Chapter 6 - Neuronal control of motor output

Question 1

Rhythmic motor pattern examples

Answer 1

• Respiration
• Chewing
• Swimming
• Walking

Question 2

Generation of rhythmic movements models

Answer 2

• Model 1 (Charles Sherrington):
  – Chain of reflexes
• Model 2 (Graham Brown):
  – Central pattern generator

Question 3

Central pattern generator

Answer 3

• Two basic mechanisms
  – Individual neurons act as pacemakers
  – Rhythmicity is result of synaptic interactions among neurons
• Play an eminent role in the production of rhythmic motor patterns
• The neural network underlying these oscillators can fulfill their function even in the absence of sensory input, or input from higher central sites conveying timing cues
• The central pattern generator controlling the rhythmic muscle contractions during swimming in hatching tadpoles consists of two half-centers in the spinal cord and three types of neurons –> motoneurons, excitatory interneurons, and inhibitory interneurons
  – During swimming, the rhythmic activity produced in the half-centers of the central pattern generators progresses from head to tail along the spinal cord; the resulting rostrocaudal delay in motor output is caused by:
  — Synaptic contact of excitatory interneurons with half-centers in more caudal segments of the spinal cord
  — Rostrocaudal reduction in the number of excitatory interneurons and inhibitory interneurons
  — Rostrocaudal gradient in both excitatory and inhibitory input received by motoneurons
• The central pattern generator controlling swimming in hatching embryos of the clawed-toad (Xenopus laevis) is readily accessible to physiological experimentation, because its rhythmic activity can be initiated even after blocking synaptic transmission at the neuromuscular junction, when the immobilized tadpoles perform ‘fictive’, instead of real, swimming

Question 4

Escape swimming in Xenopus tadpole

Answer 4

Swimming in the hatching tadpole is performed by lateral undulations of the body and forms part of the escape behavior

Question 5

Fictive behaviors

Answer 5

• Sequences of motor neuron activity occurring without the production of actual movement or muscular contraction
• Such behaviors can be observed in immobilized whole animals or in isolated preparations of the nervous system, in which muscles are removed

Question 6

Alpha-bungarotoxin

Answer 6

• Constituent protein of the venom of the Southeast Asian krait snake (Bungarus multicinctus)
• By binding to nicotinic postsynaptic receptor sites, this neurotoxin irreversibly blocks cholinergic transmission at the neuromuscular junction, thus producing muscle paralysis

Question 7

Rebound firing

Answer 7

The production of action potential(s) by a neuron after it has been hyperpolarized

Question 8

Physiological experiments (such as immobilizing whole animals) and anatomical studies have suggested that…

Answer 8

• Three types of neurons in the spinal cord are sufficient to explain the generation of the rhythmic pattern during the contractions of the trunk muscles during swimming
  – The motoneurons that innervate the swimming muscles
  – The excitatory interneurons that make excitatory synaptic contacts with the motoneurons on the ipsilateral side of the spinal cord
  – The inhibitory interneurons that project to the contralateral half-center of the central pattern generator, where they exert an inhibitory effect upon all three types of neurons

Question 9

During fictive swimming…

Answer 9

• A given half-center of the central pattern generator produces only one spike per swim cycles
• The spikes produced by the left and right half-centers alternate
• The spikes produced by the motoneurons are caused by excitatory postsynaptic potentials (EPSPs) evoked by input originating from the ipsilateral excitatory interneuron
• When the neurons on one side fire, all the neurons in the contralateral half-center receive, in the middle of their activity cycle, inhibitory postsynaptic potentials (IPSPs) mediated by the contralateral projection of the inhibitory interneurons
• It is primarily this inhibition, rather than the excitation, that is responsible for the rhythmic pattern produced by the central pattern generator