Nerve cell, illustration. Nerve cells, or neurons, are responsible for passing information around the central nervous system (CNS) and from the CNS to the rest of the body. Each nerve cell comprises a body surrounded by numerous extensions called dendrites. Dendrites collect information from other nerve cells or from sensory cells. Each neuron also has one process called an axon, which passes information to other nerve cells, or to effector cells such as muscle fibres. Within the cell body is the nucleus (round), its outer membrane pitted with pores. The nucleolus (sphere) is within the nucleus. Surrounding the nucleus, and continuous with the nuclear membrane, is the endoplasmic reticulum (ER). Some parts of the ER are studded with ribosomes (black dots), sites of active protein synthesis. Mitochondria (blue) are the sites of energy synthesis within the cell.