Nervous
System
Anatomy & Physiology
Nervous System
The nervous system regulates and
coordinates functions of the
body required to maintain
homeostasis.
The nervous system is made up of
the brain, spinal cord, nerves, and
sensory receptors and uses
electrical
and
signals to
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control
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Nervous System
Functions of the Nervous System:
1. Receiving sensory input
2. Integrating information
3. Controlling motor output
4. Establishing and maintaining mental activity
5. Maintaining homeostasis
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Nervous System
Divisions of the Nervous System
The nervous system consists of
two major divisions:
1 the central nervous system
and 2 the peripheral nervous
system
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The central nervous system (CNS)
receives information from and sends
information to the body.
The peripheral nervous system (PNS) is
responsible for detecting stimuli in and
around the body and sending that
information to the CNS and then
communicating messages from the
CNS to the body
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Nervous System
Divisions of the Nervous System
The CNS consists of the brain
and the spinal cord.
The PNS consists of all the
nervous tissue outside the CNS
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The PNS
has two primary divisions:
(1) the sensory division and (2) the
motor division.
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Nervous System
PERIPHERAL NERVOUS SYSTEM
The PNS
has two primary divisions:
(1) the sensory division and
(2) the
motor division.
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The sensory division transmits
electrical signals from specialized
receptors in the body toward the
CNS.
The sensory division is also called the
afferent division (afferent = toward).
The motor division, transmits electrical
signals from the CNS to effector
organs.
The motor division is also
called the efferent division (efferent =
away)
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Nervous System
The motor division
consists of two branches:
(1) the somatic nervous
system and (2) the
autonomic nervous system.
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The somatic nervous system is the
voluntary division of the motor
division.
The autonomic nervous system is the
involuntary division of the motor
division.
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Nervous System
The ANS has two
subdivisions:
the sympathetic division
and the parasympathetic
division
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The sympathetic division
readies the body for physical activity
and is called the fight-or flight
division.
The parasympathetic division
regulates resting
functions, such as digesting food or
slowing the heart rate, and is
called the rest-and-digest division.
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Nervous System
LORE
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Nervous System
Cells of the Nervous System
Neuron Structure
Neurons are the electrically excitable
cells of the nervous system.
There are three parts to most types of
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neuron:
(1) asitneuron
cell
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body, (2) dendrites, and (3) a single
axon.
Parts of the Neuron
1. Cell Body
The cell body is the enlarged and
rounded part that contains the
nucleus, mitochondria that supply ATP,
ribosomes that manufacture proteins,
and other organelles.
2. Dendrites
Dendrites are short, branched
extensions that transmit information
toward the cell body.
The number of dendrites may range
from one to thousands, and each can
receive input from many other
neurons.
Parts of the Neuron
3. Axon
The axon, also called the nerve fiber, conducts nerve impulses away from
the cell body. It is a single long extension that is finely branched at its tip.
• In most neurons, a single axon arises from a cone-shaped area of the
neuron cell body called the axon hillock.
• The initial segment is the actual beginning of the axon.
• The trigger zone is where action potentials are generated
• The cytoplasm of an axon is sometimes called the axoplasm
• The point of contact between the axon ending and its effector is
called a synapse
• The axon ending at the synapse is called the presynaptic terminal
Parts of the Neuron
TYPES OF NEURONS
Neurons can be classified based on either their function
or their structure.
There are three functional classifications based on the
direction of action potentials conduction:
1. sensory neurons (afferent neurons) conduct action
potentials toward the CNS;
2. motor neurons (efferent neurons) conduct action
potentials away from the CNS toward muscles or
glands.
3. Interneurons conduct action potentials within the
CNS from one neuron to another
Neurons Based on Functions
1. Sensory / Afferent Neuron
Sensory neurons bring information to the central nervous system from
the rest of the body.
2. Interneuron
Interneurons connect one neuron to another within the spinal cord and
brain.
3. Motor / Efferent Neuron
Motor neurons conducts its message from the central nervous system
toward a muscle or gland cell.
Neurons Based on Functions
Neurons Based on Structure
1. Multipolar neurons (multi-, many) have many dendrites and
a single axon. The dendrites vary in number and in their degree of
Branching. Most of the neurons within the CNS and
motor neurons of the PNS are multipolar.
