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LECTURE 1 INTRODUCTION TO NEUROPHYSIOLOGY

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PREPARED BY MOSES KAZEVU
INTRODUCTION TO NEUROPHYSIOLOGY
 The nervous system is a complex network that allows an organism to
communicate with its environment.
 The environment includes both external (world outside the body)
and internal (components and cavities inside the body)
environment.
 The nervous system includes:
 Sensory components: detect changes in environmental stimuli.
 Motor components: generates movement, contractions which
cause cardiac and smooth muscle to carry out their respective
functions as well as glandular secretions.
 Integrative components: receive, store and process sensory
information and then orchestrate the appropriate motor
response.
 The nervous system is composed of 2 divisions:
1. Central nervous system
 This includes the brain and spinal cord.
 It is formed by neurons and supporting cells called neuroglia.
 Structures of brain and spinal cord are arranged into gray
matter (representing nerve cell bodies and the proximal parts
of the nerve fibers, arising from nerve cell body) and white
matter (representing the remaining parts of nerve fibers).
 In the brain white matter is on the inner part and gray matter
is on the outer part, in the spinal cord white matter is on the
outer part and gray matter is in the inner part.
2. Peripheral nervous system
 Has both afferent inputs (sensory) and efferent outputs
(motor).
 This includes sensory receptors, sensory neurons, ganglia
outside the CNS, somatic motor neurons and autonomic
motor neurons.
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 It consists of cranial nerves arising from the brain and spinal
nerve
 The peripheral nervous system can be divided into sensory
(afferent) and motor (efferent) divisions.
 The motor (efferent) division consists of a somatic component
and an autonomic component.
o The autonomic nervous system consists of the sympathetic
division (thoracolumbar) and the parasympathetic division
(craniosacral).
 The peripheral nervous system (PNS) provides an interface between
the environment and the CNS.
 The sensory/afferent division brings information into the nervous
system usually beginning with events in sensory receptors in the
periphery.
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 These receptors include, but are not limited to visual receptors,
auditory receptors, chemoreceptors and somatosensory (touch)
receptors.
 This afferent information is then transmitted to progressively higher
levels of the nervous system and finally to the cerebral cortex.
 The motor or efferent division relays information out of the nervous
system to the periphery. This results in skeletal/cardiac/smooth muscle
contraction or glandular (endocrine and exocrine) secretion.
 The CNS includes the brain and spinal cord.
 The major divisions of the CNS are the spinal cord, brain stem
(medulla, pons and midbrain), cerebellum, diencephalon (thalamus
and hypothalamus) and cerebral hemispheres (cerebral cortex, white
matter, basal ganglia, hippocampal formation and amygadala).
 The brain consists of 3 major divisions:
 Prosencephalon (forebrain)
o Divided into the telencephalon (cerebral hemispheres,
basal ganglia, hippocampus, amydaloid nuclus) and
diencephalon (thalamus, hypothalamus, metathalamus,
subthalamus)
 Mesencephalon (midbrain): is a part of the brain stem.
 Rhombencephalon (hind brain)
o Divided into the metencephalon (pons and cerebellum)
and myelencephalon (medulla oblongata)
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 The spinal cord is the most caudal portion of the CNS, extending from
the base of the skull to the first lumbar vertebra. It consists of 31 pairs
of spinal nerves which have both sensory and motor nerves.
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 Information travels up and down within the spinal cord through
pathways. The ascending pathways in the spinal cord carry sensory
information from the periphery to higher levels of the CNS.
Descending pathways in the spinal cord carry motor information from
higher levels of the CNS to the motor nerves that innervate the
periphery.
 The medulla, pons and midbrain are collectively called the brain stem.
The 12 cranial nerves arise from the brain stem.
 The medulla is the rostral extension of the spinal cord. It
contains autonomic centers that regulate breathing, blood
pressure and reflexes such as swallowing, coughing and vomiting.
 The pons is rostral to the medulla and together with centers in
the medulla participate in maintenance of posture and in
regulation of breathing. Information from cerebral hemispheres
is also relayed to the cerebellum by the pons.
 The midbrain is rostral to the pons and participates in control of
eye movements. It also contains relay nuclei of the auditory and
visual systems.
 The cerebellum is attached to the brain stem and lies dorsal to the
pons and medulla. It functions in coordination of movement, planning
and execution of movements, maintenance of posture and
coordination of head and eye movements.
 The diencephalon is made up of the thalamus and the hypothalamus.
 The thalamus processes almost all sensory information going to
the cerebral cortex and almost all motor information coming
from the cerebral cortex to the brain stem and spinal cord.
 The hypothalamus lies ventral to the thalamus and contains
centers that regulate body temperature, food intake and water
balance. The hypothalamus also has an endocrine function.
 The cerebral hemispheres consist of the cerebral cortex, an underlying
white matter and 3 deep nuclei (basal ganglia, hippocampus and
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amygdala). The functions of the cerebral hemispheres are perception,
higher motor functions, cognition, memory and emotion.
