NEUROANATOMY & NEUROPHYSIOLOGY 1

Neurohistology

CELLS OF THE NERVOUS SYSTEM  Nissl bodies

 Characteristic of nerve cells
CNS PNS  Consists of rosettes of polysomes and rough
Neurons Ganglion cells endoplasmic reticulum

Neuroglia Schwann cells  Absent in axons

 More in motor nerves
 When the nerve cell is injured, or fatigued Nissl
bodies disappears, and the process is known as
NEURONS
Chromatolysis
 Neuron - Structural & functional unit of nervous system
 Functions
 It is an excitable cell which is concerned with the
1. Protein synthesis
receiving information from the environment or other
2. Maintains continuous axonal flow
nerve cells, process information, send information to
other neurons or effector
Axon
 Neurons are known for
 Arise from either the cell body or the dendrite
 No mitosis
 Originate from the Axon Hillock (Point of generation
 No proliferation
of electric impulse)
 High metabolic activity
 Transmit impulses away from the cell body
 Has a long central core of cytoplasm  Axoplasm
STRUCTURE OF NEURON
 Covered by Axolemma (continuation of cell
 Neuron consists of
membrane)
 Nerve cell body/Soma/Perikaryon
 Axoplasm with axolemma is called axis cylinder of
 Processes of two kinds
nerve fiber
o Dendrites
 Axolemma is surrounded by
o Axons
 CNS  Oligodendrocytes
 Apart from the usual organelles like Nucleus &
 PNS  Schwann cells (Neurilemma or Sheath of
Mitochondria, Neuron contains Neurofibrils,
Schwann)
Neurotubules & Nissl bodies
 Nissl bodies are absent in the axon, so the necessary
proteins are synthesized in the soma & transmitted
Nerve cell body/Soma/Perikaryon
to the axon via axonal flow
 Cell membrane
 Axons of some nerve fibers are insulated by myelin
 Like other cells
sheath  Myelinated nerve fibers
 Trilaminar – 75 A˚
 Have branches (collaterals) that further divide &
 Nucleus
terminate in swellings called synaptic knobs or
 Central, large, vesicular, prominent nucleolus
terminal buttons
 At Eccentric position in injury, fatigue, healthy
 End distally in terminal boutons in synapses with
sympathetic ganglion
neurons, muscle cells, and glands
 Mitochondria
 Golgi apparatus
Myelin Sheath
 Lysosomes
 Thick lipoprotein sheath that insulates the axons
 Neurofibrils
of myelinated nerve fibers
 Arranged in plexiform manner in the cell body
 Imparts white color to the myelinated nerve
and extends in all processes.
fibers
 Crowded at axon hillock
 It is not continuous & the areas where it is
 Pigments
absent are called – Nodes of Ranvier
 Lipochrome - gives yellow color to neuron,
 Segment of nerve fiber between two nodes is
Abundant in old age
called Internode
 Melanin - Dopamine synthesis, Substantia Nigra
 Formed of
of mid brain
o Concentric layers of proteins
 Lipofuscin: wear and tear
 NEUROANATOMY & NEUROPHYSIOLOGY 2
Neurohistology

o Alternating with phospholipids  Eg: Mesencephalic nucleus of

(cholesterol, lecithin & cerebroside – trigeminal nerve
sphingomyelin)  Axon & dendrite arise from 2
 Functions different poles
1. Faster conduction Bipolar  Spindle shaped
2. Insulating capacity (which restricts nerve  Eg: Olfactory cells, Ganglion cell of
impulse within the nerve fiber & prevents auditory nerve, some cells of Retina
stimulation of neighboring nerve fibers)  One pole gives rise to axon & other
 Myelinogenesis Multipolar poles gives rise to dendrites
o Formation of myelin sheath  Eg: Motor neurons
o It is formed by
Depending on the Function
- In the peripheral nerves  by
Schwann cells in neurilemma Motor Neurons Sensory Neurons
- In the CNS  by Oligodendrocytes (efferent nerve cells) (afferent nerve cells)
o Schwann cells wrap up & rotate around the  Carry motor impulses  Carry the sensory
axis cylinder in many concentric layers, from CNS to peripheral impulses from periphery
which fuse to produce the myelin sheath effector organs like to the CNS
o Outermost membrane of Schwann cells muscles, glands etc.  They have short axons &
remains as neurilemma  They have long axons & long dendrites
o Nucleus of these cells remains in between short dendrites
myelin sheath & neurilemma Depending upon the Length of the Axon

