Propagation of an action potential in a skeletal muscle cell links the signal from a motor neuron to contraction of the muscle cell. An action potential in a muscle cell is propagated by the same mechanism as in neurons, the sequential opening and closing of voltage-gated Na+ and K+ channels in the plasma membrane. However, in muscle cells, the topography of the plasma membrane is quite different than in neurons, and this difference is critical to the function of muscle cells?

Which of the following statements correctly describe(s) T tubules and their role in conducting action potentials in muscle cells?

a. T tubules have receptor proteins that bind neurotransmitters released from the synaptic terminal of the motor neuron.
b. T tubules are infoldings of the plasma membrane that encircle the myofibrils and are in contact with the sarcoplasmic reticulum.
c. T tubules lack the voltage-gated Na+ and K+ channels that are present in the plasma membrane.
d. T tubules carry action potentials into the interior of the muscle cell via voltage-gated Na+ and K+ channe
e. T tubules are extensions of the sarcoplasmic reticulum that are in contact with the plasma membrane.
f. Without T tubules, the muscle cell would not be able to contract.