Human nervous system

HUMAN NERVOUS SYSTEM
JAI NARAIN VYAS UNIVERSITY, JODHPUR
ASSISTANT PROFESSOR:- ASHWIN SINGH
CHOUHAN
DEPARTMENT:- PHARMACOLOGY
E-mail:- anshukavya1993@gmail.com

JNVU PHARMACY, JODHPUR
HUMAN NERVOUS SYSTEM that conducts stimuli from
sensory receptors to the brain and spinal cord and
conducts impulses back to other parts of the body. As
with other higher vertebrates, the human nervous
system has two main parts:- the central nervous system
(the brain and spinal cord) and the peripheral nervous
system (the nerves that carry impulses to and from the
central nervous system). In humans the brain is
especially large and well developed.

FUNCTIONS OF THE NERVOUS SYSTEM
 Gathers information from both inside and outside the
body - Sensory Function.
 Transmits information to the processing areas of the
brain and spine.
 Processes the information in the brain and spine –
Integration Function.
 Sends 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 Nervous System is divided into Two Main
Divisions: Central Nervous System (CNS) and the
Peripheral Nervous System (PNS).

Monitoring changes. Much like a sentry, it uses its
millions of sensory receptors to monitor changes
occurring both inside and outside the body these changes
are called stimuli, and the gathered information is called
sensory input.
Interpretation of sensory input. It processes and
interprets the sensory input and decides what should be
done at each moment, a process called integration.
Effects responses. It then effects a response by
activating muscles or glands (effectors) via motor output.
Mental activity. The brain is the centre of mental
activity, including consciousness, thinking, and memory.
Homeostasis. This function depends on the ability of
the nervous system to detect, interpret, and respond to
changes in the internal and external conditions. It can
help stimulate or inhibit the activities of other systems to
help maintain a constant internal environment.

Basic Cells of the Nervous System
Neuron
Basic functional cell of nervous system
Transmits impulses (up to 250 mph)
Parts of a Neuron
Dendrite – receive stimulus and
carries it impulses toward the cell
body
Cell Body with nucleus– nucleus
& most of cytoplasm
Axon – fibre which carries impulses away from cell
body
Schwann Cells- cells which produce myelin or fat layer
in the Peripheral Nervous System
Myelin sheath – dense lipid layer which insulates the
axon – makes the axon look gray

Node of Ranvier – gaps or nodes in the myelin sheath
Impulses travel from dendrite to cell body to axon
Three types of Neurons
Sensory neurons – bring messages to CNS
Motor neurons - carry messages from CNS
Interneuron's – between sensory &
motor neurons in the CNS

IMPULSES
A stimulus is a change in the environment with sufficient
strength to initiate a response.
Excitability is the ability of a neuron to respond to the
stimulus and convert it into a nerve impulse
All of Nothing Rule – The stimulus is either strong
enough to start and impulse or nothing happens
Impulses are always the same strength along a
given neuron and they are self-propagation – once it
starts it continues to the end of the neuron in only one
direction- from dendrite to cell body to axon
The nerve impulse causes a movement of ions across
the cell membrane of the nerve cell.

SYNAPSE
Synapse - small gap or space between the axon of one
neuron and the dendrite of another - the neurons do
not actually tough at the synapse.
It is junction between neurons which uses
neurotransmitters to start the impulse in the second
neuron or an effectors (muscle or gland)
The synapse insures one way transmission of impulses.
Neurotransmitters
Neurotransmitters– Chemicals in the junction which
allow impulses to be started in the second neuron.

Reflex Action
The whole mechanism of reflex action occurs in such a
fashion that there is no conscious control of the brain.
Stimulation occurs through the peripheral nervous
system and the response to this peripheral nerve
stimulation is involuntary. In a reflex action, the spinal
cord along with the brain stem is responsible for the
reflex movements.

A few examples of reflex action are:
When light acts as a stimulus, the pupil of the eye
changes in size.
Sudden jerky withdrawal of hand or leg when pricked by
a pin.
Coughing or sneezing, because of irritants in the nasal
passages.
Knees jerk in response to a blow or someone stamping
the leg.
The sudden removal of the hand from a sharp object.
Sudden blinking when an insect comes very near to the
eyes.
The whole process of reflex action involves some
important components. They are receptor organs,
sensory neurons, nerve centre, associated neurons,
motor neurons and effectors neurons.

