4 2 Rhythmicity Imagine you have a circuit that consists of only two neurons Neuron A and Neuron B Their axons point towards each other when Neuron A fires it stimulates Neuron B to fire which stimulates Neuron A to fire and so on Using the principles of neurons and neural circuits we have learned so far we can create a simple circuit with a fascinating emergent property one that is not present in any components but present in the system rhythmicity There are many rhythmic behaviors in animals including walking running swimming chewing breathing blinking and vomiting These simple rhythmic behaviors are controlled by neural circuits loosely referred to as pattern generators These groups of neurons will depolarize in patterns that are self sustaining Although their patterns can be modified by sensory input they do not require sensory input For example walking is a rhythmic behavior in which limbs are sequentially moved forward at a steady pace For the basic pattern of walking an animal does not need to know anything about its environment You can see this basic pattern when a dog is running in its sleep However many adjustments to that pattern are possible If an animal senses a predator it will quicken its pace and actually switch to a different pattern which produces running If the ground is uneven the animal will adjust the length and timing of its strides to regain its balance Rhythmic behaviors can be thought of as fitting the following criteria 1 Two or more processes that interact such that each process sequentially increases and decreases 2 As a result of this interaction the system repeatedly returns to its starting condition Behaviors can be rhythmic over different time scales Walking and running are rhythmic on a short time scale and do not last long they are episodic behaviors Waking and sleeping are rhythmic on a longer daily time scale they are continuous This lesson will focus on short term rhythmic behaviors The next lesson will look at longer time scales such as circadian and circannual rhythms Some pattern generators comprise neurons that are reciprocally connected as in the simple example with Neuron A and Neuron B Here rhythmicity is an emergent property of the network Other pattern generators include a cell that is inherently rhythmic spontaneously depolarizing at regular intervals called a pacemaker cell For example there are pacemaker cells in the hearts of many animals that set the heart rate Again this heart rate can be modified by sensory input but its rhythmicity is inherent Recall that the voltage gated K channel responsible for repolarization during an action potential is one of several voltage gated K channels Taxa vary widely in the number of genes and thus channels they have The K channel family is particularly diverse but there are also slow voltage gated Na channels channels that allow both Na and K through and many others Some of these channels confer unusual properties on the neurons that express them Four unusual possibilities are shown in the figure below For each one figure out how the neuron is being stimulated first by looking at the current that is being injected Then look at how the voltage of the membrane changes in response In the first example no current is being injected However the membrane still fires two bursts of action potentials Going through this figure step by step is good practice for reading graphs from experimental manipulations of the membrane as they are not particularly intuitive The second panel deconstructs plateau potentials into the ions that are responsible for the observed change in Vm in response to a brief depolarizing current You can see that there are actually four different channels in the neuron membrane that open at different times and their combined effect is a plateau of action potentials Connecting this neuron to an effector organ will result in a rhythmic output in response to an initial input like pulses of a metabolic hormone being released in response to food availability Alternatively rhythmicity can be an emergent property as in the simple Neuron A and Neuron B model at the beginning Some reciprocally connected neurons are actually reciprocally inhibitory In this case the two neurons are spontaneously active but not rhythmic They are connected via inhibitory synapses When one fires it inhibits the other for a brief period of time after which the neuron spontaneously fires again This inhibits the first neuron for a brief period of time and so on The graphs below are shown over a longer period of time than you have seen before then compressed so each spike represents an entire action potential Note that even without any current injected they are spontaneously firing action potentials Once they are wired together they reciprocally inhibit each other and rhythmicity emerges If each of these neurons is also connected to an effector organ those targets will receive rhythmic alternating input like swinging your left leg then right leg forward while walking When spontaneously active neurons reciprocally inhibit each other the circuit will be spontaneously active However if the neurons reciprocally activate each other as in the simple Neuron A and Neuron B example something must get the neurons started Many rhythmic circuits are initially activated by sensory input that feeds into a single neuron called a command neuron This command neuron integrates the sensory input and activates the rhythmic neurons controlling motor output as in the wiring diagram shown below This diagram shows the neurons involved in the escape reflex of the sea slug Tritonia When Tritonia is startled perhaps by being touched by a predatory sea star it will escape through the water by flexing back and forth in dorsal ventral flexions The startling stimulus is detected by the S cells which activate the dorsal ramp interneuron DRI which activates the central pattern generator CPG via the dorsal swim interneuron DSI If you follow the wiring diagram you will see that DSI activates the dorsal flexion motor neurons via C2 Thus when C2 is activated the sea slug will flex dorsally C2 feeds back to DRI creating a positive feedback loop that continues to activate the CPG However it also activates the ventral swim interneuron VSI causing the slug to flex ventrally VSI reciprocally inhibits V2 as well as the dorsal portions of the CPG This temporarily interrupts activity of the dorsal side of the CPG and the dorsal flexions This is only temporary
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