OCR A2 Biology F214 - Nerves

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NERVOUS COMMUNICATION
Sensory (afferent) neurons ­ Transmits impulses from a receptor to the central nervous system.
Motor (efferent) neurons ­ Transmits impulses from the central nervous system to an effector such as a muscle or
gland.
Intermediate neurons ­ Receives impulses from a sensory neurone and transmits them to a motor neurone, found in
the brain or spinal cord.
Cell Body ­ Contains the nucleus, many mitochondria, endoplasmic reticulum, Golgi apparatus & other organelles.
Group of ribosomes involved in protein synthesis are also present.
Dendron ­ Long cytoplasmic processes that conduct impulses towards the cell body: longest in sensory neurones.
Dendrites ­ Short cytoplasmic processes that conduct impulses towards the cell body.
Axon ­ Long cytoplasmic processes that conduct impulses away from the cell body.
Synaptic Bulb ­ Found at the tips of the terminal branches of the axon. They contain many mitochondria and vesicles
containing transmitter substances.
Nodes of Ranvier ­ Non-myelinated constrictions along the axon.
Myelin Sheath ­ Fatty insulated layer formed from Schwann cell membranes which wrap themselves around the
axon, enclosing the axon in many layers of membrane. Speeds up the transmissions of impulses and isn't present in all
neurones.
Membrane of Schwann Cell ­ Thin layer surrounding the myelin sheath.
Neurone ­ A nerve cell specialised for the conduction of action potentials.
Nerve ­ A bundle of many different neurones surrounded by a sheath (perineurium)/
A reflex action ­ A fast, stereotyped response to a particular stimulus. Reflex actions help us to avoid danger by
allowing us to respond to a potentially harmful situation without using time to think about it.
A Reflex Arc ­ Is a pathway along which nerve impulses are carried from a receptor to an effector, without involving
any conscious thought.
Compared to hormonal communication this method is electrical in nature, faster, more precise and short
lived.
Motor Neurone
Sensory Neurone

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Cell Body ­ Contains the nucleus, many mitochondria, endoplasmic reticulum, golgi apparatus and other organelles.
Groups of ribosomes involved in protein synthesis are also present.
Dendron ­ Long cytoplasmic processes that conduct impulses towards the cell body. They are the longest in sensory
neurones.
Dendrites ­ Short cytoplasmic processes that conduct impulses towards the cell body.
Axon ­ Long cytoplasmic processes that conduct impulses away from the cell body. They are the longest in motor
neurones.…read more

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Ventral root of spinal nerve
9. Motor neurone
10. Motor end plate
11. Effector (muscle that moves away from pain)
Sensory Receptors
Are energy transducers that convert one form of energy to another. Teach type of transducer is adapted to detect
changes in a particular form of energy. This may be a change in light levels or pressure on the skin etc. Each change in
the energy levels in the environment is called a stimulus.…read more

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Action potential
1. The membrane starts in its resting state ­ polarised with the inside of the cell being -60mV compared to the
outside (inside more negative).
2. Sodium ions channels open and some sodium ions diffuse into the cell.
3. The membrane depolarises - it becomes less negative with respect to the outside and reaches the thresh
hold value of -50mV.
4. Voltage-gated sodium ion channels open and many sodium ions flood in.…read more

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A synapse is a junction between two or more neurones. It is where one neurone can communicate with, or signal to,
another neurone. An action potential is produced by the movements of ions across the neurone membrane. The
action potential cannot bridge the gap between two neurones. Instead, presynaptic action potential causes the
release of a chemical transmitter (the transmitter substance) that diffuses across the gap and generates a new action
potential in the postsynaptic neurone.…read more

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Summation can also occur when several presynaptic neurones
release small numbers of vesicles into one synapse.
The creation of specific pathways within the nervous system is thought to be the basis of conscious thought
and memory.
The Frequency of Transmissions
The brain can determine that a receptor is being stimulated, for example it knows that light is falling on the eye, but it
cannot determine the intensity. When a stimulus is at a higher intensity, the sensory receptor will produce more
generator potentials.…read more

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