Neuron Circuits

Neuron circuits or pathways will determine the direction a signal will travel. In the brain, neurons are organized into regional blocks that receive, integrate, and then send out signals.

The circuits may be divergent, convergent, or reverberating:

• Convergent circuit - Signals from many neurons are sent on to just a few.
• Divergent circuit - Signals from one neuron are sent out to many others.
• Reverberating circuit - A circular circuit that returns a signal to its source.

Signals between brain or spinal cord and body regions travel by nerves. Axons of sensory neurons, motor neurons, or both, are bundled together in a nerve. Within the brain and spinal cord, such bundles are called nerve pathways, or "tracts."

Many axons are covered by a myelin sheath derived in part from Schwann cells. Each section of the sheath is separated from adjacent ones by a node where the axon membrane (plentiful in gated sodium channels) is exposed. The action potentials jump from node to node, which is fast and efficient.

This animation (Audio - Important) describes action potential propagation.

Reflex Arcs

Reflexes are simple, stereotyped movements made in response to sensory stimuli.

• In the simplest reflex, the reflex arc, sensory neurons directly synapse on motor neurons. Most reflexes involve an interneuron.
• In the stretch reflex, receptors of sensory neurons (muscle spindles) transmit impulses to the spinal cord where direct synapses with motor neurons occur.
• In the withdrawal reflex, interneurons in the spinal cord can activate or suppress motor neurons as necessary for a coordinated response.

These two animations (Audio - Important) describe the stretch reflex
stretch reflex 1,
stretch reflex 2.

Take a look at the Information Processing in the Spinal Cord Tutorial.