Download Nerves and how they work File

NERVES AND HOW THEY
WORK
Pedro Amarante Andrade, PhD
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BIOSCIENCES
FOR SPEECH AND LANGUAGE THERAPY
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DISORDERS AFFECTING NERVES,
MUSCLE CELLS AND THE FUNCTION
OF THE NEUROMUSCULAR JUNCTION
•
•
•
•
•
•
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Multiple Sclerosis
Motor neurone disease
Myasthenia gravis
Muscular dystrophies
Guillain-Barre´ syndrome
Parkinson’s disease
Alzheimer’s disease
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ANATOMICAL COMPONENTS OF
THE NERVOUS SYSTEM
Central Nervous System
CNS
Peripheral Nervous System
PNS
Sensory
Division
Brain
Somatic
Spinal
Cord
Autonomic
(visceral)
AFFERENT
Motor
Division
Somatic
Autonomic (visceral)
Sympathetic
Division
Parasympathetic
Division
EFFERENT
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NEURONS
Recap
hubpages.com
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STRUCTURAL CLASSIFICATION OF
NEURONS
Nervous system:
• Neurons
• NeuroGlial cells
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GLIAL CELLS IN THE PNS:
SCHWANN CELLS AND SATELLITE CELLS
http://medicaldictionary.thefreedictionary.com/neurilemma
www.studyblue.com
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GLIAL CELLS IN THE CNS
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GLIAL CELLS IN THE CNS
Facepunch.com
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MYELINATED vs. UNMYELINATED
NEURONS
The majority of neurons are myelinated
Even so-called unmyelinated neurons have a
thin coat of neuroglial plasma membrane
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JACK OSBOURNE, 27
MULTIPLE SCLEROSIS
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MS: A DEMYELINATING DISEASE
• Progressive destruction of the myelin sheath
in the CNS
myhealth.alberta.ca
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NEURONAL
ACTIVATION
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NEURONS
EXCITABLE CELLS
• Neurons are one of the types of excitable cell
in the body
• Muscles are also excitable cell
• Communication between a neuron and
another neuron, or a neuron or
muscle/glandular cell, is in the form of
electrical signals – called action potentials and
graded potentials
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MEMBRANE POTENTIAL
www.leinonen.net
Humanbiologylab.pbworks.com
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ACTION POTENTIAL
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THE STORY SO FAR
• O2, CO2, H20 and certain lipid soluble substances
can pass directly through the membrane
• Channels exist in the cell membrane to allow the
passage of substances including ions and water
– Integral proteins contain pores that allow certain
substances through with their concentration gradient
e.g. K+ (these are also called leakage channels)
– Other proteins act as transporters
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TWO MORE TYPES OF ION
CHANNELS IN THE MEMBRANE
OF NEURONS AND MUSCLE
FIBRES:
Voltage-gated channels and
ligand-gated channels
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VOLTAGE GATED CHANNELS
Open in response to changes in voltage
(changes in the current flowing along the axon)
www.sciencejon.blogspot.com
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LIGAND GATED CHANNELS
Opened by extracellular chemical transmitters
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ACTION POTENTIALS
• Waves of electrical activity propagated down
the axons
• SUDDEN depolarisation of the membrane
• i.e. the membrane potential is decreased and
eventually reversed
• Followed by repolarisation ie restored to its
previous state
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ACTION POTENTIALS
www.psychlopedia.wikispaces.com
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ACTION POTENTIALS
Summary
www.hyperphysics.phy-astr.gsu.edu
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ACTION POTENTIAL:
PROPAGATION
• Depolarisation is local at first
• Spreads down the axon in one direction
• current flows passively but as it moves along the
axon it opens the voltage-gated Na+ channels
• Action potential is propagated along the axon
• Na+ channels close quickly and the membrane is
then unresponsive or refractory
• This means the action potential can only travel
in one direction
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ACTION POTENTIAL:
PROPAGATION
• Action potential is self-propagating i.e. once it
starts in carries on moving down the axon
• Action potential stays at the same strength as
it travels down the axon
• Above is the method of propagation in
unmyelinated neurons
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MYELINATED NEURONS
• The myelin sheath forms an insulating layer
• At intervals (the nodes of Ranvier) there are many
Na+ channels
• Depolarisation at one node passes current to next
node without any loss of current as the neuron is
so well insulated
• This apparent ‘leaping’ of the current is called
saltatory conjunction
• As a result, rapid conduction velocity in
myelinated neurons
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INTENSITY OF RESPONSE
• Two factors: the frequency of impulses
generated at the trigger zone (hillock)
• The number of individual neurons activated
by the stimulus
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INTENSITY OF RESPONSE
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INTENSITY OF RESPONSE
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Communication between neurons and
between neurons and target tissue
• Neurons do not directly touch one another
• Neither to their directly touch their target tissue i.e. a
muscle cell or glandular tissue
• There is a minute gap called the …………………?
