the muscles themselves. The term
which means “weak”
or “soft,” is often used to describe hypotonic muscles.
Maintenance of Upright Posture
The skeleton supporting the body is a system of long bones
and a many-jointed spine that cannot stand erect against the
forces of gravity without the support provided through coor-
dinated muscle activity. The muscles that maintain upright
posture—that is, support the body’s weight against gravity—
are controlled by the brain and by reﬂ ex mechanisms “wired
into” the neural networks of the brainstem and spinal cord.
Many of the reﬂ
ex pathways previously introduced (e.g., the
stretch and crossed-extensor reﬂ exes) are active in posture
Added to the problem of maintaining upright posture
is that of maintaining balance. A human being is a very tall
structure balanced on a relatively small base, with the center
of gravity quite high, just above the pelvis. For stability, the
center of gravity must be kept within the base of support the
feet provide (
). Once the center of gravity has
moved beyond this base, the body will fall unless one foot is
shifted to broaden the base of support. Yet, people can oper-
ate under conditions of unstable equilibrium because complex
maintain their balance.
The afferent pathways of the postural reﬂ exes come from
three sources: the eyes, the vestibular apparatus, and the recep-
tors involved in proprioception (joint, muscle, and touch recep-
tors, for example). The efferent pathways are the alpha motor
neurons to the skeletal muscles, and the integrating centers
are neuron networks in the brainstem and spinal cord.
In addition to these integrating centers, there are centers
in the brain that form an internal representation of the body’s
geometry, its support conditions, and its orientation with
respect to vertical. This internal representation serves two
purposes: (1) it provides a reference frame for the perception
of the body’s position and orientation in space and for plan-
ning actions, and (2) it contributes to stability via the motor
controls involved in maintaining upright posture.
There are many familiar examples of using reﬂ exes to
maintain upright posture; one is the crossed-extensor reﬂ ex.
As one leg is ﬂ exed and lifted off the ground, the other is
extended more strongly to support the weight of the body,
and the positions of various parts of the body are shifted to
move the center of gravity over the single, weight-bearing leg.
This shift in the center of gravity, as
strates, is an important component in the stepping mechanism
It is clear that afferent input from several sources is neces-
sary for optimal postural adjustments, yet interfering with any
one of these inputs alone does not cause a person to topple over.
Blind people maintain their balance quite well with only a slight
loss of precision, and people whose vestibular mechanisms have
been destroyed have very little disability in everyday life as long
as their visual system and somatic receptors are functioning.
The conclusion to be drawn from such examples is that
the postural control mechanisms are not only effective and
ﬂ exible, they are also highly adaptable.
Walking requires the coordination of hundreds of muscles,
each activated to a precise degree at a precise time. We initiate
walking by allowing the body to fall forward to an unstable
position and then moving one leg forward to provide support.
When the extensor muscles are activated on the supported
side of the body to bear the body’s weight, the contralateral
extensors are inhibited by reciprocal innervation to allow the
nonsupporting limb to ﬂ ex and swing forward. The cyclical,
alternating movements of walking are brought about largely
by networks of interneurons in the spinal cord at the local
level. The interneuron networks coordinate the output of the
various motor neuron pools that control the appropriate mus-
cles of the arms, shoulders, trunk, hips, legs, and feet.
The network neurons rely on both plasma-membrane
spontaneous pacemaker properties and patterned synaptic
activity to establish their rhythms. At the same time, how-
ever, the networks are remarkably adaptable, and a single net-
work can generate many different patterns of neural activity,
depending upon its inputs. These inputs come from other
local interneurons, afferent ﬁ bers, and descending pathways.
These complex spinal cord neural networks can even
produce the rhythmical movement of limbs in the absence of
Center of gravity
The center of gravity is the point in an object at which, if a string
were attached and pulled up, all the downward force due to
gravity would be exactly balanced. (a) The center of gravity must
remain within the upward vertical projections of the object’s
base (the tall box outlined in the drawing) if stability is to be
maintained. (b) Stable conditions: The box tilts a bit, but the
center of gravity remains within the base area, so the box returns
to its upright position. (c) Unstable conditions: The box tilts so
far that its center of gravity is not above any part of the object’s
base—the dashed rectangle on the ﬂ oor—and the object will fall.