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Summary CCEA GCE Biology - Chapter 4 - Neurones and Synapses Mindmap
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Summary of 1 pages for the course Unit A2 1 - Physiology and Ecosystems at CCEA (Based on Chapter 4)
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Nicotine Stimulates the release of acetylcholine and otherneurotransmitters making action potentials more likely. 1. Impulse arrives at the synaptic bulb – depolarisation of the membrane causes calcium channels
to open and Ca2+ ions to enter the synaptic bulb by diffusion
Curare Blocks receptors (at neuromuscular junctions) preventing 2. Ca2+ ions cause the vesicles containing neurotransmitter substance (usually acetylcholine) to
synaptic transmission - loss of muscle function move to the pre-synaptic membrane
3. Vesicles fuse with the pre-synaptic membrane releasing acetylcholine into the synaptic cleft
Opioids Block the calcium channels in the pre-synaptic neurone. Less (exocytosis)
transmitter substance is released and action potentials less 4. Acetylcholine diffuses across the synaptic cleft – acetylcholine binds to receptors in the post–
likely. Opioids and related compounds can provide pain relief synaptic membrane
by reducing impulses coming from the pain receptors 5. This causes the opening of (sodium) ion channels in the post-synaptic membrane – positive
(sodium) ions diffuse into the post-synaptic neurone - the membrane gradually depolarises and an
excitatory post-synaptic potential (EPSP) is generated
6. If sufficient depolarisation occurs ( depending on the number of neurotransmitter molecules
• Acetylcholine is the primary transmiter substance in the central filling the receptors) the EPSP will reach the threshold intensity required to produce an AP in the
nervous system (CNS) of vertebrates, although noradrenaline (typically post-synaptic membrane
used in involuntarily nervous control, for example, the regulation of gut 7. Acetylcholine must be removed form the post-synaptic membrane, to prevent it continuously
movements) is one of several other types. However, each synaptic bulb generating a new AP in the post-synaptic neurone – the enzyme acetylcholinesterase which is
produces only one type of neurotransmitter. attached to the post-synaptic membrane, breaks down acetylcholine into choline and ethanoic acid
• GABA (aminobutyric acid) is an example of a neurotransmiter that is (acetyl) which are released into the synaptic cleft
released at inhibitory synapses. It is the chief inhibitory 8. Choline and ethanoic acid diffuse across the cleft and are reabsorbed by the pre-synaptic bulb
neurotransmitter in mammals. GABA causes negative ions to flow into -choline and ethanoic acid are resynthesised into acetylcholine and stored in the synaptic vesicles
post-synaptic neurones, thereby causing hyperpolarisation as described to be used again - the mitochondria provide ATP required for the process
above. GABA has many roles including helping reduce anxiety and 'panic
attacks' through a 'd amping down' of the ner ve pathways in the brain.
Function of synapses
Transmission of an impulse across the synapse
The ner vous system is based on a system of neurones that The diffusion of chemicals across a short gap is slower than the conduction of an impulse
transmit electrical ner ve impulses throughout the body. Fine along myelinated neurones. However, due to the very short distances involved it is still very
control and integration is provided through a system of Junctions bet ween the axon of one neurone fast! Nonetheless, the presence of synapses system many advantages.
synapses bet ween neurones that control the ner ve pathways and the dendrite/dendron/centron of the • Synapses enable nerve impulses to pass from neurone to neurone - Synapses allow nervous
The synapse next, those found bet ween neurones and communication to continue throughout the body, even though the nerve 'hardware' (neurones)
involved. Ner vous control is faster and more precise than
hormone action - it involves receptors and effectors with an muscle are – neuromuscular junctions. is not continuous.
interlinking coordinator. Receptors are sensitive to a • They ensure unidirectionality - Nerve impulses can only pass from the pre-synaptic neurone
particular type of stimulus. Effectors are parts of the body to the post-synaptic neurone, as the neurotransmitter is only made in the pre-synaptic
Synaptic transmission and the
that produce a response, eg. muscles. Co-ordination involves neurone and neurotransmitter receptors are only in the membrane of the post-synaptic
structure of synapses
the CNS (central ner vous system). neurone.
