STRUCTURE OF NEURONs

Dr.NIKHIL.G.PANPALIA Guide: Dr.K.R.NAIK

•Historical background

HISTORY OF NEURONS

• Cell theory In 1838 Theodore Schwann and Matthias Schleiden

proposed that the cell was the basic functional unit of all living things.

The complexity and small size of most neurons made elucidating the structure of the neuron the most formidable task in the field of histology, and the debate about the fine structure of the nervous system spanned most of the nineteenth century.

Purkinje’s work

• One of the first people to use the new compound microscopes to investigate nervous tissue was Johannes Evangelista Purkinje

• Dopaminergic midbrain neurons• Cerebellar cells which bear his name.• Purkinje’s speculations on the functions of the entities he had

discovered suggest that he contributed more to the Neuron Doctrine : “With reference to the importance of the corpuscles…they are probably

central structures…because of their whole organization in three concentric circles [i.e. cytoplasm, nuclear membrane and nucleolus] which may be related to the elementary brain and nerve fibres…as centres of force are related to the conduction pathways of force, or like the ganglia to the nerves of the ganglion, or like the brain substance to the spinal cord and cranial nerves. This means they would be collectors, generators and distributors of the neural organ.”

Johannes Evangelista Purkinje

Golgi contribution

• Staining method – Camillo Golgi (1843-1926, right).

• Golgi staining or Golgi impregnation - involves hardening of tissue in potassium bichromate and ammonia immersion in a silver nitrate solution.

• In 1873, Golgi published descriptions of hippocampal and cerebellar tissue that he had stained using his new technique. Golgi reasserts his belief that the nervous system consists of a reticulum .

Camillo Golgi

Hippocampus Cerebellum

Cajal work• Santiago Ramón y Cajal (1852-1934) - neuron was the

anatomical and functional of the nervous system. • Cajal examined nervous tissue from embryos, - the axon

and dendrites grow out of the cell body of the neuron.• Cajal describes the basket cells in the molecular layer of

the cerebellar cortex: “They are small, globular, and irregular…[and] supplied with numerous

protoplasmic prolongations [dendrites]. The special character of these cells is the striking arrangement of their nerve filament [axon], which arises from the cell body but also very often from any thick, protoplasmic expansion [dendrite].”

Chicken cerebellum by Santiago Ramony y Cajal

Neuron doctrine

The neuron is the fundamental structural and functional unit of the nervous system

Neurons are discrete cells which are not continuous with other cells

The neuron is composed of 3 parts – the dendrites, axon and cell body, and

Information flows along the neuron in one direction (Dendrites to the Axon, via the Cell body).

• Neuron Term introduced in 1891.

• The axis cylinder Axon was coined by RudolphVon Kollicker

• protoplasmic processes Dendrites was termed by

WilhelmHis

• Charles Sherrington described the junction between nerve and muscle, and named it the Synapse

•Introduction

What is neuron ?• Excitable cells specialized for reception of

stimuli and conduction of impulses. Brain, Spinal cord and Ganglia No division and replication The human nervous system is estimated to consist of

roughly 360 billion non-neural glial cells and 90 billion nerve cells.

• Neurons are post mitotic structures therefore they donot divide.

Neurons vs. Other Cells

• Similarities with other cells:• Neurons and other body cells

both contain a nucleus that holds genetic information.

•Neurons and other body cells are surrounded by a membrane that protects the cell.

•The cell bodies of both cell types contain organelles that support the life of the cell, including mitochondria, Golgi bodies, and cytoplasm.

• Differences that make neurons unique:

• Unlike other body cells, neurons stop reproducing shortly after birth.

•Neurons have a membrane that is designed to sends information to other cells.

• Theaxon and dendrites are specialized structures designed to transmit and receive information.

• The connections between cells are known as a synapses.

• Neurons release chemicals known as neurotransmitters into these synapses to communicate with other neurons.

Neurons consist of a main part called the cell body, which contains the nucleus and various organelles.

Cell Body

Nucleus

Nucleolus

Neurons also contain cell body “extensions” called processes or neurites, which carry impulses to and from the cell body.

axon

Dendrites

Two different types of processes can come from the cell body. The first is called the dendrite and the other the axon.