2. Bipolar neurons (bi-, two) have two processes: one dendrite
and one axon. The dendrite is often specialized to
receive the stimulus, and the axon conducts action potentials to the
CNS. Bipolar neurons are located in some sensory organs,
such as in the retina of the eye and in the nasal cavity
Neurons Based on Structure
3. Pseudo-unipolar neurons (uni-, one), also known as pseudounipolar,
start out as bipolar neurons during development, but the two
processes fuse into a single process. These neurons have a single
process extending from the cell body, which divides into two branches
a short distance from the cell body.
4. Anaxonic neurons do not have axons and only have dendrites
projecting from their cell body. Found within the brain and retina,
these neurons communicate using only graded potentials and not
action potentials.
Neurons Based on Structure
Neuroglial Cells
Neuroglia are the major supporting cells in the nervous system. These
cells help hold the nervous tissues together.
1. Astrocytes – located at the brain and spinal cord, star shaped cells
containing microfilaments giving them considerable strength to
support neurons. Processes of astrocytes wrapped around
capillaries isolate the neurons from various potentially harmful
substances in the blood maintaining selective permeability.
2. Microglia – located also at the brain and spinal cord, these cells
function as phagocytes, removing microbes and damaged nervous
tissues.
Neuroglial Cells
3. Ependymal cells – cells that line the brain ventricles and central
canal of the spinal cord (these are spaces filled with cerebrospinal
fluid, which protects and nourishes the brain and spinal cord).
Ependymal cells produce and assist in the circulation of CSF.
4. Oligodendrocytes – are responsible for forming and maintaining the
myelin sheath around the neural axons of the brain and spinal cord.
A single oligodendrocyte myelinate several axons.
Neuroglial Cells
5. Schwann cells – these are cells that form the myelin sheath around
the axons of nerves outside the brain and spinal cord. Several
Schwann cells myelinate a single axon.
6. Satellite cells – flat cells that surround the cell bodies of neurons
outside the brain and spinal cord providing structural support. These
cells also regulate exchanges of materials between cell bodies and
the interstitial fluid.
Neuroglial Cells
Neuroglial Cells
Functions of Neurons
1. Irritability
Irritability is the ability to respond to a stimulus and convert it to a nerve
impulse.
The transmission of a nerve impulse along a neuron from one end to the
other occurs as a result of electrical changes across the membrane of
the neuron.
Action Potentials – also known as a nerve impulse or nerve signals, a
pulse like wave of voltage that passes on through a neuron that
influences other neurons.
Irritability
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Irritability
Irritability
Functions of Neurons
2. Conductivity
Conductivity is the ability to transmit the impulse to other neurons,
muscles, or glands.
When a nerve impulse reaches the end of an axon, the axon releases
chemicals called neurotransmitters stored at its synaptic vesicles.
Synapse
A chemical synapse is a specialized junction at which a signal from one
neuron’s axon flows with another neuron or an effector organ such as
muscle or gland.
Conductivity
Synapse
• Synaptic Cleft
The synaptic cleft is the space between 2 communicating nerve
cells.
• Presynaptic Terminal
It consists of the end of an axon. The synaptic terminal contains
many small sacs, or vesicles, that hold neurotransmitter molecules.
• Postsynaptic Membrane
The receptor containing membrane of the postsynaptic cell
opposed to the presynaptic terminal. Postsynaptic cells are
typically other neurons, muscles or glands.
Conductivity
Conductivity
Conductivity
Neurotransmitters
A neurotransmitter is a chemical signal that travels from a “sending” cell
to a “receiving” cell. These substances may be excitatory or inhibitory.
Neurotransmitters travel across the synapse between the axon and the
dendrite of the next neuron. Neurotransmitters bind to the membrane of
the dendrite.
Conductivity
Conductivity
Removal of Neurotransmitters:
Neurotransmitters have short-term effects on postsynaptic membranes
because the neurotransmitter is rapidly destroyed or removed from the
synaptic cleft.
1. Diffusion – movement of molecules away from the synapse into the
extracellular fluid
2. Enzymatic processes – breakdown of neurotransmitters by enzymes
3. Reuptake by cells – neurotransmitters are transported back into the
presynaptic terminal where it is repackaged into the synaptic vesicles
for reuse
Nervous Tissue
1. Gray Matter
The gray matter consists of neuron cell bodies and dendrites, along with
the synapses by which they communicate with other cells. Information
processing occurs in the gray matter.
