Somatic Nervous System: Definition, Functions, Examples, Motor Neurons and Neuromuscular Junction

It consists of neurons that are associated with skeletal or striated muscle fibers and influence voluntary body movements.

The somatic nervous system (SNS), also known as the voluntary nervous system , is part of  the peripheral nervous system  (PNS).

The peripheral nervous system is made up of all the neurons that exist outside of the brain and spinal cord .

It acts as an intermediary between the central nervous system (CNS) and the muscles, skin, and sensory organs. The nerves of the PNS send electrochemical signals between the CNS and the rest of the body.

A large part of the PNS is made up of 12 pairs of cranial nerves and 31 pairs of spinal nerves.

Some of the neurons in these nerves have a sensory function and others have a motor function. The motor neurons that innervate striated muscles form the somatic nervous system.

Somatic nervous system functions

It contains afferent nerves that travel to the central nervous system and efferent nerves responsible for sending signals from the CNS to the rest of the body.

The brain and spinal cord process inputs from a variety of sources and integrate them before devising a response. This response determines the location and strength of the muscle contraction in different parts of the body.

Therefore, the main function of the somatic nervous system is to connect the CNS with skeletal organs and muscles to allow complex movements and behaviors.

Furthermore, the SNS also mediates a subset of involuntary muscular responses called reflex arcs. A reflex arc produces an extremely rapid muscle contraction in response to a stimulus, with minimal intervention from the brain.

While the drive for most voluntary muscle contractions originates in the brain or brainstem, reflex action can occur with a single sensory and motor neuron that synapses in the spinal cord.

The response of the motor is practically “wired” for a particular stimulus. The instinctual response to stimulation of the patellar ligament in the knee is an example of a reflex response.

Other examples include the immediate removal of a hand by touching a hot stove or a quick change in posture when the foot is placed on a sharp stone.

Examples of the somatic nervous system response

The somatic nervous system is intrinsically related to the central nervous system, with SOD sensory and motor neurons that communicate with the brain and spinal cord.

Striated skeletal muscles under voluntary control receive signals to contract based on stimuli transmitted to the CNS. For example, while walking in a tropical forest, look at the forest floor for fallen twigs, insects, or weeds.

As the central nervous system constantly receives visual information, it sends messages to the peripheral nervous system, in particular the SNS, to alter skeletal muscle posture and contractility and accommodate changes to the forest floor surface.

At the same time, if a leech is attached to your calf muscle, sensory neurons indicate the presence of a persistent wet sensation in the leg. Skeletal muscles work by altering their position so that the area can be visually inspected.

Upon encountering a leech, the CNS, through memory and learning, directs the skeletal muscles of the arms and fingers through the SODs to reach some salt.

Gross and fine motor skills are used to sprinkle a pinch of salt on the leech to ensure that it dislodges.

Similar events are taking place within the nervous system in very varied activities. For example, a dancer on stage is integrating her memory of music and choreography in the central nervous system to direct the movement of her skeletal muscles through the SNS.

From the motionless arrangement of your body before the music begins to the last bow and smile, SNS neurons target each group of large and small striated muscles in the body as directed by the CNS.

Motor neurons

The neural pathway that causes skeletal muscle contraction can be functionally divided into two main types of neurons: the upper motor neurons in the central nervous system and the lower motor neurons in the somatic nervous system.

Lower motor neurons can be part of the cranial or spinal nerves. They innervate the muscle fibers and directly cause them to contract.

Upper motor neurons have their cell bodies in the precentral gyrus of the brain. This region is located towards the posterior end of the frontal lobe in the cerebral cortex and is associated with the primary motor cortex.

Upper motor neuron axons related to voluntary muscle movement travel along the CNS in two pathways: the corticospinal and corticobulbar tracts.

Neurons whose axons travel along the synapse of the corticobulbar tract with lower motor neurons in the brainstem.

The axons of these lower motor neurons form cranial nerves such as the oculomotor, trochlear, or trigeminal nerves that are involved in contracting skeletal muscles in the face, neck, jaw, and tongue.