Dendrites, also called afferent processes, carry impulses TOWARDS the cell body.

Axons, also called efferent processes, carry impulses AWAY from the cell body.

Types of neurons

Types of neurons• Classified according three methods 1) Morphology 2) Size 3) Function

Morphological classification

• Unipolar • Multipolar• Bilpolar

Unipolar neurons• Unipolar neurons - cell body has a

single neurite that divides a short distance from the cell body into two branches, one proceeding to some peripheral structure and the other entering the central nervous system .

• Examples -found in the posterior root ganglion.

Unipolar neurons

Cell body

Dendrites

Multipolar neurons• Multipolar neurons have a number of

neurites arising from the cell body .• With the exception of the long

process, the axon, the remainder of the neurites are dendrites.

• Most neurons of the brain and spinal cord are of this type.

Bipolar neurons

• Bipolar neurons possess an elongated cell body, from each end of which a single neurite emerges .

• Examples of this type of neuron are found in the retinal bipolar cells and the cells of the sensory cochlear and vestibular ganglia.

According to size• Golgi type I neurons

have a long axon. – Pyramidal cells of the cerebral cortex – Purkinje cells of the cerebellar

cortex– Motor cells of the spinal cord

• Golgi type II neurons have a short axon

cerebral and cerebellar cortex

Pyramidal cells (Golgi Type I neurons

Golgi Type ii Neuron

Golgi Type ii Neuron

Functional classification

• Sensory or afferent neuron• Motor or efferent neuron• Interneuron or association neuron

Sensory neurons or Afferent neurons •Periphery of the body to the Central nervous system. •These neurons are usually concentrated in areas called ganglia and their dendrite branches extend to the skin or to sensory organs and act as sensory receptors (either directly or indirectly).

Motor neurons or Efferent neurons • These neurons carry impulses Away from the cell body and thus the central nervous system to muscles, gland, or some other “effector” to produce a certain action. •Most of the neurons in the spinal cord and many of those in the brain are motor neurons.

Interneurons or Association neurons• This type of neuron is restricted to the central nervous system. •They are also called connector neurons. •These neurons act as bridges between sensory and motor neurons or relay impulses to various functional centers of the brain or spinal cord.

Structure of neuron

Structure of neurons1. Cell body and its organelles2. Plasma membrane3. Nerve cells processes

Main structures of Cell body • Nucleus • Cytoplasmic organelles

Nucleus of cell body• Large, rounded• Appears Pale, chromatin widely

scattered; single prominent nucleolus; Barr body present in female

• Located centrally placed, displaced to periphery in cell injury

• Functions in controlling cell activity• Meshwork of neuronal and glial

processes are collectively termed as neuropil

Dendrites

Nucleus

Nucleolus

Body of nerve cell

neuropil

Photomicrograph - Section Anterior grey column of Spinal cord

Nucleolus• Structure - Small

circular structure(s) within nucleus

• Function(s) -Synthesis and partial assembly of ribosomes

Cytoplasm and its organalles

• Cytoplasm is rich in granular and agranular substance.

• Suspends the organelles within the cell

• Fills the cell and gives it shape• Allows nutrients to move

about the cell• Following are the cytoplasmic

organalles

Nissls substance

• Nissl substance consists of granules that are distributed throughout the cytoplasm of the cell body, except for the region close to the axon, called the axon hillock .

• The granular material also extends into the proximal parts of the dendrites; it is not present in the axon.

• Appearance: Nissl substance is composed of rough-surfaced endoplasmic reticulum arranged in the form of broad cisternae stacked one on top of the other.

• Many of the ribosomes are attached to the surface of the endoplasmic reticulum.

• Since the ribosomes contain RNA, the Nissl substance is basophilic and can be well demonstrated by staining with toluidine blue or other basic aniline dyes and using the light microscope.

Functions of Nissls substance

• The Nissl substance is responsible for synthesizing protein, which flows along the dendrites and the axon and replaces the proteins that are broken down during cellular activity.

