The peripheral nervous system (PNS) consists of the spinal nerves and ganglia and the cranial nerves. The nerves of the PNS contain sensory fibres which relay signals to the central nervous system (CNS) and motor fibres which relay signals from the CNS to the effector muscles/glands.
The PNS can be divided into sensory somatic and autonomic systems. The sensory somatic nervous system is voluntary and relays sensory information, of which we are conscious, from the external environment to the CNS and relays motor signals from the CNS to operate the muscles of the body. The autonomic nervous system is involuntary and relays sensory information about the internals of the body to the CNS and relays motor signals from the CNS to regulate the internal environment of the body, e.g. vessel diameter.
The sensory somatic system consists of 12 pairs of cranial nerves and 31 pairs of spinal nerves.
The cranial nerves all originate or terminate in the brain stem. All cranial nerves, apart from the first two, which are purely sensory, contain motor as well as sensory fibres and can be described as 'mixed' nerves. However, for descriptive terms each nerve is usually described in terms of its predominant fibres. The motor (afferent) fibres originate in the brain stem and terminate in muscles or glands, and the sensory (efferent) fibres originate in the sensory organs and receptors and terminate in the brain stem.
There are 31 pairs of spinal nerves each arising from dorsal and ventral rootlets from the corresponding 31 segments of the spinal cord. There are 8 pairs of cervical spinal nerves, the first set arising above the atlas (C1) and the last arising below the seventh cervical vertebra (C7). There are 12 pairs of thoracic spinal nerves, 5 sets of lumbar spinal nerves, 5 sets of sacral and spinal nerves and 1 set of coccygeal spinal nerves.
The dorsal roots arise from the posterolateral sides of the cord and carry sensory fibres from the cord. The ventral roots arise from the anterolateral sides of the cord and carry motor fibres from the cord. The dorsal and ventral roots unite to form the mixed (sensory and motor) spinal nerves. Just before the ventral and dorsal roots join, there is a small swelling known as the dorsal root ganglion. It is formed by the cell bodies of the sensory neurons.
With the exception of the 1st cervical, the sacral and the coccygeal spinal nerves, all of the spinal nerves exit the vertebral canal via the intervertebral foramina. The intervertebral foramina are found along the sides of the vertebral column, formed between the pedicles of adjacent vertebrae. The first cervical spinal nerve exits above the atlas, between it and the occipital bone, and the sacral spinal nerves exit via the ventral and dorsal sacral foramina.
After exiting through the intervertebral foramina the spinal nerves soon divide into dorsal and ventral rami (branches). The dorsal branches supply the deep muscle and the skin of the back of the trunk. The ventral branches supply the rest of the body. The ventral branches of the thoracic spinal nerves form the intercostal nerves which supply the muscles of the thoracic cage. The ventral branches of the rest of the spinal nerves interlink to form specialised nerve plexuses which supply the rest of the body.
The ventral branches of the 1st-4th cervical spinal nerves (C1-C4) interlink to form the cervical plexus. The cervical plexus gives off many branches which supply the superficial muscles of the neck and the skin over the neck and back of the head. An important branch of the cervical plexus is the phrenic nerve which travels through the thorax to innervate the movement of the diaphragm; without it we would not be able to breath.
The ventral branches of the 5th-8th cervical and the 1st thoracic spinal nerves (C5-T1) interlink to form the brachial plexus. The brachial plexus supplies the entire upper limb with motor and sensory innervation. The brachial plexus divides into lateral, posterior and medial cords before dividing into a number of large branches;
The ventral branches of the 1st to 4th lumbar spinal nerves interlink to form the lumbar plexus. The lumbar plexus supplies the thigh and skin of the buttocks and genitals;
The ventral branches of the 4th lumbar to the 4th sacral spinal nerves (L4-S4) interlink to form the sacral plexus. Because there is an overlap between the lumbar and sacral plexuses they are often described together as the lumbosacral plexus. A large division known as the lumbosacral trunk arises from the ventral branches of the L4-L5 and joins with the ventral branches of S1 to supply the lower limb.
The ventral branches of the 4th to 5th sacral spinal nerves and the coccygeal spinal nerve (S4-Co1) interlink to form the coccygeal plexus. It pierces the anococcygeal ligament to supply sensory innervation to the skin of that region.
The autonomic nervous system (ANS) maintains our internal bodily functions without us being aware of it. It consists of afferent (sensory) fibres that transmit sensory information from the periphery (viscera) to the CNS, and efferent (motor) fibres that send motor signals from the CNS to the periphery. This loop of autonomic sensory and motor fibres allows the body to monitor and control such functions as heart rate, vessel diameter, smooth muscles surrounding organs and the secretions of glands.
The ANS can be divided into two separate systems, sympathetic and parasympathetic, both of which contain motor and sensory fibres. The fibres in both systems arise in the central nervous system (brain and spinal cord) as preganglionic fibres. These synapse in ganglia with postganglionic fibres, which then travel to the target organ.
Most organs are innervated by fibres from both divisions of the ANS; the sympathetic system generally prepares the body for activity while the parasympathetic prepares it for rest.
The sympathetic system controls our 'fight or flight' responses, i.e. quickly activates our body preparing it for action by increasing the heart rate, dilating the pupils and vessels to the muscles, increases skin sweating and suppresses digestion.
The motor (efferent) sympathetic fibres (preganglionic motor neurons) arise from the intermediolateral column in the thoracic spinal cord and travel to the sympathetic ganglia listed below, where they synapse with postganglionic neurons before reaching their target organs. The sympathetic chain is found on either side of the vertebral column and consists of a number of sympathetic ganglia strung together by sympathetic trunks.
White and grey rami connect the sympathetic trunks to the spinal nerves. Fibres emerge from the chains to form sympathetic plexuses around the internal organs.
The sensory (afferent) sympathetic fibres carry sensory information from the viscera to the CNS. The afferent fibres enter the dorsal horn of the spinal cord alongside the sensory afferents from the skin, this can cause confusion with referred pain.
Because the afferent fibres enter the dorsal horn of the spinal cord alongside the sensory afferents from the skin, pain from the viscera can be perceived as originating from elsewhere in the body. This is known as referred pain and must be understood when investigating the condition of a patient, for example, when a person is experiencing a heart attack they will often feel pain in their left shoulder, neck and arm. This is due to the sensory fibres from the heart and those areas of the arm entering the spinal cord at the same level where the signals get confused.
The parasympathetic system controls the opposite responses to 'fight or flight' responses, i.e. depresses the activity of the body preparing it for rest by decreasing the heart rate, constricting the pupils and vessels to the skeletal muscles and stimulating digestion (peristalsis and salivary production).
Parasympathetic cells are located in specialised nuclei throughout the brainstem and the sacral spinal cord. Its fibres travel in four of the cranial nerves chiefly the vagus nerve as well as the splanchnic and pelvic nerves. The majority of sensory (afferent) parasympathetic fibres return to the medulla oblongata via the vagus nerve where the fibres then travel in the solitary tract to the solitary nucleus which receives sensory information about the blood pressure, CO2/O2 levels and digestive tract distension.
The motor (efferent) parasympathetic fibres travel to parasympathetic ganglia before reaching their target organ.
A reflex is a pathway with an afferent (sensory) signal that evokes an immediate efferent response (motor) without the signal being transmitted to the brain.
e.g. afferent fibres from the viscera baroreceptors and chemoreceptors in the carotid sinus and arch of the aorta are carried to the CNS by the major autonomic nerves where they control heart rate, blood pressure and respiration.