HUMAN BIOLOGY

the nerVOus system 247

What do nerves do?

• Nerves, which contain neuron axons, carry signals between
  the brain and spinal cord with the rest of the body.


• Reflexes are the simplest nerve pathways.


• Interneurons in the brain and spinal cord are organized in
  information-processing blocks that interact in circuits.

taKe-hoMe Message

Babies exhibit several reflexes that disappear as the ner-
vous system matures. One example is the “rooting” reflex
of an infant that turns its head toward something that
strokes its mouth or cheek—a response that helps guide the
baby’s mouth toward a breast for suckling.

in the brain and spinal cord,
neurons interact in circuits

In your nervous system, sensory nerves relay information
into the spinal cord, where they form chemical synapses
with interneurons. The spinal cord and brain contain only
interneurons, which integrate the signals. Many interneu-
rons synapse with motor neurons, which carry signals
away from the spinal cord and brain.
In the brain and spinal cord, blocks of hundreds or
thousands of interneurons are parts of interacting circuits.
Each block receives signals—some that excite, others that
inhibit—and then integrates the messages and responds

with new ones. For example, in some regions of the brain

the circuits diverge—the processes of neurons in one block

fan out to form connections with other blocks. Elsewhere

signals from many neurons are funneled to just a few.

And in still other brain regions, neurons synapse back on

themselves, repeating signals among themselves. These

“reverber ating” circuits include the ones that make your

eye muscles twitch as you sleep.

Figure 13.9 Animated! Reflexes are simple but important neural pathways. This diagram shows how nerves are organized in a
reflex that operates when skeletal muscle stretches. (© Cengage Learning)

In the spinal cord, axon

endings of the sensory neuron

release a neurotransmitter that

diffuses across a synapse and

stimulates a motor neuron.

Stretching stimulates sensory

receptor endings in this muscle

spindle. Action potentials propagate

toward the spinal cord.

The stimulation is strong

enough to generate action

potentials that self-propagate

along the motor neuron’s axon.

Stimulation makes the stretched

muscle contract. Ongoing stimulation

and contractions hold the bowl steady.

Fruit being loaded into a bowl

puts weight on an arm muscle and

stretches it. Will the bowl drop? No,

because muscle spindles in the

muscle’s sheath also are stretched.

Axon endings of the motor

neuron synapse with muscle

cells in the stretched muscle.

ACh released from the motor

neuron’s axon endings stimulates

muscle cells.

RESPONSE

Biceps contracts.

Biceps stretches.

muscle

spindle

neuromuscular

junction

STIMULUS

4

3

2

5

6

7

1

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