The receptor organs perceive the stimuli. They are
situated on the sense organs. The afferent neurons or the
sensory neurons carry the stimuli from receptors to the
spinal cord. The ganglion of the spinal cord has the
sensory neurons.
The spinal cord is the nerve centre, where synaptic
connections are formed. The associated neurons are
present in the spinal cord. The ventral horn of spinal cord
has the motor neurons. Effectors organs are the glands
and muscles that behave in response to the stimuli.

Reflex Arc
The neural pathway that controls the reflexes occurs
through the reflex arc. It acts on an impulse even before
it reaches the brain. There are some stimuli that require
an automatic, instantaneous response without the need
of conscious thought. The following diagram shows the
reflex arc pathway.

COMPONENTS OF A REFLEX ARC
Receptor - reacts to a stimulus
Afferent pathway (sensory neuron) - conducts
impulses to the CNS
Interneuron - consists of one or more synapses in the
CNS (most are in the spine)
Efferent pathway (motor neuron) conducts impulses
from CNS to effectors.
Effectors - muscle fibres (as in the Hamstring muscle)
or glands responds by contracting or secreting a
product.
Spinal reflexes - initiated and completed at the spinal
cord level. Occur without the involvement of higher brain
centres.

The receptor here is the sense organ that senses danger.
The sensory neurons pick up signals from the sensory
organ and send them through other neurons which are
interconnected. It is then received by the relay neuron
which is present in the spinal cord. Immediately, the
spinal cord sends back signals to the muscle through the
motor neuron. The muscles attached to the sense organ
move the organ away from danger. In reflex actions, the
signals do not travel up to the brain.

NEUROGLIA
Neuroglia, or glial cells, are cells that support neurons,
supply them with nutrients, and get rid of dead cells and
pathogens such as bacteria. They also form insulation
between neurons so that electrical signals do not get
crossed, and can also aid the formation of synaptic
connections between neurons. There are several types of
neuroglia.
Astroglial cells, also called astrocytes, are star-shaped
cells found in the brain and spinal cord. They provide
nutrients to neurons, maintain ion balance, and remove
unneeded excess neurotransmitters from the synaptic
cleft.
Ependymal cells are also found in the CNS. There are
two types of ependymal cells. Non-ciliated ependymal
cells form cerebrospinal fluid, while ciliated ependymal
cells help the cerebrospinal fluid circulate. Cerebrospinal
fluid cushions the brain and spinal cord.

Oligodendrocytes are found in the CNS and provide
physical support to neurons. They form a myelin
sheath around some neurons in the CNS. The myelin
sheath is a fatty substance wrapped around the axons of
some neurons. It provides electrical insulation.
Schwann cells also form myelin sheaths around some
neurons, but they are only found in the PNS. Neurons
that are myelinated can conduct electrical impulses faster
than non-myelinated neurons.
Microglial cells, or microglia, are small macrophage
cells in the CNS that protect against disease by engulfing
pathogens through phagocytosis (“cell eating”). They can
also destroy infected neurons and promote the regrowth
of neurons. All of the other types of neuroglia above are
larger and collectively called macroglia.

NEUROGLIA

TYPES OF NEUROGLIA CELL

STRUCTURAL CLASSIFICATION
The structural classification, which includes all of the
nervous system organs, has two subdivisions- the central
nervous system and the peripheral nervous system.
Central nervous system (CNS). The CNS consists of
the brain and spinal cord, which occupy the dorsal body
cavity and act as the integrating and command centres of
the nervous system
Peripheral nervous system (PNS). The PNS, the part
of the nervous system outside the CNS, consists mainly
of the nerves that extend from the brain and spinal cord.

CENTRAL NERVOUS SYSTEM
The part of the nervous system which in vertebrates
consists of the brain and spinal cord, to which sensory
impulses are transmitted and from which motor impulses
pass out, and which coordinates the activity of the entire
nervous system.
Brain
Brain stem – medulla, pons, midbrain
Diencephalon – thalamus & hypothalamus
Cerebellum
Cerebrum
Spine
Spinal Cord

MENINGES
Meninges are the three coverings around the brain &
spine and help cushion, protect, and nourish the brain
and spinal cord.
Dura mater is the most outer layer, very tough
Arachnoid mater is the middle layer and adheres to
the dura mater and has web like attachments to the
innermost layer, the pia mater
Pia mater is very thin, transparent, but tough, and
covers the entire brain, following it into all its crevices
(sulci) and spinal cord
Cerebrospinal fluid, which buffers, nourishes, and
detoxifies the brain and spinal cord, flows through the
subarachnoid space, between the arachnoid mater and
the pia mater

