5/18/10

What is a Neuron? The Parts of a Neuron

•! Neurons, nerve cells or neurones is a special type of cell
that is found in the bodies of most animals ( all members
of the group Eumetazoa, to be precise- this excludes
only sponges and a few other very simple animals).
•! They exhibit different shapes but all have a cell body and
one or more processes.
•! Neurons are electrically excitable cells that process and
transmits information by electrical and chemical
signalling, the latter via synapses, specialized
connections with other cells.

a Typical Neuron A Typical Neuron A Typical neuron

•! The Main Parts of a Typical Neuron:! •! The cell body of a neuron is supported by a
•! Cell Body or soma! complex meshwork of structural proteins
called neurofilaments which are resembled
•! The cell body or the soma is a nucleated body into larger neurofibrils. Some neurons also
that receives input from dendrites. It is usually contain pigment granules , such as
compact, round and a centrally located neuromelanin- a brownish pigment,
structure. A cell body contains DNA, controls ( byproduct of synthesis of catecholamines)
protein manufacturing and directs metabolism and Lipofuscin ( yellow- brown pigment that
but have no role in neural signaling. ! accumulate with age). !

A typical neuron A typical neuron A typical neuron

•! The Dendrites:! •! The dendritic growth:! •! The axon:!
•! The longest process, distinguished by the
•! Dendrites are short extensions of the cell body •! Mature neurons generally cannot divide absence of Nissl materials ( site of high
that provide an increased surface for receipt of but new dendrites can grow to protein synthesis). An axon is a special
incoming impulses fro m a number of axons . cellular filament that arises from the cell body
provide room for more connections to
Dendrites often extend for hundreds of at a swelling called axon hillock and travels
microns and branching multiple times giving other neurons. New connections are for a distance as far as 1metres in humans
rise to a complex “ dendritic tree”. ! basis for learning. ! or more in other species.!
•! A cell body may give rise to numerous
dendrites ,but never to more than one axon.!

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A typical neuron A typical neuron Structural classification of neurons

•! Axon Hillock:! •! The myelin sheath:! •! Polarity:!
•! besides being an anatomical structure, axon •! The myelin sheath is the white fatty •! Most neurons can be anatomically
hillock is also the part of the neuron that casing on axon. It acts as an electrical characterized as:!
has the greatest density of voltage- insulator . The myelin sheath is not •! Unipolar or pseudounipolar: dendrite
dependent sodium channels. This makes it present ! and axon emerging from the same
the most easily-excited part of the neuron •! When present, it increases the speed of process.!
and the spike initiation zone for the axon: neural signals down the axon.! •! Bipolar: axon and single dendrite on
in neurological terms it has the most opposite ends of the soma.!
negative action potential threshold.! •! Multipolar: more than two dendrites.!

Structure classification of neurons The kinds of Neurons The kinds of neurons

•! Multipolar – structured neurons can be •! Sensory Neurons:! •! Motor Neurons!
divided into two types:! !!Are typically classified as the neurons !!Motor neurons are classified into three broad
responsible for converting external stimuli from
•! Golgi I - neurons with long- projection the environment into internal stimuli.!
categories according to their target namely:!
axonal processes; examples are !!They are activated by the sensory input (vision,
1.! Somatic Motor Neurons!
pyramidal cells, Punkinje cells and touch, hearing, etc.), and send projections into 2.! Special Visceral Motor Neurons!
anterior horn cells.! the central nervous system that convey the 3.! General Visceral Motor Neurons!
sensory information to the brain or spinal cord. !
•! Golgi II - neurons whose axonal !!Unlike neurons of the central nervous system,
process projects locally; the best whose input come from other neurons, sensory
example is the granule cells.! neurons are activated by physical modalities
such as light, sound, and temperature.!

The somatic motor neurons BRAIN

•! the somatic neurons are further divided into two •! Portion of central nervous system contained
types: alpha efferent neurons and gamma efferent within the skull.
neurons.(both types are called efferent to indicate
the flow of information from the central nervous NERVOUS SYSTEM •! Control center for movement and emotions.
system to the periphery.
•! Alpha motor neurons- innervate extrafusal
muscle fibers (typically referred to simply as •! 1.3 kg mass
muscle fibers) located throughout the muscle.
•! Gamma motor neuron- innervate intrafusal fibers BRAINS… •! Protective membrane between the brain and
found within the muscle spindle. THE ZOMBIES cranium: dura meter, arachnoid layer and pia
mater.

