神经系统是由神经元这种特化细胞的网络所构成的。其身体的不同部位间传递信号。动物体藉神经系统和内分泌系统的作用来应付环境的变化。动物的神经系统控制着肌肉的活动，协调各个组织和器官，创建和接受外来情报，并进行协调。神经系统是动物体最重要的连络和控制系统，它能测知环境的变化，决定如何应付，并指示身体做出适当的反应，使动物体内能进行快速、短暂的消息传达来保护自己和生存。

神经组织最早是出现在五亿到六亿年前的埃迪卡拉生物群中。脊椎动物的神经系统分为二部份：分别是中枢神经系统（CNS）及周围神经系统（PNS）。中枢神经系统包括脑及脊髓，周围神经系统主要是由神经构成，是由长神经纤维或是轴突组成，连接中枢神经系统及身体各部位。发送由大脑发出信号的神经称为运动（motor）神经或是下行（efferent）神经，而将身体各部位产生信号发送到中枢神经的神经称为感觉（sensory）神经或是上行（afferent）神经。大部份的神经是双向传递信号，称为混合神经。周围神经系统可分为躯体神经系统、自律神经系统及肠神经系统。躯体神经系统处理随意运动，也就是依生物体意愿而产生的运动，自律神经系统又可分为交感神经及副交感神经，交感神经是在紧急情形时驱动，而副交感神经是在器官呈休息状态时驱动。肠神经系统则控制消化道。自律神经系统及肠神经系统都会不随意愿的自主动作。从脑部发出的神经称为脑神经，而从脊髓发出的神经称为脊神经。

The nervous system is the part of an animal's body that coordinates its voluntary and involuntary actions and transmits signals to and from different parts of its body. Nervous tissue first arose in wormlike organisms about 550 to 600 million years ago. In vertebrate species it consists of two main parts, the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS contains the brain and spinal cord. The PNS consists mainly of nerves, which are enclosed bundles of the long fibers or axons, that connect the CNS to every other part of the body. Nerves that transmit signals from the brain are called motor or efferent nerves, while those nerves that transmit information from the body to the CNS are called sensory or afferent. Most nerves serve both functions and are called mixed nerves. The PNS is divided into a) somatic and b) autonomic nervous system, and c) the enteric nervous system. Somatic nerves mediate voluntary movement. The autonomic nervous system is further subdivided into the sympathetic and the parasympathetic nervous systems. The sympathetic nervous system is activated in cases of emergencies to mobilize energy, while the parasympathetic nervous system is activated when organisms are in a relaxed state. The enteric nervous system functions to control the gastrointestinal system. Both autonomic and enteric nervous systems function involuntarily. Nerves that exit from the cranium are called cranial nerves while those exiting from the spinal cord are called spinal nerves.