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Summary 15 Control and Coordination
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EVERYTHING you need to know about "Control and Coordination" for A-level biology CIE
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Neurones03 November 2021 11:16
• The nervous system consists of:
- The central nervous system (CNS), the brain and spinal cord
- The peripheral nervous system, a network of nerves spreading throughout the body
• A nerve consists of a bundle of cells called neurones
• There are three types of neurone, sensory neurones, intermediate neurones, and motor
neurones
Structure of a Motor Neurone
• Motor neurones carry impulses from the CNS to an effector (gland or muscle)
• Cell body, contains the nucleus, many mitochondria, and large amounts of ribosomes and
rough endoplasmic reticulum
• Dendrons, small extensions of the cell body. These split down into branched dendrites
which provide a large surface area to connect with a large number of other neurones.
These carry impulses towards the cell body
• Axon, single long fibre which carries nerve impulses away from the cell body
• Myelin sheath, made of the membrane of Schwaan cells wrapped around the axon.
Schwaan cells protect the axon and provide insulation. The membranes of Schwaan cells
are high in the lipid myelin
• Neurones can be myelinated or unmyelinated
• Node of Ranvier, gap between Schwaan cells where there is no myelin sheath (occur
about every 1-3mm along the axon in humans)
• Synaptic knob, swollen end of the axon which contains many mitochondria and vesicles
containing transmitter substances (or neurotransmitters)
Sensory Neurones
• Sensory neurones carry impulses from a receptor to the CNS
• They have one long dendron that carries the impulse towards the cell body (located
somewhere along its length, often in a swelling in the spinal cord called a ganglion), and
one long axon which carries the impulse away from the cell body like a motor neurone
Intermediate Neurones
• Intermediate neurones are found in the CNS and transmit impulses between neurones.
They have numerous short dendrites
Reflex Arcs
• The nerve pathway involved in a reflex action
• The three types of neurone link receptors to the CNS to effectors (glands and muscles)
• These can be linked in a reflex arc
• This does not allow for the brain to dictate the response to a stimulus, although it can be
made aware of both the stimulus and the response
• These reflex responses are fast, involuntary, and short-lived, and they protect the body
from harm
• When a stimulus is detected by a receptor, an impulse is sent along a sensory neurone to
the CNS
• Neurones are grouped together to form nerves; those neurones running to or from the
spinal cord form spinal nerves
• Each spinal nerve has two connections to the spinal cord; a dorsal (back) root by which
the sensory neurones enter the spinal cord, and a ventral (front) root by which motor
neurones leave the spinal cord
• The dorsal root displays a swelling or ganglion, to coincide with the location of the cell
bodies of the sensory neurones
• Within the spinal cord, impulses will be sent to the brain so that the individual is made
aware of what has occurred
• The sensory neurone will transmit the impulse to an intermediate neurone (via a synapse).
Then the impulse is transmitted to a motor neurone (via a synapse) which triggers the
effector to create a response
, Nerve Impulses
08 November 2021 15:36
• A nerve impulse may sometimes be (incorrectly) referred to as an "electrical" impulse, but
it is not caused by an electrical current (flow of e-)
• Instead impulses are created by changes in ion concentrations across the axon membrane,
a potential difference
• Hodgkin and Huxley's experiments showed:
- That a potential difference (difference in charge) existed across the axon membrane
- The inside of the axon was more negative with respect to the outside (-70 mV)
- Therefore the membrane is said to be polarised
- This potential difference is known as the resting potential and is actively maintained by
the neurone
Resting Potential
• The potential difference of -70 mV is maintained by the movement of Na+ and K+ across
the membrane
• Phospholipid bilayer prevents ions diffusing across it
• Ion channels can be opened or closed to change the permeability of the membrane to Na+
and K+
• Sodium-potassium pumps move Na+ and K+ across the membrane
How is the Resting Potential Established?:
• Sodium-potassium pumps actively transport 3 Na+ out of and 2 K+ into the axon
• Therefore more Na+ move into the surrounding tissue fluid compared with K+ entering the
cytoplasm
• This creates a potential difference across the membrane (the inside is more negative than
the outside). Large negatively charged molecules (eg, proteins) inside the axon also
contribute towards this
• Some Na+ will diffuse back into the axon and K+ also diffuse back out down their
concentration gradients through channel proteins
• However, the axon membrane is more permeable to K+ ions than Na+ (there are more K+
channels)
• This maintains the potential difference across the membrane. The membrane is said to be
polarised
• However, both sodium and potassium ions are positively charged so they also create an
electrical gradient
• Therefore, when establishing the resting potential, this creates a potential difference, but
also sets up an electrochemical gradient for Na+ and K+
Action Potentials
• An action potential is a temporary local reversal of the resting potential. The membrane is
said to be depolarised
Positive Feedback:
• As a few Na+ diffuse into the axon, the cytosol becomes more positive, so triggering some
voltage gated Na+ channels to open
• This triggers more voltage gated channels to open, so even more Na+ diffuse in, making it
even more positive
• This is an example of positive feedback
Formation of an Action Potential:
• A stimulus causes voltage sensitive gated Na+ channels to open
• Na+ rapidly diffuse into the axon down their electrochemical gradient
• Axon membrane becomes depolarised (becomes ≈ +30 mV)
• At peak of the action potential the depolarisation causes the Na+ channels to close and K+
channels to open
• K+ ions diffuse out of their axon down their electrochemical gradient and the interior of
the axon becomes less positive and then negative (the membrane is repolarised)
• Slight hyperpolarisation occurs (inside becomes more negative than the resting potential)
due to a slight delay in the K+ channels closing
• An action potential will only occur if the potential difference reaches the threshold
potential
• This has a value of about -60 mV to -50 mV
• If the potential difference does not reach this threshold potential, then an action potential
will not occur
Transmission of Impulses
• A nerve impulse is the movement of a self-propagating action potential along the
neurone
• As Na+ diffuse in (and cause an action potential at one point on the axon) this sets up a
small localised circuit with the adjacent part of the axon membrane
• The Na+ ions which have entered are attracted to the adjacent negatively charged part of
the axon
• This causes a slight depolarisation inside the axon
• This stimulates voltage sensitive Na+ channels to open further down the membrane and
triggers an action potential
• This localised current occurs with regions of the membrane on both sides of the action
potential
• However, since the membrane behind the action potential is still recovering and
redistributing its ions, an action potential cannot occur there, so will only travel in one
direction (ahead)
• It is therefore incapable of producing another action potential for a short time, known as
the refractory period
• The refractory period means that:
- Action potentials are discreet events, they do not merge into one another
- The length of the refractory period limits the maximum frequency of action potentials
• Strong stimulus, frequent action potentials
• Weak stimulus, fewer action potentials per unit time
• The brain interprets the frequency of these action potentials, and the number of neurones
carrying them to determine the strength of the stimulus
• The location of the receptor and sensory neurone tells the brain the nature of the stimulus
(eg, from the eye = light, etc)
Speed of Impulses
• Schwaan cells create a myelin sheath around the axon of a neurone, with small gaps
between them (the nodes of Ranvier)
• The myelin sheath prevents the movement of ions at these points
• Action potentials can only occur at the nodes of Ranvier, so as a result the localised circuit
is set up between each node
• The nerve impulse therefore 'jumps' from one node to the next. This is known as saltatory
conduction
• This speeds up the rate of a nerve impulse
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