Summary

Neural Control of Skeletal Muscle Is the Basis of Animal Behavior

• The pattern of motor output of a nervous system produces behavioral actions. The nervous system can generate motor patterns centrally or in response to discrete stimuli. Simple, stimulus-evoked responses are often reflexive: Stronger stimulation evokes stronger responses.
• Neural circuits that generate simple patterns of behavior in invertebrates typically involve relatively small numbers of identifiable neurons. The circuits for similar acts of vertebrates involve many more individual neurons, which are not uniquely identifiable.
• Vertebrate spinal reflexes (such as the stretch reflex and the flexion reflex) have the simplest and best-understood neural circuits for a vertebrate behavior. Even the simplest vertebrate reflexes, however, have large numbers of neurons in a circuit.
• The primary synaptic input of spinal motor neurons is from the central nervous system (CNS); sensory (reflex) input is secondary. Many reflexes mediate adjustments of centrally programmed movements, such as load compensation.

Neural Generation of Rhythmic Behavior

• Most rhythmic patterns of animal behavior (walking, swimming, flying, and so on) involve a central pattern generator (CPG), which can produce the basic motor pattern without requiring sensory input at particular times in the cycle. The CPG interacts with sensory feedback from the cyclic movements, which can entrain the CPG.
• CPG circuits may depend on cellular oscillators, network oscillators, or a combination of both, as in the crustacean stomatogastric ganglion.
• CPG circuits are subject to neuromodulation, in which a neurotransmitter/neuromodulator can alter circuit function to generate or alter rhythmic output.

Control and Coordination of Vertebrate Movement

• The vertebrate brain and spinal cord interact in the generation of behavior patterns such as locomotion and voluntary movements.
• Tetrapod vertebrates have a spinal CPG for stepping during walking. Descending commands from the brain can activate the CPG, and sensory feedback can modulate it.
• Several brain areas are important in generating and coordinating movements in mammals. In the cerebral cortex, the primary motor cortex directly activates spinal motor centers to generate movements; premotor cortical areas are involved in planning and organizing movements.
• The cerebellum and the basal ganglia are connected to the cerebral cortex in looping circuits. The cerebellum is active in coordinating movements and in motor learning. The basal ganglia are involved in the selection and initiation of movements by disinhibition. Parkinson’s and Huntington’s diseases stem from abnormalities in function of the basal ganglia.