introduction• A neuron ( also known as a neurone or nerve cell) isan electrically excitable cell that processes andtransmits information by electrical andchemical signallnig.• Neurons are the core components of the nervoussystem, which includes the brain, spinal cord, andperipheral ganglia.• Human brain comprises tens of billions ofneurons, each linked to thousands of other neuronsvia the chemical channels called synapse.
structure• There are many, many different types of neurons butalmost all have certain structural and functionalcharacteristics in common.• A neuron consists of three main parts the cell body orperikaryon or soma, dendrites and axons.• The cell body is the central region which is the mostimportant part of the neuron containing the nucleus ofthe cell.• The soma is, the site of major metabolic activity inthe neuron.
• The size of neuronal somas range widely from 0.005mm to 0.1 mm in mammals.• Collections of cell bodies (somas) give the greyishappearance to the gray matter of the brain.
• The protoplasm of cell body contains peculiar angulargranules, which stain deeply with basic dyes, such asmethylene blue; these are known as Nissl’s granules.• These granules disappear (chromatolysis) during fatigue orafter prolonged stimulation of the nerve fibers connectedwith the cells. They are supposed to represent a store ofnervous energy, and in various mental diseases aredeficient or absent.• Thought to be involved in the synthesisof neurotransmitters such as acetylcholine.
• Dendrites are extensions that carry impulses toward the cell bodyand are referred to as being afferent fibers.• They effectively increase the surface area of a neuron to increaseits ability to communicate with other neurons.
• An axon is one of two types of protoplasmic protrusions thatextrude from the cell body of a neuron .• Unlike dendrites axons are long, slender projection of a nervecell, or neuron, that conducts electrical impulses away from theneurons cell body or soma.• Axons are distinguished from dendrites by severalfeatures, including shape, length , and function.• The point where the axon arises from a cell body is termed asaxon hillock.• Axoplasm is the cytoplasm within the axon of a neuron.
• The axolemma is the cell membrane surrounding an axon. It isresponsible for maintaining the membrane potential of theneuron, and it contains ion channels through which ions can flow.• In vertebrates, the axons of many neurons are sheathedin myelin, which is formed by either of two types of glialcells: Schwann cells ensheathing peripheral neurons andoligodendrocytes insulating those of the central nervous system• The myelin sheath functions to:– Protects the axon and electrically isolates it– Increases the rate of Action Potential transmission (saltation)• Along myelinated nerve fibers, gaps in the sheath knownas nodes of Ranvier occur at evenly-spaced intervals.
• Terminally the Axon branch sparsely, forming collaterals. Eachcollateral may split into telodendria which end in a synapticknob, which contains synaptic vesicles – membranous bags ofNTs.• Axons make contact with other cells via the synaptic knob—usually on dendrites of other neurons but sometimes muscle orgland cells—at junctions called synapses.• The region between the two connecting neurons is known as thesynaptic gap or snaptic cleft or neural junction.
Classification of neuronsSTRUCTURAL CLASSIFICATION BASED ON POLARITY Unipolar : type of neuron in which only one protoplasmicprocess (neurite) extends from the cell body.– Found mostly in inverterbrate– In humans mostly found in dorsal root ganglia Pseudounipolar : contains an axon that has split into twobranches; one branch runs to the periphery and the other to thespinal cord. Bipolar: An axon and a single dendrite on opposite ends of thesoma– are specialized sensory neurons for the transmission ofspecial senses,hence abundant in sensory pathways forsmell, sight, taste, hearing and vestibular functions
Multipolar: An Axon along with more than two dendrites– Multipolar neurons constitute the majority of neurons inthe brain– Subdivided in to golgi I and golgi II types– Includes motor neurons and interneurons.
FUNCTIONAL CLASSIFICATIONBASED ON CONDUCTION DIRECTION Afferent neurons –– Also called sensory neurons.– Convey information from tissues and organs into thecentral nervous system Efferent neurons –– Also called as motor neurons.– Carry nerve impulses away from the central nervoussystem to effectors such as muscles or glands.– According to their targets, motor neurons are classifiedinto three broad categories: Somatic motor neurons,Special visceral motor neurons ,General visceral motorneurons.–Somatic motor neurons are further divided in to α motorneuron (innervating extrafusal muscle fibre) and γ motorneuron (innervating intrafusal muscle fibre)
Interneuron-– also called as relay neuron or local circuit neuron.– connects afferent neurons and efferent neurons in neuralpathways.
BASED ON NEUROTRANSMITTER PRODUCTION Cholinergic neurons —secreting acetylcholine GABAergic neurons — secreting gamma aminobutyric acid. Glutamatergic neuron — secreting glutamate Dopaminergic neurons — secreting dopamine . Loss of dopamineneurons in the substantia nigra has been linked to Parkinsonsdisease Serotonergic neurons — secreting serotonin. A lack of serotoninat postsynaptic neurons has been linked to depression. BASED ON UNIQUE SHAPE AND FUNCTION Betz cells – large motor neurons located within the fifth layer ofthe grey matter in the primary motor cortex, M1. Purkinje cells - some of the largest neurons in thehuman brain, found within the Purkinje layer in the cerebellum.