Ribosomes• Structure - Dot-like

organelles attached to the rough ER or free in the cytosol

• Function(s) – Reads RNA to synthesize proteins

Rough Endoplasmic Reticulum• Structure – a system of

membranous tubes and sacs covered with dark dots (ribosomes)

• Function(s)- Makes alterations to proteins produced by ribosomes- Folds proteins into the correct shape

Smooth Endoplasmic Reticulum• Structure - A system of

membranous tubules and sacs

• Function(s) - helps move lipids, steroids, and fatty acids within the cell (intracellular highway); regulates calcium levels; breaks down toxic substances

Golgi complex

• The Golgi complex, when seen with the light microscope after staining with a silverosmium method, appears as a network of irregular wavy threads around the nucleus.

• In electron micrographs, it appears as clusters of flattened cisternae and small vesicles made up of smooth-surfaced endoplasmic reticulum.

Functions of Golgi complex• The protein produced by the Nissl substance is

transferred to the inside of the Golgi complex in transport vesicles, where it is temporarily stored.

• Golgi complex is specialized for different types of enzymatic reaction.

• At the trans side of the complex, the macromolecules are packaged in vesicles for transport to the nerve terminals.

• The Golgi complex is also thought to be active in lysosome production and in the synthesis of cell membranes.

Mitochondria • Mitochondria are found scattered throughout the cell body,

dendrites, and axons . • They are spherical or rod shaped. • In electron micrographs, the walls show a characteristic double

membrane . The inner membrane is thrown into folds or cristae that project into the center of the mitochondrion.

• Mitochondria possess many enzymes, which are localized chiefly on the inner mitochondrial membrane. These enzymes take part in the tricarboxylic acid cycle and the cytochrome chains of respiration.

• Therefore, mitochondria are important in nerve cells, as in other cells, in the production of energy.

Mitochondria

Electron micrograph of a NeuronNucleus & Number of cytoplasmic organelles

Neurofibrils

• Neurofibrils, as seen with the light microscope after staining with silver, are numerous and run parallel to each other through the cell body into the neurites .

• With the electron microscope, the neurofibrils may be resolved into bundles of Neurofilaments each filament measuring about 10 nm in diameter .

• The neurofilaments form the main component of the cytoskeleton.

Figure 2-13 Photomicrograph of a silver-stained section of a neuron showing thepresence of large numbers of neurofibrils in the cytoplasm of the cell body andthe neurites.

Silver stained section of a Neuron showing presence of large numbers of neurofibrils in cytoplasm of the cell body & neurites

Microfilaments

• Microfilaments measure about 3 to 5 nm in diameter and are formed of actin.

• Microfilaments are concentrated at the periphery of the cytoplasm just beneath the plasma membrane where they form a dense network.

• Together with microtubules, microfilaments play a key role in the formation of new cell processes and the retraction of old ones.

• They also assist the microtubules in axon transport.

Microtubules

• They measure about 25 nm in diameter and are found interspersed among the neurofilaments .

• They extend throughout the cell body and its processes

• The microtubules and the microfilaments provide a stationary track that permits specific organelles to move by molecular motors.

• They are responsible for cell transport.

• Cell transport involves the movement of membrane organelles, secretory material, synaptic precursor membranes, large dense core vesicles, mitochondria, and smooth endoplasmic reticulum.

• Cell transport can take place in both directions in the cell body and its processes.

• It is of two kinds: rapid (100 to 400 mm per day) and slow (0.1 to 3.0 mm per day).

• Rapid transport (100 to 400 mm per day) is brought about by two motor proteins associated with the microtubule adenosine triphosphate (ATP)-ase sites;

• Kinesin for anterograde (away from the cell) movement

• Dynein for retrograde movement.

• Slow transport (0.1 to 3.0 mm per day) involves the bulk movement of the cytoplasm and includes the movement of mitochondria and other organelles.

• Slow axonal transport occurs only in the anterograde direction.

