The nervous system is divided into the central nervous system (brain and spinal cord) and peripheral nervous system. It coordinates the body's activities and transmits signals via neurons, which are composed of a cell body, dendrites, and an axon. Neuroglia provide support and protection to the neurons. The nervous system consists of sensory neurons that receive information, interneurons that communicate within the central nervous system, and motor neurons that activate muscles and glands. A nerve impulse is transmitted through neurons via changes in electrical charges across the cell membrane.

1. The Nervous System
Topics to Study…

2. NERVOUS SYSTEM
CENTRAL NERVOUS SYSTEM PERIPHERAL NERVOUS
SYSTEM
Brain Spinal Cord Autonomic nervous
system
Somatic nervous
system
Sympathetic
nervous system
Parasympathetic
nervous system

3.  The Nervous system is the part of an animal's body
that coordinates its behavior and transmits signals
between different body areas
- Mainly two types of nervous tissue found – Neuron &
Neuroglia.
What are Neurons?
 Neurons are the structural and functional unit of
nervous system. They help in the conduction of nerve
impulse. Neurons are made up of three main parts –
axon, dendrites and cell body. Sensory neurons carry
information from the sensory receptors and transmit it
to the brain. The motor neurons transmit information
from the brain to the body.

4. What are Neuroglia?
 Neuroglia are non-neuronal cells that support and
protect the neurons. Neuroglia in the central nervous
system include astrocytes, oligodendrocytes,
microglial cells, and ependymal cells. Schwann cells
and satellite cells are the neuroglia in the peripheral
nervous system.

5. The Nervous System
Fig.: Structure of a Neuron
Nissl
bodies

6. Structure of Neuron
 Cell body: also called soma or perikaryon
contains all cell organelles, nissle bodies
and neurofibrils
 Neurites: cytoplasmic extension of the cell
body of neuron,
1. Axon: A single long process
arises from the cell body of a typical neuron,
and carries impulses away from the cell body.
2.Dendrite: short processes ,
carrying impulses towards the cell body.

7. Classification of Neuron:
 On the basis of Polarity:
Based on Length of Axon:
Golgi Type I neurons:
– Have long axons
– Cell body situated in CNS and their axon reaches remote peripheral organs
• Golgi type II neurons:
– Have short axons
– Present in cerebral cortex and spinal cord

8. 2. Depending upon the functions
• Motor or efferent neurons:
– Carry impulses from CNS to peripheral effector
organs e.g., muscles/glands/blood vessels
– Generally each motor neurons has long axon and short
dendrites
• Sensory or afferent neurons:
– Carry impulses from periphery to CNS
– Generally each neuron has short axon a long
dendrites.
• Interneuron: connective neuron between sensory
neuron and motor neuron mainly in CNS.

9. CLA SSIFICATION OF
NEUROGLIAL CELLS
1. Central Neuroglial cells
– Astrocytes:
– Fibrous astrocytes
– Protoplasmic astrocytes
– Microglia
– Oligodendrocytes
– Ependymal cells.
2. Peripheral neuroglial cells
– Schwann cells
– Satellite cells

10. CLA SSIFICATION OF NERVE
FIBERS
1. Depending upon structure
– Myelinated nerve fibers
– Non myelinated nerve fibers
2. Depending upon distribution
– Somatic nerve fibers (supply skeletal muscles)
– Visceral or autonomic (supply internal organs)
• Depending upon origin
– Cranial nerve (arising from brain)
– Spinal nerve (arising from spinal cord)

11. Nervous System and Senses
The nervous system consists of two types of cells.
Nerve cells are called neurons.
Various support cells are associated with the
neurons, most typically, Schwann cells.
The parts of a neuron include the dendrite which
receives the impulse (from another nerve cell or
from a sensory organ), the cell body (numbers of
which side-by-side form gray matter) where the
nucleus is found, and the axon which carries the
impulse away from the cell.

12. Wrapped around the axon are the Schwann cells,
and the spaces/junctions between Schwann cells
are called nodes of Ranvier.
Collectively, the Schwann cells make up the
myelin sheath (numbers of which side-by-side
form white matter).
Schwann cells wrap around the axon (like the
camp food, “pigs in a blanket”).
Having an intact myelin sheath and nodes of
Ranvier are critical to proper travel of the nerve
impulse.

