The nervous system is an incredibly complex yet vital part of the correctly functioning human body. With all the different parts and connections between them, this neurological system is carefully balanced and coordinated. There are chances that the system functions incorrectly and causes debilitating disorders. Understanding the ‘nervous system’ requires knowledge of its various parts. This article will describe the details of the nervous system, its types and also the clinical impact of its dysfunction.
The development of the nervous system is one of the first to begin in the foetus. A simple neural plate folds in order to form a neural fold (week 3-4), which then forms a tube. This tube is open ended at both ends that are identified as cranial and caudal neuropores. Cranial neuropore closes on the 25th day while closure of caudal occurs three days later. The failure of either neuropore to close results in a host of developmental abnormalities.
Crucial steps in the development of the nervous system are the flexures that form in the brain. These are the pontine flexure (between the metencephalon and myelencephalon), the cervical flexure (between the brainstem and the spinal cord), and the midbrain flexure (which raises the midbrain superiorly).
The neural tube eventually develops into the brain, and gives rise to the three primary vesicles i.e. the prosencephalon, mesencephalon, and rhombencephalon. The prosencephalon goes on to form two secondary vesicles i.e. the telencephalon and diencephalon. The telencephalon forms the cerebrum (the cerebral hemispheres), basal ganglia (the caudate nucleus for cognition and the globus pallidus interna and externa for motor control), amygdala (the danger detector and main input of olfactory information) and hippocampus (our main store of episodic and spatial memory).
The diencephalon forms the thalamus (the gateway to the cerebral cortex), hypothalamus (the area that controls the master gland i.e. the pituitary), subthalamus, the third ventricle (the two thalami are said to ‘kiss’ across the third ventricle) and pineal gland (melatonin release and sleep wake cycles).
The mesencephalon forms the cerebral aqueduct (connects third and fourth ventricles), tectum (roof of the midbrain), and cerebral peduncle (connects the brainstem to the cerebrum). Finally, the rhombencephalon forms the secondary vesicles named the metencephalon and myelencephalon. The metencephalon forms the cerebellum (the ‘little brain’ for coordination and smoothness of movement) and pons (give rise to the pons and other cranial nerve nuclei). The myelencephalon forms the medulla oblongata (the control of vital respiratory centres and cranial nerves).
There a variety of nerve cells. The cell body is where the neurotransmitters are generated and they are transported to the terminal part of the nerve with protein carriers. Neurons consist of a central axon and myelin for insulation. Myelin in nerves of the central nervous system is formed of oligodendrocytes and of Schwann cells in the peripheral nervous system. Not all nerve fibres are myelinated e.g. Group C pain fibres.
Sensory neurons are specialized to signal sensory stimuli. They are connected to a peripheral receptor of some sort e.g. paccinian corpuscle sends a signal when under physical pressure. These transmit to interneurons that lie within the spinal cord . These signal to motor neurons, which connect to muscles and leave via the ventral horn of the spinal cord.
Stimulus is generated from the receptors (pressure, temperature etc.). This causes a sodium influx that causes a local depolarization. Once the membrane potential rises above -45 mV, the sodium voltage gated channels open, and there is a sharp influx. This is stopped once the membrane potential reaches +40mV. The signal is spread and jumps along the nerve. There is more sodium influx at the Nodes of Ranvier and the word saltatory relates to the ‘leaping’ of the sodium ions along the myelinated segments of nerve cell. This speeds up transmission.
The arrival of the Na ions to the terminal part of the nerve causes a depolarization. This triggers a calcium influx. This causes pre-stored vesicles containing acetylcholine (in somatic nerves, interneurons and motor end plates) or noradrenaline (in postsynaptic sympathetic nerves) to fuse with the presynaptic membrane and be released into the synaptic cleft. The neurotransmitter now binds with postsynaptic receptors, causing depolarization, and the signal is sent on.
It is a junction between motor neuron and skeletal muscle. These have the same basic structure as synapses but rather than sending the signal onto another nerve, it is sent to a motor end plate, and to the muscle fibres. This is achieved via a complex structure of T Tubules and specially adapted receptors, the net result of which is calcium release from the sarcoplasmic reticulum to promote contraction.
Central Nervous system
The brain is the master organ of the central nervous system. It coordinates the functioning of our muscles and limbs, as well as the hormones we release to adapt, grow and change with our environment. It is composed of several divisions, called lobes, as follows:
- Frontal lobe: This contains the orbitofrontal cortex which is the main area of inhibition of impulsive behaviors. It also contains the pre-central gyrus i.e. the primary motor cortex and the Broca’s area (on the left side), which enables us to form words. Broca’s homologue on the right side enables us to interpret body language.
