Ch 9: Motor Unit and Neuromuscular Junction

synaptic vesicle

Small membranous containers for neurotransmitter substances which are formed by a Golgi apparatus and deliver the neurotransmitter into the synaptic cleft by a process of exocytosis.

neurotransmitter

A chemical substance, e.g., acetylcholine, dopamine, etc., a signal molecule which is released from the presynaptic neuron's axon end bulbs upon the arrival of an action potential/nerve impulse to transmit a stimulus across the synapse by exocytosis and diffusion and then binding to a receptor site on a chemically gated ion channel, opening those channels; the influence of this stimulus on the postsynaptic cell may be excitatory or inhibitory.

action potential = nerve impulse

A momentary change in electrical voltage potential (a wave of depolarization) on the surface of a nerve or muscle cell or gland cell which takes place when the cell is stimulated at or above its threshold level; the all-or-none action potential is capable of transmitting information (inhibition or excitation) to the next cell(s) in the communication pathway.

acetylcholine receptor

An integral membrane protein which is a gated Na+ channel which opens in response to the reversible binding of the neurotransmitter acetylcholine; this receptor is located in the post-synaptic cell membranes of skeletal muscle cells, cardiac muscle cells, some smooth muscle cells, some glandular cells, all autonomic postganglionic cells, and a variety of of CNS neurons.

motor end plate

The area on the sarcolemma within the synapse which contains acetyl choline receptors and various gated ion channels and which contains the enzyme cholinesterase which can catabolize the neurotransmitter, acetyl choline; the binding of acetyl choline (released from the motor neuron's axon terminal bulb) to the gated Na+ channels initiates depolarization of the sarcolemma at this postsynaptic communication point (the motor end plate).

depolarization

The change in membrane voltage potential which triggers the action potential or nerve impulse; the change is due to the opening of gated Na+ channels which allow Na+ ions to enter the cell cytoplasm causing the resting potential of -70 mV to change to +30 mV in the cell interior.

acetylcholinesterase

The enzyme located at the motor end plate which breaks down the neurotransmitter acetyl choline in the synapse and which is responsible for stimulating terminating the stimulation of the skeletal muscle cell to make it contract.

motor unit

The functional unit of a skeletal muscle (organ), composed a voluntary motor neuron and the one or more skeletal muscle fibers which it innervates; muscles with large ratio motor units (1 neuron: many muscle fibers) can provide powerful contractions but cannot provide delicate control for precision movements while muscles with small ratio motor units (1 neuron: few muscle fibers) do not typically provide powerful contractions but can provide delicate control for very precise movements.

neuromuscular junction = myoneural junction

The junction or connection, the synapse between the end bulbs of the axon terminals of a nerve fiber and the skeletal muscle cell(s) it stimulates to contract using acetyl choline as the excitatory neurotransmitter to open the chemically-gated Na+ channels to depolarize the sarcolemma; the location for the beginning (excitation) of excitation-coupling of muscle contraction.

synapse = synaptic cleft

The junction, space, cleft or gap across which a nerve impulse passes through the action of a neurotransmitter released from an axon terminal to stimulate the next excitable cell in the pathway, either another neuron, or a muscle cell or gland cell; transmission may be by an electrochemical depolarization event or by release of neurotransmitter molecules into the space.

acetylcholine (ACh)

The peripheral excitatory neurotransmitter released at the neuromuscular junction and responsible for stimulating skeletal muscle cells to contract; ACh also serves as a neurotransmitter in the periphery in the autonomic nervous system and in the central nervous system.

axon terminal = synaptic end bulb

The swollen or knob-like distal endings to the terminal branches of axons (and axon collaterals) which house the secretory components of the neuron; the site of synthesis, packaging, storage and release of neurotransmitter molecules.

motor neuron

The type of neuron whose cell body is located in the brain or spinal cord, and whose axon runs along a peripheral nerve to synapse with an individual effector cell or group of cells to stimulate the effector into action (contraction or secretion); somatic motor neurons innervate skeletal muscle and activate contraction of a group of skeletal muscle cells, i.e., a motor unit; visceral or autonomic motor neurons innervate visceral effectors, i.e., smooth muscle, cardiac muscle, endocrine glands or exocrine glands.

axon

The usually long process of a nerve fiber that conducts impulses (action potentials) away from the cell body of the nerve cell and to the next cell in the control pathway.