• The action potential does not jump across the gap or
cleft
• Rather, it is converted into a chemical ‘signal’ by a
neurotransmitter
• Then the action potential is re-activated in the postsynaptic neuron if the neurotransmitter is excitatory
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NEUROTRANSMITTERS
• When sufficiently stimulated, pre-synaptic axons
discharge neurotransmitter substances into the
synaptic cleft
• The neurotransmitter is a chemical substance
• Acts like a key
• The post-synaptic neuron will respond if the ‘key’ fits
its ‘lock’ i.e. the receptor site
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NEUROTRANSMITTERS
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SYNAPSES
http://www.agentur-tagtraum.ch
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SYNAPSES
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WHAT HAPPENS WHEN THE ACTION POTENTIAL
REACHES THE END OF THE AXON?
www.anthropology.net
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NEUROTRANSMITTERS
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NEUROTRANSMITTERS
• There are many types
• Some are found in the CNS (the …?..& the
……?…..)
• Others in the peripheral NS (the …?..& the
……?…..)
• Some in both CNS & PNS
• Some are EXCITATORY and some are
INHIBITORY
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DIRECTED STUDY 2
Neurotransmitter
Where found
Excitatory/inhibitory
Acetylcholine
motor end plate
excitatory
Dopamine
Adrenaline
Noradrenaline
Glutamate
γ-aminobutyric acid (GABA)
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EXCITATORY AND INHIBITORY
ACTIONS OF NEUROTRANSMITTERS
• Excitatory post-synaptic potential (EPSP) is set
up if the neurotransmitter activates Na+ channels
on the post synaptic membrane
• An EPSP depolarises the post-synaptic membrane
• Inhibitory post-synaptic potential (IPSP) is set up
if the neurotransmitter activates Cl- channels in
the post-synaptic membrane
• And IPSP hyperpolarises the post-synaptic
membrane
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IN SUMMARY
• EPSPs depolarise the post-synaptic membrane
and the action potential is generated in the
next cell
• IPSP’s hyperpolarise the post-synaptic
membrane, and no AP is generated
• See pages 77 in A & McH
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CHRISTOPHER, 6
MYASTHENIA GRAVIS
For more info: www.mga-charity.org
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MYASTHENIA GRAVIS
• https://www.youtube.com/watch?v=XdpW5R
Mp-T0
• Directed study:
• What is the name of the medication available
to help with the symptoms of MG? how does
it work?
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SAMPLE EXAM QUESTIONS
• Which of the following neurotransmitters are
excitatory and which are inhibitory?
Excitatory (E)
or Inhibitory (I)?
epinephrine
acetylcholine
gamma-aminobutyric acid
glutamate
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WHAT TO DO NEXT
• Revise the topics so far
• Answer the directed study questions posed in this
lecture
• Ensure you understand the structure and the
function of the cell membrane, and how these are
adapted in the case of neurons and muscle fibres
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RECAP
• Depolarisation wave arrives at …..…terminal
axon
• …………-gated Ca2+ open
• Results in influx of ………….
• Triggers release of neurotransmitter
• Neurotransmitter crosses the synaptic ……..
And binds to ………….gated channel on the
…….synaptic membrane
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DIRECTED STUDY 1: PERIPHERAL
NERVE CLASSIFICATION AND
FUNCTION
• The thickness of the neuron, and whether or
not it is myelinated, determines its specific
function in the nervous system.
• For example, Aδ fibres are thin, myelinated
slow-conducting afferents for pain &
temperature
• See page 73 in Atkinson & McHanwell (2002)
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