Muscle Contraction

The nervous system performs its functions as neurons (nerve cells that transmit nerve impulses) innervate muscle structures. For a muscle to contract, the nervous system must first receive sensory inputs. During this process, sensory neurons take nerve impulses from sensory receptors in the skin and other organs to the central nervous system (i.e. brain and spinal cord) in response to external and internal stimuli. For example, to open a book by arm muscle contraction, continual sensory input to the central nervous system from the eyes and hands must be made to inform the position of the book. In a neuron, the dendrites (short extensions) receive signals from sensory receptors or other neurons, and the signal pass to the cell body before reaching an axon (portion of a neuron that conducts nerve impulses). Nerve impulses from sensory receptors (distal end of the long axon of a sensory neuron) of eyes travel through the sensory neuron from the dendrite-like structures projecting from the peripheral (outside of the CNS) end of the axon to the CNS axon. The central nervous system then performs integration- it sums up the input it receives from all over the body before commanding the muscle contraction in the arm to proceed. In the brain, the cerebral cortex, or a thin but convoluted outer layer of gray matter that covers the cerebral hemispheres contains motor, sensory, and association areas that are important for muscle contractions. The primary motor area in the frontal lob is the beginning of skeletal muscles. Then the primary motor area sends signals to the cerebellum, which integrates them. The primary somatosensory area dorsal to the central sulcus in the parietal lobe is where the sensory information from the skin and skeletal muscles arrive. There, a primary visual area receives information from the eyes. Association areas are places where integration occurs. The somatosensory association area processes and analyzes sensory information from the skin and muscles. Through the processes in the CNS, the system decides whether to send the command for muscle contraction. At any time, integration with other sensory data might cause the CNS to command a different motion instead. Finally, motor neurons (muptipolar neurons with many dendrites and a single axon) carry nerve impulses from the central nervous system to the muscles, and cause them to contract.

Action potential, or nerve impulse, occurs after a threshold value, or the minimum change in polarity across the axomembrane that is required for an action potential.