Animal Response to Stimuli
The Nervous System
Contents
Definition
Nervous Vs Endocrine System
The human nervous system
Neurons
Three Types of Neuron
Movement of a nerve impulse
The Central Nervous System
The Brain – its parts and
functions
Reflex Actions
Reflex arc
More complex reflexes
Nervous System Disorders
Paralysis
Parkinson’s disease
Other Nervous System
Disorders
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Definition
nervous system: a collection of nerve cells in an
animal used to detect changes (stimuli) in both
the internal and external environments and to
coordinate a rapid response to the stimuli.
The other coordinating system in the body is the
endocrine system.
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Definition
endocrine system: all the endocrine (ductless)
glands responsible for coordinating a slower and
more widespread response than the nervous
system.
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Nervous Vs Endocrine system
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The human nervous system
Made of two parts: -
The central nervous system (CNS) and
The peripheral nervous system (PNS).
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Central Nervous System
The central nervous system (CNS) is composed
of the brain and spinal cord
and processes information it receives from the
receptors, makes decisions and sends
messages to the effectors
i.e. it co-ordinates all the activities.
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Peripheral Nervous System
The peripheral nervous system (PNS) is composed
of all nerves outside the central nervous system
(CNS)
i.e. all nerves except the brain and spinal cord –
they carry messages to and from the CNS.
These nerves are composed of bundles of nerve
cells called neurons.
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The human nervous system
Neurons
Are nerve cells.
all neurons basically similar in structure.
Each consists of one axon, one or more dendrons
and a nucleus contained in the cell body.
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The structure of a neuron
Parts of the neuron
Cell body – controls activities of neuron – contains
the nucleus, mitochondria and other organelles
and produces neurotransmitter chemicals.
Dendrites – allow interconnection – receive
impulses and transmit them towards the cell
body.
Axon – transmits impulses away from cell body.
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Parts of the neuron
Schwann cells – secrete the myelin sheath.
Myelin sheath – insulation on nerve, which
speeds up impulse transmission.
Nodes of Ranvier – separate Schwann cells speeds up impulse transmission.
Synaptic knobs – bulbs at end of branches of the13
axon.
Three types of Neuron
Motor neurons – carry messages from the
central nervous system (CNS) to an effector –
cell body located at end of axon, inside CNS.
Sensory neurons – pick up and carry
messages from sense organs (receptors) to the
central nervous system (CNS) – cell body at end
of a short branch to one side of the axon –
outside CNS.
Interneurons – carry messages from one nerve
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cell to another – found within the CNS.
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The types of neurons
Motor nerve cells in spinal cord
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Motor nerve ending on striated muscle
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Peripheral nerve, Nodes of Ranvier
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Single nerve cell
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Movement of a nerve impulse
(1/5)
STIMULUS = any change in the environment
(internal or external) which causes a cell or
organism to respond.
The stimulus must be of a certain minimum size
before an impulse is generated - threshold level
- obeys the 'all or nothing law' - domino effect.
The strength of the impulse is maintained along 20
the length of the axon.
Movement of a nerve impulse
(2/5)
The stimuli, which generate nerve impulses, may
differ but all impulses are the same.
Transmission of an impulse requires energy ATP (mitochondria - respiration).
Myelinated nerves transmit impulses faster (100
m/sec) than non-myelinated nerves (0.5 m/sec).
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Movement of a nerve impulse
(3/5)
The impulse moving along a nerve is electrical in
nature – it involves the movement of ions.
The region where two neurons come into close
contact is called the synapse.
When it reaches the synaptic knob the message
has to cross a tiny gap – the synaptic cleft – to
the next neuron.
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The transfer of a message at a synapse
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Movement of a nerve impulse
(4/5)
When the impulse arrives at the synapse they
cause neurotransmitters (e.g. acetylcholine) to
be released into the synaptic cleft for a very
short time.
These neurotransmitters travel across the
synaptic cleft and cause an impulse to start in
the next neuron.
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Movement of a nerve impulse
(5/5)
After transmission, the neurotransmitters are
inactivated by an enzyme and reabsorbed by the
presynaptic neuron and used again to make new
neurotransmitter substances.
As a result of this only one impulse is sent each
time a neurotransmitter is released across a
synapse.
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The transfer of a message at a synapse
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The Central Nervous System
Develops from a hollow tube found on the dorsal
(back) side of the embryo.
The top of this tube enlarges to form the brain
The rest becomes the spinal cord.
The CNS is hollow and filled with cerebrospinal fluid.
The CNS is surrounded by
a triple layered membrane – the meninges, and
bone for protection.
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The meninges
The innermost layer – is very fine.
The middle layer – is fibrous and filled with
cerebrospinal fluid (similar to blood plasma)
and the arteries for the brain or spinal cord.
The outermost layer – is very tough.
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Brain – grey & white matter
(1/2)
CNS composed of neurons and other
specialised cells.
Groups of neuron cell bodies have a greyish
colour.
Myelin sheaths surrounding the axons give them
a white colour.
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Brain – grey & white matter
(2/2)
Hence the grey and white matter of the CNS.
The grey areas make decisions (cell bodies).
The white areas carry messages (axons).