Golgi type 1 Golgi type 2

Dendrites
 Branched processes that transmit impulses towards Arrangement: Cell bodies in Arrangement: Have Short
the nerve cell body (receptive) CNS, with long axons axons
 Contain cytoplasm similar in composition to that of Eg: Reach peripheral organs Eg: Cerebral cortex, Spinal
the cell body cord
 Golgi apparatus is not present
Anaxonic Neurons
 Small, stellate (star-shaped) cells with processes that
all look alike with no apparent axon
 Can be found in CNS, Retina, and Adrenal Medulla

CLASSIFICATION OF NEURONS

Depending upon the Number of Poles

 One process divides in ‘T’ shaped

manner into dendrite and axon.
Pseudo Unipolar
 Eg: dorsal root ganglia of all spinal
nerves
 Contains only one process i.e. axon
Unipolar
 Found in lower animals
 NEUROANATOMY & NEUROPHYSIOLOGY 3
Neurohistology

SYNAPSE Synaptic delay is very less

Synaptic delay is relatively
 Site where the axon of a neuron communicates with because of direct flow of
longer
effectors or other neurons current
 It is a functional connection between Impulses are transmitted in
 Two neurons (axoaxonic / axodendritic / Impulse is transmitted in one direction only – from
axosomatic) either direction pre-to postsynaptic neuron
 Neuron & effector cell (Bell – Magendie law)
 The junction of a neuron that allows transfer of Impulses can be
Vesicles fuse with axon
message to "postsynaptic cell" (e.g. another neuron, regenerated without
membrane & NT released
muscle fiber, gland etc.) interruption in adjacent
by exocytosis
1. Axodendritic – axon terminal  dendrite cells
2. Axosomatic – axon terminal  neuron cell body Responsible for the
3. Axonaxonic – axon terminal  another axon conduction of impulses
Number of NTs released
4. Dendrodendritic – dendrite  dendrite between neighboring
depends upon frequency of
5. Dendrosomatic – dendrite  neuron cell body smooth or cardiac muscle
AP
6. Neuromuscular junction – axon terminal  fibers, epithelial or glial
Switching elements of
muscle cells (sometimes between
nervous system
7. Neuroglandular junction – axon terminal  gland neurons in the retina & in
8. Presynaptic neuron – "before" the synapse; the the CNS)
neuron that is sending the signal
9. Postsynaptic neuron – "after" the synapse; the  Chemical synapses also serve as switching elements
affected cell receiving the signal for the nervous system. They can facilitate or inhibit
 Since the postsynaptic membrane normally does not the neuronal transmission of information or process
release neurotransmitters, nerve impulses can pass them with other neuronal input.
the synapse in one direction only.
 So, the synapse, Properties of Synapses
 Acts like a valve that ensures the orderly 1. Convergence and divergence
transmission of signals 2. Law of forward or one-way conduction
 They are also the sites at which neuronal signal 3. Fatigue
transmissions can be modified by other 4. Synapses are more susceptible to hypoxia
(excitatory or inhibitory) neurons 5. Synaptic delay
6. Synaptic plasticity
Types of Synapse 7. Subliminal fringe
 Based on Impulse Transmission 8. Summation
Electrical Synapse Chemical Synapse

