Introduction to Immune-Mediated Neuropathies: A Brief Overview of the Nervous System

Abstract

The human nervous system is organized into the central and peripheral nervous systems, CNS and PNS, respectively. The brain and spinal cord constitute the CNS. The PNS links the CNS to various organs of the body. Spinal nerves are in direct communication with the spinal cord. Each spinal nerve (peripheral nerve) contains a bundle of nerve fibers (axons) that extend from the posterior (dorsal) and anterior (ventral) roots of the spinal cord. The spinal nerve roots connect via the interneurons. The spinal nerve roots join together within the intervertebral foramina to form the spinal nerve. Each spinal nerve has two components: sensory and motor. The sensory component is called the afferent component (Latin: afferēns means to bring toward). The afferent component carries the information from the various organs of the body and conveys it to the information to the CNS. The afferent component of the peripheral nerve comprises sensory nerve fibers and receptors. The dorsal root is composed of afferent sensory nerve fibers (axons) that transmit visceral and somatic sensory information from peripheral receptors back to the central nervous system. The peripheral receptors are biological structures closely associated with the distal portion of the sensory neuron. Their function is to convert external stimuli into electrical impulses (action potentials) in the associated axons. Sensory nerve fibers are a group of axons situated at the distal end of dorsal roots. The cell bodies of sensory nerve fibers are grouped that constitute the dorsal root ganglia, also known as sensory or spinal ganglia. These ganglia convey the inputs from the receptors via the sensory nerve fibers to the spinal cord. Similarly, the cranial nerves transmit the sensory information from the various body parts through the cranial nerves to the brain. The efferent (Latin: efferre means to bear off) component of the peripheral nerve comprises motor nerve fibers. They carry the information from the spinal cord to the effector organs. The cell bodies of the motor nerve fibers are localized within the anterior horns of the spinal cord. The efferent component of the PNS has somatic and autonomic divisions. The somatic (Greek: soma meaning body) division is under the direct control of the brain and controls the voluntary (skeletal) muscle. Electrical impulses from the brain are relayed to the skeletal muscle via the neuromuscular junction (NMJ), finally resulting in muscle contraction. Unlike, somatic division, autonomic division is not under the direct control of the brain and innervates the voluntary (smooth and cardiac muscles). The cranial nerves transmit the outputs of the brain to various body parts like the peripheral nerves. Functionally, the neurons are highly organized into specific neuronal circuits that process various inputs and control the desired output. The most simplified circuit is the reflex arc. Reflex arc is the output (effector response) that involves the interaction between the sensory and motor neurons. The first component of the reflex arc is a receptor that when stimulated initiates an electrical impulse (action potential) in afferent nerve fibers. The second component is an afferent neuron which transmits the impulse to the CNS. The third component is an interneuron (relay neuron) within the CNS, which passes the information to the efferent neuron. In some reflexes, known as monosynaptic reflexes (such as a stretch reflex, this component is absent. The fourth component of the reflex arc is the efferent (motor) neuron, which delivers information from the CNS to the effector organ via the synapse. Finally, the effector organ will respond to the initial stimulus. Interruption of the components of the reflex arc will abolish the response.