NEURONSANATOMICAL AND

PHYSIOLOGICAL PERSPECTIVE

introduction

• A neuron ( also known as a neurone or nerve cell) is an electrically excitable cell that processes and transmits information by electrical and chemical signallnig.

• Neurons are the core components of the nervous system, which includes the brain, spinal cord, and peripheral ganglia.

• Human brain comprises tens of billions of neurons, each linked to thousands of other neurons via the chemical channels called synapse.

structure• There are many, many different types of

neurons but almost all have certain structural and functional characteristics in common.

• A neuron consists of three main parts the cell body or perikaryon or soma, dendrites and axons.

• The cell body is the central region which is the most important part of the neuron containing the nucleus of the cell.

• The soma is, the site of major metabolic activity in the neuron. 

• The size of neuronal somas range widely from 0.005 mm to 0.1 mm in mammals.

• Collections of cell bodies (somas) give the greyish appearance to the gray matter of the brain. 

• The protoplasm of cell body contains peculiar angular granules, which stain deeply with basic dyes, such as methylene blue; these are known as Nissl’s granules.

• These granules disappear (chromatolysis) during fatigue or after prolonged stimulation of the nerve fibers connected with the cells. They are supposed to represent a store of nervous energy, and in various mental diseases are deficient or absent.

• Thought to be involved in the synthesis of  neurotransmitters such as acetylcholine.

• Sections of motor neuron spinal cord showing Nissl bodies on nissl staining.

• Dendrites are extensions that carry impulses toward the cell body and are referred to as being afferent fibers.

• They effectively increase the surface area of a neuron to increase its ability to communicate with other neurons.

• An axon is one of two types of protoplasmic protrusions that extrude from the cell body of a neuron .

• Unlike dendrites axons are long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma.

• Axons are distinguished from dendrites by several features, including shape, length , and function.

• The point where the axon arises from a cell body is termed as axon hillock.

• Axoplasm is the cytoplasm within the axon of a neuron.

• The axolemma is the cell membrane surrounding an axon. It is responsible for maintaining the membrane potential of the neuron, and it contains ion channels through which ions can flow.

• In vertebrates, the axons of many neurons are sheathed in myelin, which is formed by either of two types of glial cells: Schwann cells ensheathing peripheral neurons and oligodendrocytes 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 known as nodes of Ranvier occur at evenly-spaced intervals.

• Terminally the Axon branch sparsely, forming collaterals. Each collateral may split into telodendria which end in a synaptic knob, which contains synaptic vesicles – membranous bags of NTs.

• Axons make contact with other cells via the synaptic knob—usually on dendrites of other neurons but sometimes muscle or gland cells—at junctions called synapses.

• The region between the two connecting neurons is known as the synaptic gap or snaptic cleft or neural junction.

Classification of neuronsSTRUCTURAL CLASSIFICATION BASED ON POLARITY Unipolar : type of neuron in which only one

protoplasmic process (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 two branches; one branch runs to the periphery and the other to the spinal cord.

Bipolar: An axon and a single dendrite on opposite ends of the soma– are specialized sensory neurons for the

transmission of special senses,hence abundant in sensory pathways for smell, sight, taste, hearing and vestibular functions

Multipolar: An Axon along with more than two dendrites– Multipolar neurons constitute the majority of

neurons in the 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

the central nervous system Efferent neurons –

– Also called as motor neurons.– Carry nerve impulses away from the central

nervous system to effectors such as muscles or glands.

– According to their targets, motor neurons are classified into three broad categories: Somatic motor neurons, Special visceral motor neurons ,General visceral motor neurons.

–Somatic motor neurons are further divided in to α motor neuron (innervating extrafusal muscle fibre) and γ motor neuron (innervating intrafusal muscle fibre)

Interneuron-– also called as relay neuron or local circuit

neuron.– connects afferent neurons and efferent

neurons in neural pathways.

BASED ON NEUROTRANSMITTER PRODUCTION

Cholinergic neurons —secreting acetylcholine GABAergic neurons — secreting gamma aminobutyric

acid. Glutamatergic neuron — secreting glutamate Dopaminergic neurons — secreting dopamine . Loss of

dopamine neurons in the substantia nigra has been linked to Parkinson's disease

Serotonergic neurons — secreting serotonin. A lack of serotonin at postsynaptic neurons has been linked to depression.

BASED ON UNIQUE SHAPE AND FUNCTION

Betz cells – large motor neurons located within the fifth layer of the grey matter in the primary motor cortex, M1.

