hot plate, a local reﬂ ex arc will be initiated by pain receptors
in the skin of your hands, normally causing you to drop the
plate. If it contains your dinner, however, descending com-
mands can inhibit the local activity, and you can hold onto the
plate until you can put it down safely.
Local Afferent Input
As just noted, afferent ﬁ bers usually impinge upon the local
interneurons (in one case that will be discussed shortly, they
synapse directly on motor neurons). The afferent ﬁ bers bring
information from sensory receptors located in three places:
(1) in the skeletal muscles controlled by the motor neurons,
(2) in other nearby muscles, such as those with antagonistic
actions, and (3) in the tendons, joints, and skin of body parts
affected by the action of the muscle.
These receptors monitor the length and tension of the
muscles, movement of the joints, and the effect of movements
on the overlying skin. In other words, the movements them-
selves give rise to afferent input that, in turn, inﬂ uences how
the movement proceeds. As we will see next, their input some-
times provides negative feedback control over the muscles,
and also contributes to the conscious awareness of limb and
Stretch receptors embedded within muscles monitor muscle
length and the rate of change in muscle length. These recep-
tors consist of peripheral endings of afferent nerve ﬁ bers
wrapped around modiﬁ ed muscle ﬁ bers, several of which are
enclosed in a connective-tissue capsule and are collectively
). The modiﬁ ed muscle
ﬁ bers within the spindle are known as
skeletal muscle ﬁ bers that form the bulk of the muscle and
generate its force and movement are the
Within a given spindle are two kinds of stretch recep-
tors. One responds best to how much the muscle has been
(nuclear chain ﬁ
whereas the other responds
to both the magnitude of the stretch and the speed with
which it occurs
(nuclear bag ﬁ
Although the two kinds
of stretch receptors are separate entities, we will refer to them
collectively as the
muscle-spindle stretch receptors.
The muscle spindles are parallel to the extrafusal ﬁ bers.
Thus, an external force stretching the muscle also pulls on the
intrafusal ﬁ bers, stretching them and activating their receptor
). The more or the faster the muscle is
Converging inputs to local interneurons that control
motor neuron activity. Plus signs indicate excitatory
synapses and minus signs an inhibitory synapse. Neurons
in addition to those shown may synapse directly onto
Many spinal cord interneurons release the
neurotransmitter glycine, which opens chloride channels
on postsynaptic cell membranes.
Given that the plant-
derived chemical strychnine blocks glycine receptors,
predict the symptoms of strychnine poisoning.
Answer can be found at end of chapter.
Excitatory and inhibitory
A muscle spindle and Golgi tendon organ. The muscle spindle is
exaggerated in size compared to the extrafusal muscle ﬁ bers. The
Golgi tendon organ will be discussed later in the chapter.
Adapted from Elias, Pauly, and Burns.