Physiological continuity Synapse through which the

NEUROGLIA
between pre-and post- signals are transmitted by
synaptic neurons is the release of chemical  Non-neuronal cells of the CNS and the PNS
provided by gap junctions transmitter  Arise from the neural tube and neural crest
There is direct exchange of There is no continuity  Capable of cell division throughout life
ions between the two between pre-and post-  Best revealed with gold and silver impregnation stains
neurons through channels synaptic neurons due to
(connexons) in the region of the presence of synaptic CLASSIFICATION OF NEUROGLIA
gap junctions cleft Glial Cell Type Origin Location Functions
The action potential Action potential reaching
Myelin
reaching the terminal the presynaptic terminal
productio
portion of presynaptic causes release of Neural
Oligodendrocyte CNS n
neuron directly enters the neurotransmitter from the tube
Electric
postsynaptic neuron vesicles insulation
 NEUROANATOMY & NEUROPHYSIOLOGY 4
Neurohistology

Astrocytes  Whole nerve is covered by Epineurium (a fibrous

Fibrous Astrocytes: Neural Structural dense connective tissue with adipocytes and blood
Contains Gliofibrils tube support vessels)
which extend into Repair  Each nerve fiber is covered by Endoneurium (loose
processes processes connective tissue with reticular fibers and
Situated in white Blood– fibroblasts)
CNS
matter of CNS brain
Protoplasmic barrier
Astrocytes: Metabolic
Lack Gliofibrils exchanges
Situated in grey
matter of CNS
Lining
cavities of
Neural
Ependymal cell CNS central
tube
nervous
system

Bone Macropha
Microglia CNS
marrow gic activity

Myelin
productio CLASSIFICATION
Neural
Schwann cell PNS n
tube Erlanger – Gasser Classification
Electric
Based on diameter of nerve fibers & conduction of
insulation
Protects Fiber
Neural Conduction
Satellite Cells PNS neuron Diamete
tube Type Velocity Function
cell bodies r
(m/s)
(m)
Proprioception;
Aα 12 - 20 70 - 120
Somatic motor
Touch,
Aβ 5 - 12 30 - 70
Pressure
Motor to
Aγ 3-6 15 - 30 Muscle
spindles
Pain, cold,
Aδ 2-5 12 - 30
Touch
Pre-ganglionic
B <3 3 - 15
autonomic
NERVE FIBERS C
 Consist of axons, dendrites, and their glial investments Pain,
Temperature,
ORGANIZATION OF NERVES some mechano-
Dorsal root 0.4 - 1.2 0.5 - 2
Many axons together form a bundle called Fasciculus reception,
 reflex
Many fasciculi together form a Nerve responses
Postganglionic
Sympathetic 0.3 – 1.3 0.7 – 2.3
Connective Tissue Coverings Sympathetics
 Each fasciculus is covered by Perineurium (more  A & B fibers are Myelinated; C fibers are unmyelinated
fibroblasts)  Velocity of conduction ∞ thickness of the fibers
 NEUROANATOMY & NEUROPHYSIOLOGY 5
Neurohistology

Numerical Classification
Spinal or Sensory or Autonomic or Sympathetic
Fiber
Number Origin Dorsal Root Ganglia Ganglia
Type
Muscle spindle, Annulo-spiral Surrounded by thick Surrounded by thin capsule
Ia Aα capsule
ending
Ib Golgi tendon organ Aα Peripherally arranged big Irregularly arranged
round pseudo unipolar Multipolar neurons - star
Muscle spindle, flower-spray
II Aβ neurons shaped of varying sizes in
ending; touch, pressure
sections
Pain & cold receptors; some touch
III Aδ Each neuron is enveloped by Few satellite cells
receptors
a layer of small cuboidal incompletely surround the
Dorsal
cells - satellite cells cell body
IV Pain, temperature & other receptors root
Neurons in groups Neurons widely spaced &
C
separated by bundles of separated by axons /
Physio-Clinical Classification
nerve fibres dendrites which pass
Least through without being
Susceptibilit Most Intermediat
Susceptibl
y to Susceptible e involved in synapse
e
Regularly arranged nerve Nerve fibers are irregularly
Hypoxia B A C
fibers entering and leaving scattered
Pressure A B C the ganglion
Local No synapse Synopsis is present
C B A
anesthetics
Ex: dorsal roots of the spinal Ex: sympathetic ganglia
Depending on Presence or Absence of Myelin Sheath nerves, sensory ganglia of along sympathetic chain,
Myelinated Nerves Unmyelinated Nerves cranial nerves V, VII, IX, X otic ganglion, ciliary

