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Neurons

Aims:

Must be able to describe and label neuron.

Should be able to relate the structure of neurons
to the functions they perform.

Could be able to outline the process by which a
nerve impulse is passed along a neuron.
Nerve cell (neuron)

Nerve cells are the basic unit
of the nervous system.

Axons and dendrites may
vary in length.

A number of neurons bound
together is called a nerve.
Types of Neurons
1.
Sensory (Affector) neurons

PNS to CNS
2.
Motor (Effector) neurons

CNS to PNS
3.
Inter/connecting neurons

Found in CNS
Activity

Answer the questions from pages 95 and
96 in Biozone book.
Communication between
neurons

The cells (receptors) of a sensory nerve detect an
environmental disturbance.

Sensory neurons generate a signal that is passed as an
electrical message between neurons to particular effector
cells.

Adjacent neurons do not actually touch each other.

There is a gap between them called the synaptic cleft.

The connection between the axon of one neuron and the
dendrite of another is called a synapse.
The Synapse

As the impulse reaches the
axon terminal the terminal
produces chemicals known as
neurotransmitters.

Neurotransmitters diffuse
across the synaptic space.

Neurotransmitters bind to the
dendrites of the next neuron.

Neuron then generates and
carries an electrical impulse to
it’s axon terminals.
Neurotransmitters

Neurotransmitters are specialised chemical messengers:




produced by nerve cells
transported in the axon
released at the synapse
 cause chemical and electrical changes in adjacent
cells.
Most neurotransmitters play different roles throughout the
body, many of which are not yet known.
Activity

Answer the questions from page 101 in
Biozone book.
Neurohormones

A neurohormone is any hormone secreted
by or acting on a part of the nervous
system.

Neurohormonal activity is distinguished
from that of classical neurotransmitters as it
can have effects on cells distant from the
source of the hormone.
Neurohormone action:
Neurohormone action:

Neurons in the hypothalamus
in the brain secrete
neurohormones into blood
capillaries that become
associated with the pituitary.

Signals received by cells of
the pituitary are transduced
and the pituitary responds by
producing a hormone as
instructed.

This hormone enters the
bloodstream and is
transported to its target cells.
The Receptors

Sensory neurons serve as the body’s receptors as
they are able to detect disturbances in the
environment:

Chemoreceptors:
 chemicals

Mechanoreceptors:
 pressure

Photoreceptors:
 light

Thermoreceptors:
 temperature

Pain receptors:
 pain
The Nerve Impulse

Cell membrane of an axon is
polarised:
K


(difference in charge between
the inside and outside of the
cell).
An axon at rest (not
transmitting an impulse)
negatively charged on the
inside relative to the outside.

This is called a resting
potential.
Na
Na
K
The Nerve Impulse

To become polarised: sodium
– potassium pumps actively
pump sodium ions out of the
cell and potassium ions into
the cell.
K
Na
Na

Three sodium ions are
expelled for every two
potassium ions pumped in.
K
The Nerve Impulse

Na+ accumulates outside
and K+ inside.

They are both positive
ions - but more ++
outside – so: negative
inside and positive
outside.
K
Na
Na
K
The Nerve Impulse

As the impulse moves
along the axon, the
permeability of the
membrane changes…
K
Na

… positive ions – Na+
move into the cell –

This results in the outside
being more negative than
the inside…
Na
K
The Nerve Impulse



A nerve impulse involves a
change in the charge across the
axon membrane.
A nerve impulse is a wave of
electrical change (an action
potential) that passes rapidly
along an axon.
After the nerve impulse has
been transmitted – the
distribution of ions across the
cell membrane is restored.
K
Na
Na
K
Action Potential
Activity

Video:

Answer the questions from pages 100 in
Biozone book.
After Nerve Impulses
 After
the nerve impulse has been
transmitted along the axon of one
neuron it is transmitted further to:
 Another
neuron
 An effector - Muscle
 An effector - Gland
Neuron pathways

A neuron cell body
may have hundreds of
axon terminals
synapsing with it.

Two kinds of pathways:
1.
2.
Diverging pathway
Converging pathway
Diverging Pathway

Single axon links to
several postsynaptic
neurons

One cell can influence
many others.
Converging Pathway

Several axons link to
few postsynaptic
neurons

One cell can be
influenced by many
others.
Neurotoxins.

Homeostatic mechanisms, hormonal and
nervous, can be brought to a halt by
adverse event.

Toxins often act on the nervous system,
causing paralysis and pain, even
death, unless treatment is obtained
immediately…
Reflex Arc

Happens in situations with
extreme stimulus.

Not enough time to involve the
brain without serious injury.

Relay neurones in the spinal
cord pass message from
sensory neuron straight to
motor neuron, they also
transmit a pain signal to the
brain.

React to the stimulus – Then
feel pain after.
Comparing Responses
Reflex Arc
Receptors
Involved
Transmission to
CNS
General Pain
Voluntary
Response
Any
Involuntary
Response
Any
Sensory (afferent) Sensory (afferent) Sensory (afferent)
nerves
nerves
nerves
CNS Involved
Spinal Cord
Brain
Transmission
from CNS
Motor Neurons
Motor Neurons
Brain Hypothalamus
Autonomic
Nerves
Activity

Answer the questions from page 99 in
Biozone book.