13. Diseases which destroy the myelin sheath
(demyelinating disorders) can cause paralysis or
other problems.
Schwann cells are analogous to the insulation on
electrical wires, and just as electrical wires short
out if there’s a problem with the insulation, so
also, neurons cannot function properly without
intact myelin sheaths.

14. The nervous system has three basic functions:
1. Sensory neurons receive information from the
sensory receptors.
2. Interneurons transfer and interpret impulses.
Motor neurons send appropriate impulses/
3. instructions to the muscles and glands.

15. A nerve impulse is an electrical charge that travels
down the cell membrane of a neuron’s dendrite
and/or axon through the action of the Na-K
pump.
The inside of a neuron’s cell membrane is
negatively-charged while the outside is positively-
charged.
When sodium and potassium ions change places,
this reverses the inner and outer charges causing
the nerve impulse to travel down the membrane.

16. A nerve impulse is “all-or-none:” it either goes or
not, and there’s no halfway.
A neuron needs a threshold stimulus, the
minimum level of stimulus needed, to trigger the
Na-K pump to go and the impulse to travel.
A neuron cannot immediately fire again; it needs
time for the sodium and potassium to return to
their places and everything to return to normal.
This time is called the refractory period.

17. A junction between two nerve cells or a nerve and
a muscle cell is called a synapse.
In a synapse, various chemicals are used to
transfer the impulse across the gap to the next cell.
These are collectively known as
neurotransmitters, and include such chemicals
as dopamine (brain levels of which are low in
Parkinson’s disease), serotonin, and
acetylcholine (levels of which are low in
myasthenia gravis).

21. Nervous System Function
Somatic NS
voluntary muscles
and reflexes
Vs
Autonomic NS
visceral/smooth and
cardiac muscle
Sympathetic
NS
increases
energy
expenditure
prepares for
action
Parasympa
thetic NS
decreases
energy
expenditure
gains stored
energy
These have the opposite
effects on the same organs

22. Peripheral NS
(PNS)
sensory and motor
neurons
Vs Central NS (CNS)
interneurons: brain and spine
covered with three membranes,
the meninges
inflammation of these is called
meningitis
brain has gray matter on
outside and white in center
spine has white matter on
outside and gray in center

24. General Functions of the Nervous
System
Sensory receptors at the ends of peripheral nerves
gather information and convert it into nerve
impulses.
When sensory impulses are integrated in the brain
as perceptions, this is the integrative function of
the nervous system.
Conscious or subconscious decisions follow,
leading to motor functions via effectors.

26. Neuron Structure
A neuron has a cell body with mitochondria,
a Golgi apparatus, Nissl bodies
rough endoplasmic reticulum, and
lysosomes,
containing
neurofibrils.

27. Nerve fibers include a solitary axon and numerous
dendrites.
Branching dendrites carry impulses from other
neurons (or from receptors) toward the cell
body.
The axon transmits the impulse away from the
axonal hillock of the cell body and may give off
side branches.
Larger axons are enclosed by sheaths of myelin
provided by Schwann cells and are myelinated
fibers.

28. The outer layer of myelin is surrounded by a
neurilemma (neurilemmal sheath) made up of
the cytoplasm and nuclei of the Schwann cell.
Narrow gaps in the myelin sheath between
Schwann cells are called nodes of Ranvier.
The smallest axons lack a myelin sheath and are
unmyelinated fibers.
White matter in the CNS is due to myelin sheaths in
this area.
Unmyelinated nerve tissue in the CNS appears gray.

29. Types of Neurons & Neuroglial Cells
Neurons can be grouped in two ways: on the basis
of structural differences (bipolar, unipolar, and
multipolar neurons), and by functional differences
(sensory neurons, interneurons, and motor
neurons).

30. Classification of Neurons:
Bipolar neurons are found in the eyes, nose, and
ears, and have a single axon and a single dendrite
extending from opposite sides of the cell body.
Unipolar neurons are found in ganglia outside the
CNS and have an axon and a dendrite arising
from a single short fiber extending from the cell
body.
Multipolar neurons have many nerve fibers
arising from their cell bodies and are commonly
found in the brain and spinal cord.

31. Sensory neurons (afferent neurons) conduct
impulses from peripheral receptors to the CNS
and are usually unipolar, although some are
bipolar neurons.
Interneurons are multipolar neurons lying within
the CNS that form links between other neurons.
Motor neurons are multipolar neurons that
conduct impulses from the CNS to effectors.