Purkinje cells - some of the largest neurons in the human brain, found within the Purkinje layer in the cerebellum.

Renshaw cells - neurons with both ends linked to alpha motor neurons. Target of the toxin of Clostridium tetani

Pyramidal neurons (pyramidal cells) - type of neurons with triangular soma found in areas of the brain including cerebral cortex, the hippocampus, and in the amygdala.

Basket cells - inhibitory GABAergic interneurons found in several brain regions: the molecular layer of the 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 a group of axons bathed in endoneurial fluid.

• Between the fascicles is a fatty material called the interfascicular  epineurium.  The nerve is then wrapped in the main epineurium

Neuronal communication

Neurons are the information/signal relay system of our nervous system

Once stimulated neurons need to conduct information in two ways:1. From one end of a neuron to the other end.2. Across the minute space separating one neuron

from another neuron/muscle end plate (synaptic cleft).

The 1st is accomplished electrically via Action Potential generation.

The 2nd is accomplished chemically via neurotransmitters

Electrical conductionRESTING MEMBRANE POTENTIAL• The relatively static membrane potential of

quiescent cells is called the resting membrane potential.

• Resting membrane potential of nerve cell = -70 mV

• Resting membrane potential is maintained by the ionic distribution 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 voltage gated channels.

GENERATION OF ACTION POTENTIAL AND IT’S CONDUCTION IN NEURON:• A excitatory stumulus generates at a dendrite of

neuron• This transmitter acts on the membrane excitatory

receptor to increase the membrane’s permeability to Na+.

• Influx of Na+ inside the cell, causing resting membrane potential to move toward positive side.

• This positive increase in voltage above the normal resting neuronal potential—that is, to a less negative value—is called the graded potential or excitatory postsynaptic potential (or EPSP).

• Dendrites and somata typically lack voltage-gated channels, which are found in abundance on the axon hillock and axolemma.So Action potential is not generated here.

• The positive charge carried by the Na+ spreads as a wave of depolarization through the cytoplasm in the form EPSP.

• If the initial amplitude of the EPSP is sufficient, it will spread all the way to the axon hillock where Voltage-gated channels resides in high number.

• At the axon hillock If the arriving potential change is suprathreshold, an Action potential will be initiated and it will travel down the axon to the synaptic knob where it will cause NT exocytosis.

• If the potential change is subthreshold, then no AP will ensue and nothing will happen.

• The EPSP can undergo spatial summation or temporal summataion so as to reach suprathreshold level and excite an action potential.

• Temporal summation :The same presynaptic neuron stimulates the postsynaptic neuron multiple times in a brief period. The depolarization resulting from the combination of all the EPSPs may be able to cause an AP.

• Spatial summation : Multiple neurons all stimulate a postsynaptic neuron resulting in a combination of EPSPs which may yield an AP

• If an AP gets generated at the axon hillock, it will travel all the way down to the synaptic knob.

• The manner in which it travels depends on whether the neuron is myelinated or unmyelinated.

• Unmyelinated neurons undergo the continuous conduction of an AP whereas myelinated neurons undergo saltatory conduction of an AP.

• In continous conduction, the wave of de- and repolarization simply travels from one patch of membrane to the next adjacent patch.

• Action potential propagation in unmyelinated axon

• Saltatory conduction (from the Latin saltare, to hop or leap) is

the propagation of action potentials along myelinated axons from one node of Ranvier to the next node.

• It ncreasing the conduction velocity of action potentials without needing to increase the diameter of an axon.

• MS destruction of mylin sheath by own immune system (progressive loss of signal conduction, muscle control & brain function)

Chemical conduction

• One neuron will transmit information to another neuron or to a muscle or gland cell by releasing

chemicals called neurotransmitters.• 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 axon terminal of the presynaptic cell and causes V-gated Ca2+ channels to open.

• Ca2+ rushes in, binds to regulatory proteins & initiates NT exocytosis.

• NTs diffuse across the synaptic cleft and then bind to receptors on the postsynaptic membrane and initiate some sort of response on the postsynaptic 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 a graded depolarization.

• Some NTs cause anion channels to open, which results in a graded hyperpolarization.

• A graded depolarization will bring the neuronal VM closer to threshold. Thus, it’s often referred to as an excitatory postsynaptic potential or EPSP

• Graded hyperpolarizations bring the neuronal VM farther away from threshold and thus are referred to as inhibitory postsynaptic potentials or IPSPs

• EPSP and IPSP

THANX FOR THE PATIENCE VIEWINGA PRESENTATION BY ……. Dr ANWAR H

SIDDIQUI JR II