2. White Matter
The white matter consists of myelinated axons transmitting information
throughout the nervous system.
The white matter forms the nerve tracts or conduction pathways of the
brain and spinal cord which propagate impulses. Outside the brain and
spinal cord, the bundles of axons and their connective tissue sheaths
are called nerves.
Nervous Tissue
Nervous System Divisions
Central Nervous System
The CNS consists of the brain and spinal cord, which act as the
integrating and command centers of the nervous system.
Brain
The brain is the largest and the most complex mass of nerve tissue in the
body. It functions to exert centralized control over the other organs of
the body.
The brain acts on the rest of the body both by generating patterns of
muscle activity and by driving the secretion of chemicals called
hormones.
Brain Subdivisions
Hindbrain
1. Cerebellum
The cerebellum is the largest part of the hindbrain. The neurons of the
cerebellum refine motor messages and coordinate muscle movements
for posture and balance subconsciously.
2. Medulla Oblongata
Pathway for ascending and descending nerve tracts. This region not only
regulates blood vessel diameter, breathing, blood pressure, and heart
rate, but it also contains reflex centers for vomiting, coughing, sneezing,
defecating, swallowing, and hiccupping.
3. Pons
The pons which means “bridge”, connects the forebrain with medulla
and cerebellum enabling relaying of information.
Hindbrain
Hindbrain
Midbrain
1. Brainstem
The brainstem is made up of the midbrain and parts of the hindbrain
(medulla oblongata and pons). It is a stalk-like lower portion of the brain
that regulates essential survival functions such as breathing and
heartbeat. It connects the spinal cord to the brain.
2. Reticular Formation
It is nuclei scattered throughout the brainstem; controls many brainstem
activities, including motor control, pain perception, rhythmic activities,
and the sleep wake cycle (Circadian rhythm).
Midbrain
Forebrain
1. Cerebrum
It controls the activities of the lower parts of the brain. The cerebrum
contains the cortex enabling it to perform higher functions like
interpreting touch, vision and hearing, as well as speech, reasoning,
emotions, learning, and fine control of movement. The cerebrum
controls the qualities of what we consider the “mind” that is
personality, intelligence, learning, perception, and emotion.
2. Diencephalon
It connects the brainstem to the cerebrum; has many relay and
homeostatic functions.
Forebrain
Cerebrum
The entire surface of the cerebrum exhibits elevated ridges called gyri
(gyrus), separated by shallow grooves called sulci (sulcus). Both are
considered anatomical landmarks and function to increase the brain’s
surface area.
It is divided into 2 hemispheres that gather and process information
simultaneously.
• Left hemisphere: speech, language skills, mathematical ability and
reasoning
• Right hemisphere: spatial, intuitive, musical and artistic abilities
Each hemisphere controls the opposite side of the body.
Cerebrum
Corpus Callosum
The corpus callosum is a very large fiber tract that connects the
cerebral hemispheres. The cerebral hemispheres work together,
interconnected by a thick band of nerve fibers called the corpus
callosum.
Parts of the Cerebrum:
1. Cerebral Cortex
2. Limbic System
3. Basal Nuclei
Cerebral Cortex
The cerebral cortex is the outer layer of the cerebrum that consists of
gray matter that processes information.
• Frontal Lobe – primary motor area that allows the conscious
movement of the skeletal muscles.
• Parietal Lobe – somatic sensory area. The body’s sensory receptors
are localized and interpreted in this area of the brain. Allows
recognition of pain, coldness and light touch.
• Temporal Lobe – auditory and olfactory area.
• Occipital Lobe – visual area.
Cerebral Cortex
Cerebral Cortex
Limbic System
The Limbic System found also in the cerebrum is a loosely defined
collection of brain structures that is sometimes called the emotional
center of the brain. It plays a central role in basic survival functions such
as memory, reproduction and nutrition.
• Hippocampus – participates in long term memory formation.
• Amygdala – center of emotions such as pleasure or fear.
The amygdala sends signals to the hypothalamus which activates the
autonomic nervous system and coordinates the physical sensations
that accompany strong emotions.
Basal Nuclei
Basal Nuclei or Basal Ganglia
It is located bilaterally in the inferior cerebrum, controls muscle activity
and posture, largely inhibits unintentional movement when at rest.