MENINGES

REGIONS OF THE BRAIN
Cerebellum – coordination of movement and aspects
of motor learning
Cerebrum – conscious activity including perception,
emotion, thought, and planning
Thalamus – Brain’s switchboard – filters and then
relays information to various brain regions
Medulla – vital reflexes as heart beat and respiration
Brainstem – medulla, pons, and midbrain (involuntary
responses) and relays information from spine to upper
brain
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

Midbrain
Spinal cord

BRAIN STEM
The midbrain, pons, and the medulla oblongata are
collectively referred to as the brain stem The structure
connects the brain to the spinal cord. Attached to the
brain stem, but considered a separate region of the adult
brain, is the cerebellum. The midbrain coordinates
sensory representations of the visual, auditory, and
somatosensory perceptual spaces. The pons is the main
connection with the cerebellum. The pons and the
medulla regulate several crucial functions, including the
cardiovascular and respiratory systems and rates. The
cranial nerves connect through the brain stem and
provide the brain with the sensory input and motor
output associated with the head and neck, including most
of the special senses. The major ascending and
descending pathways between the spinal cord and brain,
specifically the cerebrum, pass through the brain stem.

MIDBRAIN
The midbrain is the most superior portion of the
barinstem. It is located posterior to the hypothalamus
and superior to the pons.
MIDBRAIN
BRAIN STEM

It contains reflex centers for the head, eye, and body
movements in response to visual and auditory
stimuli. For example, reflexively turning the head to hear
better or see better is activated by the midbrain.
PONS
The word pons comes from the Latin word for bridge. It is
visible on the anterior surface of the brain stem as the
thick bundle of white matter attached to the cerebellum.
The pons is the main connection between the cerebellum
and the brain stem. The bridge- like white matter is only
the anterior surface of the pons; the gray matter beneath
that is a continuation of the tegmentum from the
midbrain. Gray matter in the tegmentum region of the
pons contains neurons receiving descending input from
the cerebrum and thalamus that is sent to the
cerebellum. The pons works closely with the medulla to
regulate respiratory activities.

MEDULLA
The medulla oblongata is the most inferior portion of the
brain, and it’s connecting link with the spinal cord. It
consists of ascending and descending tracts that are
entering the brain for sensory integration and exiting the
brain for motor responses. The medulla contains 3
integration centers that are vital for homeostasis:
(1) The respiratory center that controls the rhythm of
breathing and reflexes such as coughing and
sneezing (2) The cardiac control center that regulates
the rate and force of hear contractions (3) The
vasomotor center that regulates blood pressure
through vasoconstriction of blood vessels and vasodilation
of blood vessels. Another area that spreads throughout
the brain stem from the medulla up to the thalamus is
the reticular formation. The reticular formation is
responsible for regulating general brain activity and
attention. It is related to sleep and wakefulness.

DIENCEPHALON
The diencephalon is the one region of the adult brain that
retains its name from embryologic development. The
etymology of the word diencephalon translates to
“through brain.” It is the connection between the
cerebrum and the rest of the nervous system, with one
exception. The rest of the brain, the spinal cord, and the
PNS all send information to the cerebrum through the
diencephalon. Output from the cerebrum passes through
the diencephalon. The single exception is the system
associated with olfaction, or the sense of smell, which
connects directly with the cerebrum. The diencephalon is
deep beneath the cerebrum and can be described as any
region of the brain with “thalamus” in its name. The two
major regions of the diencephalon are the thalamus itself
and the hypothalamus

THALAMUS
The thalamus is a collection of nuclei that relay
information between the cerebral cortex and the
periphery, spinal cord, or brain stem. All sensory
information, except for the sense of smell, passes
through the thalamus before processing by the cortex.
Axons from the peripheral sensory organs synapse in the
thalamus, and thalamic neurons project directly to the
cerebrum. It is a requisite synapse in any sensory
pathway, except for olfaction. The thalamus does not just
pass the information on, it also processes that
information. For example, the portion of the thalamus
that receives visual information will influence what visual
stimuli are important, or what receives attention. The
cerebrum also sends information down to the thalamus,
which usually communicates motor commands.

