardiac, respiratory, vomiting, and vasomotor
centers and regulates autonomic, involuntary
functions such as breathing, heart rate, and blood
pressure.

• It lies in the back part of the skull, resting
on the part of the base known as the
clivus, and ends at the foramen magnum, a
large opening in the occipital bone.
• The brainstem continues below this as the
spinal cord, protected by the vertebral
column.
• Ten of the twelve pairs of cranial nerves
emerge directly from the brainstem.

• The brainstem also contains many cranial nerve
nuclei and nuclei of peripheral nerves, as well as
nuclei involved in the regulation of many essential
processes including breathing, control of eye
movements and balance.
• The reticular formation, a network of nuclei of ill-
defined formation, is present within and along the
length of the brainstem.
• Many nerve tracts, which transmit information to
and from the cerebral cortex to the rest of the
body, pass through the brainstem.

2- Spinal Cord
• The spinal cord is a long, thin, tubular structure made
up of nervous tissue, which extends from the medulla
oblongata in the brainstem to the lumbar region of the
vertebral column.
• It encloses the central canal of the spinal cord, which
contains cerebrospinal fluid.
• In humans, the spinal cord begins at the occipital
bone, passing through the foramen magnum and
entering the spinal canal at the beginning of the
cervical vertebrae.

Segments of Spinal cord
• In most adults, the spine is composed of 33 individual back
bones (vertebrae).
• Just as the skull protects the brain, vertebrae protect the
spinal cord.
• The vertebrae are separated by disks made of cartilage,
which act as cushions, reducing the forces generated by
movements such as walking and jumping.
• The vertebrae and disks of cartilage extend the length of
the spine and together form the vertebral column, also
called the spinal column.
• Vertebral segments of spinal cord are divided into three
regions
1. Cervical
2. Thoracic
3. Lumbal

• Like the brain, the spinal cord is covered by three layers of tissue
(meninges).

• Spinal nerves: Emerging from the spinal cord between the vertebrae
are 31 pairs of spinal nerves. Each nerve emerges in two short
branches (roots):
• One at the front (motor or anterior root) of the spinal cord
• One at the back (sensory or posterior root) of the spinal cord
• The motor roots carry commands from the brain and spinal cord to
other parts of the body, particularly to skeletal muscles.
• The sensory roots carry information to the brain from other parts of
the body.
• Cauda equina: The spinal cord ends about three fourths of the way
down the spine, but a bundle of nerves extends beyond the cord. This
bundle is called the cauda equina because it resembles a horse’s tail.
The cauda equina carries nerve impulses to and from the legs.

3-Neurons
• An single nerve cell is called a neuron. There are about a trillion neurons in the
human nervous system!
• These important cells enable communication within the nervous system. To carry
out this function, neurons possess certain crucial properties:
• All neurons are very excitable, meaning that they are able to respond to
environmental stimuli very well.
• Neurons conduct electricity very well. This allows them to respond to stimuli by
producing electrical signals that travel very quickly to cells that may be at a
distance.
• Neurons are secretory cells. This means that when an electrical signal is
transmitted to the end of the neuron, the cell secretes a particular chemical
messenger called a neurotransmitter. The neurotransmitter then stimulates other
cells around the neuron.

• Atypical neuron has three anatomical structures
• Cell body. As the name suggests, this is the main body part of the cell. The key
organs needed for cell survival are located in the cell body.
• Dendrites. These are similar to antenna projecting outwards from the cell
body. They increase the surface area available to receive signals from other
neurons. A neuron can sometimes have up to 400,000 dendrites!
• Axon. The axon is also known as the nerve fibre. It is an enlongated tubular
structure that extends from the cell body and ends at other cells. It conducts
electrical signals called action potentials away from the neuron. Axons can
vary in length, ranging from less than a millimetre to longer than a metre.
• The axon hillock is the first portion of the axon, and the region of the cell
body from which the axon leaves. The axon hillock is also known as the trigger
zone, because this is where action potentials are started.
• The axon terminal is the end of the axon where action potentials are
conducted down to. It is here that neurotransmitters are released.