Nuclei – clusters of gray matter located deep within the brain.
Diencephalon
1. Thalamus
The thalamus is a mass of gray matter located between the midbrain
and the cerebrum. This central relay station processes incoming sensory
information and sends it to the appropriate part of the cerebrum. It also
influences mood and movement.
2. Subthalamus
Small area inferior to the thalamus. It contains nerve tracts and nuclei.
The subthalamic nuclei are associated with the basal ganglia and are
involved in controlling motor functions.
Diencephalon
3. Epithalamus
Small area superior and posterior to the thalamus. It contains nuclei
responding to olfactory stimulation and contains the pineal gland for
sleep wake cycle and other biorhythms.
Pineal gland secretes melatonin.
4. Hypothalamus
Neural and hormonal signals from the hypothalamus regulate body
temperature, heartbeat, water balance, and blood pressure, along with
hunger, thirst, sleep, and sexual arousal. It also functions for mood,
motivation, and emotion. It also interacts with the reticular activating
system coordinating the sleep wake cycle.
Forebrain
Spinal Cord
It is the major communication link between the brain and the
peripheral nervous system to the head. It integrates incoming
information and produces responses through reflex mechanisms.
The spinal cord tapers to form a cone like region called the conus
medullaris. The numerous roots of spinal nerves extending from the
conus medullaris resemble a horse’s tail is called the cauda equina.
Spinal Cord
Spinal Cord
Spinal Cord Gray Matter – consists of neuron cell bodies, dendrites and
unmyelinated axons
3 horns of the gray matter:
1. Ventral (Anterior) Horn – the anterior (ventral) gray horns contain
somatic motor nuclei, which are clusters of cell bodies of somatic motor
neurons that provide nerve impulses for contraction of skeletal muscles.
2. Dorsal (Posterior) Horn – the posterior (dorsal) gray horns contain cell
bodies and axons of interneurons as well as axons of incoming sensory
neurons.
3. Lateral Horn – the lateral gray horns contain autonomic motor nuclei,
which are clusters of cell bodies of autonomic motor neurons that
regulate the activity of cardiac muscle, smooth muscle, and glands.
Spinal Cord
Spinal Cord White Matter – consists of myelinated axons, which form
nerve tracts
3 columns of the white matter:
1. Ventral (Anterior) Column – the ventral
(and ventrolateral or anterolateral) columns carry both ascending
information about pain and temperature, and descending motor
information.
2. Dorsal (Posterior) Column – the dorsal columns carry ascending
sensory information from somatic mechanoreceptors.
3. Lateral Column – the lateral columns include axons that travel from
the cerebral cortex to contact spinal motor neurons. These pathways are
also referred to as the cortico-spinal tracts.
Spinal Cord Tracts
1. Sensory (Ascending) Tracts – consist of axons that conduct nerve
impulses toward the brain.
2. Motor (Descending) Tracts – consists of axons that carry nerve
impulses from the brain.
Sensory and motor tracts of the spinal cord are continuous with sensory
and motor tracts in the brain.
Spinal Cord Tracts
Reflex Arc
Reflex Arc – the nerve pathway involved in a reflex action including
at its simplest sensory nerve and a motor nerve with a synapse
between. Smallest and simplest portion of the nervous system
capable of receiving a stimulus and producing a response.
Reflex – an action that is performed as a response to a stimulus and
without conscious thought produced by the reflex arc. Many reflexes
are integrated within the spinal cord.
Reflex Arc
1. Stretch Reflex – simplest reflex in which muscles contract in
response to stretching force applied to them. The purpose of the
stretch reflex is muscle protection.
When a muscle length increases, the muscle spindle within that
muscle stretches, and its nerve motor activity will increase. These
neurons will cause the muscle to contract, and therefore reduce
the stretching of the muscle. The whole system functions as an
autonomic regulation of muscle length.
Reflex Arc
2. Withdrawal Reflex – or flexor reflex, removes a limb or another
body part from a painful stimulus.
Reflexes do not operate as isolated entities. Because of the
convergent and divergent pathways, their activities are integrated
with the functions of the nervous system as a whole. Diverging
branches of the sensory neurons or interneurons in a reflex arc send
action potentials along ascending nerve tracts to the brain.