HYPOTHALAMUS
Inferior and slightly anterior to the thalamus is
the hypothalamus, the other major region of the
diencephalon. The hypothalamus is a collection of nuclei
that are largely involved in regulating homeostasis. The
hypothalamus is the executive region in charge of the
autonomic nervous system and the endocrine system
through its regulation of the anterior pituitary gland.
Other parts of the hypothalamus are involved in memory
and emotion as part of the limbic system.

The diencephalon is composed primarily of the thalamus
and hypothalamus, which together define the walls of the
third ventricle. The thalami are two elongated, ovoid
structures on either side of the midline that make contact
in the middle. The hypothalamus is inferior and anterior
to the thalamus, culminating in a sharp angle to which
the pituitary gland is attached. JNVU PHARMACY, JODHPUR

CEREBELLUM
The cerebellum, as the name suggests, is the “little
brain.” It is covered in gyri and sulci like the cerebrum,
and looks like a miniature version of that part of the
brain. The cerebellum is largely responsible coordinating
the interactions of skeletal muscles. It controls posture,
balance, and muscle coordination during movement.
Descending fibers from the cerebrum have branches that
connect to neurons in the pons. Those neurons project
into the cerebellum, providing a copy of motor commands
sent to the spinal cord. Sensory information from the
periphery, which enters through spinal or cranial nerves,
is copied to a nucleus in the medulla known as
the inferior olive.

Fibers from this nucleus enter the cerebellum and are
compared with the descending commands from the
cerebrum. If the primary motor cortex of the frontal lobe
sends a command down to the spinal cord to initiate
walking, a copy of that instruction is sent to the
cerebellum. Sensory feedback from the muscles and
joints, proprioceptive information about the movements
of walking, and sensations of balance are sent to the
cerebellum through the inferior olive and the cerebellum
compares them. If walking is not coordinated, perhaps
because the ground is uneven or a strong wind is
blowing, then the cerebellum sends out a corrective
command to compensate for the difference between the
original cortical command and the sensory feedback.
Damage to the cerebellum may result in a loss of
equilibrium, muscle contractions, and muscle tone.

The cerebellum is situated on the posterior surface of the
brain stem. Descending input from the cerebellum enters
through the large white matter structure of the pons.
Ascending input from the periphery and spinal cord
enters through the fibers of the inferior olive. Output
goes to the midbrain, which sends a descending signal to
the spinal cord.

CEREBRUM
Is the largest portion of the brain encompasses about
two-thirds of the brain mass -It consists of two
hemispheres divided by a fissure – corpus callosum It
includes the cerebral cortex, the medullary body, and
basal ganglia.
cerebral cortex is the layer of the brain often referred
to as gray matter because it has cell bodies and
synapses but no myelin.
The cortex (thin layer of tissue) is gray because nerves
in this area lack the insulation or white fatty myelin
sheath that makes most other parts of the brain appear
to be white.
The cortex covers the outer portion (1.5mm to 5mm)
of the cerebrum and cerebellum.
The cortex consists of folded bulges called gyri that
create deep furrows or fissures called sulci.

The folds in the brain add to its surface area which
increases the amount of gray matter and the quantity of
information that can be processed.
Medullary body – is the white matter of the cerebrum
and consists of myelinated axons.
Commisural fibers – conduct impulses between the
hemispheres and form corpus callosum.
Projection fibers – conduct impulse in and out of the
cerebral hemispheres.
Association fibers – conduct impulses within the
hemispheres.
Basal ganglia – masses of gray matter in each
hemisphere which are involved in the control of
voluntary muscle movements.

LOBES OF THE
CEREBRUM
Frontal – motor area
involved in movement
and in planning &
coordinating behavior
Parietal – sensory
processing, attention, &
language
Temporal – auditory
perception, speech, &
complex visual perceptions
Occipital – visual center –
plays a role in processing
visual information

SPECIAL REGIONS
Broca’s area – located in the frontal lobe – important
in the production of speech
Wernicke’s area – comprehension of language and the
production of meaningful speech
Limbic System – a group of brain structures
(aamygdala, hippocampus, septum, basal ganglia, and
others) that help regulate the expression of emotions
and emotional memory

BRAIN WAVES
Brain waves are rhythmic
fluctuation of electric potential
between parts of the brain as seen on
an electroencephalogram (EEG).
To measure brain waves
electrodes are placed onto the
scalp using the EEG.
There are four types of
brainwaves:
 Beta
 Alpha
 Theta
 Delta

THE SPINAL CORD
The description of the CNS is concentrated on the
structures of the brain, but the spinal cord is another
major organ of the system. The spinal cord is continuous
with the brain. It descends from the medulla through the
foramen magnum of the occipital bone and extends to
the lumbar vertebrae. A cross-sectional view of the
spinal cord reveals both gray matter and white matter.
The gray matter has the shape of a butterfly with
outstretched wings and is centrally located to the white
matter. The spinal cord has two basic functions. It
transmits nerve impulses to and from the brain, and it
serves as a reflex center for spinal reflexes.