• They prevent the overstimulation of effectors (for example, muscles) - Too many impulses
• Motor neurones > carry impulses from the CNS to effectors. Neurones + synapses passing along the same neurone in a short period of time will exhaust the supply of the
• Sensory neurones > carry impulses from receptors to the CNS. neurotransmitter more quickly than it can be built up - the synapses fatigue.
Important factors of ner ve impulses • Connector (relay, immediate or intermediate) neurones > connect neurones within the CNS. • They provide integration - This may involve a number of pre-synaptic neurones forming
junctions with one post-synaptic neurone. In effect, synapses provide flexibility - fi there were
Neurones have a cell body
Cell body
no synapses, nervous activity would be little more than a series of reflexes with a particular
:
• Threshold
·
Schwann cells
An impulse is generated when a stimulus reaches and an extended ner ve Myelin sheath stimulus producing an automatic and never changing response. Integration is aided through the
critical level called threshold stimulus: fibre. The cell body Dendrites
Dendron Axon
process of summation, which is discussed in the next section.
Stimulus < threshold there will be NO impulse (centron) contains a Synaptic bulbs Direction of impulse
isn
+19,
nucleus, mitochondria and
&
Stimulus > threshold all impulses are the same size Axon
Sensory neurone
• The All or Nothing Law other organelles, as well Nucleus
Summation is important in providing the complexity and flexibility that synapses demonstrate. For example, an infrequent action
Once the threshold stimulus is reached the action as Nissl's granules (large Cell body
Nodes of ranvier
Direction of impulse &
= potential reaching a synapse may not be sufficient to caues an action potential in the adjacent post-synaptic neurone. However, a
ribosome groups). An axon
isn
+19 series of impulses traveling along the same neurone or a number of pre-synaptic neurones operating in unison, each releasing
potential occurs. All impulses are the same size, no
, Direction of impulse
Motor neurone
isn
+19
Connector
,
matter how big the stimulus is. This is known as carries impulses away neurone neurotransmitter chemicals, may be enough to cause a sufficient EPSP to trigger an impulse in the post-synaptic neurone.
the all-or-nothing-law. Strong stimuli produce a from the cell body. The synapses discussed so far refer to excitatory synapses - neurotransmitter chemicals are released with the function of causing
greater frequency of action potentials A dendron carries impulses to the cell body. Dendrites are very small extensions. Axons terminate in an EPSP and a subsequent action potential. Inhibitory synapses have the function of making it more difficult for synaptic
synaptic bulbs. Many ner ve fibres are myelinated eg. Covered w/ an insulating myelin sheath - which is transmission to take place. The neurotransmitter they release makes it more difficult for an EPSP to form in the post-synaptic
Propagation of the nerve impulse rich in the lipid myelin and made from schwaan cells, they are arranged at inter vals along the ner ve fibre membrane. The inhibitory neurotransmitters lead to an influx of negative ions in the post-synaptic membrane, making the inside of
• (Generation starts the impulse, propagation is how it moves with gaps in bet ween the cells, called nodes of ranvier.it is protective and speeds up ner vous conduction. the membrane even more negative, thus creating an inhibitory post-synaptic potential (IPSP) in the post-synaptic neurone which is
along the neurone) even more negative than the normal resting potential. Consequently, this hyper-polarisation makes it even more difficult than
• Once generated the AP moves rapidly along the neurone as a Resting potential > Neurones have a potential difference across their cell surface normal for excitatory synapses to produce an EPSP that reaches threshold level.
wave of depolarisation. membrane called a resting potential, ie. the neurones are polarised as there is an Whether an impulse will actually take place inthe post-synaptic neurone depends on the relative contribution excitatory and
----
• The area immediately behind the depolarised zone of the AP electrochemical gradient across the membrane. The potential difference is caused by inhibitory synapses make in promoting or inhibiting depolarisation
quickly repolarises -
.....
there being an excess of positively charged ions (NA+) outside the membrane Why have inhibitory synapses? They can help by reducing the input of background stimuli that would clutter up the ner vous
• As one part of the membrane depolarises it sets up local compared to the inside. At rest the outside of the neurone is positive relative to the activity in the brain or may prevent some reflex actions.
