Nervous Control in Humans
The Nervous System:
Central Nervous System (CNS):
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Brain – Controls the whole nervous system by receiving messages from and sending messages to different
parts of the body
Spinal Cord – A great number of nerve cells grouped together
Peripheral Nervous System (PNS):
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Cranial nerves – connected to the brain and carry messages directly to and from the brain
Spinal Nerves – connected to the spinal cord and carry messages from the receptors to the spinal cord and
the brain and from the spinal cord and the brain to the receptors.
Neurons:
Neurons are nerve cells specially adapted to conduct messages in the form of electric signals called nerve impulses
Main Parts:
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Cell Body – contains the cell organelles: nucleus, mitochondria, ribosome’s, endoplasmic reticulum
Dendrites - carry impulses to cell body
Sensory Neuron
Axon – carries impulses away from cell body
Types of Neurons:
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Sensory Neurons:
Function:
 Carry impulses from receptors to the
Properties:
 Single long Dendron
 Relatively short axon
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CNS
Motor Neuron
Motor Neurons:
Function:
 Carry impulses from the CNS to the
effectors such as muscles and glands
Properties:
 A number of short dendrites
 A single long axon
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Relay Neurons (Interneurons)
Function:
 Carry impulses inside the CNS from a sensory neuron either to a motor neuron or to another
relay neuron.
Properties:
 A number of short dendrites
 A short axon
Detailed Structure of a Motor Neuron
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Long axon
 Can be up to one meter in length
 Carries impulses away from the cell body and to the effector, such as a muscle
Myelin Sheath
 Surrounds the axon
 Consists of Swann Cells
 Myelin is a fatty substance that insulates the axon and speeds up the transmission of an electrical
impulse along the axon.
Nodes of Ranvier
 Gaps in the Myelin Sheath between Swann cells
 No insulation at the gaps
 The impulse jumps from one gap to the next
 Increase the rate of transmission of the electrical impulse
Dendrites
 Short
 Unmyelinated
 Carry impulses from other neurons to the cell body
Reflex Actions:
Characteristics:
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The simplest responses to stimuli
Inborn – cannot be learned, but are there from birth
Rapid, Automatic and involuntary responses to specific stimuli
Each reflex always follows a specific pattern and gives the same response
Spinal Cord:
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Protected by backbone
Path taken by an impulse:
o Sensory Nerve  Receptor Cells in Spinal Cord  Relay Neuron  Motor Neuron Effector Muscle
The relay neuron allows the impulse to be transmitted to other motor neurons and different effectors
Reflex Arc:
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The pathway a nerve impulse travels along to bring about a reflex action
Simplest reflex arc (two neuron spinal reflex arc) involves only sensory and motor neurons
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Knee jerk reflex:
1. The hit on the tendon is the stimulus
2. Special stretch receptor cells in the thigh muscle detect this stimulus
3. A nerve impulse is sent along sensory neurons to the spinal cord
4. In the spinal cord, the nerve impulses from the sensory neurons pass to the motor neurons
5. The motor neurons carry the nerve impulses from the spinal cord to the same thigh muscles
Most reflex arcs involve relay neurons and trigger both voluntary and involuntary actions.
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Synapses:
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The junction of two or more nerve cells
Between any two communicating neurons is a
synaptic gap
Nerve impulses are transmitted across the
synaptic gap by means of a chemical
substance which is released.
Neurons on either side of the synapse are
described as presynaptic and postsynaptic
neurons.
The presynaptic neuron communicates with
the postsynaptic neuron by releasing a,
chemical messenger, a neurotransmitter such
as acetylcholine, across the synapse.
Transmission across a synapse:
1. The vesicles of the axon terminal release a neurotransmitter into the synaptic gap
2. The neurotransmitter diffuses across the gap and combines with receptor molecules on the postsynaptic
membrane
3. This triggers a nerve impulse in the postsynaptic neuron
4. The neurotransmitter in the postsynaptic membrane is then broken down by enzymes
Types of Synapses:
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Excitatory – They transmit a nerve impulse
Inhibitory – They block a nerve impulse
The type of synapses depends on the kind of receptor molecules of the postsynaptic membrane
One Way Transmission
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The synapses, not the neurons are responsible for the one way transmission of impulses
Impulses travel from the axon of the presynaptic neuron, across the synapses to the dendrites of the post
synaptic neurones
Only the presynaptic neurons can produce the neurotransmitter and only the post synaptic neurons have
the receptors for them.
Synapses and Integration
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One neuron may synapse with many other neurons
o e.g. A motor neuron in the spinal cord may have hundreds of synapses with other neurons
The dendrites of the postsynaptic neurons need to sort-out and integrate all the incoming messages from
the axons of the presynaptic neurons to ensure the appropriate response.
Effect of opiate drugs on the synapses
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Opiate drugs contain opium, or a derivative, morphine.
Opiates work by blocking the release of neurotransmitters at the synapses.
This prevents impulses from travelling across the synaptic gap and reaching the effectors.
Effects:
o Numbness + Sleepiness
o Reduces pain
Uses:
o Medicinally as a sedative, to encourage relaxation and sleep
o As painkillers
Mimic the activity of endorphins, naturally occurring painkillers which are released by the body
Caffeine and Nicotine have the opposite effect of opiate drugs:
o They act as stimulants
o Enable impulses to pass more easily across the synapses.
Nervous and Hormonal Control
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The nervous and endocrine systems share the same basic function of controlling and co-ordinating the
majority of activities in the body.
Nervous System
Endocrine System
 Sends electrical impulses along high speed
 Hormones travel more slowly from one place
nerve cells
to another through the blood stream
 Carry message to one particular part of the
 Effects of hormones are widespread and
body
diffuse
 Information is precise and controlled with
 Only the hormone’s target cells will
a high degree of accuracy
respond to its stimulus
 The effect of a nerve impulse is immediate
 Effects of hormones are long lasting
and final.
 They exert their effect over their target
 Any further stimulation or activity will
cells for several hours or even days
require another impulse from the nervous
system