35: Spinal Reflexes

Introduction

• Reflexes Are Adaptable to Particular Motor Tasks

• Spinal Reflexes Produce Coordinated Patterns of Muscle Contraction

  • Cutaneous Reflexes Produce Complex Movements That Serve Protective and Postural Functions

  • The Stretch Reflex Resists the Lengthening of a Muscle

• Local Spinal Circuits Contribute to the Coordination of Reflex Responses

  • The Stretch Reflex Involves a Monosynaptic Pathway

  • Ia Inhibitory Interneurons Coordinate the Muscles Surrounding a Joint

  • Divergence in Reflex Pathways Amplifies Sensory Inputs and Coordinates Muscle Contractions

  • Convergence of Inputs on Ib Interneurons Increases the Flexibility of Reflex Responses

• Central Motor Commands and Cognitive Processes Can Alter Synaptic Transmission in Spinal Reflex Pathways

  • Central Neurons Can Regulate the Strength of Spinal Reflexes at Three Sites in the Reflex Pathway

  • Gamma Motor Neurons Adjust the Sensitivity of Muscle Spindles

• Proprioceptive Reflexes Play an Important Role in Regulating Both Voluntary and Automatic Movements

  • Reflexes Involving Limb Muscles Are Mediated Through Spinal and Supraspinal Pathways

  • Stretch Reflexes Reinforce Central Commands for Movements

• Damage to the Central Nervous System Produces Characteristic Alterations in Reflex Response and Muscle Tone

  • Interruption of Descending Pathways to the Spinal Cord Frequently Produces Spasticity

  • Transection of the Spinal Cord in Humans Leads to a Period of Spinal Shock Followed by Hyperreflexia

• An Overall View

During purposeful movements the central nervous system uses information from a vast array of sensory receptors to ensure that the pattern of muscle activity suits the purpose. Without this sensory information movements tend to be imprecise, and tasks requiring fine coordination in the hands, such as buttoning one's shirt, are impossible.

Charles Sherrington was among the first to recognize the importance of sensory information in regulating movements. In 1906 he proposed that simple reflexes—stereotyped movements elicited by activation of receptors in skin or muscle—are the basic units for movement. He further posited that complex sequences of movements can be produced by combining simple reflexes. This view guided motor physiology for much of the 20th century.

The view that reflexes are automatic, stereotyped movements in response to stimulation of peripheral receptors arose primarily from laboratory studies of reflexes in animals with central nervous system lesions. Once investigators began to measure reflexes in intact animals engaged in normal behavior, ideas about reflexes changed. We now know that reflexes are flexible, that under normal conditions they can be adapted to a task. The prevalent view today is that reflexes are integrated by centrally generated motor commands into complex adaptive movements.

In this chapter we consider the principles underlying the organization and function of reflexes, focusing on spinal reflexes. The sensory stimuli for spinal reflexes arise from receptors in muscles, joints, and skin, and the neural circuitry responsible for the motor response is entirely contained within the spinal cord.

Reflexes Are Adaptable to Particular Motor Tasks

A good example of the adaptability of reflexes is how certain reflexes change in response to stretching the wrist ...