Gradation of sensory signals – cellular communication
The input signal (a receptor or synaptic potential) is graded in amplitude, proportional to
the amplitude and duration of the stimulus.
The trigger zone integrates the input signal – the receptor potential in sensory neurons,
or synaptic potential in motor neurons – into a trigger action that produces action
potentials that will be propagated along the axon. An action potential is generated only if
the input signal is greater than a certain spike threshold. Once the input signal
surpasses this threshold, any further increase in amplitude of the input signal increases
the frequency with which the action potentials are generated, not their amplitude. The
duration of the input signal determines the number of action potentials. Thus, the
graded nature of input signals is translated into a frequency code of action potentials at
the trigger zone.
Action potentials are all-or-none. Every action potential has the same amplitude and
duration, and thus the same wave form on an oscilloscope. Since action potentials are
conducted without fail along the full length of the axon to the synaptic terminals, the
information in the signal is represented only by the frequency and number of spikes, not
by the amplitude.
When the action potential reaches the synaptic terminal, the cell releases a chemical
neurotransmitter that serves as the output signal. The total number of action potentials
in a given period of time determines exactly how much neurotransmitter will be released
by the cell.
The anatomical location of muscle spindles and Golgi tendon organs underlies their roles as
length and tension sensors, respectively.
Length and tension are extremely important – proprioception
Embedded in belly of muscle (muscle spindle)
A force sensor would exist between the tendon and muscle (Golgi tendon organ)
Have efferent neurons
Muscle spindles – important length sensors
Muscle spindles sense changes in muscle length
Muscle length changes with joint position
Muscle spindles, therefore, can provide information about