WHITE AND GREY MATTER
WHITE MATTER
GREY MATTER

CEREBROSPINAL FLUID
Definition:
Formation:
Function Of Cerebrospinal Fluid
Physical Characteristics

FLOW OF THE CEREBROSPINAL FLUID

PERIPHERAL NERVOUS SYSTEM
the part of the nervous system that is outside the
central nervous system and comprises the cranial nerves
excepting the optic nerve, the spinal nerves, and the
autonomic nervous system.
The peripheral nervous system (PNS) includes the
nerves and ganglia outside of the brain and spinal cord.
These nerves control automatic functions of the body and
provide information to the central nervous system about
the external environment.
The PNS is divided into two distinct divisions: the
sensorimotor division which provides awareness of and
response to surroundings and the external environment
to the through sensory and motor nerves, and the
autonomic system which is largely responsible for
monitoring and maintaining the internal environment of
the body.

The autonomic nervous system controls the automatic
functions of the body, including regulating heart rate and
blood pressure, bronchial dilation and contraction, among
other functions of the internal organs. The autonomic
nervous system is then further divided to the
sympathetic, parasympathetic, and enteric divisions. The
sympathetic and parasympathetic divisions are
responsible for automatic functions that help the body to
either prepare the body to respond to a stimulus or help
the body conserve energy. Typically, actions of the
sympathetic division are those that are related to the
“fight or flight” response while the parasympathetic
division is associated with the “rest and digest” state.
Both divisions are equally important and exist in a crucial
balance. The third portion, the enteric division, is also
known as the intrinsic division and exists in the lining of
the gastrointestinal system and is the focus of the field of
neurogastroenterology

PERIPHERAL NERVOUS SYSTEM
Cranial nerves
12 pair
Attached to under surface of brain
Spinal nerves
31 pair
Attached to spinal cord

CARNIAL NERVE

SPINAL NERVE

The peripheral nervous system consists of the somatic
nervous system (SNS) and the autonomic nervous
system (ANS). The SNS consists of motor neurons that
stimulate skeletal muscles. In contrast, the ANS consists
of motor neurons that control smooth muscles, cardiac
muscles, and glands. In addition, the ANS monitors
visceral organs and blood vessels with sensory neurons,
which provide input information for the CNS.
The ANS is further divided into the sympathetic nervous
system and the parasympathetic nervous system. Both of
these systems can stimulate and inhibit effectors.
However, the two systems work in opposition—where one
system stimulates an organ, the other inhibits. Working
in this fashion, each system prepares the body for a
different kind of situation, as follows:

ANATOMY AND PHYSIOLOGY OF ANS

THE SYMPATHETIC NERVOUS SYSTEM prepares the
body for situations requiring alertness or strength, or
situations that arouse fear, anger, excitement, or
embarrassment (“fight‐or‐flight” situations). In these
kinds of situations, the sympathetic nervous system
stimulates cardiac muscles to increase the heart rate,
causes dilation of the bronchioles of the lungs (increasing
oxygen intake), and causes dilation of blood vessels that
supply the heart and skeletal muscles (increasing blood
supply).
The adrenal medulla is stimulated to release epinephrine
(adrenalin) and norepinephrine (noradrenalin), which in
turn increases the metabolic rate of cells and stimulates
the liver to release glucose into the blood. Sweat glands
are stimulated to produce sweat. In addition, the
sympathetic nervous system reduces the activity of
various “tranquil” body functions, such as digestion and
kidney functioning.

THE PARASYMPATHETIC NERVOUS SYSTEM is active
during periods of digestion and rest. It stimulates the
production of digestive enzymes and stimulates the
processes of digestion, urination, and defecation. It
reduces blood pressure and heart and respiratory rates
and conserves energy through relaxation and rest.
In the SNS, a single motor neuron connects the CNS to
its target skeletal muscle. In the ANS, the connection
between the CNS and its effector consists of two neurons
the preganglionic neuron and the postganglionic neuron.
The synapse between these two neurons lies outside the
CNS, in an autonomic ganglion. The axon (preganglionic
axon) of a preganglionic neuron enters the ganglion and
forms a synapse with the dendrites of the postganglionic
neuron.

