THE SOMATIC MOTOR SYSTEM
The muscles in the body can be described according to two broad categories:
o Lines the digestive tract, arteries and related structures
o Innervated by nerve fibers from the ANS
o Plays a role in peristalsis and the control of blood pressure and blood flow
Striated – there are two types of striated muscles
o Cardiac muscle
is the heart muscle
innervations of the heart from the ANS functions to accelerate or slow
down the heart rate
o Skeletal muscle
Constitutes the bulk of muscle mass in the body
Functions to move bones around joints
Is enclosed in a connective tissue sheath that forms tendons
Contains hundreds of muscle fibers, each innervated by a single axon
branch from the CNS (see Fig. 13.3)
Derived from 33 paired somites
Controlled by the somatic motor system
Under voluntary control and it generates behaviour
Consider elbow joint (see Fig. 13.2)
Movement in the direction that closes the knife is called flexion
o The muscles that cause flexion are called flexors. Because they work together,
they are called synergists.
Movement in the direction that opens the knife is called extension
o The muscles that cause extension are called extensors.
Flexors and extensors pull on a joint in opposite directions and are antagonists.
Muscles that are responsible for movements:
of the trunk are called axial muscles
o important for maintaining posture
of the shoulder, elbow, pelvis, and knee are called the proximal muscles
o important for locomotion
of the hands, feet, and digits are called the distal muscles
o important for specialized manipulation of objects
THE LOWER MOTOR NEURON
Somatic musculature is innervated by the somatic motor neurons in the ventral horn of the
spinal cord (see Fig. 13.3).
These cells are sometimes called lower motor neurons to distinguish them from the
upper motor neurons of the brain that supply input to the spinal cord.
Only the lower motor neurons directly command muscle contraction. The Segmental Organization of Lower Motor Neurons
The axons of lower motor neurons bundle together to form ventral roots. Each ventral root joins
with a dorsal root to form a spinal nerve that exits the cord through the notches between the
Skeletal muscles are not distributed evenly throughout the body, nor are lower motor neurons
distributed evenly within the spinal cord. For example, innervations of the more than 50 muscles
of the arm originate entirely from spinal segments C3-T1. The ventral horns in this region of the
spinal cord appear swollen to accommodate the large number of motor neurons that control the
arm musculature (see Fig. 13.4).
The lower motor neurons are also distributed within the ventral horn at each spinal segment in a
predictable way, depending on their function (see Fig. 13.5).
Alpha Motor Neurons
There are two categories of lower motor neurons of the spinal cord: alpha and gamma motor
neurons. Alpha motor neurons trigger the generation of force by muscles.
One alpha motor neuron and all the muscle fibers it innervates collectively make up the
motor unit (the elementary component of control).
Muscle contraction results from the individual and combined actions of these motor
units. The collection of alpha motor neurons that innervates a single muscle is called a
motor neuron pool.
Graded Control of Muscle Contraction by Alpha Motor Neurons
The nervous system uses several mechanisms to control the force of muscle contraction in a