Electron micrograph of dentrites showing presence of neurofilament & microtubules within their cytoplasm

(B) Transverse section of a dentrites(A) Longitudional section of two adjacent dentrites

Lysosomes • Lysosomes are membrane-bound vesicles measuring about

8 nm in diameter. • They serve the cell by acting as intracellular scavengers

and contain hydrolytic enzymes. • Lysosomes exist in three forms: (1) primary lysosomes, which have just been formed; (2) secondary lysosomes, which contain partially digested

material (myelin figures)(3) residual bodies, in which the enzymes are inactive and the

bodies have evolved from digestible materials such as pigment and lipid.

Digesting enzymes

Centrioles

• Structure - Cylindrical bodies that appear during cell division

• Function(s) – Help move DNA during division in animal cells

• Found In - Animal cells only

cell membranecell boundarycontrols movement

of materials in & outrecognizes signals

cytoplasmjelly-like material holding

organelles in place

vacuole & vesiclestransport inside cellsstorage

mitochondriamake ATP energy

from sugar + O2

nucleusprotects DNAcontrols cell

ribosomesbuilds proteins

ERhelps finish proteinsmakes membranes

Golgi apparatusfinishes, packages

& ships proteins

lysosomefood digestiongarbage disposal &

recycling

2) Plasma membrane• It is the site for the initiation and conduction of the nerve

impulse• The plasma membrane is composed of an inner and an outer

layer of very loosely arranged protein molecules separated by a middle layer of lipid .

• Certain protein molecules lie within the phospholipid layer and span the entire width of the lipid layer. These molecules provide the membrane with hydrophilic channels through which inorganic ions may enter and leave the cell.

Resting state

Excited state

Returns to resting state

Ion Channels and the AP

Properties of the Action Potential

• The action potential: – Is an “all or none” event: membrane potential either

passes threshold or doesn’t– Is propagated down the axon membrane– Has a fixed amplitude: AP’s don’t change in height to

signal information (nondegremental)– Has a conduction velocity (meters/sec)– Has a refractory period in which stimulation will not

produce an AP (limits the firing rate)

.

Saltatory Conduction• The “jumping” of an impulse between the “Nodes of Ranvier”

dramatically increasing it’s speed.

3) Nerve cell processes

• Dendrites • Axons

Dendites• The membrane of the neuron functions as a

receptive surface over its entire extent; however, specific inputs (termed afferents) from other cells are received primarily on the surface of the cell body and on the surface of the specialized processes known as dendrites.

• The dendritic processes may branch extensively and are often covered with projections known as dendritic spines.

• Spines provide a tremendous increase in the surface area available for synaptic contacts.

• Dendrites contain numerous orderly arrays of microtubules and fewer neurofilaments.

• The microtubule associated proteins (MAPs) in the dendrite have a higher molecular weight than those found in the axon. An example is MAP2.

Why do dendrites have so much branches?

• Dendrites provide an enlarged surface area to receive signals from the terminal buttons of other axons allowing for a chemical signal to pass simultaneously to many target cells.

Dendrites

Figure :Diagrammatic representation of the neuron dendrite, emphasizing the areas of contact by other afferent inputs to the neuron.

Axon hillock• The cone-shaped region of the cell body where the

axon originates is termed the axonhillock. This area is free of ribosomes and most other cell organelles, with the exception of cytoskeletal elements and organelles that are being transported down the axon.

• The region between the axon hillock and the beginning of the myelin sheath is known as the initial segment. In many cases, this region is the anatomical location for the initiation of the action potential.

.

Axons • The other type of process in the

idealized neuron is the axon. Each neuron has only one axon and it is usually straighter and smoother than the dendritic profiles.

• Axons also contain bundles of microtubules and neurofilaments and scattered mitochondria.

• Microfilaments within the axon are usually associated with an area adjacent to the plasmalemma and often are the most dense at the nodes of Ranvier.

Axon Hillock

Initial segment of axon

Microtubules

• The axon itself is often surrounded by a membranous

material, called the myelin sheath, formed by glia cells. • The myelin sheath acts to insulate the plasmalemma of

the axon in a way that necessitates the more rapid spread of the depolarization of the plasmalemma and increases the speed of conduction of the nerve impulse.

• At the distal-most end of the axon and its collaterales are small branches whose tips are button-shaped cytoplasmic enlargements called terminal boutons or nerve endings

AXON vs DENDRITE

AXON

• Only one axon is present in a neuron.

