The central nervous system is our body’s chief commander. It makes decisions, stores memories, creates associates. But to do all that it needs its sidekick, the peripheral nervous system.
In this post I would like to discuss the peripheral nervous system, its divisions, nerve fibres, ganglia, motor and sensory nerve endings that help our bodies perceive and react to the environment.
Peripheral vs Central Nervous System
The human nervous system divides into the central nervous system (CNS) which has the brain and the spinal cord and the peripheral nervous system (PNS) which has everything else. CNS processes, organises and stores information, creates associations and issues motor commands. It is the PNS that sends information about the external world to the brain for processing and relays commands from the brain to the rest of the body.
The peripheral nervous system the nervous tissue that exists outside the brain and the spinal cord. It doesn’t exist on its own, rather it connects to the CNS via nerves that enter or leave the spinal cord. Nerves are a cable-like bundle of axons that run together outside the brain and the spinal cord. Cells bodies of those axons are usually clumped together in clusters called ganglia. Similar clusters of cell bodies in the CNS are known as nuclei. Nerves can be afferent (carrying information from the receptors to the spinal cord via dorsal roots) or efferent (carrying the commands from the spinal cord or the brain to the motor endings).
Unlike CNS the peripheral nervous system is far less protected. There are no bones of the skull or vertebrae to protect it, no protective meningeal layers or cerebrospinal fluid, and no blood-brain barrier. Trauma, toxic medications, diabetes, infections, and even the lack of nutrients can damage peripheral nerves.
There is a striking difference in the ability of the peripheral nerves to regenerate as compared to the CNS and is mostly related to different glial cells. In peripheral nerves Schwann cells myelinate and envelop axonal fibres. Satellite glial cells also intimately surround the nervous tissue.
When a peripheral nerve is injured or severed, it starts shooting out new axons attempting to reconnect the severed ends. Often it succeeds because it is believed that Schwann cells secrete growth factor helping new fibres to grow.
Unfortunately in the CNS the axons are myelinated by oligodendrocytes and surrounded by astrocytes and microglia. When an axon is injured, astrocytes start forming long cytoplasmic processes which form a tangled mass, preventing axonal regrowth. Combined with the lack of growth factor, this means that most injuries in the CNS are irreparable.
Divisions of the Peripheral Nervous System
The peripheral nervous system divides into the somatic nervous system (SNS) and the autonomic nervous system (ANS).
The somatic (voluntary) nervous system is the part of PNS that is under the voluntary control. Walking, singing, chewing, squinting or holding your breath are all examples of voluntary control.
The autonomic nervous system supplies smooth muscles and glands and affects the internal organs (viscera). It is not under direct voluntary control. A fast beating heart in response to a fright, pupils constricting in bright light, sweating or getting sexually aroused are examples of involuntary action.
To make things even more complicated, the autonomic nervous system further divides into the sympathetic and parasympathetic divisions. The sympathetic division connects with the spine in the thoracic and lumbar segments, while the parasympathetic divisions emerges in the cranial nerves and in the sacral region of the spinal cord.
Let’s look at the differences between the sympathetic and parasympathetic nervous systems.
|Sympathetic Division||Parasympathetic Division|
|Function||Fight or Flight. Shuts down functions not critical for survival.||Rest and Digest. Promotes rest and digestion.|
|How fast||Fast acting||Slow acting|
|Cardiovascular||Increases heart rate||Decreases heart reate|
|Gastrointestinal||Reduces motility and secretions||Promotes motility and digestion|
|Glands||Secretes adrenaline, decreases saliva||Increase saliva|
|Muscles||Pupils constrict, skeletal muscles tense up.||Pupils dilate, skeletal muscles relax.|
Did you know that there is are slow and fast types of pain? That local anaesthetics act faster on thinner nerve fibres? Nerve fibres are not created equal. Each nerve fibre is an axon enveloped in a myelin sheath together with the ensheathing glial cell. The speed with which the nerve fibre conducts an impulse depends on its diameters. Larger axons are well myelinated and conduct nerve signals at much higher velocities than thin and unmyelinated axons.
Nerve fibres were classified in groups A, B and C by Erlanger and Gasser in 1941. Let’s review these groups and their functions, for more information please refer to this article.
|Aα||Yes||Large, fast neurons controlling extrafusal muscle fibres, i.e. skeletal muscles.|
|Aβ||Yes||Control intrafusal fibres with collaterals to extrafusal muscle fibres.|
|Aγ||Yes||Pre-tune intrafusal muscle fibres to improve proprioception.|
|Aα (Ia)||Yes||Also known as type Ia fibres, receive proprioceptive signals from muscle spindles.|
|Aα (Ib)||Yes||Receive proprioceptive (used to determine body’s balance, position) signals from Golgi tendon organ.|
|Aβ (II)||Yes||Receive proprioceptive signals from muscle spindles, and sensory information from mechanoreceptors.|
|Aδ (III)||Thin||Receive sensory signals from small hair follicles and free nerve endings (touch, pressure, pain), cold receptors|
|C (IV)||No||Pain receptors, warmth receptors, olfaction|
Just to give you an idea of the diameter and the conduction velocity of these fibres, type Aα fibres are 20 μm in diameter and conduct signals at the speed of up to 120 m/sec. On the other hand, type C fibres conduct pain and temperature information at the speed of just 2.5 m/s and are only around 1 μm in diameter.
