Neuron & its structural & functional type by Murtaza Syed

NEURON, TYPES OF NEURON, STRUCTURE OF NEURON,

1. NEURON AND IT’S structural & functional TYPES BY: Syed Irshad Murtaza Technologist Neurophysiology Dept AKUH Karachi Date: 05/06-08-2015

2. 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.

3. Functional types of neurons

4. 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

5. How neurons function?

6. 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.

7. Structural types of neuron

8. 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

9. 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…..

10. 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

11. 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

12. Functional classification Pyramidal neurons/Cells Like 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

13. Types of nerve fibers The 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.

14. 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

15. Properties of Neuron Irritability To initiate the nerve impulse in response to stimuli Conductivity Ability 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

16. 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

17. Types of Synapses/ cOMMUNICATION There are 3 types of synaptic connections between 2 neurons: A ) Axodendritic B ) axosomatic C ) axoaxonic

18. 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 rush inside until the action potential reaches its peak and the sodium gates close. If the depolarisation is not great enough to reach threshold , then an action potential and hence an impulse are not produced. Action Potential

19. Excitatory neurotransmitters

20. Inhibitory neurotransmitters

21. Action Potential

22. 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 stage In it the neuron is polarised due to different concentration of ion across membrane. Inside – ve membrane. Outside + ve membrane Na+ and K+ Channels are close Na+ major extracellular cation K+ major intracellualr cation Membrane is at rest. This is resting membrane potential = -70mV .

23. Depolarization Threshold stimulus (about -60 to -55mV) Excite the Na-channels. Na+ Channels Open Influx of Na+ Starts Na+ ions Rush in Inside became more + ve membrane Outside - ve membrane Membrane potential rises from -70mV to +40mV Action Potential or nerve impulse is then generated. Phases of Action Potential

24. Repolarization Na+ Channels Close. At the same time K+ Channels Open. Efflux of K+ Starts. Inside again became – ve membrane And Outside + ve membrane. Membrane potential fall to -80mV (called Undershoot) Then Na-K-Pumps Work actively using ATP, to retain the Equilibrium. Sending 3 Na+ out and 2 K+ in to the cell. And restore the membrane potential of -70mV. Phases of Action Potential

25. 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

26. 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