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A diagram of the brain
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The Brain – its parts and functions
Cerebrum: - largest part – divided into two halves
= cerebral hemispheres (grey) – joined by the
corpus callosum
Function: - responsible for consciousness, reason
and intelligence. LHS responsible for language
and mathematical ability. RHS – musical and
spatial sense.
Nerves entering the cerebrum through the corpus
callosum cross over so the RHS of the cerebrum
controls the LHS of the body and vice versa. 32
(1/3)
Areas controlled by different
parts of the cerebrum
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The Brain – its parts and functions
Hypothalamus: - located beneath the cerebrum.
Function: - homeostasis. It controls osmoregulation,
appetite, sleep, thirst and sexual activity.
Pituitary gland: - attached to the underside of the
hypothalamus.
Function: - is controlled by the hypothalamus –
master gland – controls other glands – is the link
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between the nervous and hormonal systems.
(2/3)
The Brain – its parts and functions
Cerebellum: - beneath and to the back ot the
cerebrum.
Function: - coordinate voluntary muscles activity
and balance.
Medulla oblongata: - connects the brain to the
spinal cord.
Function: - controls involuntary muscle activities –
breathing, heartbeat, saliva production and
swallowing.
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(3/3)
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Spinal cord, cat, lumbar region
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The Spinal Cord
(1/2)
Runs down the middle of the vertebral column.
Acts as a coordinating centre.
Sends messages from the body to the brain.
Controls most of the reflex actions.
Note the grey and white matter, hollow central
canal filled with …
Spinal cord also protected by the meninges (triple
layered …)
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T.S. of spinal cord
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The Spinal Cord
(2/2)
The spinal nerves of the PNS lead into the spinal
cord,
through the dorsal root into the back of the cord.
Note: - (a) the dorsal root ganglion – this contains
cell bodies of sensory neurons.
The ventral root comes out the front of the cord,
contains motor neurons (cell bodies in grey
matter) – no ganglion.
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Ganglion
ganglion: a mass of nervous tissue, containing the
cell bodies of the neurons, lying outside the CNS
(brain and spinal cord); or
a collection of cell bodies within the PNS e.g. the
dorsal root ganglion.
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The knee-jerk reflex
Reflex Actions
reflex action: automatic involuntary response to
an internal or external stimulus - not under
conscious control e.g. knee jerk, ankle jerk.
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Reflex arc
(1/2)
reflex arc: The pathway from the point of stimulation to
the responding effector.
A reflex arc has five components. The stimulus is picked
up by a receptor that sends the message along an
afferent (sensory) neuron to the spinal cord.
The reply to this stimulus is sent from the spinal cord along
an efferent (motor) neuron to an effector (muscle or
gland).
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Pick out the five components
of the reflex arc
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Reflex arc
(2/2)
This reflex is used to keep you upright.
If you are standing and begin to fall backwards the
stretch receptor is stimulated as you pull on your
kneecap and this reflex brings you upright again.
The blinking reflex of the eye uses the brain and
not the spinal cord as the eye is connected to
the brain directly.
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More complex reflexes e.g.
(1/3)
If you pick up a hot plate
Heat receptors in skin register this
Message sent to spinal cord
Decisions made
Reply sent to muscles of arm and hand
Withdraw hand from hot plate.
Another message may be sent to larynx – shout.
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More complex reflexes e.g.
(2/3)
So a message was sent along the spinal cord to
different motor neurons.
The brain is not involved
Brain can overide the reflex
e.g. if your dinner was on the plate you may try and
catch it before it lands on the ground.
The second action will occur after the first.
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More complex reflexes e.g.
(3/3)
You pull away but then make a grab for the plate a
fraction of a second later.
The second response demonstrates the time it
takes for the message to travel to the brain and
back.
Reflexes can be prevented from happening.
If you know the plate is hot you can stop yourself
from dropping it.
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Nervous System Disorders
One of the following: -
Paralysis or
Parkinson’s disease
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Paralysis
Cause
Physical damage to spinal cord.
A protein that prevents growth surrounds neurons,
which run up and down the white matter of the
spinal cord.
Damage to these neurons cannot be repaired.
Crushing or severing of the spinal cord leads to
loss of function of the nerves lower down the
cord.
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Treatment
No cure for such damage at the moment.
Experimental work on animals being carried out to
bridge gaps with other neurons or
by splicing the broken neurons through the grey
matter, which will allow growth.
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Prevention
Reduce damage to spine of accident victims when
they are being moved.
Immobilise the head and neck.
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Parkinson’s disease
Cause
Patients are missing the neurotransmitter
dopamine, due to loss or damage of the tissue
in the brain that makes it.
Dopamine regulates the nerves controlling muscle
activity.
Lack of dopamine results in tremor, stiff joints and
a slow walk.
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Treatment
No cure for the disease.
Symptoms can be reduced by drug levodopa,
which the body converts into dopamine.
Some experiments have been done where
dopamine-producing tissue has been
transplanted into the patient’s brain. The results
have been variable.
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Parkinson’s disease
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Other Nervous system disorders
Not examinable
for information only
Multiple sclerosis
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Meningitis
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Brain abscesses and tumours
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END
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