The axon of the postganglionic neuron emerges from the
ganglion and travels to the target organ. There are three
kinds of autonomic ganglia:
The sympathetic trunk, or chain, contains sympathetic
ganglia called paravertebral ganglia. There are two
trunks, one on either side of the vertebral column along
its entire length. Each trunk consists of ganglia connected
by fibers, like a string of beads.
The prevertebral (collateral) ganglia also consist of
sympathetic ganglia. Preganglionic sympathetic fibers
that pass through the sympathetic trunk (without forming
a synapse with a postganglionic neuron) synapse here.
Prevertebral ganglia lie near the large abdominal arteries,
which the preganglionic fibers target.
Terminal (intramural) ganglia receive parasympathetic
fibers. These ganglia occur near or within the target
organ of the respective postganglionic fiber.

Sympathetic nervous system. Cell bodies of the
preganglionic neurons occur in the lateral horns of gray
matter of the 12 thoracic and first 2 lumbar segments of
the spinal cord. (For this reason, the sympathetic system
is also called the thoracolumbar division.) Preganglionic
fibers leave the spinal cord within spinal nerves through
the ventral roots (together with the PNS motor neurons).
The preganglionic fibers then branch away from the nerve
through white rami (white rami communicantes) that
connect with the sympathetic trunk. White rami are white
because they contain myelinated fibers. A preganglionic
fiber that enters the trunk may synapse in the first
ganglion it enters, travel up or down the trunk to synapse
with another ganglion, or pass through the trunk and
synapse outside the trunk. Postganglionic fibers that
originate in ganglia within the sympathetic trunk leave
the trunk through gray rami (gray rami communicantes)
and return to the spinal nerve.

which is followed until it reaches its target organ. Gray
rami are gray because they contain unmyelinated fibers.
Parasympathetic nervous system. Cell bodies of the
preganglionic neurons occur in the gray matter of sacral
segments S 2–S 4 and in the brainstem (with motor
neurons of their associated cranial nerves III, VII, IX, and
X). (For this reason, the parasympathetic system is also
called the craniosacral division, and the fibers arising
from this division are called the cranial outflow or the
sacral outflow, depending on their origin.) Preganglionic
fibers of the cranial outflow accompany the PNS motor
neurons of cranial nerves and have terminal ganglia that
lie near the target organ. Preganglionic fibers of the
sacral outflow accompany the PNS motor neurons of
spinal nerves. These nerves emerge through the ventral
roots of the spinal cord and have terminal ganglia that lie
near the target organ.

THE SOMATIC NERVOUS SYSTEM
The efferent portion of the peripheral nervous system
consists of the somatic nervous system and the
autonomic nervous system. The autonomic nervous
system controls the function of glands, smooth muscle,
cardiac muscle, and the neurons of the GI tract. It is
composed of two neurons in series that can either excite
or inhibit the target organ. In contrast, the somatic
nervous system contains single neurons that excite
skeletal muscles. The movements controlled by the
somatic nervous system can be voluntary or involuntary
(reflexes).

MOTOR UNIT
The axons of motor neurons are myelinated and have
large diameters for fast conduction of action potentials.
As the axon approaches a skeletal muscle fiber (muscle
cell) it usually branches to form synapses with anywhere
from three to one thousand muscle fibers. However, each
muscle fiber is usually innervated by only a single
neuron. A motor unit consists of a neuron and all of the
muscle fibers it innervates. A single neuron innervates
fibers from only one muscle and the innervated muscle
fibers are usually spread throughout the muscle

The portion of the skeletal muscle fiber plasma
membrane that synapses with the motor neuron axon is
called the motor end plate. Once an action potential
arrives at the axon terminal, the depolarization of the
membrane opens voltage-gated calcium channels. An
increase in intracellular calcium at the terminal causes
release of acetylcholine vesicles into the neuromuscular
junction. The acetylcholine binds nicotinic channels at the
motor end plate which causes them to open and allow
sodium to enter .The sodium entry triggers voltage-gated
sodium channels near the motor end plate, initiating an
action potential which is propagated in all directions along
the plasma membrane of the muscle fiber.
SPINAL CORD ANATOMY The cell bodies of the neurons
that innervate skeletal muscle of the body are found in
the ventral horn of the spinal cord .The neurons that
innervate the skeletal muscle of the head are in the
brainstem.

