Anatomy, composition and physiology of neuron, dendrite, axon, and synapses

Glial cells/ Astrocyets, Oligodendrocyt , Schwan cells/

Support neuronal cells Produce myelin Act as scavengers Takes up released neurotransmitters Guide migrating neurons and the out growth of axons Form BBB Release growth factor and help nourish nerve cells

Neurons type

Unipolar cells: single process serving as receptor and releasing terminal e.g. In autonomic nervous system

Biplar cells : two process dendrite and axone.g. in retina , olfactory nerves

Pseudo_unipolar cells: e.g. dorsal root ganglia

Multipolar cells : most common typee.g. spinal motor

neurons ,pyramidal cells,pukrinje cells NERONS COULD ALSO BE CLASSIFIED AS :

SENSORY MOTOR

LOCAL INTERNEURONPROJECTION

INTERNEURONNEUROENDOCRINE CELL

Structure of a neuron

Cell body Dendrites:

apical and basal types, are input elements together with the cell body contains nucleus and gives rise to axon

and dendrite

Axons: transmitting element, could be longer than 3m, covered with myelin interrupted by node of Ranvier is the out put element of neuron single axon may form synapses with as many as 100000 neurons

Axon hillock: initial segment of neuron Synapse:

presynaptic terminal, synaptic cleft , postsynaptic membrane

dendrites cell body axon hillock muscle

synaptic connections could be divergent or convergent

Functional organization of neurons

Input component /receptor or synaptic potential/ : signal electrical input signal is graded in amplitude and duration, proportional to amplitude and duration of stimulus

Integrative component : signal electrical action potential is generated only if input signal is greater than

spike threshold stimulus intensity is represented by frequency of action

potential duration of stimulus is represented by number of action

potentials

Functional organization of neurons

Conductile component/action potential/ : Signal is electrical action potentials are all or none every action potentials have same amplitude and duration

information in the signal is represented by frequency and duration Out put component :

signal chemical transmitter total number of action potential determine how much

neurotransmitter should be released

Comparison of local and propagated signals

Input signals

amplitude smallduration brief

summationgraded signal effect depolarizing propagation

passive

Action potential

amplitude largeduration brief

summationall/none signal effect depolarize

propagation

active

Cytology of Neurons

Nucleus Nuclear envelope Cytoplasm

cytosol including cytoskeletal matrix membranous organelle

Plasmalemma myelin

Membranous organelles

Mitochondria and peroxisomes Rough endoplasmic reticulum/smooth endoplasmic reticulum Golgi complex Secretory vesicles, endosomes, lysosomes Most of these structures are abundant in the cell body and

dendrite and there are no synthetic function differences between cell body and dendrite.

The axon has few mitochondria and smooth endoplasmic reticulum with abundant secratory vesicles

Synthesis and trafficking of neural proteins

Most proteins are synthesized in the cell body The neuron express more the total genetic material than any

other organ Neural cells are engaged in protein synthesis more often than

other cells and hence their chromosomes uncoiled Ribosomal and m RNA are synthesized in the nucleus and

exported through nuclear pore Some genetic information is also contained in the mitochondria Protein synthesis occurs in cytosol where mRNA ribosome and

tRNA form complex Secretory proteins and vacuolar apparatus and plasmalemma

are synthesized and modified in the endoplasmic reticulum Secretory proteins are processed further in the golgi complex

and then transported

cytoskeleton

Microtubules largest diameter fibershelical cylinders

made of protofilaments undergo cycles of

polymerization and depolymerization

Neurofilaments monomers twist to form dimerprotofilamentprotofibrilfilament

Microfilaments polymerized actin monomerssmallest-diameter fibers

undergo cycles of polymerization and depolymerization

Anterograde Axonal Transport

Fast phase of axonal transport 70-400 mm/d 20-70mm/d 4-20 mm/d Convey mainly plasma membrane proteins such as

acetyl cholinesterase mitochondria multivesicular bodies and secretoty vesicles ATPases

The physiological properties of fast phase axonal transport has been important for tracing connections in the brain