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ANATOMY SUBDIVISIONS OF CEREBRUM 2. PARIETAL LOBE

- somatesthetic interpretation (cutaneous and muscular
•! CEREBRUM senstaions)
•! Cerebral cortex 1. FRONTAL LOBE – voluntary control of - understanding and utterance of speech (wermcke’s
•! Longitudinal fissures area)
skeletal muscles 3. TEMPORAL LOBE
•! CEREBELLUM
•! Vermis -! Personality (with limbic system) - interpretation of auitory sensations
•! Cerebral peduncles -! Intellectual processes (concentration, - auditory and visual memory
•! THALAMUS AND HYPOTHALAMUS planning, decision – making) 4. OCCIPITAL LOBE
•! BRAIN STEM -! Verbal communication
- visual center
•! Pons - integration of eye mov’mts, correlation of visual images
•! Medulla oblongata with previous experience, concious seeing
•! Reticular formation
•! midbrain

SUBDIVISIONS COMPONENTS
SUBDIVISION OF BRAIN AND MAJOR COMPONENTS DIENCEPHALON
TECTUM
MESENCEPHALON TEGMENTUM
SUBDIVISIONS CHIEF COMPONENTS CEREBRAL AQUEDUCT
•! EPITHALAMUS
"!Pineal- club-shaped of knoblike organ.
RHINECEPHALON CEREBELLUM
:lampreys- photoreceptor
TELENCEPHALON CEREBRAL HEMISPHERE METENCEPHALON TEGMENTUM
:gnathostomes-endocrine organ that is stimulated in
LATERAL VENTRICLES 4TH VENTRICLE
part by light that first enters the body through the retina
PROSENCEPHALON RHOMBENCEPHALON
:absent in few vertebrate
EPITHALAMUS MYELENCEPHALON MEDULLA OBLAGATA
:large in primates and hagfishes
DIENCEPHALON THALAMUS 4TH VENTRICLE
"! Parapineal- constant features of bony fishes of the
HYPOTHALAMUS Devonian and of ancestral amphibians and reptiles,
THIRD VENTRICLE serves as parietal eye
:present in few fishes, lampreys, larval and
adult anuran and some lizards.

•! HABENULAE CHONDRICHTHYES OSTEICHTHYES

: largest in sharks and bloodhounds
: inconspicuous in most birds
: developed permanently in mammals
•! THALAMUS
:hidden in amniotes
•! HYPOTHALAMUS
: optic chiasma- cephalic boundary of diencephalon
where optic nerves reach the brain.
: saccus vasculosus- vascularized thin-walled ventral
evagination of the diencephalon floor of elasmobranch
and ray-finned fish
•! THIRD VENTRICLE
- continuous caudal with cerebral aqueduct of the

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d. metencephalon REPTILES
-muscular coordination and equilibrium
e. myencephalon
-cardio-vascular and respiratory central
(heart, lungs, blood vessels)
-glandular secretion
*cranial nerves – 10 pairs
*spinal nerves – 1 pair in each intervertebral discs
*spinal cord – terminates in the anterior part of the urostle

AVES Cerebellum
d. myelencephalon
- has increased surface and has many superficial folds
-spread laterally below the cerebellum •! Brain is proportionately larger than in reptiles, and is
and narrows into the spinal nerve cord - many activities of motor coordination
short and broad.

* Cranial nerves – 12 pairs d. Metencephalon

a. Telencephalon
- behind cerebellum
-large and smooth
* Spinal nerves – 1 pair each somite -intelligence, memory, reasoning
*cranial nerves - 12 pairs
-olfactory lobes – small - poor olfactory sense
*spinal nerves – 1 pair each somite
b. Optic lobes
-in the midbrain, conspicuously developed (keen sight)

MAMMALS . Diencephalon
. Metencephalon
1. THALAMUS – relay center for sensory impulses
1. CEREBELLUM – equilibrium and motor coordination
2. HYPOTHALAMUS – “F” center
2. PONS – relay center, respiratory nuclei-depth of
3. PITUITARY GLAND – regulation of endocrine glands respiration
- RH and IH e. Myelencephalon
c. Mesencephalon * medulla oblongata
- corpora quadrigemina - relay center
1. SUPERIOR COLLICULUS- visual reflexes -visceral autonomic center
2. INFERIOR COLLICULUS – auditory reflexes *RR
3. CERBRAL PEDUNCLES – coordinating reflexes, with *HR
many motor fibers *Dilation & constriction of blood
vessels

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Anamniotes have 10 pairs of

cranial nerves, amniotes have 12.

