Physiologically, the degree of skeletal muscle contraction is controlled by: (choose all that apply) Select one or more: a. the number of motor units that are activated. b. the frequency of motor neuron action potentials in each motor unit. c. the amount of tonus. d. positive feedback from the muscle fibers to the motor neuron.

Answer is A:The number of motor units that are activated

Explanation:

The frequency of motor neuron action potentials in each motor unit .

The motor unit consists of motor neuron and skeletal muscle fibers and together group of motor unit work in co-ordination to undergo contraction of the muscle. The motor neuron is a type of neuron which helps in sending signal to muscles and hence helps in its muscle contraction. These motor neurons originate within the spinal cord and hence helps in controlling the contraction and relaxation of the muscle. Depending on the number of motor units activated, it will define the degree of muscle contraction. When a greater number of motor units are activated, it will increase the rate of contraction. The frequency and rate at which each of the motor unit generates and send an action potential also helps in determining the contraction degree. When it generates higher action potential, it will relay the signal faster leading to varying degree of muscle contraction.

a. the number of motor units that are activated.

b. the frequency of motor neuron action potentials in each motor unit.

Explanation:

When the need arise to increase the rate of contraction of skeletal muscles, input signals are sent to the brain. Output signal from the brain   arrive as  a simultaneous increase of active motor units within the muscles.This is represented as  increase in Acetycholine neurotransmitters secretions  diffusion  across the synapse, more  Ligand gated  ion channels  opening,  more depolarisation, and   action potentials  propagation and generation. As a results there is rapid increase in  Excitatory Post Synaptic Potential( EPSP) and increase in degree of  Skeletal muscle contraction.

 An increase in amount of  depolarization, and therefore action potential generated, increases the frequencies of action potential at the receptors(receptor potential).This results in the degree of skeletal muscle contraction, because the increased action potential frequency  is  above the threshold levels to elicits responses in the muscle cells.

many other transport proteins, often called pumps, move materials against a concentration gradient. active transport drives molecules across a membrane from a region of lower concentration to a region of higher concentration. this process, shown in figure 5.1, uses transport proteins powered by chemi- cal energy.