32. Nerve Impulse
A nerve impulse is conducted as action potential
is reached at the trigger zone and spreads by a
local current flowing down the fiber, and adjacent
areas of the membrane reach action potential.

33. Impulse Conduction
Unmyelinated fibers conduct impulses over
their entire membrane surface.
Myelinated fibers conduct impulses from node
of Ranvier to node of Ranvier, a phenomenon
called saltatory conduction.
Saltatory conduction is many times faster than
conduction on unmyelinated neurons.

34. All-or-None Response
If a nerve fiber responds at all to a stimulus, it
responds completely by conducting an
impulse (all-or-none response).
Greater intensity of stimulation triggers more
impulses per second, not stronger impulses.

35. The Synapse
The junction between two communicating neurons is
called a synapse; there exists a synaptic cleft
between them across which the impulse must be
conveyed.
Synaptic Transmission
The process by which the impulse in the presynaptic
neuron is transmitted across the synaptic cleft to the
postsynaptic neuron is called synaptic transmission.
When an impulse reaches the synaptic knobs of an axon,
synaptic vesicles release neurotransmitter into the synaptic
cleft.
The neurotransmitter reacts with specific receptors on the
postsynaptic membrane.

36. Classification of Synapse:
A. Anatomical classification

40. Excitatory and Inhibitory Actions
Neurotransmitters that increase postsynaptic
membrane permeability to sodium ions may trigger
impulses and are thus excitatory.
Other neurotransmitters may decrease membrane
permeability to sodium ions, reducing the chance
that it will reach threshold , and are thus inhibitory.
The effect of the postsynaptic neuron depends on
which presynaptic knobs are activated.

41. Neurotransmitters
At least 50 kinds of neurotransmitters are produced
by the nervous system, most of which are synthesized
in the cytoplasm of the synaptic knobs and stored in
synaptic vesicles.
When an action potential reaches the synaptic knob,
calcium ions rush inward and, in response, some
synaptic vesicles fuse with the membrane and release
their contents to the synaptic cleft.
Enzymes in synaptic clefts and on postsynaptic
membranes rapidly decompose the neurotransmitters
after their release.
Destruction or removal of neurotransmitter prevents
continuous stimulation of the postsynaptic neuron.

44. Impulse Processing
How impulses are processed is dependent upon? how
neurons are organized in the brain and spinal cord?
Neuronal Pools
Neurons within the CNS are organized into neuronal
pools with varying numbers of cells.
Each pool receives input from afferent nerves and
processes the information according to the special
characteristics of the pool.
Facilitation
A particular neuron of a pool may receive excitatory or
inhibitory stimulation; if the net effect is excitatory but
subthreshold, the neuron becomes more excitable to
incoming stimulation (a condition called facilitation).

45. Divergence
Impulses leaving a neuron in a pool may be passed into
several output fibers (divergence), a pattern that serves
to amplify an impulse.
Convergence
A single neuron within a pool may receive impulses
from two or more fibers (convergence), which makes
it possible for the neuron to summate impulses from
different sources.

46. Types of Nerves
A nerve is a bundle of nerve fibers held together
by layers of connective tissue.
Nerves can be sensory, motor, or mixed, carrying
both sensory and motor fibers.

47. Nerve Pathways
The routes nerve impulses travel are called
pathways, the simplest of which is a reflex arc.
Reflex Arcs
A reflex arc includes a sensory receptor, a sensory
neuron, an interneuron in the spinal cord, a motor
neuron, and an effector.
Reflex Behavior
Reflexes are automatic, subconscious responses to
stimuli that help maintain homeostasis (heart rate,
blood pressure, etc.) and carry out automatic
responses (vomiting, sneezing, swallowing, etc.).

48. The knee-jerk reflex (patellar tendon reflex) is an
example of a monosynaptic reflex (no interneuron).
The withdrawal reflex involves sensory neurons,
interneurons, and motor neurons.
At the same time, the antagonistic extensor muscles
are inhibited.

51. Spinal Cord
The spinal cord begins at the base of the brain and
extends as a slender cord to the level of the
intervertebral disk between the first and second
lumbar vertebrae.

52. Structure of the Spinal Cord
The spinal cord consists of 31segments, each of
which gives rise to a pair of spinal nerves.
A cervical enlargement gives rise to nerves leading to
the upper limbs, and a lumbar enlargement gives rise
to those innervating the lower limbs.
Two deep longitudinal grooves (anterior median
fissure and posterior median sulcus) divide the cord
into right and left halves.
White matter, made up of bundles of myelinated
nerve fibers (nerve tracts), surrounds a butterfly-
shaped core of gray matter housing interneurons.
A central canal contains cerebrospinal fluid.

