The Human Nervous System controls voluntary and involuntary actions using the central nervous system (CNS) and the peripheral nervous system (PNS). The central nervous system consists of the brain and the spinal cord and serves as the main control center. The peripheral nervous system consists of the associated nerve networks, whose function is to connect parts of the body with to CNS. The PNS is divided into the autonomic nervous system, which controls the involuntary activities like digestion and breathing, whilst the somatic nervous system controls voluntary actions by transmitting stimuli information to the CNS, and sending back response signals to muscles. The autonomic nervous system is further divided into the sympathetic, parasympathetic and enteric nervous system – sympathetic nervous system deals with fight-or-flight signals, parasympathetic maintains body conditions, and enteric nervous system oversees gastrointestinal functions. The divisions of the Nervous System are displayed in the diagram:
There are two types of nerves in the PNS, which are sensory and motor neurons. Sensory neurons carry information from receptors that detect stimuli in the surrounding environment to the motor neurons. The motor neurons are located in the spinal cord, it transfers signals from sensory neurons to the brain, and back to effector muscles to produce a response. Neurons communicate to each other by releasing neurotransmitters at axons, which travel through the synapse to bind to receptors on the dendrites of the adjacent neuron – this creates an action potential. There are two types of receptors, voltage-gated ion channels and second messenger systems. When the voltage of the post-synaptic membrane reaches the threshold, the voltage-gated ion channels open selectively allowing specific ions to enter. In second messenger systems, molecular alterations take place which increase or decrease the cell sensitivity to a certain stimuli.
A reflex arc is when a stimuli-associated signal is transported to the motor neuron in the spinal cord by the sensory neuron, it bypasses the brain and the motor neuron then sends a signal to effector muscles to respond. An example is when a hand responds to a heat source, and withdraws immediately.