HUMAN BIOLOGY

expLoreon your oWn

patellar tendon

motor

neuron

reflex arc

extensor muscle of knee

(quadriceps femoris)

muscle

spindle

The “knee jerk” patellar reflex diagrammed in Figure 13.27 is a

familiar example of a reflex arc. A doctor will often use a small rubber-

tipped instrument to test for this reflex, but you can easily trigger it yourself

just by tapping the base of one of your kneecaps (the patella) with a knuckle.

The reflex occurs when stretch receptors in a tendon attached to the patella

are activated, leading to a contraction of the quadriceps femoris, the exten-

sor muscle of the knee.

Try tapping the base of one of your kneecaps (not too forcefully) and see

if you can elicit the patellar reflex. Then write a short paragraph describing

the path of the reflex arc, including the location and kind of neurons that inte-

grated the sensory information and ordered a muscle response.

Figure 13.27 a reflex arc operates in the patellar reflex. (© Cengage Learning)

262 Chapter 13

suMMary

section 13.1 The nervous system

detects, processes, and responds to stimuli.

Sensory neurons respond directly to external

or internal stimuli. Interneurons in the brain

and spinal cord receive sensory signals,

process them, and then send outgoing signals that influence

other neurons. Motor neurons relay messages away from the

brain and spinal cord to muscles or glands. Neuroglia provide

various forms of physical or chemical support for neurons.

Neurons have extensions called axons and dendrites. Axons

carry outgoing signals, and dendrites receive them. A resting

neuron has a steady voltage difference across its plasma

membrane. This difference is called the resting membrane

potential.

A neuron maintains concentration gradients of various

ions, notably sodium and potassium, across the membrane.

Changes in this difference allow a neuron to send signals

(nerve impulses).

section 13.2 When the voltage

difference across the membrane exceeds a

threshold level, gated sodium channels in

the membrane open and close rapidly and

suddenly reverse the voltage difference.

This reversal is a nerve impulse, or action

potential. A sodium–potassium pump restores ion gradients

after an action potential fires. Action potentials propagate

away from the point of stimulation.

section 13.3 Action potentials selfpro-

pagate along the neuron membrane until

they reach a synapse with another neuron,

muscle, or gland. The presynaptic cell

releases a neurotransmitter into the synapse.

The neurotransmitter excites or inhibits the

receiving (postsynaptic) cell. Synaptic integra-

tion sums up the various signals acting on a neuron. Neuro-

modulators boost or reduce the effects of neurotransmitters.

section 13.4 Nerves consist of the long

axons of motor neurons, sensory neurons, or

both. A myelin sheath formed by Schwann

cells insulates each axon, so that action

potentials propagate along it much more

rapidly. Nerve pathways extend from neurons in one body

region to neurons or effectors in different regions.

A reflex is a simple, stereotyped movement in response to

a stimulus. In the simplest reflexes, sensory neurons directly

signal motor neurons that act on muscle cells. In more complex

reflexes, interneurons coordinate and refine the responses.

.

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