Reflex Arc
Other CNS Structures
1. Meninges
The meninges are layered membranes that jacket the central nervous
system: brain and spinal cord.
• Pia Mater – innermost membrane, closely adhering to the surface of the
brain and spinal cord; well supplied with blood vessels that carry food
and oxygen.
• Arachnoid Mater – middle membrane, made up of fibrous and elastic
connective tissues; separated to the pia mater by the cerebrospinal fluid
• Subarachnoid Space – the interval between the arachnoid mater
and the pia mater. It is occupied by delicate connective tissue
and intercommunicating channels containing cerebrospinal fluid.
• Dura Mater – outermost membrane; thick and tough linings
Meninges
Meninges
Other CNS Structures
2. Ventricles
It is a hollow tube or cavity formed by the CNS. The ventricles are a
communicating network of cavities that produce and is filled with CSF.
The brain and the spinal cord contain fluid filled cavities.
• Lateral Ventricles – large cavity in each cerebral hemisphere, 1st and
2nd ventricles
• Third Ventricle – a smaller midline cavity at the center of the
diencephalon between two halves of the thalamus
• Fourth Ventricle – inferior of the pons and superior of the medulla
oblongata at the base of the cerebellum. It is continuous with the
central canal of the spinal cord.
Ventricles
Other CNS Structures
3. Cerebrospinal Fluid
CSF is a clear, colourless fluid found in CNS either intracerebrally in the
ventricular system of the brain (making up 20 % of the total CSF volume)
or extracerebrally in the subarachnoid space (the remaining 80 % of the
total volume).
The CSF bathes and cushions the brain and the spinal cord. This fluid
further insulates the CNS from injury. It nourishes the brain and removes
metabolites as well as regulates intracranial pressure. The total volume of
cerebrospinal fluid is approximately 150 ml and it is produced at
a rate of 450 ml per day (thus replacing itself three times a day).
Cerebrospinal Fluid
Choroid Plexus – is a plexus of cells that produces the cerebrospinal
fluid in the ventricles of the brain. The choroid plexus consists of
modified ependymal cells.
80 – 90% of the CSF is produced by the ependymal cells within the
lateral ventricles, with the remainder produced by similar cells in the
third and fourth ventricles.
Cerebrospinal Fluid
Other CNS Structures
4. Blood-Brain Barrier
The blood-brain barrier lines the brain’s capillaries and helps protect
the brain from harmful chemicals. It consists mainly of tight junctions
that seal together the endothelial cells of brain blood capillaries and
a thick basement membrane that surrounds the capillaries.
Blood Brain Barrier
Peripheral Nervous System
The PNS consists of all nervous tissue outside the CNS.
It includes nerves (cranial and spinal), sensory receptors, ganglion
(collection of neuron cell bodies and clusters of gray matter outside
the CNS that act as relay stations) and plexus (a branching network of
intersecting afferent and efferent nerves outside the CNS innervating
the same area of the body for sensory and motor functions)
Nerve
Nerve – an enclosed cable like bundles of axon nerve fibers and their
sheaths (Schwann cells) in the peripheral nervous system. It connects
the CNS to sensory receptors, muscles and glands. It provides a
common pathway for the electrochemical nerve impulses. Each axon
within the nerve is an extension of an individual neuron along with other
supportive cells such as the Schwann cells that coats the myelin sheath.
Connective Tissue Layers:
• Endoneurium – is a layer around the myelin sheath of each
myelinated axon nerve fiber.
• Perinerurium – surrounds a group of axons to form nerve fascicles
(bundle of axon nerve fibers).
• Epineurium – binds the nerve fascicles together to form a nerve.
Nerve
Cranial Nerves
Cranial Nerves – emerge directly from the brain and carry impulses
to and from the brain. These nerves control movements of the eyes,
face, neck, and mouth along with the senses of taste and hearing.
Cranial nerves have sensory, somatic motor and autonomic motor
functions.
