NEURON AND IT’S STRUCTURAL & FUNCTIONAL TYPES

BY: Syed Irshad Murtaza Technologist

Neurophysiology Dept AKUH Karachi

Date: 05/06-08-2015

NEURONS The nervous system consists of vast

number of cells called neurons. The are the functional unit of the nervous

system. Each neuron consists of cell body(soma),

axon and dendrites. Neurons are commonly referred to as

nerve cells. Bundles of axons bound together and are called nerves.

FUNCTIONAL TYPES OF NEURONS

FUNCTIONS OF THE NERVOUS SYSTEM

1. Sensory input – gathering information· To monitor changes occurring inside and outside

the body (changes = stimuli)2. Integration –

· to process and interpret sensory input and decide if action is needed.

3. Motor output· A response to integrated stimuli· The response activates muscles or glands

HOW NEURONS FUNCTION?

STRUCTURAL TYPES OF NEURON Neurons are divided on the basis of structures and size Three General Types of Neurons 1. Unipolar:   A neuron from which only a single axon

leaves the cell body Most sensory neurons have this shape. It’s just one.   some books call it a pseudobipolar nerve.

2. Bipolar:   A neuron from which two processes leave the cell body.  It’s found in the retina of our eyes

3. Multipolar: A neuron from which multiple branches leave the cell body It got many neuronal processes.  Multi means many. This is a very important nerve because all motor neurons have this shape and many interneurons also have.

STRUCTURAL TYPES OF NEURON

Structural Classification of Neurons

· Multipolar neurons – many extensions from the cell body

· Bipolar neurons – one axon and one dendrite

· Unipolar neurons – have a short single process leaving the cell body

Neurons can also be classified into various categories, depending on what criteria are used. For example

Functional classification

sensory neurons, that receive sensory signals from sensory organs and send them via short axons to the central nervous system

Morphological classification based on the number of extensions from the cell body:

pseudo-unipolar neurons with a short extension that quickly divides into two branches, one of which functions as a dendrite, the other as an axon…..

Functional classification

motor neurons that conduct motor commands from the cortex to the spinal cord or from the spinal cord to the muscles

Morphological classification

multipolar neurons that have short dendrites emanating from the cell body and one long axon

Classification …. Cont’d

Functional classification

interneurons that interconnect various neurons within the brain or the spinal cord

Morphological classification

bipolar neurons that have two main extensions of similar lengths

Classification …. Cont’d

Functional classificationPyramidal neurons/CellsLike most neurons, pyramidal neurons have multiple dendrites and a single axon, but both dendrites and axons branch extensively.

Morphological classification Pyramidal neurons/Cells

are neurons with a pyramidal shaped cell body (soma) and two distinct dendritic trees. The basal dendrites emerge from the base and the apical dendrites from the apex of the pyramidal cell body.

Classification …. Cont’d

TYPES OF NERVE FIBERSThe individual nerve fibers have different

diameters related to different functions. Each nerve fiber arises from soma of the neuron through a long extended process called the axon.

Not all signals move at the same conduction velocities. This is due to the type of fiber that is conducting the signal. Most fibers fall under one of the three different fiber types: A fibers, B fibers, and C fibers. These classifications are based on their diameters, and other physiological characteristics

Note: Larger the diameter of the neuron faster the speed of the action potential.

TYPE A FIBERS TYPE B FIBERS TYPE C FIBERS

Thickest and fastest conducting fibers.

Medium in size Smallest and thinnest

Myelinated. Myelinated. Non-myelinated.

diameter 1.5-20 micron

 diameter 1.5-3.5 microns.

diameter 0.1-2 microns.

speed of conduction is 4-120 m/sec

speed of conduction is 3-15 m/sec

speed of conduction is 0.5-2 m/sec

PROPERTIES OF NEURON IrritabilityTo initiate the nerve impulse in response to stimuli ConductivityAbility to transmit the response.

How neurons communicate? Neurons communicate by means of an electrical

signal called the Action Potential Action Potentials are based on movements of ions

between the outside and inside of the cell When an Action Potential occurs a molecular

message is sent to neighboring neurons

AXONS AND NERVE IMPULSES· Axons end in axonal terminals· Axonal terminals contain vesicles with

neurotransmitters· Axonal terminals are separated from

the next neuron by a gap called· Synaptic cleft – gap between adjacent neurons

· Synapse – junction between nerves

TYPES OF SYNAPSES/COMMUNICATION

There are 3 types of synaptic connections between 2 neurons:     

A) Axodendritic  B) axosomatic   C) axoaxonic

All or None Principle : All or None Principle A law stating that once

the membrane depolarized to a threshold value, action potentials occur maximally or not at all.

Throughout depolarisation, the Na+ continues to rushinside until the action potential reaches its peak and

thesodium gates close.If the depolarisation is not great enough to reachthreshold, then an action potential and hence an

impulseare not produced.

ACTION POTENTIAL

EXCITATORY NEUROTRANSMITTERS

INHIBITORY NEUROTRANSMITTERS

ACTION POTENTIAL

PHASES OF ACTION POTENTIAL “An action potential (also known as a nerve impulse or a spike

potential) is a self-regenerating wave of electrochemical activity (in response to stimuli) that allows excitable cells (such as nerve cells) to carry a signal over a distance.”

Stages of Action potential : Resting stagei. In it the neuron is polarised due to different concentration of

ion across membrane.ii. Inside –ve membrane. iii. Outside +ve membraneiv. Na+ and K+ Channels are closev. Na+ major extracellular cationvi. K+ major intracellualr cationvii. Membrane is at rest. viii. This is resting membrane potential =-70mV.

Depolarization i. Threshold stimulus (about -60 to -55mV)ii. Excite the Na-channels.iii. Na+ Channels Open iv. Influx of Na+ Starts v. Na+ ions Rush invi. Inside became more +ve membranevii. Outside -ve membraneviii. Membrane potential rises from -70mV to +40mVix. Action Potential or nerve impulse is then generated.

PHASES OF ACTION POTENTIAL

Repolarization i. Na+ Channels Close. ii. At the same time K+ Channels Open.iii. Efflux of K+ Starts. iv. Inside again became –ve membranev. And Outside +ve membrane. vi. Membrane potential fall to -80mV (called Undershoot) vii. Then Na-K-Pumps Work actively using ATP, to retain the

Equilibrium. viii. Sending 3 Na+ out and 2 K+ in to the cell. And restore the

membrane potential of -70mV.

PHASES OF ACTION POTENTIAL

Hyperpolarization This is the Refractory period in which a second stimulus

will not produce a second action potential (no matter how strong that stimulus is)

In which neuron returns to resting potential In it Na+ is expel outside the membrane while K+

inside the membrane. RELATIVE - Refractory period : RELATIVE - Refractory period, Another action potential

can be produced, but only if the stimulus is greater than the threshold stimulus

ACTION POTENTIAL

Propagation of Action potential Through Myelinated Neurons : Insulatroy properties of the myelin sheath prevent the

movement of the ions Exchange of ions takes place only in gaps called Nodes of

Ranvier. When depolarization occurs one node it moves across along

myelin sheath to the next nodes. This movement called Saltatory conduction. Propagation of Action potential Through Un-myelinated

Neurons : Propagation of Action potential Through Un-Myelinated

Neurons through each and every part of the membrane. That’s why it is called Continuous conduction with slow speed.

In unmyelinated fibres, the entire axon membrane is exposed and impulse conduction is slower.

ACTION POTENTIAL