• It is a thin long process of uniform thickness and smooth surface.

• The branches of axon are fewer and at right angles to the axon.

• Axon contains neurofibrils and no Nissl’s granules.

• It forms the efferent component of the impulse.

DENDRITE

• Dendrites are usually multiple in number in a neuron.

• These are short multiple processes. Their thickness diminishes as these divide repeatedly. The branches are studded with spiny projections.

• The dendrites branch profusely and are given off at acute angles.

• Dendrites contain both neurofibrils and Nissl’s granules.

• Dendrites form the afferent component of the impulse

Synapse

Synapse• It is the site of interneuronal communication• There is a physical gap between pre- and post-synaptic

membranes.• Presynaptic membrane is typically an axon and it contains

• Mitochondria that provide energy for axon functions• Vesicles (round objects) that contain neurotransmitter• Cisternae that are a part of the Golgi apparatus: recycle

vesicles• Postsynaptic membrane can be

• A dendrite (axodendritic synapse)• A cell body (axosomatic synapse)• Another axon (axoaxonic synapse)

Most neurons make synaptic connection to 1000 or more neurons and receive upto 10,000 connections from other neurons.

Types of synapses

• There are two fundamentally different types of synapses:

1. Electrical synapse2. Chemical synapse.

Electrical synapse• In an electrical synapse, the presynaptic and postsynaptic

cell membranes are connected by special channels called gap junctions that are capable of passing electric current, causing voltage changes in the presynaptic cell to induce voltage changes in the postsynaptic cell.

• The main advantage of an electrical synapse is the rapid transfer of signals from one cell to the next.

• They are rare in human CNS.• They can be bidirectional while chemical synapse are

unidirectional.

Chemical synapse

• In a chemical synapse, electrical activity in the presynaptic neuron is converted (via the activation of voltage-gated calcium channels) into the release of a chemical called a neurotransmitter that binds to receptors located in the plasma membrane of the postsynaptic cell.

• The neurotransmitter may initiate an electrical response or a secondary messenger pathway that may either excite or inhibit the postsynaptic neuron.

• Chemical synapses can be classified according to the neurotransmitter released:

glutamatergic(often excitatory), GABAergic (often inhibitory), cholinergic (e.g. vertebrate

neuromuscular junction), and adrenergic (releasing norepinephrine).

Neurotransmitter Release

• Vesicles lie “docked” near the presynaptic membrane• The arrival of an action potential at the axon terminal opens

voltage-dependent CA++ channels– CA++ ions flow into the axon– CA++ ions change the structure of the proteins that bind the vesicles

to the presynaptic membrane– A fusion pore is opened, which results in the merging of the

vesicular and presynaptic membranes• The vesicles release their contents into the synapse

– Released transmitter then diffuses across cleft to interact with postsynaptic membrane receptors

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Postsynaptic Receptors

• Molecules of neurotransmitter (NT) bind to receptors located on the postsynaptic membrane– Receptor activation opens postsynaptic ion channels– Ions flow through the membrane, producing either depolarization

or hyperpolarization – The resulting postsynaptic potential (PSP) depends on which ion

channel is opened• Postsynaptic receptors alter ion channels– Directly (ionotropic receptors)– Indirectly, using second messenger systems that require

energy (metabotropic receptors)

Postsynaptic Potentials

• PSPs are either excitatory (EPSP) or inhibitory (IPSP)– Opening NA+ ion channels results in an EPSP– Opening K+ ion channels results in an IPSP

• PSPs are conducted down the neuron membrane• Neural integration involves the algebraic summation

of PSPs– A predominance of EPSPs at the axon will result in an

action potential– If the summated PSPs do not drive the axon membrane

past threshold, no action potential will occur

Neuromodulator

• Modulate and modify the activity of postsynaptic neuron

• They donot have direct effect rather they enhance, prolong and inhibit the effect of principal neurotransmitter on postsynaptic membrane.

• Act on G protein coupled receptors• Ex. Acetycholine, serotonin, histamine and

adenosine

Source of information

• Snells neuroanatomy

• Grays anatomy

Thank you