In the body, sensory signals come into the spinal cord
from the dorsal root ganglia, which contain the cell bodies
of sensory neurons .These neurons can excite motor
neurons in the spinal cord. Motor neuron axons travel
through tissues as nerves and synapse on skeletal muscle
cells. Excitation of motor neurons causes acetylcholine to
be released at the neuromuscular junction causing
contraction of the muscle. The muscle relaxes when the
motor neuron is no longer excited.

CONTROL OF MOVEMENT The spinal cord is not just a
conduit connecting the peripheral nervous system and
the brain. Instead, movements such as walking as well as
reflexes are organized by the spinal cord. In situations
when control by upper motor neurons in the brain is
necessary, they act on the lower motor neurons of the
spinal cord to influence reflexes and voluntary
movements. Two types of lower motor neurons. The
portion of a skeletal muscle that controls posture and
movement, the extrafusal muscle fibers, are innervated
by alpha motor neurons. A specialized type of skeletal
muscle fiber, the intrafusal muscle cell, resides in the
muscle spindle in the interior of the muscle .The
intrafusal muscle fibers are innervated by gamma motor
neurons. During muscle contraction, alpha and gamma
motor neurons are coactivated. Stretching of the
intrafusal fibers in the muscle spindle is sensed by stretch
receptors and sent via afferent sensory neurons

Intrafusal fibers
Extrafusal
fibers
to the spinal cord. This allows for
monitoring of the length of the muscle
which helps control muscle tone.
Two types of muscle sensory receptors.
In order for the body to be able to
control muscle contraction properly,
there must be feedback about the
contractile status of individual muscles.
The muscle spindle is an important
muscle sensory receptor that provides
information about muscle length and the
rate of change of muscle length.
In addition, Golgi tendon organs are encapsulated
sensory receptors situated in tendons near the junction
with the muscle .They detect changes in muscle tension
instead of changes in muscle length. Both types of
sensory receptors send information to the spinal cord and
the brain that is usually subconscious.
Golgi
tendon
organ
muscle spindle

MUSCLE STRETCH REFLEX. If a muscle spindle within a
muscle is quickly stretched, the muscle stretch reflex
causes contraction of the muscle as well as nearby
muscles. This is what occurs when the patellar tendon is
struck during a physical exam. The afferent sensory
neuron relays the stretch signal from the muscle spindle
to its cell body in the dorsal root of the spinal cord. The
sensory neuron synapses with the motor neuron in the
spinal cord that controls that muscle. In addition, the
sensory neuron activates an inhibitory neuron which
inhibits the motor neuron (reducing its likelihood of firing
an action potential) leading to the muscle on the opposite
side of the limb, causing it to relax. The muscle spindle
reflex is important in allowing maintenance of the length
of a certain muscle.

MUSCLE STRETCH REFLEX. If a muscle spindle within a
muscle is quickly stretched, the muscle stretch reflex
causes contraction of the muscle as well as nearby
muscles. This is what occurs when the patellar tendon is
struck during a physical exam. The afferent sensory
neuron relays the stretch signal from the muscle spindle
to its cell body in the dorsal root of the spinal cord. The
sensory neuron synapses with the motor neuron in the
spinal cord that controls that muscle.

In addition, the sensory neuron activates an inhibitory
neuron which inhibits the motor neuron (reducing its
likelihood of firing an action potential) leading to the
muscle on the opposite side of the limb, causing it to
relax. The muscle spindle reflex is important in allowing
maintenance of the length of a certain muscle.
Inhibitory
neuron
Golgi tendon organ
reflex
contract
GOLGI TENDON REFLEX. If the
Golgi tendon organs of a
muscle are stretched and
stimulated, muscle contraction
is inhibited through inhibition
of the motor neuron leading to
that muscle. This is because
the afferent neuron activates
an inhibitory neuron which is
synapsing with the motor
neuron .

In addition, the opposing muscle is stimulated to contract
through the interaction of an excitatory interneuron with
the afferent neuron .The Golgi tendon reflex acts to
protect muscles and tendons from damage due to
excessive tension. In addition, they may play a role in
equalizing the load across different parts of a muscle.
WITHDRAWAL REFLEXES. A withdrawal reflex occurs
when a part of the body such as a portion of a limb is
subjected to a painful stimulus. The flexor reflex causes
contraction of one muscle and relaxation of the opposing
muscle to move that portion of the body away from the
insult. A short time after the flexor reflex initiates, the
crossed extensor reflex initiates on the opposite side of
the body. This reflex allows for the opposite side of the
body to support the body’s weight or to push the body
out of the way of the painful stimulus.