• There are three types of neurons in the nervous system –
1. Afferent neurons carry signals towards the CNS – afferent means
“towards”. They provide information about the external environment
and the regulatory functions being carried out by the nervous system.
2. Efferent neurons are mainly located in the peripheral nervous system,
but their cell bodies orginate in the CNS. Many incoming signals from
the CNS converge onto the efferent neurons, which then affect the
outgoing signals to various organs in the body. These organs then
carry out the appropriate response.
3. Interneurons are located entirely within the CNS. They make up about
99% of all neurons.
Types of neurons based on Function

Afferent Neurons Efferent NeuronsInterneurons

Glial Cells
• in addition to neurons, glial cells are the other major cell type that make up
the nervous system.
• Glial cells are also called neuroglia.
• Although they are not as well known as neurons, they make up about 90%
of cells within the CNS. However, they only occupy about half of the space
in the brain because they do not have extensive branching like neurons.
• Unlike neurons, glial cells do not conduct nerve electrical signals. They
instead serve to protect and nourish the neurons.
• There are four major types of glial cells in the CNS –
1. astrocytes,
2. oligodendrocytes,
3. microglia
4. ependymal cells.

• Astrocytes --“Astro” means “star” and “cyte” means cell.
Astrocytes are so named because they have a star-like shape.
They are the most abundant glial cells and have the act as a
“glue” to hold neurons together in their proper positions and
play a role in neurotransmitter activity
• Oligodendrocytes form sheaths around the axons of the CNS
that serve as insulation. These sheaths are made of myelin,
which is a white material that enables the conduction of
electrical impulses.
• Microglia act as the immune defence cells of the CNS. They
are made of the same tissues as monocytes, which are a type
of white blood cell that leaves the blood and sets up a front-
line defence against invading organisms throughout the
body.
• Ependymal cells line the internal cavities of the CNS. The
ependymal cells that line the cavities of the brain also
contribute to the formation of cerebrospinal fluid (CSF).
These cells have tail-like projections called cilia. The beating
of this cilia assists the flow of CSF throughout the brain
cavities. Ependymal cells also act as stem cells in the brain,
and have the potential to form other glial cells and new
neurons which are only produced in specific site of the brain.
Neurons in most of the brain are considered to be
irreplaceable.

There are also two types of glial cells in the PNS –
1. Schwann cells
2. and satellite cells.
• Schwann cells are wound repeatedly around nerve
fibres in the peripheral nervous system, producing a
myelin sheath similar to the membrane produced by
oligodendrocytes in the CNS. They also play a role in
the regeneration of damaged fibres.
• Satellite cells surround the cell bodies of neurons in
the ganglia of the PNS. Their function has not been
properly defined yet.

4-Nerve Fibers
• The axonal extended fibers of nervous system
• According to the size, myelination and speed of conduction, the nerve
fibers are classified into three different categories, namely A, B and C.
These are described in detail below:

Type A fibers:
• These fibers are the thickest and fastest conducting.
• They are heavily myelinated.
• They have a diameter of 1.5-20 micron
• Their speed of conduction is 4-120 m/sec, which shows that they have a really fast conduction of impulse.
• Examples of type A fibers are skeletomotor fibers, fusimotor fibers and afferent fibers to skin.
Type B fibers:
• These fibers are medium in size, i.e. they are smaller than type A fibers but larger than type C.
• They are moderately myelinated.
• They have a diameter of 1.5-3.5 microns.
• Their speed of conduction is 3-15 m/sec, which shows that they are slower than type A fibers.
• Examples of type B fibers are preganglionic autonomic efferents.
Type C fibers:
• These fibers are the smallest and thinnest.
• They are non-myelinated.
• They have a diameter of 0.1-2 microns.
• Their speed of conduction is 0.5-4 m/sec, which shows that they have the slowest conduction.
• Examples of type C fibers are postganglionic autonomic efferents and afferent fibers to skin.

4. nervous system

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