Page 492 - Fundamentals of anatomy physiology

between the sensory neuron and the motor neuron. These
types are called polysynaptic reflexes. Polysynaptic reflexes have
a longer delay between stimulus and response. The length of
the delay is proportional to the number of synapses involved.
Spotlight Figure 13–14 highlights these reflexes.
Processing Sites
In spinal reflexes, the important interconnections and pro-
cessing events occur in the spinal cord. We discuss these reflexes
further in the next section. We consider reflexes processed in
the brain, called cranial reflexes, in Chapters 14, 16, and 17.
Checkpoint
15. Define reflex.
1 6. What is the minimum number of neurons in a reflex
arc?
1 7. One of the first somatic reflexes to develop is the
suckling reflex. Which type of reflex is this?
See the blue Answers tab at the back of the book.
13-7 Spinal reflexes vary
in complexity
Learning Outcome Distinguish among the types of motor responses
produced by various reflexes, and explain how reflexes interact to produce sponsible for the resting muscle tone and, at greater levels of
complex behaviors.
Spinal reflexes range in complexity from simple monosynap-
tic reflexes involving a single segment of the spinal cord to
polysynaptic reflexes that involve many segments. In the most
complicated spinal reflexes, called intersegmental reflex arcs,
many segments interact to produce a coordinated, highly vari-
able motor response.
Monosynaptic Reflexes
In monosynaptic reflexes, there is little delay between sensory
input and motor output. These reflexes control the most rapid,
stereotyped (preexisting, mechanically repetitive) motor responses
of the nervous system to specific stimuli.
The Stretch Reflex
The best-known monosynaptic reflex is the stretch reflex,
which automatically regulates skeletal muscle length. The knee-
jerk or patellar reflex is an example. When a physician taps
your patellar tendon with a reflex hammer, receptors in the
quadriceps muscle are stretched (Spotlight Figure 13–14). The
distortion of the receptors in turn stimulates sensory neurons
that extend into the spinal cord, where they synapse on motor
neurons that control the motor units in the stretched muscle.
This leads to a reflexive contraction of the stretched muscle that
extends the knee in a brief kick.
Chapter 13 The Spinal Cord, Spinal Nerves, and Spinal Reflexes 479
To summarize: The stimulus (increasing muscle length)
activates a sensory neuron, which triggers an immediate motor
response (contraction of the stretched muscle) that counteracts
the stimulus. The entire reflex is completed within 20–40 msec
because the action potentials traveling toward and away from
the spinal cord are conducted along large, myelinated Type
A fibers.
The receptors in stretch reflexes are called muscle spindles.
(We describe the sensory mechanism in the next section.) The
stretching of muscle spindles produces a sudden burst of activ-
ity in the sensory neurons that monitor them. This in turn leads
to stimulation of motor neurons that control the motor units
in the stretched muscle. The result is rapid muscle shortening,
which returns the muscle spindles to their resting length. The
rate of action potential generation in the sensory neurons then
decreases, causing a drop in muscle tone to resting levels.
Muscle Spindles
The sensory receptors involved in the stretch reflex are muscle
spindles. Each consists of a bundle of small, specialized skeletal
13muscle fibers called intrafusal muscle fibers (Figure 13–16).
The muscle spindle is surrounded by larger skeletal muscle
fibers, called extrafusal muscle fibers. These fibers are re-
stimulation, for the contraction of the entire muscle.
Figure 13–16 A Muscle Spindle. The location, structure,
and innervation of a muscle spindle.
Gamma
efferent
from CNS
Extrafusal To CNS
fiber
Gamma
Sensory efferent
region from CNS
Intrafusal
fiber
Muscle
spindle