Download Sensory Receptors

Sensory Receptors
Miss Tagore
A2 Biology
Learning Outcomes
• Outline the roles of sensory receptors in
mammals in converting different forms of
energy into nerve impulses.
• Describe, with the aid of diagrams, the
structure and functions of sensory and motor
neurones.
Sensory Receptors
• Changes in our surroundings
are detected by specialised
cells called sensory receptors.
• Sensory receptors convert one
form of energy into another.
• E.g. a change in pressure in
the skin.
• Change in energy levels in the
environment is called a
stimulus.
Light-sensitive cells (rods
and cones) in the retina
of the eye
Sound receptors in the inner ear
(cochlea)
Vibrations in the air
Light intensity and
range of wavelengths
(colour)
Pressure receptors
(Pacinian corpuscles)
in the skin
Olfactory cells lining the
inner surface in the nasal
cavity
Presence of volatile chemicals
Taste buds in the tongue,
hard palate, epiglottis and
the first part of the
oesophagus
Pressure on skin
Presence of soluble chemicals
Summary
• Sensory receptors detect changes from surroundings;
• Sensory receptors transduce signals – they convert one
type of energy into a form of electrical energy;
• Changes in energy levels in the environment are called
stimuli;
• Stimuli are converted by sensory receptors into
electrical energy called a nerve impulse.
Voltage gated ion channels
• Some of the proteins found on
the surface of the cell
membrane are channels that
allow ions to move across.
• Open channels allow ions to
diffuse from an area of high
concentration to lower
concentration until they are
evenly spread out.
• Channel proteins found in
neurons are more specific
than this.
Voltage gated ion channels
• Protein channels in neurones are specific to either sodium or
potassium ions;
• These protein channels have gates that open or close the channel.
They are usually kept closed;
• When open, the permeability of the ions is increased and they
flow through the channel. When closed, the permeability is
reduced.
NOTE! These types of
proteins simply let sodium or
potassium ions to pass in or
out of the cell
Summary
• There are a type of protein found on the cell
surface called voltage-gated ion channels.
• When the channel/gate is open, proteins
allow sodium or potassium ions to pass in to
or out of the cell.
• Channels are normally kept closed.
Ion pump proteins
• Another type of protein in the cell membrane is one
that actively pumps ions in or out of the cell.
• These pumps transport sodium (Na+) and potassium
(K+) ions extra- and intracellularly respectively.
• This means lots of Na+ is pumped out of the cell, while
K+ is pumped into the cell.
• This creates an overall negative charge inside the cell
with respect to the outside.
Polarisation
• As the inside of the cell is “more negative”
than outside, we say the cell membrane has
become polarised.
Positive
charge
Negative
charge
Depolarisation
• Altering the permeability of sodium ions creates a nerve impulse.
• When sodium channels open, sodium ions flow down the
concentration gradient from high to low, flowing back into the cell
(These were the first proteins we talked about).
• The movement of positive Na ions going back into the cell makes
the inside of the cell “less negative” than before.
• This creates a change in the potential difference, or charge, across
the cell.
• This change of charges across the membrane of neurones is called
depolarisation.
Sodium channel is open, allowing
sodium ions to flow back into the
cell.
Sodium ions flowing back into the
cell makes it “less negative” than
before
Generator Potentials
• Receptor cells respond to changes in the environment.
• If a small number of Na+ cross the channel, this is
called a generator potential.
• The larger the stimulus (the change in energy levels in
the environment) the more gated channels will open.
• If there are enough Na+ entering the cell, the potential
difference changes significantly and will initiate an
impulse, known as an action potential.
http://www.youtube.com/watch?v=7EyhsOewnH4
Large stimulus
creates large
change in potential
difference
Small stimuli create
small changes in
potential difference
Different types of neurone
Transfer of information
• Stimuli detected have energy. This energy is
converted in order to depolarise the membrane
of a neurone (make the receiving neuron
membrane “less negative”). Once this happens,
an impulse is transmitted to other parts of the
body.
• The impulse is transmitted through neurones as
action potentials from one part of the body to
the other.
Neurone Specialisations
• Very long axons – transmit over long distances
• Many gated ion channels on cell membrane surface to allow Na, K
and Ca ions through
• Na/K pumps to facilitate active transport of ions into/out of cell
• Maintain a potential difference across cell membrane (+ve outside,
-ve inside)
• Myelin sheath (Schwann cells) insulate neurone from electrical
activity of nearby cells
• Cells have a nucleus, many mitochondria and ribosomes