Cranial Nerves
I. Olfactory – sensory: sense of smell
II. Optic – sensory: sense of vision
III. Oculomotor – motor: eye movement, pupil size, lens shape
IV. Trochlear – motor: eye movement
V. Trigeminal – sensory: sensation of the head and face including eyes
and oral cavity
motor: middle ear muscle, mastication muscle for chewing, palatine
muscle and throat muscle;
VI. Abducens – motor: eye movement
VII. Facial – sensory: sense of taste in the anterior 2/3 tongue
motor: facial expressions, submandibular and sublingual salivary
glands of the mouth, lacrimal glands of the eye
Cranial Nerves
VIII.Vestibulocochlear – sensory: sense of hearing and balance
IX. Glossopharyngeal – sensory: sense of taste in the posterior 1/3 tongue,
tactile sensation of posterior tongue, middle ear, pharynx, sensory
receptors from carotid arteries that monitor BP, blood pH, oxygen and
carbon dioxide levels
motor: swallowing, parotid salivary glands
X. Vagus – sensory: sensation in the larynx, neck, thorax, abdominal
organs, sensory receptors in the aortic arch that monitor BP, blood pH,
oxygen and carbon dioxide levels
motor: muscles of palate, pharynx, larynx, lungs, kidneys, heart rate
and digestion
XI. Accessory – motor: head and shoulder movement
XII. Hypoglossal – motor: tongue movement
Cranial Nerves
Spinal Nerves
Spinal Nerves – are the paths of communication between the spinal cord
and specific regions of the body. They carry impulses to and from the spinal
cord to the appropriate glands or muscles.
• Ventral (Anterior) Root – the ventral root of the spinal nerve contains
outgoing, efferent fibers that carry information destined to control motor
or glandular function.
• Dorsal (Posterior) Root – the dorsal root of the spinal nerve contains
incoming, afferent fibers that carry sensory information from the sensory
receptors.
• Dorsal Root Ganglion – or spinal ganglion, a cluster of sensory neuron cell
bodies functioning as a relay station located at the dorsal root of a spinal
nerve.
Spinal Nerves
Spinal Nerves
The spinal cord appears to be segmented because the 31 pairs of
spinal nerves emerge at regular intervals from intervertebral foramina.
Spinal Nerves:
• Cervical Nerves – 8 pairs (C1 to C8)
• Thoracic Nerves – 12 pairs (T1to T12)
• Lumbar Nerves – 5 pairs (L1 to L5)
• Sacral Nerves – 5 pairs (S1 to S5)
• Coccygeal Nerves – 1 pair (Co1)
Spinal Nerves
Plexuses
• Cervical Plexus – spinal nerves from C1 to C4 innervating superficial
neck structures and muscles, skin of the neck and posterior portion of
the head.
• Brachial Plexus – spinal nerves from C5 to T1 supply the skin and
muscles of the body’s upper limbs.
• Lumbar and Sacral Plexuses – overlapping spinal nerves of the lumbar
plexus from L1 to L4 and sacral plexus from L4 to S4, supplying the skin
and musculature of the lower limb.
• Coccygeal Plexus – from spinal nerve S5 and coccygeal nerve,
supplying motor innervation to the muscles of the pelvic floor and
sensory innervation of the skin over the coccyx. For bladder and
bowel control of muscles, external genitalia sensation.
Plexuses
Myotome & Dermatome
PNS Functional Classifications
1. Sensory or Afferent Division
The sensory or afferent division consists of nerve fibers that convey
impulses to the central nervous system from sensory receptors located in
various parts of the body.
2. Motor or Efferent Division
The motor or efferent division carries impulses from the CNS to the
effector organs, the muscles and glands.
PNS Functional Classifications
Efferent Division
1. Somatic Nervous System
The somatic nervous system allows conscious or voluntary control on
skeletal muscles. Otherwise known as the Voluntary Nervous System.
2. Autonomic Nervous System
The autonomic nervous system regulates events that are automatic or
involuntary, such as the activity of the smooth and cardiac muscles and
glands. Otherwise known as the Involuntary Nervous System. It is
responsible for the control of visceral functions: cardiovascular,
respiratory, digestive, urinary, reproductive, and response to stress.
Autonomic Nervous System
1. Sympathetic Nervous System
The sympathetic nervous system is activated during stress and
emergencies. Neurons of the sympathetic nervous system slow digestion
and boost blood flow toward vital organs like the heart, brain, and
muscles necessary for “fight or flight”.
2. Parasympathetic Nervous System
The parasympathetic nervous system returns body systems to normal;
heart rate and respiration slow and digestion resumes for “rest and
digest”.
Autonomic Nervous System