The basic unit of structure and function in the nervous system is
the neuron.
a. true
b. false
17
The endocrine system produces chemical messengers called enzymes.
a. true
b. false
18
The thyroid is a link between the nervous and endocrine systems.
a. true
b. false
19
Axons connect sensory neurons to motor neurons.
a. true
b. false
20
The parathyroids regulate the level of calcium in the blood.
a. true
b. false

17 - True, neurons are the most significant or basic unit for transferring nervous signals throughout the body. Nerves which are linked act as wires for connection. The brain wouldn't be able to send or receive signals if there were no neurons

18 - False, hormones are just chemical molecules or messengers that are sent towards tissues or organs through the bloodstream to help regulate the growth or metabolism through homeostasis process

19 - True, axons are the longest part of a neuron which helps for neurotransmission process throughout the whole nerve to the end of the motor neuron

the answer that best describes the relative refractory period in the neuron is the resting membrane potential is restored, but concentrations of sodium and potassium are not restored to their original state.

the refractory period is the time during which another stimulus given to the neuron will not lead to a second potential. thus, because na+ channels are inactivated during this time, additional depolarizing stimuli do not lead to new action potentials.  

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i think the answer would be d