Anatomy of a Reflex Arc

Reflex Arc Components

• A reflex arc consists of five main components: sensory receptor, sensory neuron, integrating center, motor neuron, and effector.
• The sensory receptor responds to internal or external stimuli, and the effector responds to nerve signals to counteract the stimulus or remove the body part from the stimulus.

Classification of Reflexes

• Reflexes can be classified into five types: spinal reflex or cranial reflex, somatic reflex or visceral reflex, monosynaptic reflex or polysynaptic reflex, ipsilateral reflex or contralateral reflex, and intersegmental reflex or non-intersegmental reflex.

Types of Reflexes

• Stretch reflex: initiated by muscle spindle proprioceptors, contracts muscles in response to being stretched, and is indirectly involved in reciprocal inhibition.
• Withdrawal (flexor) reflex: initiated by nociceptors, contracts muscles to withdraw the body part from a painful stimulus.
• Crossed-extensor reflex: initiated by nociceptors, contracts antagonistic muscles in the opposite leg to maintain balance.

Conduction Pathways

• Conduction pathways are series of neurons and their associated structures that relay nerve signals between the brain and a specific part of the body.
• There are two types of conduction pathways: sensory pathways and motor pathways.
• Sensory pathways are also known as ascending pathways, while motor pathways are also known as descending pathways.

Sensory Pathways

• There are two categories of sensory pathways: somatosensory and viscerosensory pathways.
• Sensory pathways use a series of two or three neurons to transmit nerve signals from sensory receptors to the brain.
• Sensory tracts are found within all funiculi.
• Primary (first-order) neuron extends from the sensory receptor to the brainstem or spinal cord.
• Secondary (second-order) neuron extends from the primary neuron to the thalamus.
• Tertiary (third-order) neuron extends from the secondary neuron to the parietal lobe.

Somatosensory Pathways

• There are three major somatosensory pathways: posterior column medial-lemniscus pathway, anterolateral (spinothalamic) pathway, and spinocerebellar pathway.
• Posterior column medial-lemniscus pathway provides information about touch, pressure, vibration, and proprioception for conscious interpretation.
• Anterolateral (spinothalamic) pathway provides information about pain, temperature, touch, and pressure for conscious interpretation.
• Spinocerebellar pathway provides information about body position and body movements for subconscious interpretation that helps in maintaining balance and posture.

Motor Pathways

• There are two major types of motor pathways: direct (pyramidal) pathways and indirect (extrapyramidal) pathways.
• Motor tracts are found within the anterior and lateral funiculi.
• Upper motor neuron extends from the cerebral cortex, cerebral nuclei, or a specific nucleus in the brainstem to the lower motor neuron.
• Lower motor neuron extends from the anterior horn of the spinal cord to skeletal muscle fibers.

Direct Motor Pathways

• There are two major direct motor pathways: lateral corticospinal tract and anterior corticospinal tract.
• Lateral corticospinal tract innervates appendicular skeletal muscles that control precise, agile, skilled movements in the distal parts of the upper and lower limbs.
• Anterior corticospinal tract innervates axial skeletal muscles like those in the trunk, neck, and shoulders.

Indirect Motor Pathways

• There are two major indirect motor pathways: rubrospinal tract and tectospinal tract.
• Rubrospinal tract regulates and controls fine, precise, voluntary movements in the distal parts of the upper limbs.
• Tectospinal tract regulates reflexive positional changes of the skeletal muscles of the head and trunk due to auditory and visual stimuli.