- Temporal lobe: It lies just under the lateral fissure on each cerebral hemisphere. It contains the transverse temporal gyri, which interpret auditory information. The left temporal lobe enables us to understand words, and comprehend information.
- Partietal lobes, which lie on the superoposterior surface of the brain and are the main site of visual interpretation. They also have a crucial role in the pursuit eye movements we perform e.g. following an object across the horizon, as well as the saccades which draw our eyes to different parts of an object. It also contains the post central gyrus of primary sensory strip. Wernicke’s area lies in the boundary between this lobe and the temporal lobe.
- Finally at the posterior side of the brain, we have the occipital lobe that contains the primary visual cortex and association visual areas.
The brainstem attaches to the inferior aspect of the brain. It consists of the midbrain superiorly, the pons in the middle, and the medulla oblongata inferiorly. The brainstem lies within the cranial cavity and lies against the clivus on the inferior aspect of the cranial vault, and is continuous with the spinal cord.
The cerebellum or ‘little brain’ is responsible for balance and coordination. It gives smoothness to our movements, and reprograms itself with a feed forward system according to the stimuli it faces.
The spinal cord lies within the vertebral canal. It lies deep to all three layers of the meninges , and gives off the 31 pairs of spinal nerves. These nerves exit via the intervertebral foramina, and merge to form plexi and go on to innervate different muscles.
Peripheral Nervous System
There are 31 pairs of spinal nerves. They consist of an incoming sensory component that enters at the dorsal horn, and an outgoing motor component that leaves via the ventral horn. Both the sensory and motor components are contained within the spinal nerve along with the autonomic signals.
Once spinal nerves leave the intervertebral foramen, they form anterior and posterior rami. The anterior rami supply the limbs and trunk, while the posterior rami supply a few structures, such as the back muscles.
The Cranial Nerves
They originate from the brainstem and brain, but they are in fact part of the peripheral nervous system. There are twelve pairs of nerves that are as follows:
- Olfactory (sense of smell)
- Optic (sense of sight)
- Oculomotor (moves the eye, and eyelid. Constricts the pupil)
- Trochlear (moves the eye down and out, innervates superior oblique)
- Trigeminal (V1- Ophthalmic, V2- Maxillary, V3- Mandibular, sensation to face and innervates chewing muscles)
- Abducens (moves the eye laterally. Innervate the lateral rectus)
- Facial (moves the face, sense of taste of anterior 2/3 of tongue , together with other functions)
- Vestibulocochlear (hearing and balance)
- Glossopharyngeal (taste for posterior 1/3 tongue, sensation to pharynx)
- Vagus (parasympathetic to whole body down to splenic flexure; motor part of cough reflex)
- Accessory (innervates the sternocleidomastoid and trapezius muscles)
- Hypoglossal (innervates all tongue muscles, except palatoglossus)
Somatic Nervous System
The word somatic means ‘relating to the body’ and it explains the function of this system well. The nerves that supply our arms and legs, as well as our neck muscles and trunk, all originate from this system. It is considered a part of the peripheral nervous system and is responsible for carrying sensory and motor information. Both cranial and spinal nerves contribute to the somatic nervous system.
The ventral rami of the spinal nerves (except T2-T12) coalesce and form plexi, resulting in final nerves that go to innervate muscles and provide sensation.
The cervical plexus supplies the neck region and it is formed by the ventral rami of the first four cervical nerves. It mostly gives cutaneous branches to the area of head, neck and chest. Its muscular branches are to rectus capitis lateralis and anterior, longus capitis and longus colli muscles. It has a loop of nerves called the ansa cervicalis, which gives off branches to a number of strap muscles of the neck (superior belly of omohyoid, inferior belly of omohyoid, sternohyoid and sternothyroid). Perhaps most important of all, it gives rise to the phrenic nerve; which innervates the diaphragm; hence C3, 4, 5- keeps the diaphragm alive.
The Brachial plexus
The brachial plexus is formed by the ventral rami of C5-T1 and supplies the muscles and sensation of the upper limb. It has numerous branches, but the major ones to remember are:
Radial nerve (C5-T1): Comes from the posterior cord. It supplies all posterior arm and forearm muscles, as well as the majority of posterior sensation.
Median Nerve (C5-T1): It is formed by the unification of the medial and lateral cords. It supplies almost all of the forearm muscles (except the flexor carpi ulnaris and ulnar head of flexor digitorum profundus), the thenar eminence and lateral two lumbricals.
Ulnar Nerve (C8-T1): Comes from the medial cord. It supplies all the intrinsic hand muscles (all the interossei and the medial two lumbricals) as well as the flexor carpi ulnaris and ulnar head of flexor digitorum profundus in the forearm.
