Download Structure of a Neuron Transmission of “Information” Nerve Impulse

Recapitulation
Nervous Tissue
and
Peripheral Nerves
• Central Nervous System (CNS)
– Brain and spinal cord
• Peripheral Nervous System (PNS)
– Peripheral nerves (cranial and spinal)
– Ganglia
• Cellular basis
– Neurons : Generate and propagate impulses
(action potentials)
– Neuroglia : supporting cells
ANHB 2212 – 2008
Avinash Bharadwaj
Structure of a Neuron
Transmission of “Information”
• Electrical transmission
• Body (‘soma’)
– Very fast
– However, not like conduction through electrical
wires!
– Main cytoplasmic mass with nucleus
• Nucleus euchromatic, nucleolus prominent
• Cytoplasm rich rER – seen as granules (Nissl
granules or substance)
• Chemical messengers
• Branched, ‘tree-like’ processes
– Dendrites
– Bring information towards the soma
– Across narrow gaps between cells
• Single, long process
–
–
–
–
Axon
Takes information away from the soma
Unbranched except at its end
Terminal part – swellings
Nerve Impulse – Fundamentals
• Resting membrane potential
– Interior – ve : ‘polarisation’
– Difference approx 70 mV,
expressed as –70mV
+++
+
- - - - - -+++
- - ++
-+
• Action potential
– Begins at one point, progresses like a wave
– Previously depolarised regions ‘repolarise’.
70 mV
More on Processes
• Dendrites
– Resemble the soma
– Can summate small changes
• Axons
– Long, thin
– Can only conduct action potential
Exceptions exist!
Processes that bring an impulse to the soma,
yet structurally and functionally are like axons.
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Communication Between Neurons
Chemical Synapse
• Neuron meets neuron!
• ‘Synapse’ : junction
• Majority of neuronal synapses use a chemical
mechanism for transmission.
N1
• May be between
• Presynaptic membrane
• Synaptic cleft
• Postsynaptic membrane
– Axon and a dendrite
– Axon and soma
– Axon and axon
(Only axo-dendritic shown here)
• Depolarisation, e.g. – 60
OR
hyperpolarisation e.g. – 85
N2
• Vesicles
• Neurotransmitter
• Receptors (Not shown)
• Great variety of neurotransmitters e.g.
Acetylcholine, Noradrenaline, γ-amino butyric acid
More on Structure
• Myelin sheath of an axon
• Schwann cell –
– membrane rolled around,
myelin deposited
Speed of Conduction
•
•
•
•
Non-myelinated fibres (Ø < 1.5µm) : < 2.5 m/sec
Thinly myelinated (Ø < 1 – 3 µm) : 5 – 15 m/sec
Medium
(Ø < 5 – 15 µm) : 30 – 80 m/sec
Thick
(Ø < 12 – 20 µm): 70 – 120 m/sec
Consider that the longest axons in the body measure
about 1 metre!
• Nodes of Ranvier (arrows)
• Action potential ‘jumps’
from node to node (saltatory conduction)
Maintenance Department!
•
•
•
•
Maintenance of ‘distant’ structures
Transport along axons
Fast and slow
Bidirectional
Neurons – Shapes and Sizes
– Unipolar / bipolar /
multipolar
– Stellate /
pyramidal … and
many more!
– 1 to 2 µm…
… 25 µm or more
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Neuroglia
Peripheral Nerves
• Glia : glue
• A variety of supporting cells
• In CNS
• Nerve fibre bundles with
connective tissue
• Epineurium
• Perineurium
• Endoneurium
• Myelin sheath
• “Axis cylinder”
– Astrocytes
– Microglia
– Oligodendrocytes…. And
more!
• Peripheral nerves
– Schwann cells – myelination
• No connective tissue in
CNS!!
A Typical Spinal Nerve
Roots
Rami
Dorsal
Ventral
(Motor)
Dorsal
(Sensory)
With ganglion
Ventral
Autonomic connections … later!
Last Slide
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