Schwann cell membrane is Axons are buried in the ganglion, Submandibular

ganglion
coiled many times around Schwann cell, Myelin –
axon – multiple layers absent
SYMPATHETIC GANGLION CELLS
Eg: Preganglionic fibers of Eg: Post ganglionic fibers of
 Multipolar neurons that reside entirely within the
ANS ANS
Faster conduction Slower conduction PNS in sympathetic chain ganglia and “pre-aortic”
More number of voltage Less number of voltage ganglia
gated Na channels gated sodium channels
PARASYMPATHETIC GANGLION CELLS
 Multipolar neurons that also reside entirely within
the PNS in the wall of the innervated organ
GANGLIA
 Are aggregations of the neuronal cell bodies in PNS HISTOLOGY OF CNS
 Ovoid structures surrounded by a capsule
 2 types  Myelinated axons and the myelin-producing
1. Sensory ganglia White Oligodendrocytes
 Two types matter  Does not contain neuronal cell bodies
i. Cranial ganglia (associated with cranial  Present in more central regions
nerves)  Contains neuronal cell bodies, dendrites
ii. Spinal ganglia (dorsal root ganglia)  Initial unmyelinated portions of axons and
2. Autonomic ganglia Glial cells
Gray matter
 Consists of  Region at which synapses occur prevalent at
 Neuronal cell bodies the surface of the cerebrum and cerebellum
 The nerve fibers leading to and from them  Forms the cerebral and cerebellar cortex
 Satellite cells
 Connective tissue
 NEUROANATOMY & NEUROPHYSIOLOGY 6
Neurohistology

 Outer White o Deep layer of large pyramidal cells

 Inner Gray 4. Internal Granular Layer
 Central (H shaped) horizontal bar of this H  Composed of stellate cells
is an opening  Interspersed within the layer are
 Central canal - a remnant of the lumen of horizontal myelinated fibers forming
Spinal Cord
the embryonic neural tube. external band of Baillarger (thalamo-
 Limbs / horns of the H forms the anterior cortical ramifications)
horns (ventral motor) 5. Internal Pyramidal or Ganglion Layer
 Arms forms the posterior horns (dorsal  Giant Pyramidal cells of Betz (origin of
sensory) the pyramidal tract)
 Interspersed are granule cells and cells of
 Have three layers Martinotti, and horizontal fibers forming
1. An outer molecular layer the internal band of Baillarger
2. Central layer of large Purkinje cells 6. Multiform or Fusiform Layer
3. Inner granule layer  Contains spindle-shaped cells, granule
1. Molecular Layer cells, Martinotti cells, and stellate cells
 Few cells and few Myelinated fibers; more  The layer is pervaded by fiber bundles
dendrites
 Stellate cells (superficial) & basket cell
(deep)
 Reason for the sparseness of nuclei
2. Purkinje or Middle Ganglionic Layer
 Conspicuous cell body and their dendrites
Cerebellum are highly developed, assuming the aspect
(Cerebellar of a fan
Cortex) 3. Granular Layer
 Closely packed chromatic nuclei
resembling lymphocytes, irregular light
spaces that constitute the “islands” or
“glomeruli”
 Granule Cells: Small, Multipolar cells with
3 or 4 dendrites, have unmyelinated axons
 Golgi Type II cell: Vesicular nuclei and
chromophilic bodies
 Mossy fibers: thick fibers and synapse with
the granular layer
 Climbing Fibers: granular layer to
terminate of Purkinje cell

 Composed of six layers

Cerebrum 1. Molecular or Plexiform - Composed of
(Cerebral horizontal cells of Cajal & Golgi type II
Cortex) 2. Outer / External Granular Layer - Small
pyramidal or triangular cells whose
dendrites terminate on the molecular layer
3. Outer / External Pyramidal Layer
 Composed of typical well-formed
pyramidal neurons
 2 sub layers
o Superficial layer of medium sized
pyramidal cells: horizontal
Myelinated fibers