Associate Professor, Department of Veterinary Physiology
Transcript of Associate Professor, Department of Veterinary Physiology
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