The nervous system has two major division, the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS is made up of the brain and the spinal cord. The CNS is protected by three layers of tissue called meninges – the dura mater, the arachnoid mater and the pia mater. The subarachnoid space is filled with cerebrospinal fluid which acts as a shock absorber and protects the brain. The PNS consists of the cranial nerves (12 pairs) spinal nerves (31 pairs) and ganglia, which are outside the dura of the brain and spinal cord. 
Peripheral nerves are either sensory (afferent) or motor (efferent). Sensory nerves conduct impulses into the CNS. Motor nerves transmit impulses from the CNS to the muscles and glands. The motor nerve fibers are divided into two subgroups , somatic which end in skeletal muscle and we can consciously control, autonomic fibers which we have no control over and innervates cardiac, smooth muscle and glands. The autonomic nervous system is subdivided into sympathetic (speeds up the body's systems) and parasympathetic (slow down the body's systems) nerve routes.
The body's nervous system are organized into a number of reflex arcs. The simplest is the monosynaptic, which occurs at a single level of the spinal cord.
Activating stretch receptors such as tapping on a tendon, sends sensory (afferent) impulses to the spinal cord. They synapse with the axons of the motor (efferent) neurons which branch to the muscle whose tendon was tapped.
Polysynaptic reflexes involve several segments of the spinal cord and brain. These have a series of at least three neurons. Interneurons (excitatory / inhibitory) connect the sensory and motor neurons. This provides a check system to whether a specific muscle contraction is made or not. Having both stimulatory and inhibitory activities is called reciprocal innervation.
Mass limb reflex involves multiple segmental interactions of the spinal cord. This involves movements such as withdrawal from painful stimuli with flexion of one limb (the one with pain) and extension of the other.
Erect postural tone is based in the brain stem and balancing structures in the inner
ear (vestibular apparatus).
The cerebrum controls selective motor control. This level of functioning and control is required for normal gait. Selective control allows for changes in the reflex patterns so that one joint or muscle can be moved independently of the normal reflex patterns. In walking this allows the muscles at the hip, knee , and / or ankle to be ,moved in response to the ground reaction forces.
In the brain, motor activity is controlled by the interaction of three major regions:
the cerebral cortex, the cerebellum and the basal ganglia. Simply the motor control can be subdivided into selective (pyramidal) and automatic (extra – pyramidal).


Upper / Lower Motor Neuron

Looking at the classification of the nervous system, pathological conditions can be divided into two groups. Lower Motor Neuron (LMN) and Upper Motor Neuron (UMN) lesions. LMN disorders affect the cell bodies, anterior nerve roots and / or motor axons (outside the spinal cord). This loss of nerve impulses from the spinal cord to the muscle results in a decrease in muscle tone and flaccid paralysis. In these cases the stretch reflexes is absent. Examples are – polio, muscular dystrophy, spina bifida. UMN disorders result from damages to the cerebral cortex, brain or spinal cord. In these cases spastic paralysis is seen. This shows an inability to control the muscle voluntarily. Examples are cerebral palsy, cerebral vascular accident (CVA) and spinal cord injuries.