MJF College of Veterinary & Animal Science, Chomu, Jaipur (Rajasthan)

Dr. Vijay Domple, Ph.D.

Associate Professor,

Department of Veterinary Physiology

THE NEURON

Introduction

• Nervous system consists of neurons andsupportive cells.

• Neurons are excitable cells that are able totransmit an electrical impulse along theirlength.

• Supportive cells are responsible for makingthe myelin sheath surrounding many neurons,providing nutrients, as well as performing aphagocytic role.

• Neurons come in many sizes and shapes.

• They have a long life span and are generally

considered amitotic, meaning that they no

longer divide.

• However, recent evidence has revealed that

within certain sites in the brain, neurons do

reproduce.

Introduction

Cell body

• The cell body, or soma, consists of a large, roundnucleus 5–10 μm in diameter surrounded by cytoplasm,also called perikaryon (karyon = nucleus).

• Within the cytoplasm are the normal cell organellesexcept for the lack of centrioles that are responsiblefor formation of the mitotic spindle associated withcell division.

• The cytoplasm contains free ribosomes, smooth andrough endoplasmic reticulum (ERs), mitochondria, andGolgi apparatus.

• The rough ER is also known as Nissl substance thatstains darkly in the presence of basic dyes called Nisslstains.

• The rough ER is the major site of protein synthesisdestined for insertion into the membrane of thecell or an organelle.

• Free ribosomes are responsible for the synthesis ofproteins destined for the cytosol.

• Smooth ER can be continuous with rough ER andacts as a site where newly synthesized proteins arefolded into their three-dimensional structure.

• Smooth ER can also regulate cytosolicconcentrations of ions such as calcium.

Cell body

• Also within the cytosol is an internalscaffolding called the cytoskeleton.

• The cytoskeleton consists of severalcomponents.

• The largest are the microtubules measuringabout 20 nm in diameter and runninglongitudinally down the neurites (axons anddendrites).

• They are formed through polymerization ofmolecules of the protein tubulin.

Cell body

• Associated with the microtubules is anotherclass of proteins called microtubuleassociated proteins (MAPs).

• These proteins help anchor microtubles toother parts of the neuron and to each other.

• The second component of the cytoskeleton isneurofilaments, which are called intermediatefibers in other cell types

• Neurofilaments measures 10 nm in diameter.

Cell body

• A third component is the microfilaments

measuring 5 nm in diameter.

• They consist of two actin protein strands

• Microfilaments run longitudinally down the

neurites.

Cell body

Dendrites

• They look like the branches of a tree originating from the

cell body- All of the dendrites collectively make up the

dendritic tree.

• The dendrites on some neurons also have on their surface

dendritic spines.

• Dendrites act as the receptive region of the neuron.

• Combination of dendritic tree and dendritic spines

makes for a large surface area that facilitates this

function.

• The cytoplasm of the dendrites resembles that of the soma.

Axon

• While the structures discussed earlier are common

to most cells, the axon is unique to neurons.

• The axon is specialized to allow an impulse to be

transmitted along its length and thus carried from

one location to another.

• A long axon is sometimes called a nerve fiber.

• A bundle of axons within the CNS is called a

nerve tract- it is called a nerve in the periphery.

• A neuron has a single axon that originatesfrom the soma in a region called the axonhillox.

• This is where the nerve impulse originatesand, therefore, is called the trigger zone.

• The axon is unique in that it contains norough ER

• Its membrane has a different proteincomposition from that of the soma.

Axon

• While the axoplasm contains neurofibrils,

neurotubules, lysosomes, mitochondria, and

small vesicles

• It lacks rough ER and ribosomes.

• Therefore, protein synthesis does not occur

in the axon.

• Instead, proteins must be synthesized in the

soma and transported along the axon.

Axon

• Axons may extend less than a millimeter or over ameter in length.

• Axons typically branch, forming collaterals thatenable one neuron to communicate with severalother sites.

• The diameter of an axon can range from less than1 μm to as large as 1 mm.

• The thicker the axon, the faster the speed ofconduction of the nerve impulse down itslength.

Axon

• The nerve fibers of many nerves are coveredwith a whitish, fatty sheath called myelin.

• Myelin acts to protect and insulate the axonand increases the speed of conduction of theimpulse.

• Whereas the speed of conduction may be 1 m/sin unmyelinated fibers, it can be 150 m/s inmyelinated fibers.

• The dendrites are always unmyelinated.

Axon

• In the PNS, the myelin is produced by theSchwann cells.

• The Schwann cell spirals around the axon,producing many concentric circles enclosing theaxon and forming the myelin sheath.

• During the spiraling process, the nucleus andcytoplasm of the Schwann cell gets squeezed tothe outer layer of the cell and appears as a bulgeon the outer surface.

• The outer layer is the neurilemma.

Axon

• Occasionally, the Schwann cell does not spiral

around the axon, but instead encloses many axons

at one time

• Such axons are said to be unmyelinated.

• Adjacent Schwann cells do not touch one another,

but instead form a space called the node of

Ranvier, or neurofibral nodes, in which the

axonal membrane (axolemma) is exposed.

Axon

• In the CNS, myelin is produced by another typeof cell called an oligodendrocyte.

• Oligodendrocytes are a type of glial, orsupportive, cell in the CNS.

• Do not spiral around axon- oligodendrocytesform end feet that surround the axon andform the myelin sheath.

• One oligodendrocyte can thereby myelinatemany axons, whereas a Schwann cellmyelinates only a single axon.

Axon

• In the CNS, areas containing myelinated fibersare referred to as white matter and generallyconsist of fiber tracts.

• Areas containing cell bodies are referred to asgray matter; collections of cell bodies are callednuclei.

• The collaterals off the main axon trunk end in aseries of fine extensions called telodendria.

• The telodendria end in a knoblike expandedstructure called the axon terminal, terminalbouton, or synaptic knob.

Axon

• Microtubules do not extend into the axon

terminal- it contains synaptic vesicles.

• Synaptic vesicles are small membrane-bound

spheres measuring 50 nm in diameter and

containing neurotransmitters.

• The axon terminal will end at another neuron

or cell such as an endocrine gland or muscle

cell.

Axon