Page 927 - Fundamentals of anatomy physiology

914 Unit 5 Environmental Exchange

Figure 24–5 The Regulation 2
of Digestive Activities. Neural Control Mechanisms

The movement of materials along the digestive tract, as well as many secretory

CNS functions, is primarily controlled by local factors. Short reflexes are triggered by
Long
reflex chemoreceptors or stretch receptors in the walls of the digestive tract; the

Myenteric controlling neurons are located in the myenteric plexus. These reflexes are often
plexus
Short called myenteric reflexes. Long reflexes involving interneu-
reflex
rons and motor neurons in the CNS provide a higher level of
Stretch receptors,
chemoreceptors control over digestive and glandular activities, generally

controlling large-scale peristalsis that

moves materials from one region of the

Peristalsis and digestive tract to another. Long reflexes
segmentation may involve parasympathetic motor fibers
movements in the glossopharyngeal (N IX), vagus (N X),
or pelvic nerves that synapse in the

myenteric plexus.

Buffers, acids, Secretory 3
enzymes released cells Hormonal Control Mechanisms

1 Enteroendocrine Carried by The digestive tract produces at
Local Factors cells bloodstream least 18 hormones that affect
almost every aspect of
Local factors are the primary stimulus for Hormones digestion, and some of them
digestion. They coordinate the responses released also affect the activities of
to changes in the pH of the contents of other systems. These
the lumen, physical distortion of the wall hormones are peptides
of the digestive tract, or the presence of produced by enteroendocrine
chemicals—either specific nutrients or cells, endocrine cells in the
chemical messengers released by cells of epithelium of the digestive
the mucosa. tract. We will consider these
hormones as we proceed down
the length of the digestive tract.

Neural Mechanisms 1 digestive glands in response to the arrival of a bolus. Many

24 Neural mechanisms control digestive tract movement ( 2 ). For neurons are involved. The enteric nervous system has about
example, sensory receptors in the walls of the digestive tract trig-
ger peristaltic movements that are limited to a few centi3meters. as many neurons as the spinal cord, and as many neurotrans-
The motor neurons that control smooth muscle contraction
and glandular secretion are located in the myenteric plexus. mitters as the brain. The specific functions and interactions
These neurons are usually considered parasympathetic, because
some of them are innervated by parasympathetic pregangli- of these neurotransmitters in the enteric nervous system are
onic fibers. However, the plexus also contains sensory neurons,
motor neurons, and interneurons responsible for local reflexes unknown.
that operate entirely outside the control of the central nervous
system. As we noted in Chapter 16, local reflexes are called short Sensory information from receptors in the digestive tract
reflexes. p. 575
is also distributed to the CNS. There it can trigger long reflexes,
In general, short reflexes control localized activities that
involve small segments of the digestive tract. For example, which involve interneurons and motor neurons in the CNS.
they may coordinate local peristalsis and trigger secretion by
p. 575 1

Hormonal Mechanisms 2

Digestive hormones can enhance or diminish the sensitivity of

the smooth muscle cells to neural commands ( 3 ). These hor-

mones, produced by enteroendocrine cells in the digestive tract,

travel through the bloodstream to reach their target organs.