The reflex arc

Components of a reflex arc

A reflex arc has five components, connected in sequence so the signal travels one way from stimulus to response:

1. Receptor. It detects the stimulus and converts it into an electrical signal.
2. Sensory (afferent) neuron. It carries the signal to the spinal cord or brainstem.
3. Integration center. It processes the signal and determines the response.
4. Motor (efferent) neuron. It carries the command to the effector.
5. Effector. This is the muscle or gland that produces the response.

Let’s see those components in more detail.

Receptor: a specialized sensory structure that detects the initial stimulus and converts it into an electrical signal through the process of transduction. Receptors are generally located in peripheral tissues, such as the skin, joints, muscles, and sensory organs. Different receptors respond to specific types of stimuli. For example, receptors in the skin detect pressure, vibration, pain, and temperature. Internal receptors track the body's own conditions: baroreceptors in blood vessels sense blood pressure, chemoreceptors sense blood oxygen, and proprioceptors in muscles and joints sense stretch and limb position. The stimulus produces a graded receptor potential whose size matches the stimulus strength. When the receptor potential rises above threshold, the sensory neuron fires an action potential.

Sensory (afferent) neuron: it transmits the signal from the receptor to the central nervous system (CNS). Its cell body is usually located in the dorsal root ganglion. The afferent fiber enters the spinal cord to transmit the signal to the integration center. This ensures that the sensory information is rapidly conducted.

Integration Center: it is located in the spinal cord or brainstem and processes the incoming sensory information for an appropriate motor response to be determined. In monosynaptic reflexes, the sensory neuron synapses directly with the motor neuron, allowing a very rapid response. In polysynaptic reflexes, the response is more complex and finely modulated, since one or more interneurons are involved.

Motor (efferent) neuron: once the response is determined, the motor neuron transmits the command from the integration center to the effector. Its cell body is located in the ventral horn of the spinal cord, and its axon exits through the ventral root to reach the target tissue.

Effector: the effector is the part of the body that carries out the response to the initial stimulus. The effector can be a skeletal muscle, smooth muscle, cardiac muscle, or a gland.

Types of reflex arcs

Reflex arcs can be classified based on several criteria:

Effector type: reflexes are classified into somatic and autonomic (visceral). In somatic reflexes, skeletal muscles are the effectors, responding with automatic responses (e.g., stretch reflex). In autonomic reflexes, smooth muscles, cardiac muscle, or glands serve as effectors for the regulation of involuntary physiological functions (e.g., baroreceptor reflex).

Site of integration: spinal reflexes are integrated in the spinal cord, without any involvement of brain centers (e.g., flexor withdrawal reflex), while cranial reflexes are integrated in the brainstem with the involvement of cranial nerves (e.g., pupillary light reflex).

Number of synapses: reflexes are classified into monosynaptic, which involve a direct connection between a motor and sensory neuron, without interneurons, allowing for faster response (e.g., patellar (or knee-jerk) reflex), and polysynaptic, involving interneurons between sensory and motor neurons, allowing for more complex slower responses (e.g., withdrawal reflex).

Origin: An innate (unconditioned) reflex is present from birth and genetically defined, while an acquired (conditioned) reflex develops through experience and repetition.

Reflex arcs are grouped by the number of synapses between the sensory and motor neuron. The two types are compared below.