The Nervous System
OR…
Why you are able to poke yourself
in the eye.
Functions of the Nervous
System
Receives sensory
information from
internal and external
environment
Processes
information and
transmits it around
the body.
Organises a
coordinated response
to information
Structure of the Nervous System
The Central Nervous System
Brain – protected by the cranium.
Spinal cord – protected by the vertebra
The Peripheral Nervous System
Consists of all of the nervous system
apart from the brain and spinal cord.
Divided into:
1. Somatic Nervous System
2. Autonomic Nervous System
Somatic Nervous System
Voluntary movement.
Conveys sensory messages from the
sense organs to the CNS…..AND
Conveys motor messages from the CNS
to the muscles and glands.
Autonomic Nervous System
Involuntary movement.
Regulates internal environment.
Controls functioning of the heart,
intestines and other organs.
Divided into Sympathetic and
Parasympathetic Nervous System.
Nerve Cells
Basic units of the
nervous system.
Also known as
neurons.
4 basic parts to
a neuron.
Neurons
1. Dendrites = Receive messages from
other neurons.
2. Soma/Cell body = Accepts impulse
travelling from the dendrites.
3. Axon = Impulses travel from the soma
and are carried along this thin fibre.
4. Axon terminals = Branches at the end of
the axon that link with the dendrites of
other neurons.
Neurons
Neurons do not touch each other.
Instead a chemical, called a
neurotransmitter, is released from the
axon terminals into a gap called a
synapse.
The chemical is then picked up by the
dendrites of the next neuron.
i.e. nerve impulses are electrochemical.
Myelin Sheath
Fatty layer that protects axon, and conducts
messages faster.
Gaps in myelin is called nodes of Ranvier.
Match the terms with their definitions
Term
Definition
Neuron
Axon
Soma
a. A fatty layer coating some axons.
b. The main cell body of a neuron.
c. Fibre that carries information away from
the cell body of a neuron.
Dendrite
Myelin
Nerve
d. A bundle of neuron fibres.
e. Branching fibre at the end of an axon.
f. Carries chemical message across the
synapse.
Synapse
g. An individual nerve cell.
Axon terminal h. Microscopic space between two neurons.
NT
i. A neuron fibre that receives incoming
messages
True or False?
1. A neuron and a nerve are structurally the same
thing.
2. An axon is about 0.1mm long.
3. The transmission of a message within a
neuron is electrochemical.
4. The release of NT’s occurs at the axon.
5. Axons are coated with myelin in order to
protect them from damage.
6. A synapse is a gap between the axon terminal
of one neuron and an adjacent neuron.
Activity: Making a Neuron
1. Use pipe cleaners to construct a model
of a neuron.
2. Label the main parts of a neuron and
state the function of each.
3. Indicate the direction of the nerve
impulse.
Sensory (Afferent) Neuron
Detect changes in the external or internal
environment and transmit that information
to the CNS, via the PNS.
Motor (Effector) Neuron
Carry impulses away from the CNS to muscle
cells or glands and cause a response.
Interneuron (Relay Neuron)
Link sensory
and effector
neurons.
Normally found
in CNS
(particularly PNS)
S-A-M-E
Sensory = Afferent + Motor = Efferent
Reflex Arcs
Reflex Arcs
Putting it all together…
Create a flowchart to illustrate:
1. How sensory information is transmitted
from the PNS to the CNS.
2. How motor information is transmitted
from the CNS to the PNS.
The Nerve Impulse
Ions are electrically charged chemical
molecules, e.g. K+, Na+, Cl- etc.
In and around a neuron there will be
positive and negative ions.
When a neuron is resting it has a negative
charge.
This is known as its resting potential.
The neuron is said the be polarised.
The Resting Neuron
The Action Potential
Along the axon membrane there are
numerous ion channels.
Normally these channels are blocked, but
if a neuron is sufficiently stimulated the ion
channels open and positive ions rush in.
The neuron becomes positively charged
and an action potential is been
generated.
The Action Potential
An action potential is a very rapid change in
polarity.
The neuron moves from the resting potential to
some positive value in a few milliseconds.
The Action Potential
If a nerve cell is stimulated past the
threshold (about -30mV in humans) ion
channels open and positive ions rush into
the axon.
This causes a region of the axon to have
a more positive charge.
This is called depolarisation.
The Action Potential
Soon after the positively charged ions are
pushed back outside.
The neuron moves back to its resting
potential.
This is known as repolarisation.
The ion channels are only located at the
nodes of Ranvier.
The action potential jumps from node to
node.
The Action Potential
Once an action potential has occurred
there is a period when the neuron is
unable to conduct another nerve impulse.
This is known as the refractory period.
This ensures that the nerve impulse
moves in one direction along the axon.
All-or-None Law
Action potentials occur maximally or not at
all.
I.e. There is no such thing as a partial or
weak action potential.
Either the threshold potential is reached
and an action potential occurs, or it isn't
reached and no action potential occurs.
Bridging the Gap
The axon ends with many small swellings
called axon terminals.
The small gap or space between the axon
of one neuron and the dendrites or cell
body of the next neuron is called the
synapse or synaptic gap.
A nerve impulse cannot go backward
across a synapse.
Bridging the Gap
The axon terminals contain tiny vesicles
filled with chemicals known as
neurotransmitters.
When an impulse reaches the axon
terminal, NT’s are released into the
synaptic gap.
The NT moves through the synapse and
bind to receptor molecules on the postsynaptic neuron.
Bridging the Gap
Bridging the Gap
NT’s attach only to specific receptors.
NT’s may excite or inhibit the next neuron.
If the neuron receives enough excitatory
messages an action potential is
generated.
Any leftover NT’s are rapidly removed or
destroyed.
SYNAPTIC CLEFT OR GAP