1. STRUCTURE & CLASSIFICATION Dr. Ayisha Qureshi
NEURON/ NERVE
STRUCTURE & CLASSIFICATION
Dr. Ayisha Qureshi

2. OBJECTIVES By the end of the lecture, you should be able to:
Define nerve/neuron.
Draw, label and identify different parts of a neuron.
Classify neurons on the basis of function and structure.
Differentiate b/w afferent & efferent nerves
Explain the process of myelination
Enumerate the factors that affect the rate of conduction of a nerve impulse

3. WHAT PARTS DO YOU KNOW THAT ARE IN THE NERVOUS SYSTEM?
Brain
Spinal Cord
Peripheral Nerves
The brain, spinal cord and the nerves form the Nervous system that makes human superior than animals….which enables us to think, to judge, to memorize, to learn from experience, to be smart….and so on…. Thus, the way humans act and react depends on the complex organization of the nervous system.

4. A system that controls all of the activities of the body.
Nervous System
A system that controls all of the activities of the body.
The nervous system is made of:
The brain
The spinal cord
The nervous system is organized into the CENTRAL NERVOUS SYSTEM which is comprised of the brain and the spinal cord and the PERIPHERAL NERVOUS SYSTEM which is made up of the nerve fibers that carry information to different parts of the body.
The nerves
The senses

5. CENTRAL NERVOUS SYSTEM PERIPHERAL NERVOUS SYSTEM
Brain
Spinal Cord
PERIPHERAL NERVOUS SYSTEM
Sensory Input
Motor Output
As the name suggests the Sensory input portion receives input from the environment, takes it to the CNS where it is integrated, action is decided upon and then the reaction is send through the Motor output portion which is supplying the muscles….E.g: touching a hot stove….

6. A diagram showing the sensory neuron which will receive the stimulus or the change in the environment (e.g, skin), takes it to the CNS where a reaction is decided and the Motor neuron which then instructs the muscle involved to take the appropriate action…..(e.g: withdrawing the hand after touching a hot surface)

7. What makes up the brain, the spinal cord or your peripheral nerves?
The neuron is the basic, functional unit of the brain, spinal cord and the peripheral nerves…..

12. NEURONS

13. INTRODUCTION What is a neuron?
It is a basic structural and functional unit of the nervous system.
It is a highly differentiated and specialized excitable tissue.
The Human NS contains 100 billion neurons.
(Nerve cells and neurons are the same.)

14. The nerves allow you to react to a stimulus.
A stimulus is a change in the environment.
Example: A hot stove
Or… tripping over a rock

15. Functions of the Neurons
Reception of the stimulus
Generation of the nerve Impulse
Transmission of the nerve Impulse

16. Structure of the A Typical Neuron
A typical neuron thus has the following parts:
Soma or Nerve Cell body
Axon with the axon terminals
Dendrites

17. Its like a tree that is lying on its side…… The roots are the axon terminals, the trunk is the axon while the dendrites emerge as branches from the soma or the cell body.

19. SOMA (Nerve cell body) Different shapes Different sizes
Fusiform, stellate, oval, rounded, pyramidal.
Different sizes
– 5 to 135 micrometers
Nucleus: typically large
one nucleolus (usually)
Perikaryon= cytoplasm which has:
Nissl bodies
Neurofibrils
All organelles:
– mitochondria, ribosomes, endoplasmic reticulum, lysosomes & Golgi apparatus

20. Nerve Cell Body Nissl bodies Neurofibrils
Are rough endoplasmic reticulum with ribosomes
Stained with basic dyes
Composed of RNA & polysomes.
Tigroid substance (due to striped appearance)
Not present in the axon
Synthesis of proteins
Dissolve & disappear if cell injured (nerve cut, injured, fatigued, poisoned)
Neurofibrils
Formed by clumping of neurotubules & neurofilaments
Delicate threads running from cytoplasm of the nerve cell body into the axon and the dendrite
Functions:
Neuronal microtubules transport substances from the cell body to the distal cell processes.
Neurofibrils give support and shape to the neuron.

22. Point to remember:
Nerve cell body is the most vital part-if it is destroyed the entire neuron dies!

23. Axon Also called axis cylinder or nerve fiber. Longest process
A single axon arises from a cone-shaped area of the neuronal cell body called the axon hillock
Axon hillock & first 100 µm of axon (no myelin sheath) is called Initial segment.
Trigger zone: is the name given to the axon hillock & the initial segment. It is an area that shows high excitability and a nerve impulse is generated here.

24. Axon AXON IS MADE UP OF: AXON ENDS IN:
Jelly-like semi fluid substance called Axoplasm
Plasma membrane called Axolemma
Mitochondria and ER
No Nissl granules so Does NOT synthesize proteins.
AXON ENDS IN:
Terminal Buttons (Synaptic knob or Bouton Terminaux)
Axon break up into no. of terminal branches called Telodendria or Terminal filaments
At their end is a small swelling called Terminal knob.
These knobs contain granules or vesicles with neurotransmitter substance

26. Dendrites
Short, tree-like, highly branched tapering processes of the nerve cell
Receive and then carry impulses to the cell body
Small knob-like projections called dendritic spines
Have all the components of the cell body

29. Questions:

30. What are Afferent and Efferent fibers?
When nerve fiber carries impulses from the periphery towards the CNS, it is called an Afferent nerve fiber.
Efferent
When the nerve fiber carries impulses from the CNS to the periphery, it is called an Efferent nerve fiber.