53. Functions of the Spinal Cord
The spinal cord has two major functions: to
transmit impulses to and from the brain, and
to house spinal reflexes.
Tracts carrying sensory information to the
brain are called ascending tracts; descending
tracts carry motor information from the
brain.
The names that identify nerve tracts identify
the origin and termination of the fibers in
the tract.
Many spinal reflexes also pass through the
spinal cord.

54. BRAIN
FORBRAIN
(Procencephalon)
MIDBRAIN
(mesencephalon)
HIND BRAIN
(Rhomencephalon)
Cerebrum
• Hemisphere
• Grey & white matter
• Sulcus & Gyrus
• Lobes
i. Frontal
ii. Temporal (2)
iii. Parietal
iv. Occipital
Diencephalon
1. Thalamus
2. Hypothalamus
3. Epithalamus
4. Subthalamus
1. Cerebellum
2. Pons varolli
3. Medulla oblogata
Central Nervous System
Spinal cord

55. BRAIN

56. Brain
The brain consists of the cerebrum which is the
large, anterior portion.
The cerebellum which is the wrinkled-looking,
posterior part.
The pons which is the closest, larger bulge at the
top of the spinal cord.
The medulla which is the farther, smaller bulge
between the pons and the top of the spinal cord.
Then the spinal cord starts after the medulla.
Also note under the cerebrum, the optic
chiasma, the place where the optic nerves cross
to the other side of the brain.

57. The cerebellum, medulla, and pons are
collectively referred to as the hindbrain.
Their functions
coordination
are involved in homeostasis,
of movement, and
maintenance/control of breathing and heart rate.
While a stroke in the cerebrum might result in
partial paralysis, a stroke in the hind brain is
actually, potentially more dangerous because it
could knock out coordination of the cerebrum’s
activities, or worse yet, automatic control of
breathing and/or heart beat.

58. The midbrain is responsible for receiving and
integrating of information and sending/routing
that information to other appropriate parts of the
brain.
The forebrain is composed of the cerebrum and
related parts, and functions in pattern and image
formation, memory, learning, emotion, and motor
control. In addition, the right side functions more
in artistic and spatial concepts, while the left side
controls speech, language, and calculations.
Left-brain stroke would cause paralysis on the
right side of the body. It might cause problems
with speech while a right-brain stroke is more
likely to cause abnormal emotional responses.

59. Cerebrum:
 The cerebrum is the largest and most developed of the five
major divisions of the brain.
 The brain contains two hemispheres, the left and the right,
connected by a bundle of nerve fibers called the corpus
callosum.
 The cerebrum directs the conscious or volitional motor
functions of the body. Damage to this area of the brain can
result in loss of muscular power and precision rather than total
paralysis.
 The primary sensory areas of the cerebral cortex receive and
process visual, auditory, somatosensory, gustatory, and
olfactory information.
 Each hemisphere of the mammalian cerebral cortex can be
broken down into four functionally and spatially defined lobes:
frontal, parietal, temporal, and occipital.

60. Key Terms
 sulci: Any of the grooves that mark the convolutions of the surface of the
brain (plural of sulcus).
 cerebral cortex: The cerebrum’s outer layer of neural tissuecomposed of
folded gray matter. The cerebral cortex plays a key role in memory, attention,
perception, awareness, thought, language, and consciousness.
 olfactory bulb: A neural structure of the vertebrate forebrain involved in
olfaction (sense of smell).
 Broca’s area: A region in the frontal lobe of the dominant hemisphere of the
human brain with functions linked to speech production.
 Wernicke’s area: Involved in the comprehension or understanding of written
and spoken language.
 aphasia: A combination speech and language disorder often caused by a
stroke.
 gyri: A ridge on the cerebral cortex (plural of gyrus).

61. Each lobe contributes to overall functionality of the brain and each lobe has
many different roles.
 The frontal lobe is involved in conscious thought.
 The parietal lobe is important for spatial reasoning.
 The occipital lobe is required for visual processing.
 The temporal lobe contributes to language and face recognition.
Key Terms
 Frontal lobe: The frontal lobe is an area in the brain of mammals, located at
the front of each cerebral hemisphere and positioned anterior to the parietal
lobe and superior and anterior to the temporal lobes. In humans, it
contributes to a number of higher cognitive functions including attention,
planning, and motivation.