LOCOMOTION. Walking and running require the legs to
alternate between forward flexion (the swing phase) and
backward extension (the stance phase). The repetition of
this pattern is synchronized with the other leg so that the
two legs remain in opposite phases. In most animals, if
the spinal cord is separated from the brain the four legs
can still make coordinated walking motions. This is
accomplished through central pattern generators which
are oscillatory neural circuits of lower motor neurons in
the spinal cord. Even though these circuits control the
basic movements of walking, they are greatly influenced
by the brain. For instance, running occurs when the brain
causes the central pattern generators to shorten the
stance phase. In addition, posture and goal directed
locomotion require input from the brain. However, the
central pattern generators allow relatively simple
modifications by the brain to control a very complicated
process such as locomotion

DISORDERS OF THE
NERVOUS SYSTEM

Epilepsy - common and diverse set of chronic
neurological disorders characterized by seizures.
Seizures - the physical findings or changes in behavior
that occur after an episode of abnormal electrical
activity in the brain and are caused by abnormal
electrical discharges in the brain
Alzheimer’s Disease - a degenerative disease of the
brain that causes dementia, which is a gradual loss of
memory, judgment, and ability to function. - the most
common form of dementia- affects an estimated 1 in
10 people over age 65
Multiple Sclerosis - an autoimmune disease that
affects the brain and spinal cord (central nervous
system) - body's immune system eats away at the
protective myelin sheath that covers the axons of the
neurons and interferes with the communication - MS
can affect vision, sensation, coordination, movement,
and bladder and bowel control.

Shingles (herpes zoster) - painful, blistering skin
rash due to the varicella -zoster virus, the virus that
causes chickenpox – the virus remains inactive
(becomes dormant) in certain nerves in the body.
Shingles occurs after the virus becomes active again
Cerebral Palsy - group of disorders that can involve
brain and nervous system functions such as movement,
learning, hearing, seeing, and thinking resulting from
damage to certain parts of the developing brain
Glaucoma - a group of eye conditions that lead to
damage to the optic nerve due to increased pressure in
the eye - the eye’s drainage system becomes clogged so
the intraocular fluid cannot drain and as the fluid builds
up, it causes pressure to build within the eye. High
pressure damages the sensitive optic nerve.

Pink eye (Conjunctivitis) – infection of the
conjunctiva of the eye.
Parkinson’s Disease - disorder of the brain that leads
to shaking (tremors) and difficulty with walking,
movement, and coordination. People with Parkinson's
disease have low brain dopamine concentrations.

EFFECTS OF DRUGS ON THE
NERVOUS SYSTEM
Alcohol - central nervous system depressant – cell
membranes are highly permeable to alcohol so once in
the bloodstream it can diffuse into almost all body
tissues. It is absorbed in the stomach so it gets into the
blood stream quickly and slows down function of the
nervous system
Caffeine - acts as a central nervous system stimulant
- caffeine suppresses melatonin for up to 10 hours and
also promotes adrenalin. Melatonin is strongly
associated with quality sleep, while adrenalin is the
neurotransmitter associated with alertness.

Nicotine - small doses of nicotine have a stimulating
action on the central nervous system – it is highly
addictive nicotine's effects on the brain cause an increased
release of neurotransmitters associated with pleasure.
The brain quickly adjusts to repeated nicotine consumption
by decreasing the amount of neurotransmitters released.
The effect of this increased tolerance is that the smoker
must continue to use nicotine in order to avoid the feelings
of discomfort associated with withdrawal from the drug.
Irritability and anxiety often ensue during nicotine
withdrawal.
Marijuana- Tetrahydrocannabinol, the main active
ingredient in marijuana, binds to membranes of nerve
cells in the centra system that have protein nervous
receptors. After binding to nerve cells, THC initiates a
chemical reaction that produces the various effects of
marijuana use. One of the effects is suppression of
memory and learning centers (called the hippocampus) in
the brain.

THANK
YOU

Human nervous system

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to Human nervous system

Similar to Human nervous system (20)

More from Jai Narain Vyas University Jodhpur Rajasthan India 342003

More from Jai Narain Vyas University Jodhpur Rajasthan India 342003 (20)

Recently uploaded

Recently uploaded (20)

Human nervous system