Alpha Motor Neurons and the Neuromuscular Junction
Lower Motor Neurons
Each effector of the motor system (glands, smooth muscle, cardiac muscle, striated muscle) is controlled by a motor neuron.
In the somatic (voluntary) motor system, each α motor neuron controls between several and thousands of individual muscle fibers. This α motor neuron and the group of muscle fibers it controls are collectively called the motor unit (Unit 10 Objective 5).
The α motor neuron is the last point of contact between the nervous system and the effector organ. You can think of it as the point where the baton is passed from one type of tissue to another.
In the case of somatic motor systems, the effector organ that receives the baton is skeletal muscle. The baton is the neurotransmitter acetylcholine and it acts on nicotinic acetylcholine receptors in the muscle’s postsynaptic membrane. These nicotinic acetylcholine receptors are excitatory; they cause muscle contraction (Units 10 & 13). Remember that the entire process, from α motor neuron activation to muscle contraction, is called excitation-contraction coupling. Anatomically, the structure where nerve contacts muscle is called the neuromuscular junction or motor end-plate. It’s just a special name for the synapse between muscle and nerve.
Upper Motor Neurons
All the other neurons in the brain and spinal cord that influence movement, but do not make direct contact with a skeletal muscle fiber, are called upper motor neurons. Examples of these upper motor neurons are neurons of the precentral gyrus (primary motor cortex) of the brain; neurons in the basal ganglia (basal nuclei); and neurons of the cerebellum.
The neurons which make up the pathway from Brodmann area 4 (primary motor cortex) to the cell bodies of α motor neurons in the anterior horn of spinal cord, which we will see later in this objective and name the corticospinal (or pyramidal) tract, are examples of upper motor neurons.