o The Withdrawal reflex is one type of reflex, however they all have the same five general
components: First, a receptor (pain receptors – nociceptors (free dendrites of the sensory
neuron) – in the case of the withdrawal reflex).
o Then, a sensory neuron (in this case it is also the nociceptor) relays the information to an
interneuron in the spinal cord.
o The interneuron takes the information, integrates it and sends that information to other
cells, including a motor neuron which relays the information to a target organ which can
then elicit the appropriate response (in the withdrawal reflex it is a muscle body).
o Perception of the painful stimulus begins with tissue damage around the injury which
release chemicals that bind to the pain receptors (nociceptors). This begins eliciting the
response as the binding causes the electrical signal which drives the response.
o This electrical signal is in the form of a depolarization. The reflex is elicited if this
depolarization achieves a voltage greater than the threshold (the minimum
intensity/strength of a stimulus required to create an action potential – the active state of
o Nociceptors are present within the dermis. o The depolarization of a neuron can be visualized with technology. Two electrodes (the
microelectrode within the cell and the reference electrode outside of the cell) carry a
potential difference that can be detected to allow us to visualize an action potential.
o This voltage difference across the membrane is known as the membrane potential (V)
and the membrane potential of a neuron in a resting state is about -70 mV.
o The threshold voltage (indicated by the dotted line) is the voltage required after a
depolarization to induce an action potential (it is at about -55 mV).
o A stimulus that fails to depolarize a neuron to its threshold voltage is known as a
o Examples of stimuli that can be subthreshold include tissue damage, noxious chemicals
(for example, those created by a foreign microbe) or neurotransmitters.
o These stimuli depolarize the neuron by binding to ligand-gated channels. For example,
the chemicals secreted by the cells after
tissue damage has occurred, act as the
ligands. These ligands will bind to
specific sodium ligand-gated channels
and sodium enters the cell.
o This makes the cell’s interior more
positive and slightly increases the
membrane potential. The cells are no
longer at rest, however, they are not yet
active if the stimulus is subthreshold.
o The membrane potential goes back to
resting state once the stimulus is gone if
the threshold voltage was not achieved.
o The stimuli were too small and so no
action potential occurs (as if nothing happened).
o A stimulus that does reach the threshold (suprathreshold “bigger than required” stimulus)
will induce an action potential. The same types of stimuli can induce an action potential. If
depolarization to the threshold voltage does occur, an action potential will always happen
the same way (“all-or-none”).
o Once the threshold voltage is achieved, a different channel becomes involved. These are
the voltage-gated channels which induce a rapid depolarization which generally
overshoots the 0 mV mark and becomes positive (~35 mV). This is the action potential.
o A stimulus that is strong enough to just reach threshold and a stimulus that is 10x stronger
than said stimulus both create the same action potential. Strength of the suprathreshold
stimulus not a factor. o White channel on slide – voltage-gated sodium channel. Orange channel – voltage-gated
o In the normal resting state, both channels are closed. The voltage-gated sodium channel
has two gates – the activation gate (membrane protein portion) is the actual tube/channel
and is closed at resting state; the inactivation gate (represented as a cork-like loop), which
is open at resting state.
o However, only one of the two channels need to be closed to restrict the movement of
sodium across the channel.
o Thevoltage-gatedpotassiumchannelis composedonlyofanactivationgate.Thissingular
channel is important for the refractory stage of an action potential. The potassium
activation gate is closed at rest and so no potassium ions can move across at rest.
o Since they are voltage-gated channels, a specific membrane potential value is needed to
open and close the gates.
o For sodium the membrane potential needed to open the activation gate is the threshold
voltage: -55 mV.
o The inactivation gate closes at around +35 mV which is the same membrane potential
needed to open the voltage-gated potassium channels (beginning of the refractory period). o The initial stage of an action potential involves a suprathreshold stimulus inducing a rapid
depolarization to the threshold voltage by opening enough ligand-gated sodium
channels to depolarize the cell.
o Once threshold is reached, the voltage-gated