Reflex Arcs - Anatomy & Physiology

Patellar Reflex

Introduction

A reflex arc represents a mechanism by which a physiological function is automatically managed or regulated. Reflex arcs can be found throughout the body, ranging from skeletal muscles to smooth muscle in glands. Reflex arcs are initiated via the excitation or stimulation of specific sensory cells that are directly connected to motor neurons thus enabling motor nerve impulses to be automatically passed on to that particular muscle or gland. Therefore a basic reflex arc consists of sensory cells and their associated nerve fibers, motor nerve fibres and the ultimate muscle or gland. Some reflex arcs can include a coordination centre within the spinal cord or brain prior to stimulation of the motor nerve. Reflex arcs can involve a single or mutliple segments up and down the body, although reflex arcs do not require brain input in order to function. However, the brain can act to modulate reflexes. The brain obtains its afferent information via the ascending sensory tracts of the spinal cord. The descending tracts originate from the brain to allow responses to be modulated. These tracts constitute the white matter of the spinal cord.

A number of different sensory inputs are utilised by reflex arcs, including; skin receptors, muscle spindles, the retina, the Organ of Corti and the olfactory mucosa. These sensory aspects of reflex arcs feed into two main types of reflex systems in the body; Autonomic reflexes and Somatic reflexes.

Autonomic Reflexes

Autonomic reflexes control and regulate smooth muscle cells, cardiac muscle cells and glands. In general these reflexes contain the same basic components as somatic reflexes but a key difference is that autonomic reflexes have the ability to both stimulate or inhibit the smooth muscle/gland.

For further information regarding the basic principles of the autonomic reflex arcs and for more detailed information, please see the Autonomic Nervous System.

Somatic Reflexes

Somatic reflexes are involved in the reflex control of skeletal muscles and as such there are many different types of somatic reflexes including withdrawal reflexes, scratching reflexes, withdrawal reflexes and stretch reflexes and tendon reflexes. A few of these will be covered in the section below.