Circuit Dynamics

Transcript

0:00 – 0:30  [Basic Definition]  Neural circuit dynamics are the patterns of activity produced by a group of neurons.  The neural circuit may be small, for example, two neurons and associated muscle fibers in the monosynaptic reflex.  Or a neural circuit can be quite large, for example, the collection of neurons in the central and peripheral nervous system that work together to homeostatically regulate body temperature. The output of a neural circuit is dynamic because the response properties of individual cells can change over time and so can the balance between excitation and inhibition.

0:30-2:30  

Dynamic relationships between voltage-gated channels and synaptic transmission play an important role in neural communication and behavior. The way in which neurons are functionally connected can be described as a neural circuit, in the same way we describe electronic circuits in our gadgets. A central goal of neuroscience is to try to explain the functional logic of information processing by neural circuits.

The simplest neural circuit is one controlling the ‘knee jerk’ reflex in the spinal cord, which contains a single central synapse. Ia sensory axons innervating muscle spindles sense an unexpected stretch of the muscle and directly excite motor neurons to cause a compensatory contraction. This is why when a physician taps the tendon below your kneecap, the sudden stretch of your quadrucep causes your leg to kick forward.

By adding an inhibitory interneuron to the monosynaptic stretch reflex circuit, we can prevent unexpected stretching of one muscle from triggering the same response in an antagonistic muscle. This type of relationship is called reciprocal inhibition. Without this unexpected stretching of one muscle would continue to perpetuate cyclical activity and spasm out of control.

Reciprocal inhibition of antagonistic muscles is not only useful to prevent spasms, but also for generating rhythmic activity in the spinal cord during locomotion. In this case, continuous excitation from the brain activates spinal excitatory interneurons, which in turn synapse on spinal inhibitory interneurons that form reciprocal connections. So when synergistic motor neurons are active, antagonistic motor neurons are silent, and vice versa. Think about alternating contractions between flexors and extensors.

Inhibitory circuits not only organize simple motor behaviors, but also sensory processing. In this case, lateral inhibition in the brainstem helps to sharpen the responses of sensory afferents reporting mechanical stimuli in the periphery.

The principle of using lateral inhibition to improve neural processing is also observed in the retina, where horizontal cells inhibit photoreceptors to enhance contrast sensitivity. Thus the same circuit dynamics can be found throughout the nervous system to explain diverse functions.

2:30-3:00  [Parallel Vocabulary] In introductory classes you learned about many examples of neural circuit dynamics, which is an umbrella term that includes a huge variety of pathways.  Here are two examples… In the visual system, circuit dynamics within the retina produce lateral inhibition when the activity of inhibitory interneurons enhances visual contrast.  In the thalamocortical pathway, a balance between excitation and inhibition relates to the transition from being asleep to being awake.  In both examples, the output of a circuit is dynamically altered in ways that affect cognition or behavior.

3:00-4:00  [Here’s a real world example]  Circuit dynamics are thought to play an important role in almost every aspect of neural communication including sensation, motor control, and cognition.  For example, did you know that many movement disorders such as Parkinson’s disease, can be treated by adjusting the balance between excitation and inhibition in specific neural circuits.  Parkinson’s can be caused by a decrease in dopaminergic signaling that simultaneously increases inhibition and decreases excitation in the basal ganglia circuit.  The overall result is a decrease in excitatory motor control signals and this in turn disrupts voluntary movement. One treatment for Parkinson’s disease is deep brain stimulation that blocks neural activity in the subthalamic nucleus. Recall that within the basal ganglia, the subthalamic nucleus excites the globus pallidus internal segment, which in turn inhibits motor circuits.  By blocking activity in the subthalamic nucleus, deep brain stimulation is thought to reduce inhibitory output of the basal ganglia and this in turn helps to restore the balance between excitation and inhibition in motor circuits.

4:00-6:00 [Follow along with this example]

6:00-6:30 [Here are a few readings to help you review]
1) Neuroscience Exploring the Brain (Bear)

• Chapter 13: “Spinal Control of Movement” covers reflex circuits

2) Principles of Human Physiology (Stanfield)

• Chapter 14: “Brain control of movement” covers basal ganglia circuits.

Media attributions

Image from “BIO103” is licensed under a Creative Commons Attribution 3.0 United States (CC BY 3.0 US).

Extensor digitorum by Zhang MJ, Zhu CZ, Duan ZM, and Niu X is licensed under a Creative Commons Attribution Sharealike 4.0 license (CC-by-SA 4.0).

Image by Cenveo is licensed under a Creative Commons Attribution 3.0 United States (CC BY 3.0 US).

Image from “Foundations of Neuroscience” by Casey Henley is licensed under a Creative Commons Attribution NonCommercial Sharealike license (CC-by-NC-SA 4.0).