(electrical) circuits with the area immediately on either side inside, with a potential difference of around -70mV which is maintained as the cell The effects of summation and the action of inhibitory synapses provide integration and fine control through the synapse
• Positive ions from the depolarised zone diffuse passively along surface membrane is largely impermeable to the flow of sodium ions when not integrating all the diferent inputs (there may be hundreds) at synapse junctions. Synapses also have an important role in filtering
the inside of the axon towards the polarised zone in front of it Resting potential out low-level background stimuli thus preventing overload and overstimulation.
conducting an impulse (if it was permeable the Na+ ions would diffuse down the
(rather than across it) concentration gradient.
• Ions also move from the depolarised to polarised zones on the Action Potential > When a neurone is stimulated the cell surface membrane becomes
outside of the membrane – these processes occur continuously permeable to ions. There is an excess of positive ions outside the neurone relative to
creating the wave of polarisation the inside, they diffuse into the neurone down the concentration gradient. When the 1. RP inside axon -7mV relative to outside
potential difference decreases, a threshold is reached where the potential difference 2. Membrane permeable, positive ions diffuse
Factors affecting the speed of a nerve impulse is -55mV outside the cell surface membrane relative to the inside, because of this a across the membrane into the axon & depolarisation
• Myelin sheath – acts as an electrical insulator in myelinated neurones, number of gated ion channels open, rapidly increasing the rate of diffusion of ions starts
preventing depolarisation in that part of the neurone leading to depolarisation of the neurones. As positive ions flood into the cell, the 3. -55mV gated membrane channels open. Positive
• Depolarisation can occur at the nodes of Ranvier, which occur bet ween inside becomes positive relative to the outside reaching a potential difference of ions move across the membrane into the axon rapidly
adjacent Schwann cells very 1 to 2 mm – allowing local currents to be set up +40mV. At the peak of the action potential the recovery phase starts - the until inside is +40mV relative to outside. This is the
bet ween adjacent nodes, such that the impulse jumps from one node to the next refractory period - and the positive ions both diffuse and are pumped out of the ACTION POTENTIAL.
by passing sections of neurone ~ saltatory conduction and only occurs in neurone. This rapidly restores the resting potential and the cell surface membrane 4. Positive ions diffuse and are pumped out (AT) of
myelinated nerves (mammals) becomes largely impermeable again. During the refractory period, a further impulse the axon across the membrane. This is the
cannot occur as the gated ion channels are closed and the resting potential hasn’t REFRACTORY PERIOD. The membrane cannot be
1. DIAMETER OF THE AXON: thicker axons conduct impulses faster – less leakage of been restored. Refractory period is important as it; Ensures that the action depolarised until the RP is restored. There is a slight
ions across the membrane. Smaller diameters make it difficult to maintain the potentials are propagated in one direction. This is important because the axons are overshoot with the inside becoming more negative
potential gradient required to form the RP and AP. physiologically capable of transmitting an impulse in either direction, Limits the relative than the RP. This is HYPERPOLARISATION.
2. MYELINATED NERVES: can be narrower because they have relatively few ion number of action potentials that can be fired and ensures that each action potential
channels under the myelin sheath; they are concentrated Nodes of Ranvier. This is a discrete entity i.e. don’t want to mix successive action potentials up. At the end
overcomes the problem of having a small diameter, and produces faster speeds of of the refractory period there’s a slight ‘overshoot’ as the inside of the axon becomes
conduction. slightly more negative than the normal resting potential - hyper-polarisation. The
3. TEMPERATURE: affects the rate of diffusion of ions involved and therefore the entire electrochemical sequence takes 4 milliseconds.
speed of conduction; important in warm blooded animals
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