Final answer:
Ia spindle afferent nerve fibers form axo-somatic and axo-dendritic synapses in the spinal cord, which are essential for transmitting sensory information to the CNS.
Step-by-step explanation:
Ia spindle afferent nerve fibers form specific types of synapses in the spinal cord. These synapses are critical for transmitting sensory information from muscle spindles to the central nervous system. The correct answer to the types of synapses that Ia spindle afferent nerve fibers form is axo-somatic and axo-dendritic. Axo-somatic synapses are those where the axon of the presynaptic neuron connects to the soma, or cell body, of the postsynaptic neuron. Axo-dendritic synapses occur when the axon terminal of the presynaptic neuron connects to the dendrites of the postsynaptic neuron.
These connections facilitate the transmission of nerve impulses through the release of neurotransmitters at the synaptic cleft, which then bind to receptors on the postsynaptic neuron, thereby passing along the signal. As such, Ia spindle afferent nerve fibers play a crucial role in the reflex arcs and proprioceptive pathways, giving the central nervous system information about the state of muscle stretch and tension.