The brain weighs approximately 1.3 kg and is surrounded by warm shock-absorbing fluid called the cerebrospinal fluid.  It is contained in cranium, which consists of three layers of membranes, also referred to as meninges, to protect the brain.  First, it is covered by the most thickest later called the dura mater, the second layer is called archnoid membrane and the finally the thinnest and the most delicate layer is called the pia mater. 

The brain is a part of the Central Nervous System, which also includes the spinal cord.  The Peripheral Nervous System consists of 12 cranial nerves such as Olfactory, Optic, Oculomotor, Trochlear, Trigeminal, Abducens, Facial, Vestibulocochlear, Glossopharyngeal, Vagus, Accessory and Hypoglossal cranial nerves.  The peripheral nervous system also consists of various nerves extending from the spinal cord to the rest of the body. 

The brain is composed of three distinct parts:  Cerebrum, Cerebellum and Brainstem.  Cerebrum is a network of neurons that consists of billions of various neurons cells as well as glial cells.  Cerebellum is also composed of network of neurons, but is different in function-wise.  Brainstem contains fibre tracts that connects the cerebrum and cerebellum to the rest of the body. 

Spinal cord is the major information pathway for the central nervous system.  It contains two tracts:  ascending tract that carries somatosensory information to the brain and descending tract that carries motor control information to spinal motor neurons so that the information can be transmitted to the rest of the body.  Spinal cord may also control behaviours independently.  The reason is because various reflexes are still in tact even without the brain.  It realizes many reflexes such as sensory neurons that detect stimuli, then the relay of information to the brain begins; sensory neurons stimulate interneurons in the spinal cord, which stimulates the motor neurons in the various parts of the brain.

Spinal cord provides rhythmic organization of locomotive movements such as walking or wing flopping.  The complex network of neurons provide the rhythmic organization, which stimulates each other in cyclic manner.  Therefore, the stimuli will produce action potentials in a repeating pattern.  The rhythmic organization of locomotive movement is also referred to as pattern generators.