NEURON/ NERVE

STRUCTURE & CLASSIFICATION

Dr. Ayisha Qureshi

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

• Brain

WHAT PARTS DO YOU KNOW THAT ARE IN THE NERVOUS SYSTEM?

• Spinal Cord

• Peripheral Nerves

A system that controls all of the activities of the body.

The nervous system is made of:

The brain The spinal cord

The nerves The senses

What makes up the brain, the spinal cord or your peripheral nerves?

NEURONS

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.)

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

• Reception of the stimulus

• Generation of the nerve Impulse

• Transmission of the nerve Impulse

Functions of the Neurons

Structure of the A Typical Neuron

• A typical neuron thus has the following parts:

1. Soma or Nerve Cell body

2. Axon with the axon terminals

3. Dendrites

SOMA (Nerve cell body)

• Different shapes– 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

Nerve Cell Body

• Nissl bodies– 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:1. Neuronal microtubules

transport substances from the cell body to the distal cell processes.

2. Neurofibrils give support and shape to the neuron.

Point to remember:

• Nerve cell body is the most vital part-if it is destroyed the entire neuron dies!

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.

Axon

AXON IS MADE UP OF:• 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

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

QUESTIONS:

What are Afferent and Efferent fibers?

• Afferent– 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.

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

CLASSIFICATION OF NEURONS:

Neurons are classified on the basis of:

STRUCTURE:

• Unipolar• Bipolar• Multipolar

FUNCTION:

• Motor• Sensory• Interneurons

Classification of nerves

• Structural/histological classification (depending on the number of processes):– Unipolar– Bi-polar– Multi-polar

Classification

Motor neurons Sensory neurons Interneurons

Physiological/functional

MYELINATION

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.

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)

Myelination

• Outside CNS ↓

Schwann cells↓

Neurons CAN regenerate

↓Neurons can recover

after injury

• Inside CNS ↓

Oligodendrocytes↓

Neurons CANNOT regenerate

↓Neurons DIE after injury

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)

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.

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)

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.

OLIGODENDROCYTES

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.

FACTORS AFFECTING THE SPEED OF CONDUCTION OF AN ACTION POTENTIAL (NERVE IMPULSE) IN A NERVE FIBER:

• The factors that affect the rate of conduction of an action potential are:

1.Myelination

2.Diameter of the nerve fiber

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.

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!

Conduction Velocity in Neurons

• Aeroplane/ Jet

↓

MYELINATED FIBERS

(Very Fast)

• Caterpillar

↓

UNMYELINATED FIBERS

(Very Slow)

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.

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.