Page 491 - Fundamentals of anatomy physiology

478 Unit 3 Control and Regulation

are directed by your brain, involve multiple synapses and take while walking a tightrope over the Grand Canyon, you might
longer to organize and coordinate. ignore a bee sting on your hand. Under other circumstances
you would probably withdraw your hand immediately, while
Classification of Reflexes shouting and thrashing about as well.

Reflexes are classified on the basis of (1) their development, Nature of the Response
(2) the nature of the resulting motor response, (3) the complex-
ity of the neural circuit involved, or (4) the site of information Somatic reflexes provide a mechanism for the involuntary
processing. These categories are not mutually exclusive. They rep- control of the muscular system. Superficial reflexes are triggered
resent different ways of describing a single reflex (Figure 13–15). by stimuli at the skin or mucous membranes. Stretch reflexes are
triggered by the sudden elongation of a tendon, and thus of the
13 Development of Reflexes muscle to which it attaches. A familiar example is the patellar,
or “knee-jerk,” reflex that is usually tested during physical exams.
Innate reflexes result from the connections that form be- These reflexes are also known as deep tendon reflexes, or myotatic
tween neurons during development. Such reflexes generally reflexes.
appear in a predictable sequence, from the simplest reflex re-
sponses (withdrawal from pain) to more complex motor pat- Visceral reflexes, or autonomic reflexes, control the activi-
terns (chewing, suckling, or tracking objects with the eyes). The ties of other systems. We consider somatic reflexes in detail in
neural connections responsible for the basic motor patterns of this chapter and visceral reflexes in Chapter 16.
an innate reflex are genetically programmed. Examples include
the reflexive removal of your hand from a hot stovetop and The movements directed by somatic reflexes are neither
blinking when your eyelashes are touched. delicate nor precise. You might wonder why they exist at all,
because we have voluntary control over the same muscles. In
More complex, learned motor patterns are called acquired fact, somatic reflexes are absolutely vital, primarily because
reflexes. For example, an experienced driver steps on the brake they are immediate. Making decisions and coordinating vol-
when trouble appears ahead, or a professional skier makes untary responses take time. In an emergency—when you slip
equally quick adjustments in body position while racing. These while walking down a flight of stairs, or accidentally press your
motor responses are rapid and automatic, but they were learned hand against a knife edge—any delay increases the likelihood
rather than preestablished. Repetition enhances such reflexes. of severe injury. Thus, somatic reflexes provide a rapid response
that can be modified later, if necessary, by voluntary motor
The distinction between innate and acquired reflexes is not commands.
absolute. Some people can learn motor patterns more quickly
than others. The differences probably have a genetic basis. Complexity of the Circuit

Most reflexes, whether innate or acquired, can be modified Monosynaptic reflexes involve the simplest reflex arc. Most re-
over time or suppressed through conscious effort. For example, flexes are more complicated and have at least one interneuron

Figure 13–15 The Classification of Reflexes.

development response Reflexes processing site
can be classified by

complexity of circuit

Innate Reflexes Somatic Reflexes Monosynaptic Spinal Reflexes
• Genetically • One synapse
• Control skeletal muscle contractions • Processing in
determined • Include superficial and stretch reflexes Polysynaptic the spinal cord
• Multiple synapses
Acquired Reflexes Visceral (Autonomic) Reflexes (two to several hundred) Cranial Reflexes
• Learned
• Control actions of smooth and cardiac • Processing in
muscles, glands, and adipose tissue the brain