Slow phase of axonal transport 1-4 mm/d 0.2-1.2mm/d Conveys mainly

Neurofilaments and Microtubulins Actin and certain glycolytic enzymes

Retrograde Axonal Transport

Toxins, drugs, heavy metals Neurotropic viruses Nerve Growth Factors and neurotrophines Mitochondria, endosomes

Axonal vs dendrite transport

Axonal Nerofilaments abundant Microtubules are of tau type Microtubules are uniformly

arranged

Dendrite Microtubules abundant MAP-2 type Microtubules are

bidirectionaly arranged These difference in

arrangement explain the

polarization of organelles

myelin

Insulate axons and facilitates speed of action potential transmission

Arranged in concentric bimolecular layers Has a composition similar to plasma membranes Schwan cells form myelin of peripheral nerves and

Oligodendrocyts that of central nerves Schwan cells express their myelin gene in response to contact

with axon while Oligodendrocytes depend also on the presence of Astrocytes

Myelin Proteins

Myelin Basic Protein important for myelin compactionstrongly immunogenicused to produce experimental

allergic encephalomyelitis Myelin Associated glycoprotein supper family of immunoglobulin

involved in cell to cell recognitionis an adhesion molecule that

initiate myelination Protiolipids important for compaction of myelin

mutation in the gene causes hypomyelination and

degeneration Myelin protein zero major protein in peripheral myelin

immunoglobulin familyimportant for myelin compactionmice that lack the protein have poor

motor coordination Peripheral myelin protein 22 encoded by chromosome 17

DNA duplication results in CMT disease

Ion channels

Conduct ions at fast rate Selective for specific ions Ion channels are proteins that span the cell mme Flux of ions through the ion channel is passive Opening and closing of ion channel involves conformational change Open and close in response to specific stimulus

Voltage –gated Ligand –gated Mechanically –gated Gap-junction channels

The binding of exogenous ligands /toxins, poisons and drugs/can make channels open or close

Ion channels are composed of several subunits Channels are also important targets of diseases

myasthenia gravis hyperkalemic periodic paralysis

Synaptic transmission

The average neuron makes 100000 connections Two basic forms of transmission

Chemical Electrical

Electrical transmissions are Short lasting Only excitatory Do not induce long lasting postsynaptic changes Gap junction channels Bidirectional transmission

Chemical transmissions are Variable signaling :inhibitory or excitatory Produce complex behavior Longer lasting / delay in transmission Amplify signals Modify post synaptic receptors both functionally and anatomically Ionotropic receptors :conformational change that opens the channels on binding

transmitter Metabotropic receptors: act by altering intracellular metabolic reaction

Chemical transmitters

Classical Acetylcholine Cathecolamines Glutamates GABA Serotonine histamine

Peptides Substance p Enkephaline Endorphine Prolactin, oxytocine, vasopresin

Soluble gases NO

Cellular basis of connectionist approach

Principle of dynamic polarization : electrical signals within a nerve flow only in one direction

Principle of connectional specificity : nerve cells do not connect indiscriminately with one another to from a network

Specificity and modifiability of neuronal connections

Specific networks Brain has at least two types of neuronal map/ motor and sensory

maps/ which are interconnected with each other by interneuron. The neurons that make up these map do not differ greatly in their electrical properties. Rather, They have different function because of the connections they make.

Parallel processing: deployment of several neuron groups or several pathways to

convey similar information Plasticity :

functional transformation in neurons as a result of appropriate stimulation

How dose nerve cells differ ?

Lack of axon Location of synaptic in puts on the cell

cell body dendrite axon hillock type of target cell

Difference in cell body size and shape, distribution of axon and dendrite tree

Expressing different combination of ion channels providing them with different thresholds ,excitability and firing

patterns. Thus ,neurons with different ion channels encode the same class of

synaptic potential into different firing patterns and thereby convey different signals

Cont.

Chemical transmitter The type of neurotransmitter they use The type of Receptors they have

Myelin content Location in the nervous system –central/peripheral These differences and others may account for different

patterns of disease