62.  Temporal lobe: A region of the cerebral cortex that is located behind the
temples and beneath the Sylvian fissure on both cerebral hemispheres of
the human brain. This region is involved in auditory perception, speech and
vision processing, and the formation of long-term memory as it houses the
hippocampus.
 Parietal lobe: A part of the brain positioned superior to the occipital lobe
and posterior to the frontal lobe that integrates sensory information from
different modalities, particularly spatial sense and navigation.
 Occipital lobe: Located at the back of the head, this is the visual
processing center of the mammalian brain containing most of the
anatomical region of the visual cortex.

63. Effects of Cerebral injury:
 Even after a minor head injury, brain function can be temporarily
impaired and this is sometimes referred to as concussion. This can
lead to difficulties such as headaches, dizziness, fatigue,
depression, irritability and memory problems.
 While most people are symptom-free within two weeks, some can
experience problems for months or even years after a minor head
injury.
Eg. Behavioural effects:
 Disinhibition
 Impulsiveness
 Obsessive behaviour
 Irritability and aggression
 Apathy and loss of initiative
 Egocentricity (self-centredness)

64. cognitive effects
 Memory problems
 Language loss (aphasia)
 Impairments in visual-perceptual skills
 Reduced initiative and problems with motivation
 Reduced concentration span
 Reduced information processing ability
 Repitition or 'perseveration'
 Impaired reasoning
 Impaired insight and empathy
communication problems:
 Language impairment (aphasia)
 Speech difficulties
 Cognitive communication difficulties
Executive dysfunction & Hormonal imbalances

65. Meninges
https://youtu.be/CIkgQcmv0Xs
The brain and spinal cord are surrounded by
membranes called meninges that lie between the
bone and the soft tissues.
The outermost meninx is made up of tough, white
dense connective tissue, contains many blood
vessels, and is called the dura mater.
It forms the inner periosteum of the skull bones.
In some areas, the dura mater forms partitions between
lobes of the brain, and in others, it forms dural sinuses.
The sheath around the spinal cord is separated from
the vertebrae by an epidural space.

66. The middle meninx, the arachnoid mater, is thin
and lacks blood vessels.
It does not follow the convolutions of the brain.
Between the arachnoid and pia maters is a
subarachnoid space containing cerebrospinal fluid.
The innermost pia mater is thin and contains many
blood vessels and nerves.
It is attached to the surface of the brain and spinal
cord and follows their contours.

67. Brain stem
https://youtu.be/T2zjlB4ctu4
 In vertebrate anatomy, the brainstem is the posterior
part of the brain adjoining, and structurally continuous
with, the spinal cord.
 Though small, the brainstem is an extremely important
part of the brain, as the nerve connections from the
motor and sensory systems of the cortex pass through
it to communicate with the peripheral nervous system.
 The brainstem also plays an important role in the
regulation of cardiac and respiratory function,
consciousness, and the sleep cycle.
 The brainstem consists of the medulla oblongata,
pons, and midbrain.

68. Key Terms
 pons: Contains nuclei that relay signals from the forebrain
to the cerebellum, along with nuclei that 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.

69. Cerebellum
 https://youtu.be/Fir-v6EoZNE
 The cerebellum, which looks like a separate structure
attached to the bottom of the brain, plays an important role
in motor control.
 The cerebellum can be separated into three lobes: the
flocculonodular lobe, anterior lobe, and posterior lobe.
 The medial zone of the anterior and posterior lobes
constitutes the spinocerebellum, or paleocerebellum.
 There are about 3.6 times as many neurons in
the cerebellum as in the neocortex.
 Based on surface appearance, three lobes can be
distinguished in the
cerebellum: the flocculonodular lobe, anterior lobe (above
the primary
fissure), and the posterior lobe (below the primary fissure).

70.  Key Terms
 granule cells: An extremely small type of neuron that
is the the smallest cell found in the brain.
 neocortex: The largest part of the cerebral cortex of
the human brain, covering the two cerebral
hemispheres.
 Purkinje cells: A class of GABAergic neurons located
in the cerebellar cortex that are some of the largest
neurons in the human brain.
GABA is the chief inhibitory neurotransmitter
in the mammalian
central nervous system.
 cerebellum: Part of the hindbrain. In humans, it lies
between the brainstem and the cerebrum. It plays an
important role in sensory perception, motor output,
balance, and posture.

