Key points

■ A reflex is a stereotypical somatic or autonomic activity triggered by a specific stimulus. The reflex arc involves sensory input, connection in the CNS to the UMN, the LMN, neuromuscular junction and muscle.

■ Muscle tone and bulk depends on LMN function.

■ UMNs initiate, regulate, modify and terminate the activity of the LMN. UMNs may inhibit or facilitate LMNs. Loss of inhibitory UMN function results in increased muscle tone and spinal reflexes, whereas loss of facilitatory UMNs results in paresis or paralysis.

■ The UMN system originating from the motor cortex is responsible for voluntary and learned movement of the face, body and limbs using the corticonuclear and corticospinal/pyramidal tracts, respectively. It is more important in primates and humans than quadrupeds.

■ The extrapyramidal system is responsible for maintaining posture and rhythmical/semiautomatic activities including locomotion. Extrapyramidal UMN tracts originate primarily in the brainstem and their fibres do not travel in the pyramids. In quadrupeds, this system is of primary importance.

■ Specific UMN tracts are excitatory or inhibitory to LMNs. Movement is ultimately expressed through the LMNs stimulating muscles.

■ Loss of UMN input typically results in paresis or paralysis with normal to increased muscle tone and spinal reflexes caudal to the lesion. Muscle atrophy is mild and due to disuse.

■ Loss of the LMNs results in paresis/paralysis, with decreased to absent muscle tone and reflexes. Muscle atrophy can be severe and is neurogenic in origin.

■ The ‘Neuro RAT’ helps differentiate between UMN and LMN signs - Reflexes, Atrophy and Tone.

■ Planning of motor activity takes place in the forebrain and motor function is modulated by input from the cerebellum and basal nuclei.