Musculocutaneous nerve (C5-7): Comes from the lateral cord. It supplies the flexor compartment of the arm as well as the sensation to the lateral forearm.
Thoracodorsal nerve (C6-8): Comes from the posterior cord and supplies the latissimus dorsi muscle.
Suprascapular nerve (C4,5): Originates from the superior trunk of the plexus and supplies supraspinatus and infraspinatus.
This is the plexus of the lower limb. It is formed by the ventral rami of L1-L4 with a contribution of 12th thoracic. A good acronym to remember the branches is ‘I (Twice) Get Lunch On Fridays’. They are the following branches:
Iliohypogastric (L1): It supplies transversus abdominus, and internal oblique muscles. It also gives sensory distribution to the skin over part of the gluteal region and pubis.
Ilioinguinal (L1): It supplies transversus abdominus, and internal oblique muscles. It also innervates the skin over the root of the penis and upper part of the testes , as well as the skin over the mons pubis and labia majora in females.
Genitofemoral (L1,2): It has a genital branch that runs within the spermatic cord and innervates the cremaster muscle. The nerve also provides sensation to the external genitalia.
Lateral femoral cutaneous (L2,3): This supplies sensation to the lateral aspect of the thigh.
Obturator (Ventral divisions of L2-L4): Supplies the muscles of the medial/adductor compartment of lower limb.
Femoral (Dorsal divisions of L2-L4): Supplies the muscles of the anterior compartment of the thigh, as well as sensation over the thigh via the medial and anterior cutaneous nerves of the thigh.
The sacral plexus is fiendishly difficult to memorize. In general, it supplies the muscles of the gluteal region (gluteal muscles, short external rotators of the hip) and also the pelvis sphincters.
Autonomic Nervous System
The autonomic nervous system is composed of our sympathetic and parasympathetic nervous systems. The former is said to act in sympathy without emotions, hence its name. It causes flight, fight and fright reactions. The parasympathetic has rest and digest functions i.e. slows down the heart, promotes peristalsis. The sympathetic nervous system is so named because it is said to act in sympathy with the emotions. It increases its effect when we undergo the ‘flight or fight’ response.
Gray and white rami communicantes
There is of course communication between the different nervous systems. The white rami communicantes (preganglionic sympathetic neurons) are short myelinated sections of nerves that connect the spinal nerve to the sympathetic paravertebral ganglion. The latter resemble beads on a string and run along the vertebrae for a significant length of the thoracic spine. The white rami enter the sympathetic trunk, where they either terminate, pass upward or downward. They synapse with the cell bodies of postganglionic sympathetic neurons located in the sympathetic ganglia. The white rami communicant will then synapse with the grey rami communicant. This then runs with the spinal nerve to the peripheral target.
The beads on a string like structure (sympathetic trunk) described above give rise to the thoracic splanchnic nerves. The greater (T5-T9), lesser (T10-11) and least (T12-L2) splanchnics pass through the diaphragm and contribute to the coeliac, superior mesenteric and renal plexi respectively.
Loosely speaking, the sympathetic outflow can be described as ‘thoracolumbar’, as this is where the nerves originate. There are collections of sympathetic nerve fibres and many of them coalesce around the major branches of the abdominal aorta. These include the coeliac, superior mesenteric, and inferior mesenteric plexus. These plexi follow the course of the arteries and provide sympathetic innervation to the same areas of the bowel as the arteries i.e. the coeliac plexus supplies the foregut, the superior mesenteric plexus supplies the midgut, and the inferior mesenteric plexus supplies the hindgut.
Parasympathetic Nervous System
The parasympathetic nervous system fulfills our ‘rest and digest’ functions i.e. slows the heart, increases bowel contractions. The outflow can be described as ‘cranio-sacral.’ This is because there are four cranial nerves that provide parasympathetic innervation (Cranial Nerves 3,7,9 and 10):
Oculomotor: This nerve innervates constrictor pupillae (that constricts the pupil) and has a parasympathetic component.
Vagus: Vagus means ‘wanderer’ and it’s easy to see why. The nerve supplies parasympathetic innervation all the way down to the splenic flexure of the large bowel.
These are different from the thoracic splanchnics i.e. they are parasympathetic, not sympathetic. They provide parasympathetic innervation to the remainder of the large bowel, after the vagus has completed its innervation.
Enteric Nervous System
The enteric nervous system is known as the ‘Brain in the bowel.’ It works independently but some complex interaction exists with the autonomic nervous system. There are two broad groups of plexi in the wall of the gastrointestinal tract. These are Meissner’s (in the submucosa) and Auerbach’s (in tunica muscularis) plexi. These cause contraction of the bowel wall.