FUNCTIONS: Nerves I, III, VIII are purely ssensory to

Directs sense impulses the olfactory epithelium, retina and
throughout the body. membranous labyrinth.

Equilibrium
Eye movement, vision Nerves III, IV, VI supply the
Facial sensation myotomal muscles of the eyeball.
Hearing; Phonation
The cranial or cerebral nerves Respiration
are the peripheral nerves of the Nerves V, VII, IX, X supply the
Salivation; Swallowing pharyngeal arches with motor and
head that are related to the Smell sensory innervations.
brain. Twelve pairs of cranial Taste
nerves have been
distinguished in human
anatomy.

Nerve I Nerve I
Function: Smell The number of olfactory Nerve I
fibers in birds and
Filia olfactora is the platypus is small. Terminal nerve-lies
discrete bundles of close to the olfactory
olfactory nerve fibers. In sharks and some bulb and tract.
teleosts the sac
Vomeronasal containing the olfactory
In lower vertebrates it
nerve is the epithelium is distant
from the olfactory bulb. has been implicated in
separate triggering sexual
subdivision of the In mammals the responses.
olfactory epithelium is in
olfactory nerve in the upper part of the
tetrapods. nasal passage.
Terminal nerve is also
Its cell bodies are located
In marine mammals called as “nerve 0”.
in the olfactory
epithelium. the olfactory nerve is
vestigial.

Nerve II
Function: Sight Nerve III
The nerve emerges from the
rear of the eyeball and
Function: Supplies
extends to the optic the extrinsic eyeball
chiasma, beyond w/c it is muscles and certain
termed the optic tract. other myotomal
The cell bodies of the optic muscles of the eyes.
nerve fibers are in the retina.
Arises ventrally from
The term optic nerve is from the mesencephalon.
the embryological viewpoint,
a misnomer since the retina
arises from the paired optic It contains visceral
vesicles that never separate motor fibers that end
from the forebrain.
in the ganglion of the
The term tract is a bundle autonomic nervous
of nerve fibers w/n the
system
central nervous system.

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Nerve IV Nerve V Nerve V

Function: Supplies the Divided into three
superior oblique.
divisions: ophthalmic, The cell bodies for
It emerges ventrally at the maxillary, and
anterior end of the
all sensory fibers in
mandibular.
hindbrain. This nerve is a sensory
the nerve are all in
for general sensation to the trigeminal
One of the smallest of
the cranial nerves, the skin of the entire ganglion.
having the fewest fibers. head, including the
conjunctiva of the eye, The mandibular nerve
One of the smallest of the the lining of the ear is motor to the muscles
cranial nerves, having the canal, and the
fewest fibers. of the jaws.
ectodermal of the
The cell bodies of all eardrum. It also
fibers in these nerves supplies the jaw teeth, All branches contain
are in nuclei w/n the oral mucosda, nasal sensory fibers.
central nervous system. passageway and the
anterior surface of the
tongue.

Nerve VI Nerve VII Nerve VIII

Arises from the anterior The vestibulocochlear
This nerve supplies end of the hindbrain in nerve innervates the hair
the external rectus. close association with cell receptors of the inner
the fifth nerve. ear. It carries vestibular
information to the brain
It is sensory to the from the semicircular
It emerges ventrally neuromast organs on the canals, utricle, and saccule
and anterior end of head of fishes and aquatic providing the sense of
amphibians and to taste balance. It also carries
the hindbrain. buds. information from the
cochlea providing the
Nerve VII of fishes sense of hearing. This
contains sensory fibers cranial nerve branches into
One of the smallest of the Vestibular branch
for general sensation.
the cranial nerve, (balance) and the cochlear
having the fewer fibers. branch (hearing). The
In mammals, it contains cochlear fibers originate
visceral motor fibers. from the spiral ganglion. It
is pure sensory nerve fiber