71. Cerebral cortex.
 https://youtu.be/7TK1LpjV5bI
 The cortex is composed of two hemispheres, right and left,
separated by a large sulcus. A thick fiber bundle, the corpus
callosum, connects the two hemispheres, allowing information to
be passed from one side to the other. The right hemisphere
controls and processes signals from the left side of the body,
while the left hemisphere controls and processes signals from
the right side of the body.

72. Midbrain
 https://youtu.be/NsWukc8G6wE
 The midbrain is the topmost part of the brainstem, the
connection central between the brain and the spinal
cord. There are three main parts of the midbrain - the
colliculi, the tegmentum, and the cerebral peduncles.
Of the 12 cranial nerves, two thread directly from the
midbrain - the oculomotor and trochlear nerves,
responsible for eye and eyelid movement.

73. Ventricle:
 https://youtu.be/9e9Lo0OPON4
 The brain ventricles are four cavities located within the brain that
contain cerebrospinal fluid (CSF). There are two lateral ventricles—one on each
side of the cerebral cortex. The lateral ventricles are continuous with the third
ventricle, which is lower in the brain. The third ventricle is continuous with the
fourth ventricle, which runs along the brainstem.
Function:
The ventricles are all an important part of the “ventricular system.” The ventricles
are interconnected with each other, and also with the central canal of the spinal cord
and with the subarachnoid space (space between two of the linings that separate the
brain from the skull).

74.  CSF is produced by the lining of the ventricles. The CSF then circulates
throughout the ventricular system and is eventually reabsorbed in the
subarachnoid space.
 The ventricular system is critically important to the normal functioning of the
central nervous system. It protects the brain by allowing it to “float” in a fluid
bath and provides a shock absorber against head trauma. The CSF itself
also helps to provide nutrients to the brain and to keep the brain in chemical
balance.

75. Diencephalon:
 The diencephalon is made up of four main components:
the thalamus, the subthalamus, the hypothalamus, and
the epithalamus.
 The hypothalamus is an integral part of the endocrine
system, with the key function of linking the nervous
system to the endocrine system via the pituitary gland.
https://youtu.be/TVhm2rBGhB0
 The thalamus is critically involved in a number of
functions including relaying sensory and motor signals to
the cerebral cortex and regulating consciousness, sleep,
and alertness. https://youtu.be/IF8_82e9RmQ
 The epithalamus functions as a connection between the
limbic system to other parts of the brain. Some functions
of its components include the secretion of melatonin by
the pineal gland (involved in circadian rhythms) and
regulation of motor pathways and emotions.

76. Key Terms:
 subthalamus: Receives afferent connections from the substantia nigra
and striatum and regulates skeletal muscle movements.
 thalamus: Either of two large, ovoid structures of gray matter within the
forebrain that relay sensory impulses to the cerebral cortex.
 hypothalamus: A region of the forebrain located below the thalamus,
forming the basal portion of the diencephalon, and functioning to regulate
body temperature, some metabolic processes, and the autonomic nervous
system.
 epithalamus: The dorsal posterior segment of the diencephalon, involved
in the maintenance of circadian rhythms and regulation of motor pathways
and emotions.
 limbic system: A set of brain structures located on both sides of the
thalamus, right under the cerebrum. Supports a variety of functions
including emotion, behavior, motivation, long-term memory, and olfaction.

77. Reticular formation:
 The reticular formation is a set of
interconnected nuclei that are located throughout
the brainstem.
 It is not anatomically well defined, because it
includes neurons located in different parts of the brain.
 The neurons of the reticular formation make up a
complex set of networks in the core of the brainstem
that extend from the upper part of the midbrain to the
lower part of the medulla oblongata.
 The reticular formation includes ascending pathways
to the cortex in the ascending reticular activating
system (ARAS) and descending pathways to
the spinal cord via the reticulospinal tracts

78. Traditionally the reticular nuclei are divided into three columns:
 In the median column – the raphe nuclei
 In the medial column – gigantocellular nuclei (because of larger size of
the cells)
 In the lateral column – parvocellular nuclei (because of smaller size of
the cells)
 The reticular formation consists of more than 100 small neural networks,
with varied functions including the following:
Functions:
 Somatic motor control – Some motor neurons send their axons to the
reticular formation nuclei, giving rise to the reticulospinal tracts of the
spinal cord. These tracts function in maintaining tone, balance, and
posture—especially during body movements. The reticular formation also
relays eye and ear signals to the cerebellum so that the cerebellum can
integrate visual, auditory, and vestibular stimuli in motor coordination.
Other motor nuclei include gaze centers, which enable the eyes to track
and fixate objects, and central pattern generators, which produce
rhythmic signals of breathing and swallowing.

