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,SEC.B-01 : IMPULSE CONDUCTION IN THE HUMAN NERVOUS SYSTEM
Parts of a neuron
Dendrites: look like tree roots, receive messages from other neurons and surround the cell body
Cell body: also called the soma. It receives and sends messages in the form of new impulses
Cell nucleus: control centre of the cell; it controls all metabolic activities
Axon: carries messages from the soma. It is a thin fibre that differs in length
Myelin: white fatty sheath insulating the axon. It conducts much faster than un-myelinated axons. Multiple
sclerosis is the effect of axons that aren’t properly myelinated, as it attacks the myelin
Axon terminals: also called telodendria. Axon terminals branch out from the axon and end in small knobs, called
boutons (buttons in French).
Vesicles: tiny containers in the boutons which are filled with neurotransmitters
Neurotransmitters: chemical substances that play an important role in the conduction of a message from one
neuron to the next
Types of neurons
Sensory/afferent: carry messages from the environment to the brain/spinal cord. Info is detected by the senses. This
info can also come from the organs in the body
Motor/efferent: conduct messages from the spinal cord/brain to the muscles and glands
Nerve tract: bundle of axons in the brain/spinal cord
Nerve: bundle of nerves outside the brain and spinal cord
Process of impulse conduction
Stimulus is a form of energy received by the senses and converted into a form of energy understandable to the
nervous system. Impulse conduction is the basic form of sending info in the nervous system. Two main processes:
• Electrical – nerve impulse begins in the first segment of the axon and travels down the axon to the terminals
because of electrical events at the cell membrane
• Chemical – the passage of the nerve impulse from one axon to another. There is a small gap between the
axon terminals of one neuron and the dendrites of another and the chemical process will determine whether
it reaches it or not
Nerve impulse: each neuron is like a tiny battery that stores potential energy. The fluid inside and outside the cell
contains small chemical particles called ions that are electrically charged. Some are positive and some are negative.
There are more positive ions on the outside of the cell and more negative ones on the inside and they naturally
constantly move from an area of high concentration to an area of low concentration. Also, opposites attract and like
repel.
Resting membrane potential: condition of readiness before an impulse can fire. It is an electrical charge brought
about by the difference between the positive and negative ions inside and outside the cell. The neuron is ready to
receive or conduct impulses
Action potential: messages arriving from other neurons alter the resting potential. If the resting potential changes
enough the cell reaches a threshold or critical point. Each neuron has a different threshold and the stimulus has to
be intense enough to exceed the threshold and change the resting potential into action potential. In this way the
structure of the axon membrane changes: tiny openings in the cell membrane allow ions from the outside of the cell
to move inside. These channels first open near the soma and then sweep along the length of the axon as the action
potential (and thus the impulse)sweeps along.
Refractory period: immediately after an impulse has been conducted, the neuron is not ready to send another
message until the resting potential has been restored. Two types of refractory periods:
• Absolute – no impulse can be generated
• Relative – an impulse can be generated but only with very intense stiulus
Refractory periods ensure that stimulus only travel one way and also prevent over-stimulation
Characteristics of impulse conduction: ‘all or nothing’ event. The stimulus is either strong enough to result in
impulse conduction or it isn’t. the strength of the stimulus does not change the strength or speed of the impulse.
• Strength + speed: the strength + speed of impulse conduction is constant within a particular neuron but it
can vary with nerve fibres of different sizes. The larger the fibre, the stronger and faster the impulse can be
conducted.
, • Frequency: the intensity of the stimulus does make a difference in terms of the frequency at which impulses
are conducted. If the stimulus is very strong there is a shorter space between the firing of each impulse so
the frequency increases.
• Effect of myelination: myelin sheaths insulate the axons and make action potentials travel much faster than
along un-myelinated axons. There are gaps or nodes between the sheaths and it is in these nodes that ion
channels open and the impulse is conducted by jumping from node to node (saltatory conduction)
Synaptic transmission of impulses: the conduction of a nerve impulse in a neuron is electrical, but between neurons
it is chemical. There is a tiny gap between neurons called a synapse. When an action potential reaches the tip of the
axon terminals, the vesicles attach themselves to the presynaptic membrane where the membrane opens and causes
chemicals to be released into the little space between the neurons called the synaptic cleft. This is the gap between
the presynaptic membrane of one neuron and the postsynaptic membrane of another. These chemicals are called
neurotransmitters. These chemicals combine with the fluid outside the cells and receptors in the postsynaptic
membrane. Different neurons use different chemicals as their neurotransmitters but each neuron releases the same
chemical from all branches of its axon.
Postsynaptic potentials: Neurons that excite can make the next neuron more likely to produce an action potential.
The action potential in the next neuron is called postsynaptic potential. Others neurons release neurotransmitters
than inhibit the production of an action potential in the next neuron. Once the neurotransmitter excites/inhibits a
receptor in the next neuron, it can become reabsorbed by the axon that released it (re-uptake), it could diffuse
away, it could be broken up by enzymes, or it could bounce around for a while and then return to the postsynaptic
receptor again. The longer the neurotransmitter stays in the synaptic cleft, the more likely it is to affect the next
neuron. A postsynaptic potential will only be generated if the amount of neurotransmitters discharged into the
synapse are large enough.
Postsynaptic potential is a graded potential. The impulse will get weaker as it travels further from the point of
stimulation. If the impulse is no reinforced or strengthened, it may disappear before it reaches the next axon and
then no postsynaptic potential is generated. Even a weak impulse can be strengthened by additional
neurotransmitters:
• Spatial summation – action potentials from the terminals of several axons reaching the same synapse at one
time or closely together. This results in the accumulation of the neurotransmitter in the synaptic cleft and
makes more of it available.
• Temporal summation – frequent action potentials along the same axon that allow the discharge of more of
the neurotransmitter to reinforce the postsynaptic potential. Increases the chance of a neuron firing in the
case of excitatory and decreases the chance in the case of inhibitory.
Nature of neurotransmitters: whether a neurotransmitter has excitatory or inhibitory effects depends on:
• Nature of the neurotransmitter
• Place where it acts
• Quantity of the neurotransmitter in relation to the enzymes that destroy it
• Amount of inhibitory neurotransmitter in relation to the amount of excitatory neurotransmitter at a
particular synapse
What are neurotransmitters?
• Chemicals that are present in/made by neurons
• When a neuron is active the chemical is released and produces a response in the target cell
• There is a mechanism for removing the neurotransmitter form the synaptic cleft once its work is done
Classic neurotransmitters
• Acetylcholine (Ach) – released by cells in the brain and spinal cord as well as by the parasympathetic
nerves. Effects: causes skeletal muscles to contract. It is also believed to be related to memory because it
supports normal wakeful behaviour and mental alertness. An insufficiency has been found in some brain
areas of patients suffering from Alzheimers. It may explain the decline in cognitive functioning
• Adrenalin (epinephrine) – released by the sympathetic nerves and the adrenal glands. Increases heart beat
and the contraction of blood vessels, skeletal muscles and heart muscle. Speeds up metabolism and the
release of glucose in the blood
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