Exam 3 Review:  Chapter 09:  Motor Unit and Neuromuscular Junction

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.

motor unit - The functional unit of a skeletal muscle, 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.

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.

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.

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.

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.

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., which transmits a nerve impulses across a synapse by exocytosis and diffusion and binding to a receptor site on a gated ion channel; the influence on the postsynaptic cell may be excitatory or inhibitory.

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.

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).

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.

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.

neuromuscular junction = myoneural junction - The junction or connection, the synapse between a axon terminals of a nerve fiber and the skeletal muscle cell(s) it stimulates to contract using acetyl choline as the exitatory neurotransmitter; 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 the next cell in the pathway, either another neuron, or a muscle cell or gland cell.

Describe: (or you may sketch and label)

4. The depolarization of a cell membrane.

[Note:  Depolarization is a part of a larger process which includes the starting point, the resting state, followed by depolarization, repolarization, hyperpolarization, and a return to the resting state.  See Chapter 11 for more details.]

I.  Resting state:
                 a)  all voltage-gated and chemically-gated ion channels are closed
                 b)  membrane potential approximately -70 mV
                 c)  the Na⁺ K⁺ ATPase pumps are operating (3 Na⁺ out/2 K⁺ in per ATP hydrolysis)

         II.  Depolarization phase:
                 a)  a stimulus of threshold strength or greater causes the gated Na⁺ ion channels to open; Na⁺ ions diffuse into the cytoplasm
                 b)  membrane potential reverses from approximately -70 mV to approximately +30 mV
                 c)  the Na⁺ K⁺ ATPase pumps are operating (3 Na⁺ out/2 K⁺ in per ATP hydrolysis)

        III.  Repolarization phase:
                 a)  the gated Na⁺ ion channels slowly close; Na⁺ ions can no longer diffuse into the cytoplasm
                 b)  meanwhile, the gated K⁺ ion channels open; K⁺ ions diffuse out of the cytoplasm
                 c)  membrane potential reverses again, dropping from approximately +30 mV to approximately -70 mV
                 d)  the Na⁺ K⁺ ATPase pumps are operating (3 Na⁺ out/2 K⁺ in per ATP hydrolysis)

         IV.  Hyperpolarization phase:
                 a)  the gated Na⁺ ion channels have closed; Na⁺ ions can no longer diffuse into the cytoplasm
                 b)  meanwhile, the gated K⁺ ion channels slowly close; K⁺ ions can no longer diffuse out of the cytoplasm
                 c)  membrane potential becomes more negative than the resting state, dropping to approximately -80 mV
                 d)  the Na⁺ K⁺ ATPase pumps are operating (3 Na⁺ out/2 K⁺ in per ATP hydrolysis) and will restore the resting state conditions

Sketch and Label:

1. A "motor unit."

2. A neuromuscular junction.