Upper motor neurons emerge from the precentral gyrus and travel along the corticobulbar tract toward the brainstem.

The axons of other upper motor neurons travel along the corticospinal tract , passing through the medulla oblongata and reaching the ventral horns of the spinal cord.

The origin of the upper motor neurons is from the precentral gyrus, moving through the midbrain and medulla to form the lateral and anterior corticospinal tracts. The main function of these neurons is to connect the brain with the spinal cord.

In the spinal cord, upper motor neurons synapse with lower motor neurons and release glutamate in the synaptic cleft. The depolymerization of the lower motor neuron results in the transmission of the action potential to the skeletal muscles.

There are three types of lower motor neurons: alpha, beta, and gamma. Alpha motor neurons are thick, myelinated, multipolar nerve fibers that are involved in the innervation of most skeletal muscle fibers and cause them to contract.

Gam motor neurons support alpha motor neuron activity by keeping muscle spindles taut. Alpha motor neurons can receive signals from higher motor neurons for voluntary muscle movement.

At the same time, they can also receive information from sensory neurons and interneurons to initiate reflex actions. The number of alpha motor neurons that innervate a single muscle depends on the extent of fine motor control required at the site.

Therefore, the muscles of a finger will have substantially more alpha motor neurons associated with them than the muscles of the thigh or upper arm.

Neuromuscular junction

The axon terminus of an alpha motor neuron forms a neuromuscular junction with striated muscle fibers, where acetylcholine is released as a neurotransmitter.

When an action potential reaches the axon end of the alpha motor neuron, a voltage-gated ion channel allows calcium ions to enter the neuron.

These ions induce the fusion of synaptic vesicles with the plasma membrane, resulting in the release of acetylcholine at the neuromuscular junction.

Acetylcholine then binds to nicotinic receptors on muscle cells. These receptors are ion channels that open upon ligand binding, which then leads to a cascade of ions within the muscle fiber, leading to muscle contraction.

Two potent toxins that affect the neuromuscular junction are botulinum toxin and tetanus toxin. Both chemicals are produced by bacteria: the first by a bacterium called  Clostridium botulinum  and the second by Clostridium tetani.

Botulism can affect humans through inhalation or ingestion of the toxin or by ingestion of bacterial spores from contaminated food.

This is particularly true for improperly prepared canned foods, as the warm, humid, and anaerobic environment inside food containers can provide a fertile environment for bacteria to grow.

The toxin interferes with the fusion of synaptic vesicles with the neuronal plasma membrane and thus prevents the release of acetylcholine at the neuromuscular junction.

Thus, it leads to paralysis, initially of the facial muscles and in severe cases, even the smooth muscles of the diaphragm.

It is among the most potent neurotoxins known, with a lethal dose of 1 microgram for an adult. The only other toxin of this potency is tetanus toxin, and it works in a similar way.

When tetanus toxin enters the presynaptic nerve terminal, it prevents the release of neurotransmitters at the neuromuscular junction. While botulinum toxin produces flaccid paralysis, tetanus toxin produces spastic or rigid paralysis.

Sensory neurons

Afferent sensory neurons of the somatic nervous system provide information to the CNS about joint angle, muscle length, muscle tension, and the presence of noxious stimuli.

Proprioceptors

In addition to typical extrafusal muscle fibers, a muscle body also contains muscle spindles. These small sensory organs contain specialized muscle fibers that have a non-contractile central segment.

The afferent neurons of the somatic nervous system have their sensory dendrites in this area. These dendrites contain ion channels that open in response to mechanical forces in the cell.

When the muscle spindle is stretched, the opening of the ion channels generates an action potential in these sensory neurons.

The presence of mechanically closed ion channels allows these neurons to carry detailed information about the condition of the muscle and its contractile activity.

Nociceptores

Nociceptors are pain receptors found throughout the body and are an essential part of injury prevention, especially in muscle fibers. These neurons fire in response to potentially harmful stimuli, such as heat, cold, or extreme forces.

The presence of nociceptors prevents us from over-extending our joints, overstretching our muscles, and protects us from a wide range of injuries.