Introduction to the Peripheral Nervous System (PNS):
The portions of the nervous system outside the brain and spinal cord are referred to as the peripheral nervous system (PNS). The peripheral nerves, neuromuscular connections, spinal nerves and their roots and branches, and cranial nerves are all included. Technically, the anterior horn cells belong to the central nervous system (CNS), but because they are a component of the motor unit, they are commonly discussed in conjunction with the PNS.
Peripheral nervous system consists of 12 pairs of cranial nerves and 31 pairs of spinal nerves. Cranial Nerves are direct extensions of the brain. The cerebrum is from where Nerve I (olfactory) originate; the brain stem is from where other 11 pairs.
- Nerve I (Olfactory): responsible for the sensory perception of smell.
- Nerve II (Optic): responsible for the sensory perception of vision.
- Nerve III (Oculomotor): mostly responsible for motor regulation of the eye’s muscles and associated structures.
- Trochlear nerve IV: responsible for directing the eye’s muscles, mainly the motor ones.
- The trigeminal nerve, or nerve V: regulates the muscles of the eyes, the jaws, and the mixed tear glands.
- Nerve VI (Abducens): mostly for motor control of the muscles that move the eye.
- Nerve VII (facial): responsible for taste perception and the regulation of face muscles, tear glands, and mixed salivary glands.
- Nerve VIII (Vestibulocochlear): responsible for the senses of hearing and equilibrium (sensory).
- Nerve IX (Glossopharyngeal): responsible for controlling muscles in the pharynx and to control salivary glands (mixed).
- Nerve X (Vagus)- responsible for controlling muscles used in speech, and the digestive tract, and controls cardiac and smooth muscles (mixed).
- Nerve XI (Accessory): responsible for controlling muscles of soft palate, pharynx and larynx (primarily motor).
- Nerve XII (Hypoglossal): responsible for controlling muscles that move the tongue (primarily motor).
Because spinal nerves are mixed nerves with both sensory and motor nerve fibers, they can communicate with the spinal cord and the arms, legs, neck, and body trunk in both directions. Every spinal neuron forms a plexus, with the exception of the thoracic area. Nerve fibers from separate spinal nerves are recombined and sorted in each plexus, allowing diverse spinal nerve nerve fibers to reach the same areas.
Divisions of the Peripheral Nervous System:
The Peripheral Nervous System (PNS) can be divided into two parts, the Somatic Nervous System (SNS) and the Autonomic Nervous System (ANS).
Somatic Nervous System (SNS)
Skeletal muscle is the effector tissue of the SNS, which controls bodily movement (soma = body).
Autonomic Nervous System (ANS)
Heart muscle, smooth muscle, and glands are the effector tissues of the autonomic nervous system (ANS), which controls the body’s so-called automated reactions, such as blood pressure and heart rate. The SNS only controls skeletal muscle.
The two ANS divisions, the parasympathetic and the sympathetic, regulate smooth muscle, glands, and heart muscle.
Fig: Peripheral nervous system
The Somatic Nervous System:
Since the somatic nervous system regulates the body’s skeletal muscles, it essentially governs how the body moves. Actually, the frontal lobe (precentral gyrus) is the primary part of the central nervous system that communicates with the SNS.
The exception to the voluntary control of these actions are somatic reflexes, which are automated. A reflex is, by definition, a quick, automatic, and conditioned reaction to a stimulus, typically to protect the body from harm. For example, if we put our hand on something hot without realizing, but before we even perceive the hotness, we have automatically pulled your hand away from the potentially dangerous stimulus. Examples of somatic reflexes are the monosynaptic ‘patellar’ reflex and the polysynaptic ‘withdrawal’ reflex which occur without conscious control.
The Control of Skeletal Muscle
Many different areas of the brain relay their inputs to the primary motor cortex, and the information it receives aids in the planning of body movement that is facilitated by the contraction of skeletal muscle. Principally, the efferent (outward) commands move down the brain centers to stimulate spinal cord neurons, which then continue outward to the neuromuscular junction (NMJ) and act to stimulate and contract skeletal muscle.
The Autonomic Nervous System (ANS):
The autonomic nervous system is another component of the peripheral nervous system. It predominantly involves automatic or involuntary regulation of the body. The autonomic nervous system (ANS) actually has three divisions or branches: the parasympathetic and the sympathetic.
Autonomic Nervous System mode of action
Compared to the SNS, ANS is more intricately organized. The two motor neurons involved are one from the central nervous system to an autonomic ganglion and another from the ganglion to the effector tissue. In the PNS or outside the central nervous system, a ganglion is a collection of nerve cell bodies.
The ANS is a control system that acts largely unconsciously and regulates bodily a broad range of bodily functions. It regulates involuntary physiological processes including heart activity, air flow in the lungs, the activity of smooth muscle in organs and vessels, influencing blood pressure, digestion, exocrine and endocrine glands, and sexual arousal, among other bodily processes. The ANS continuously monitors various conditions within these different systems and implements changes depending on the circumstance. There are three categories of effector tissue in the ANS, they are: Cardiac muscle; smooth muscle and, glandular tissues. Both the Parasympathetic and the Sympathetic divisions act on the same effector tissue.
The Arrangements of the Parasympathetic and Sympathetic divisions
The parasympathetic division has its origins in the craniosacral regions, which means the brain and sacral regions. From there, the outward direction into the periphery goes to the ganglion, which is defined as a cluster of nerve cell bodies outside of the CNS. The effector tissue is significantly closer to the parasympathetic ganglion than it is to the central nervous system. The axon goes to the effector tissue once the parasympathetic postganglionic neuron exits the ganglion.
The thoracolumbar regions, or the thoracic and lumbar parts of the spinal cord, are where the sympathetic division first began. The sympathetic ganglion is located very close to the CNS (sympathetic ganglionic chain) and relatively far away from the effector tissue. The sympathetic postganglionic neuron’s axon leaves the ganglion and travels to the effector tissue.
The way the ANS operates as a sequential two-neuron efferent (outgoing) route is intriguing and distinct. The preganglionic neuron must go to a ganglion to synapse with the postganglionic neuron. And in both divisions the chemical signal and receptors at the ganglion are identical. This really acts like a relay station, and the area where the two division are remarkably different are at the postganglionic neuron at the effector tissue.