The nervous system in humans consists of the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS comprises the brain and the spinal cord. The PNS is composed of nerves that run from the brain and spinal cord which serve as linkages between the CNS and the body. The PNS is made up of motor neurons and sensory neurons which run from the brain and spinal cord (cranial and spinal nerves).
There are three types of neurons, the previously mentioned motor and sensory neurons and intermediate neurons.
Sensory neurons send nerve messages known as impulses from receptors in the body to the CNS. Motor neurons carry impluses from the CNS to effectors in the body. Intermediate neurons (also called interneurons or relay neurons) are neurons that connect sensory neurons to motor neurons.
Please note that adjacent neurons don't actually touch, but are actually separated by spaces called synapses. Chemicals are released into the synapses by synaptic knobs, which cause impulses to form in adjacent neurons. This ensure that impulses only travel in one direction.
When a receptor detects a stimulus, nerve cells known as neurons carry the signal from the stimulated receptor to the central nervous system to the correct effector. A sensory neuron carries the message from the receptor to the central nervous system (the spinal cord and brain). A motor neuron carries the message from the central nervous system to the effector. This is known as a reflex arc.
The reflex arc is a neural pathway that controls a reflex.
Reflexes are automatic, quick, involuntary responses to stimuli by a muscle or gland. Simple reflex actions occur without conscious thought. They are not learned, but rather are necessary for survival.
Simple reflex actions can either be cranial or spinal based on which part of the central nervous system the impulse passes through.
The impulses of cranial reflexes pass through the brain, for example, the pupil reflex, sneezing, coughing and saliva production
The impulses of spinal reflexes pass through the spinal cord, like the knee jerk reflex and the withdrawal reflex.
You may be wondering how exactly the correct connection is made between a sensory and motor neuron. This is the work of the intermediate neurons of the central nervous system, and it is called integration.
Integration is like what happens when you call a friend. The request goes to a central office at your service provider (like the impulse goes through a sensory neuron to the central nervous system). The central office directs your request to your friend (integration).
Additional Reading and Sources