Final answer:
Both pyramidal tract neurons and afferents from muscle spindle stretch receptors synapse directly on motor neurons in the spinal cord. Pyramidal neurons are part of the CNS, and their axons travel through the corticospinal tract to the spinal cord. Motor neurons in the spinal cord connect to skeletal muscle fibers and are part of the PNS, specifically functioning as efferent nerves.
Step-by-step explanation:
The question asks which neurons synapse directly on motor neurons in the spinal cord. Both pyramidal tract neurons (option A) and afferents from muscle spindle stretch receptors (option C) synapse directly on motor neurons in the spinal cord. Therefore, the correct answer is E) both B and C.
As for parts c through f from the referenced information, part c refers to the axon of the upper motor neuron (pyramidal neuron) traveling to the spinal cord, while part d describes a chemical synapse at which neurotransmitters are released into the synaptic cleft to transmit the signal from neuron A (upper motor neuron) to neuron B (lower motor neuron). Neuron A is in the central nervous system (CNS), and the axon of neuron B, which travels in a nerve to a skeletal muscle cell, is part of the peripheral nervous system (PNS) and is an efferent nerve, as it carries signals away from the CNS to effectors like muscles.