 The cerebral cortex consists of 4 lobes (frontal, parietal,
temporal and occipital) separated by sulci or grooves. The
cerebral cortex receives and processes sensory information and
integrates motor functions.
o Sensory and motor areas of the cortex are further
designated as “Primary”, “Secondary” and “tertiary”
depending on how directly they deal with sensory or motor
processing.
o The primary areas are the most direct and involve the
fewest number of synapses, the tertiary areas require the
most complex processing and involve the greatest number
of synapses.
o Association areas integrate diverse information for
purposeful actions
 The basal ganglia consist of the caudate nucleus, the putamen
and globus pallidus. The basal ganglia receive input from all
lobes of the cerebral cortex and have projections, via the
thalamus to the frontal cortex to assist in regulating movement.
 The hippocampus and amygdala are part of the limbic system.
The hippocampus is involved in memory, the amygdala is
involved with the emotions and communicates with the
autonomic nervous system via the hypothalamus.
NEURONS
NEURONS AND THEIR CLASSIFICATION
 Neurons/ nerve cells are the structural and functional unit of the
nervous system.
 A nerve consists of a collection of neurons. These terms should thus
not be used synonymously.
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PARTS OF NEURON
 They have cell bodies with a nucleus and all cytoplasmic organelles as
well as processes (axons and dendrites).
 Neurons do not have centrosomes and so do not undergo division
(terminally differentiated).
TABLE 2.1: Parts of a neuron
Part of neuron
Nerve cell body
Dendrite
Axon
Function
 Also known as the soma/ perikaryon
 Irregular shape.
 Consists of cytoplasm (neuroplasm) covered
by a cell membrane.
 Neuroplasm has a large nucleus (with 1 or
more prominent nucleoli), neurofibrils
(cytoskeleton), mitochondria, Nissl bodies
(basophilic granules, contain ribosomes) and
Golgi apparatus.
 These are short processes.
 Transmits impulses towards the nerve cell
body, usually shorter than axon.
 These are long processes.
 Transmits impulses away from the nerve cell
body.
 Longest axon is about 1 meter.
TYPES OF NEURONS
 Neurons are classified according to:
 Number of poles
 Function
 Length of axon
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TABLE 2.2: CLASSIFICATION OF NEURONS
Classification
Number of
poles (Fig 2.1)
Neuron type
Unipolar
Bipolar
Multipolar
Function
Sensory or
Afferent
Motor or
Efferent
Comment
 Only has one pole.
 Axon and dendrite arise
from one pole.
 Only present in embryonic
stage in human beings. (In
adults there is a
pseudounipolar neuron)
 Neuron has two poles.
 Axon arises from one pole
and dendrite arises from
the other pole.
 Neurons have many pole.
 One of the pole gives rise
to axon and all other poles
give rise to dendrites
 Carry sensory impulses
from periphery to central
nervous system.
 They are pseudounipolar
in nature
 Carry motor impulses
from the CNS to the
peripheral effector organs
 Most are multipolar in
nature
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Length
Golgi type I
neuron
Golgi type II
neuron
 Have long axons.
 Cell body is in different
parts of the CNS and their
axons in peripheral organs
 Have short axons.
 These neurons are present
in the cerebral cortex and
spinal cord
 Note: each nerve is formed by many bundles or groups of nerve
fibers.
 Each bundle of nerve fibers is called a fasciculus.
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CLASSIFICATION OF NERVE FIBERS
 Nerve fibers are classified by 6 different methods including:
 Structure
 Distribution
 Origin
 Function
 Secretion or neurotransmitter
 Diameter and conduction of impulse (Erlanger-Gasser
classification)
CLASSIFICATION BY STRUCTURE
 These are either
 Myelinated: covered by myelin sheath
 Non-Myelinated: not covered by myelin sheath
CLASSIFICATION BY DISTRIBUTION
 These include:
 Somatic nerve fibers: supply skeletal muscles
 Visceral or autonomic nerve fibers: control various internal
organs of the body
CLASSIFICATION UPON ORIGIN
 Consist of:
 Cranial nerve fibers: arise from the brain
 Spinal nerve fibers: arise from the spinal cord
CLASSIFICATION BY FUNCTION
 Are of two types (discussed above)
 Sensory
 Motor
CLASSIFICATION UPON SECRETION OF
NERUOTRANSMITTER
 They include
 Adrenergic nerve fibers: secrete noradrenalin
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 Cholinergic nerve fibers: secrete acetylcholine
CLASSIFICATION BY DIAMETER AND CONDUCTION OF
IMPULSE
 Erlanger and Gasser classified the nerve fibers into 3 major types,
on the basis of diameter (thickness) of the nerve fibers and velocity
of conduction of impulses.
 They are divided into:
 Type A nerve fibers: these are the thickest. These are
myelinated. They are further divided into
o Type A alpha or type I nerve fibers
o Type A beta or type II nerve fibers
o Type A gamma nerve fibers
o Type A delta or type III nerve fibers
 Type B nerve fibers: these are myelinated.
 Type C nerve fibers (also called type IV fibers): these are the
smallest and are unmyelinated
 The velocity of impulse is directly proportional to the thickness of
the fiber.
TABLE 2.3: Type of nerve fibers
Type
A alpha (Type I)
A beta (Type II)
A gamma
A delta (Type III)
B
C (Type IV)
Diameter ()
12 to 24
6 to 12
5 to 6
2 to 5
1 to 2
<1.5
Velocity of
conduction (m/s)
70 to 120
30 to 70
15 to 30
12 to 15
3 to 10
0.5 to 2
 Just like any other excitable tissue neurons have the property of
excitability, conductivity, refractory period, summation, adaptation
(Accomodation), infatigability and abide to the all or non-law.
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