79.  Cardiovascular control – The reticular formation includes the cardiac
and vasomotor centers of the medulla oblongata.
 Pain modulation – The reticular formation is one means by which pain
signals from the lower body reach the cerebral cortex. It is also the origin of
the descending analgesic pathways. The nerve fibers in these pathways act
in the spinal cord to block the transmission of some pain signals to the brain.
 Sleep and consciousness – The reticular formation has projections to
the thalamus and cerebral cortex that allow it to exert some control over
which sensory signals reach the cerebrum and come to our
conscious attention. It plays a central role in states of consciousness
like alertness and sleep. Injury to the reticular formation can result in
irreversible coma.
 Habituation – This is a process in which the brain learns to ignore repetitive,
meaningless stimuli while remaining sensitive to others. A good example of
this is a person who can sleep through loud traffic in a large city, but is
awakened promptly due to the sound of an alarm or crying baby. Reticular
formation nuclei that modulate activity of the cerebral cortex are part of the
ascending reticular activating system

81. Peripheral Nervous System
1. The peripheral nervous system (PNS) consists
of the cranial and spinal nerves that arise from
the central nervous system and travel to the
remainder of the body.
2. The PNS is made up of the somatic nervous
system that oversees voluntary activities, and
the autonomic nervous system that controls
involuntary activities.

82. 12 pairs of cranial nerves arise from the underside of
the brain, most of which are mixed nerves.
The 12 pairs are designated by number and name and
include the olfactory, optic, oculomotor, trochlear,
trigenimal, abducens, facial, vestibulocochlear,
glossopharyngeal, vagus, accessory, and hypoglossal
nerves.
Cranial Nerves

84. 1. 31 pairs of mixed nerves make up the spinal nerves.
2. Spinal nerves are grouped according to the level from
which they arise and are numbered in sequence,
beginning with those in the cervical region.
3. Each spinal nerve arises from two roots: a dorsal, or
sensory, root, and a ventral, or motor, root.
4. The main branches of some spinal nerves form plexuses.
5. Cervical Plexuses – Lie on either side of the neck and
supply muscles and skin of the neck.
6. Brachial Plexuses – Arise from lower cervical and upper
thoracic nerves and lead to the upper limbs.
7. Lumbrosacral Plexuses – Arise from the lower spinal
cord and lead to the lower abdomen, external genitalia,
buttocks, and legs.
Spinal Nerves

85. Autonomic Nervous System
The autonomic nervous system has the task of
maintaining homeostasis of visceral activities
without conscious effort.

86. General Characteristics
The autonomic nervous system includes two
divisions: the sympathetic and parasympathetic
divisions, which exert opposing effects on target
organs.
The parasympathetic division operates under
normal conditions.
The sympathetic division operates under
conditions of stress or emergency.

87. Autonomic Nerve Fibers
In the autonomic motor system, motor pathways
include two fibers: a preganglionic fiber that
leaves the CNS, and a postganglionic fiber that
innervates the effector.
Sympathetic Division
Fibers in the sympathetic division arise from the
thoracic and lumbar regions of the spinal cord, and
synapse in paravertebral ganglia close to the vertebral
column.
Postganglionic axons lead to an effector organ.

90. Parasympathetic Division – Fibers in the
parasympathetic division arise from the brainstem
and sacral region of the spinal cord, and synapse
in ganglia close to the effector organ.
Autonomic Neurotransmitters
1. Preganglionic fibers (PF) of both sympathetic and
parasympathetic divisions release acetylcholine.
Parasympathetic PF are cholinergic fibers and
release acetylcholine.
2. Sympathetic postganglionic fibers are adrenergic
and release norepinephrine.
3. The effects of these two divisions, based on the
effects of releasing different neurotransmitters to
the effector, are generally antagonistic.

91. The sympathetic
system.
Preganglionic
fibers, solid line;
post ganglionic
fibers, broken lines.

92. Control of Autonomic Activity
a. The autonomic nervous system is largely
controlled by reflex centers in the brain and spinal
cord.
b. The limbic system and cerebral cortex alter the
reactions of the autonomic nervous system
through emotional influence.