This is the natural pacemaker of the HEART, and comprises a collection of specialised muscle cells in the wall of the upper chamber (atrium) of the heart. The cells initiate electrical impulses at a rate of up to 100 a minute. These impulses stimulate the muscles of the heart to contract. The rate is altered by the effects of certain hormones and various impulses from the nervous system. Damage or disease of the node affects the regular beating of the heart. (See also CARDIAC PACEMAKER.)