31. What is an anterograde and retrograde flow?
Anterograde flow
Flow of axoplasm from the soma/ cell body to the axon.
Cell body continuously synthesizes new material which is carried to the axon!
It usually occurs along the neurotubules.
Energy is provided by ATP
400 mm/day to 0.5 mm/day
Enzymes for NT synthesis, Ca.
Retrograde flow
Occurs in the reverse direction
From the axon terminals
(peripheral regions) to the soma
Proteins, NGF, herpes virus, neurotropic proteins as Polio virus, rabies, even some used up synaptic vesicles for recycling

34. Classification of neurons:

35. Neurons are classified on the basis of:
STRUCTURE:
FUNCTION:
Unipolar
Bipolar
Multipolar
Motor
Sensory
Interneurons

36. Classification of nerves
Structural/histological classification (depending on the number of processes):
Unipolar
Bi-polar
Multi-polar

40. myelination

41. What is myelination?
Myelination is the presence of myelin around the neuron. Myelin is not part of the structure of the neuron but consists of a thick layer mostly made up of lipids, present at regular intervals along the length of the axon.
Such fibers are called myelinated fibers.
The water-soluble ions carrying the current across the membrane cannot permeate this coat, it act as an insulator, just like the white coating of the electric wires and prevents the leakage of ions from the neuron through its membrane.

42. How does the process of myelination occur?
Myelination is carried out by myelin-forming cells that wrap themselves around the axons in jelly-roll fashion. These myelin-forming cells are Schwann cells in the PNS (peripheral nervous system) and the Oligodendrocytes in the CNS (brain & the spinal cord)
The CNS consists of 2 types of cells: the neurons and the Glial cells. As the name suggests, “glia” implies that they act as glue for the nervous system which is not entirely true.
The glial cells are non-neuronal cells that perform the following functions:
To surround neurons and hold them in place
To supply nutrients and oxygen to the neurons
To insulate neurons
To destroy pathogens and remove dead and damaged neurons.

43. Myelination Outside CNS ↓ Schwann cells Neurons CAN regenerate
Neurons can recover after injury
Inside CNS ↓
Oligodendrocytes
Neurons CANNOT regenerate
Neurons DIE after injury

44. Outside the CNS: myelinated fibers
Myelination is not part of the neuron but is done by the schwann cells.
As the diagram shows, the nerve cell invaginates the schwann cell…
The schwann cell wraps around the axon in concentric spirals.
Collectively, the various layers form the myelin sheath (a patch of myelin might be made of upto 300 layers of wrapped lipid bilayers)

46. Nodes of Ranvier
In myelinated nerve fiber, the myelin sheath is not a continuous sheath, but is deficient at regular intervals.
Between the myelinated regions, at the NODES OF RANVIER, the axonal membrane is bare and exposed to the ECF.
Current can flow across the membrane only at these bare spaces to produce action potentials.
Voltage-gated Na+ channels are concentrated at these regions.

48. Fibers OUTSIDE the CNS Myelinated (WHITE MATTER)
Only single nerve fiber invaginates single cell
Concentric layers of schwann cells wrapped around the fiber
No cytoplasm as all squeezed out- process called myelination
Outermost layer called Neurilemma or sheath of schwann
White appearance (white matter)
Unmyelinated (GREY MATTER)
Small diameter fibers
The nerve fiber only invaginates
No concentric layers or wrapping
A single schwann cell is invaginated by multiple nerve fibers
Nerve fibers surrounded by Schwann cell cytoplasm
Gray appearance (gray matter)

49. Nerve fibers lying WITHIN the CNS
Myelinated fibers
Myelin sheath produced by Oligodendrocytes
Myelinates upto 6 nerve fibers at a time.
Do not aid in regeneration
Unmyelinated fibers
Not supported by Oligodendrocytes
Indirectly supported by mass of surrounding tissues.
Do not aid in regeneration.

50. OLIGODENDROCYTES

51. SALTATORY CONDUCTION
In a myelinated nerve fiber, the nerve impulse “jumps” from node to node skipping over the myelinated sections of the axons. This process is called Saltatory conduction. Basis: Saltatory conduction propagates nerve impulse more rapidly because the nerve impulse has to be generated only at the nodes of ranvier and not repeatedly. Thus, it is faster. In unmyelinated fibers, the nerve impulse is like a grasshopper walking while in a myelinated fiber, the nerve impulse is like grasshopper jumping.

52. Factors affecting the speed of conduction of an action potential (nerve impulse) in a nerve fiber:

53. The factors that affect the rate of conduction of an action potential are:
Myelination
Diameter of the nerve fiber

54. Myelination and Conduction of action potential
Myelin contains the substance “Sphingomyelin” (lipid) which is an excellent electric insulator decreasing the ion flow through the membrane by 5,000 fold and insulates against leakage.
The higher the capacitance and the better the insulation, the faster the nerve impulse will travel along the neuron.

55. Myelination increases speed of nerve impusle conduction
Action potentials race along myelinated nerve fibres at rates of up to 100 metres/second or more, but can barely manage 1 metre/second in many unmyelinated fibres.
Very very important!

56. Conduction Velocity in Neurons
Aeroplane/ Jet ↓
MYELINATED FIBERS
(Very Fast)
Caterpillar ↓
UNMYELINATED FIBERS
(Very Slow)

57. Diameter of nerve fiber and Conduction of Action Potential
When fiber diameter increases, resistance to local current decreases, SO:
The larger the diameter of the nerve fiber, the faster it can propagate action potential.

58. Layers of nerve fibers
Endoneurium: finely reticular tissue lying just next to neurilemma.
Surrounds individual fibers separating them from each other.
Forms the endoneurial tube.
Perineurium: Several nerves surrounded by layer of connective tissue.
Epineurium: Nerve trunk itself surrounded by a loose layer of elastic tissue and CT.