Renshaw cells - neurons with both ends linked to alpha motorneurons. Target of the toxin of Clostridium tetani Pyramidal neurons (pyramidal cells) - type of neurons withtriangular soma found in areas of the brain including cerebralcortex, the hippocampus, and in the amygdala. Basket cells - inhibitory GABAergic interneurons found inseveral brain regions: the molecular layer ofthe cerebellum, the hippocampus, and the cortex.
• Neurons collectively form a nerve.• a nerve is usually made up from a variety of fascicles .• each fascicle is encased by perineurium. Inside the fascicle are agroup of axons bathed in endoneurial fluid.• Between the fascicles is a fatty material calledthe interfascicular epineurium. The nerve is then wrapped in themain epineurium
Neuronal communication Neurons are the information/signal relay system of our nervoussystem Once stimulated neurons need to conduct information in twoways:1. From one end of a neuron to the other end.2. Across the minute space separating one neuron from anotherneuron/muscle end plate (synaptic cleft). The 1st is accomplished electrically via Action Potentialgeneration. The 2nd is accomplished chemically via neurotransmitters
Electrical conductionRESTING MEMBRANE POTENTIAL• The relatively static membrane potential of quiescent cells iscalled the resting membrane potential.• Resting membrane potential of nerve cell = -70 mV
• Resting membrane potential is maintained by the ionicdistribution across the neuron cell membrane• Ions involved mainly are the potassium and sodium ion.• concentration gradients of Na+ & K+– Na+ 10x greater outside– K+ 30x greater inside
• At rest more K+ move out than Na+ move in.• K+ ions diffuse out leave behind excess negative charge inside.• Sodium-potassium pump– Na+ out - K+ in (more Na+ out than K+ in)– contributes to loss of (+).
THE ACTION POTENTIAL :• The action potential is generated by ion flux through voltagegated channels.
GENERATION OF ACTION POTENTIAL AND IT’SCONDUCTION IN NEURON:• A excitatory stumulus generates at a dendrite of neuron• This transmitter acts on the membrane excitatory receptor toincrease the membrane’s permeability to Na+.• Influx of Na+ inside the cell, causing resting membrane potentialto move toward positive side.• This positive increase in voltage above the normal restingneuronal potential—that is, to a less negative value—is called thegraded potential or excitatory postsynaptic potential (or EPSP).• Dendrites and somata typically lack voltage-gatedchannels, which are found in abundance on the axon hillock andaxolemma.So Action potential is not generated here.
• The positive charge carried by the Na+ spreads as a wave ofdepolarization through the cytoplasm in the form EPSP.• If the initial amplitude of the EPSP is sufficient, it will spread allthe way to the axon hillock where Voltage-gated channels residesin high number.• At the axon hillock If the arriving potential change issuprathreshold, an Action potential will be initiated and it willtravel down the axon to the synaptic knob where it will cause NTexocytosis.• If the potential change is subthreshold, then no AP will ensueand nothing will happen.• The EPSP can undergo spatial summation or temporalsummataion so as to reach suprathreshold level and excite anaction potential.
• Temporal summation :The same presynaptic neuron stimulates thepostsynaptic neuron multiple times in a brief period. Thedepolarization resulting from the combination of all the EPSPs may beable to cause an AP.• Spatial summation : Multiple neurons all stimulate a postsynapticneuron resulting in a combination of EPSPs which may yield an AP
• If an AP gets generated at the axon hillock, it will travel all theway down to the synaptic knob.• The manner in which it travels depends on whether the neuron ismyelinated or unmyelinated.• Unmyelinated neurons undergo the continuous conduction of anAP whereas myelinated neurons undergo saltatory conduction ofan AP.• In continous conduction, the wave of de- and repolarizationsimply travels from one patch of membrane to the next adjacentpatch.
• Action potential propagation in unmyelinated axon
• Saltatory conduction (from the Latin saltare, to hop or leap) isthe propagation of action potentials along myelinatedaxons from one node of Ranvier to the next node.• It ncreasing the conduction velocity of action potentials withoutneeding to increase the diameter of an axon.• MS destruction of mylin sheath by own immune system(progressive loss of signal conduction, muscle control & brainfunction)
Chemical conduction• One neuron will transmit information to another neuron or to amuscle or gland cell by releasing chemicals calledneurotransmitters.• The site of this chemical interplay is known as the synapse.• Three Types of Synapses occur between neurons– Axodendritic Synapse• Axon to dendrite– Axosomatic Synapse• Axon to cell body– Axoaxonic Synapse• Axon to terminal endings
• An AP reaches the axonterminal of the presynapticcell and causes V-gatedCa2+ channels to open.• Ca2+ rushes in, binds toregulatory proteins &initiates NT exocytosis.• NTs diffuse across thesynaptic cleft and then bindto receptors on thepostsynaptic membrane andinitiate some sort ofresponse on thepostsynaptic cell.
• Different neurons can contain different NTs.• Different postsynaptic cells may contain different receptors.– Thus, the effects of an NT can vary.• Some NTs cause cation channels to open, which results in agraded depolarization.• Some NTs cause anion channels to open, which results in agraded hyperpolarization.• A graded depolarization will bring the neuronal VM closer tothreshold. Thus, it’s often referred to as an excitatorypostsynaptic potential or EPSP• Graded hyperpolarizations bring the neuronal VM fartheraway from threshold and thus are referred to as inhibitorypostsynaptic potentials or IPSPs