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CHAPTER

9

Reflexes

OBJECTIVES

After studying this chapter, you should be able to:

■

Describe the components of a reflex arc.

■

Describe the muscle spindles and their role in the stretch reflex.

■

Describe the Golgi tendon organs and analyze their function as part of a feedback

system that maintains muscle force.

■

Define reciprocal innervation, inverse stretch reflex, clonus, and lengthening reaction.

INTRODUCTION

The basic unit of integrated reflex activity is the
reflex arc.

This arc consists of a sense organ, an afferent neuron, one or

more synapses within a central integrating station, an efferent

neuron, and an effector. In mammals, the connection between

afferent and efferent somatic neurons is generally in the brain

or spinal cord. The afferent neurons enter via the dorsal roots

or cranial nerves and have their cell bodies in the dorsal root

ganglia or in the homologous ganglia on the cranial nerves.

The efferent fibers leave via the ventral roots or correspond-

ing motor cranial nerves. The principle that in the spinal cord

the dorsal roots are sensory and the ventral roots are motor is

known as the
Bell–Magendie law.
Activity in the reflex arc starts in a sensory receptor with a

receptor potential whose magnitude is proportional to the

strength of the stimulus (Figure 9–1). This generates all-or-

none action potentials in the afferent nerve, the number of

action potentials being proportional to the size of the genera-

tor potential. In the central nervous system (CNS), the

responses are again graded in terms of excitatory postsynaptic

potentials (EPSPs) and inhibitory postsynaptic potentials

(IPSPs) at the synaptic junctions. All-or-none responses are

generated in the efferent nerve. When these reach the effector,

they again set up a graded response. When the effector is

smooth muscle, responses summate to produce action poten-

tials in the smooth muscle, but when the effector is skeletal

muscle, the graded response is always adequate to produce

action potentials that bring about muscle contraction. The

connection between the afferent and efferent neurons is usu-

ally in the CNS, and activity in the reflex arc is modified by

the multiple inputs converging on the efferent neurons or at

any synaptic station within the reflex loop.

The simplest reflex arc is one with a single synapse between

the afferent and efferent neurons. Such arcs are

monosynaptic,

and reflexes occurring in them are called

monosynaptic

reflexes.

Reflex arcs in which one or more interneuron is inter-

posed between the afferent and efferent neurons are called

polysynaptic reflexes.

There can be anywhere from two to

hundreds of synapses in a polysynaptic reflex arc.