Nerve IX Nerve X Nerve XI

In sharks it has three major The spinal accessory nerve has
It is the longest cranial nerve
branches, pretrematic that two branches. The cranial
enters the arch in front of the
innervating many structures in
first gill slit, posttrematic the throat, including the muscles branch provides somatic motor
controls the muscles that enters of the vocal cords, thorax and innervation to some of the
inside it and pharyngeal. abdominal cavity. It provides muscles in the throat involved
sensory information (touch, in swallowing. This cranial
The glossopharyngeal temperature and pressure) from branch is accessory to CN X,
nerve innervates the the external auditory meatus (ear originating in the caudal
pharynx (upper part of the canal) and a portion of the nucleus ambiguous, with the
throat), the soft palate and external ear. It carries taste fibers of the cranial root
the posterior one-third of sensation from taste buds in the traveling the same extracranial
the tongue. It carries pharynx. It also provides sensory path as the branchial motor
sensory information information from the esophagus, component of the vagus nerve.
respiratory tract, and abdominal
(touch, temperature, and The spinal branch provides
viscera (stomach, intestines,
pressure) from the pharynx somatic motor innervation to
liver, etc.). It provides visceral
and soft palate. It carries the trapezius muscles,
motor innervation to the heart,
taste sensation from the stomach, intestines, and providing muscle movement for
taste buds on the posterior gallbladder. the upper shoulders head and
one third of the tongue. neck. It is pure motor nerve
fiber.

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Nerve XII
AUTONOMIC NERVOUS SYSTEM
- is the part of the nervous
The hypoglossal nerve system of the higher life forms Autonomic ganglia are
that is not consciously classified in three categories:
provides somatic controlled. paravertebral, colateral, and
motor innervation to -part of nervous system that terminal.
innervates glands and smooth
the muscles of the and cardiac muscles.
tongue. This pure -it is composed of (1) a
-it is entirely a visceral sympathetic system-
motor nerve originates thoraculumbar system,
motor system.
from the hypoglossal emerges from the cord via
nucleus located in the 2 motor neurons: most of the spinal nerves Representative
of the trunk and (2) a components of the
tegmentum of the 1. Preganglionic neuron-
parasympathetic
medulla. in visceral efferent nucleus autonomic nervous
in central nervous system. (craniosacral) system that
emerges from the brain via system of a mammal.
2. Postgangloic neuron- is
in an autonomic ganglion. nerves III, VII, IX, X, and
XI.

AUTONOMIC NERVOUS SYSTEM AUTONOMIC NERVOUS SYSTEM AUTONOMIC NERVOUS SYSTEM

Sympathetic nervous system
Promotes a "fight or flight" response, corresponds with arousal and energy The white rami carry pregangloic Autonomic plexuses are
generation, inhibits digestion: 1. Paravertebral ganglia fibers from the spinal nerve to the 2.Collateral ganglia are present in head
Diverts blood flow away from the gastro-intestinal (GI) tract and skin via entwined around blood (ciliary, submandibular, sphenopalatine
vasoconstriction. are interconnected by ganglion. It conducts visceral afferent
vessels, the vertebral and otic) and in the abdomen at the
Blood flow to skeletal muscles, the lung is not only maintained, but enhanced (by as longitudinal strands of fibers enroute to the spinal cord.
much as 1200%, in the case of skeletal muscles).
Cervical and sacral nerve lack white column and other base of the major branches of the
Dilates bronchioles of the lung, which allows for greater alveolar oxygen exchange. autonomic fibers to form
rami. structures around the neck abdominal aorta.
Increases heart rate and the contractility of cardiac cells (myocytes), thereby providing sympathetic trunk close to
a mechanism for the enhanced blood flow to skeletal muscles. and trunk.
Dilates pupils and relaxes the lens, allowing more light to enter the eye. the vertebral column.
Parasympathetic nervous system
Gray rami conduct postganglionic 3. Terminal ganglia found only in the
Promotes a *rest and digest" response; promotes calming of the nerves and return to fibers from the ganglion to the spinal trunk, and are embedded in the walls of
regular function, enhances digestion. It is connected by two nerve for distribution to the skin.
Dilates blood vessels leading to the GI tract, increasing blood flow. This is important the organs they innervate-heart, lungs,
small branches, a white
following the consumption of food, due to the greater metabolic demands placed on
Branches to collateral ganglia stomach, and urinary bladder.
the body by the gut. ramus communicans and
The parasympathetic nervous system can also constrict the bronchiolar diameter usually join those from adjacent
when the need for oxygen has diminished. a gray ramus
paravertebral ganglia to form 3. Terminal ganglia found only in the
During accommodation, the parasympathetic nervous system causes constriction of communicans.
the pupil and lens. discrete autonomic nerves such as trunk, and are embedded in the walls of
The parasympathetic nervous system stimulates salivary gland secretion, and the splanchnic and and cardiac
accelerates peristalsis, so, in keeping with the rest and digest functions, appropriate
the organs they innervate-heart, lungs,
PNS activity mediates digestion of food and indirectly, the absorption of nutrients. nerves that supply the heart with a stomach, and urinary bladder.
Is also involved in erection of genitals, via the pelvic splanchnic nerves2–4. sympathetic innervation.