It could be challenging to memorise all these types so let’s try to make sense of these.
- Fast fibres send motor signals (Aα, Aβ)
- Fast fibres receive proprioceptive data (Aα, Aβ)
- Slower fibres receive touch signals (Aβ, Aδ)
- Slower receive pain and temperature (Aδ, C)
Pain receptors are also known as nociceptors. They are free nerve endings and can be found everywhere in the body, but especially in the skin and in the joints. They can react to temperature (extreme heat), chemicals (e.g. acid) or mechanical injury or stretching.
As you now know pain is carried by Aδ and C fibres with the former being thicker and conducting signals faster. So it’s no surprise that there are two types of pain – fast and slow.
Fast pain is carried by Aδ fibres and is felt within 0.1 sec as a sharp localised sensation, e.g. a pin prick. It is mostly felt in the skin. Slow pain is carried by C fibres and is felt within 1 sec as a full aching feeling which is harder to localise. Slow pain can be felt anywhere in the body including the skin and internal organs.
Sensory Nerve Endings
Sensory nerve endings (receptors) can be broadly classified according to a number of categories.
- Superficially located exteroceptors (e.g. skin nerve endings)
- Proprioceptors in muscles, tendons or joints
- Interoceptors found in viscera (e.g. those detecting blood pressure)
- Mechanoreceptors (responding to stress)
- Proprioceptors (providing sense of position)
- Nocireceptors (responding to damage by pain signals)
- Chemoreceptors (detecting chemicals)
- Thermoreceptors (responding to changes in temperature)
- Baroreceptors (e.g. responding to changes in blood pressure)
- Encapsulated (specialised non-neuronal cells encapsulate the terminal part of the axon)
Adaptation to Stimulus
- Phasic receptors that quickly adapt to a stimulus
- Tonic receptors that take longer to adapt.
Let’s consider cutaneous (cutis = skin) sensory nerve endings. There are quite a few cutaneous receptors that are non-encapsulated:
- Cutaneous plexuses reacting to heat, cold, pain or pressure
- Free nerve endings which respond to various types of stimuli
- Merkel endings which are tactile sensors
- Peritrichial nerve endings are touch receptors at the base of hair follicles
And there’s a number of encapsulated receptors as well:
- Ruffini endings are slowly adapting mechanoreceptors
- Meissner’s corpuscles are sensitive to light touch
- Pacinian corpuscles respond to vibration and pressure
- End bulbs (e.g. end bulbs of Krause which are thermoreceptors)
Some of them are shown in the picture below.
The top layer is called the epidermis and you can see Merkel discs entering it. Free nerve endings extend into the middle of the epidermis.
The next layer pictured in pink is the dermis with the thin top part called the papillary layer (papula = swelling) and the bottom part called the reticular layer. Meissner’s corpuscles exist in the papillary and Ruffini’s endings are deeper in the reticular layer.
The third layer is called the hypodermis or the subcutaneous layer and consists of fatty connective tissue. This is where Pacinian corpuscles can be found.
Let’s review each of the nerve endings.
In general, nerve plexuses are networks of intersecting axons found throughout the body. They comprise afferent and efferent nerve fibres. There are five spinal plexuses as well as a number of autonomic plexuses serving the enteric system. Nerve plexuses tend to be named after the area in which plexus occurs and the body parts it serves, for example cervical plexus, lumbar plexus, celiac plexus and so on.
Upon reaching the skin, axons of spinal and cranial nerves spread out to form three cutaneous plexuses:
- The subcutaneous plexus
- The dermal plexus in the reticular layer of skin
- The papillary plexus immediately below the epidermis
Free Nerve Endings
Free nerve endings are non-encapsulated unspecialised receptors resembling the roots of an upside down plant. They can be found in cutaneous tissue, dermis and extend into the middle of epidermis (see the picture above), around hair follicles and their axons are encapsulated in Schwann cells. Free nerve endings can detect temperature, mechanical stimuli and pain. They can be slow adapting or fast adapting to stimulus. They are usually Aδ and C type fibres.
Merkel endings also known as Merkel discs are non-encapsulated mechanoreceptors found in the basal epidermis and hair follicles. They are especially dense in the fingertips. They are extremely sensitive and slowly adapting. A nerve ending is surrounded by Merkel cells which release serotonin in response to pressure causing a sustained action potential. They provide information on pressure, position and deep static touch features such as shapes and edges. It’s the Merkel endings that make it possible for someone to read Braille with their fingertips.
Peritrichial Nerve Endings
Peri = round, about. Trich = pertaining to hair. These are non-encapsulated cage-like axonal endings surrounding hair follicles. Each axon can send collaterals to more than one hair follicle and each hair follicle can be served by more than one axon. These receptors respond to movements of hair follicles and can detect, for example, a bug crawling on our skin.
Ruffini endings (also known as bulbous corpuscles) are encapsulated endings that look like elongated capsules about 1 mm long. They are mostly present in the cutaneous tissue. These are slowly adapting receptors responding to sustained pressure and are sensitive to skin stretch and contribute to the sense of finger position and movement. They can also act as thermoreceptors.
Meissner’s Tactile Corpuscle
These encapsulated sensory nerve endings are rapidly adapting mechanoreceptors sensitive to shape and texture in the exploratory touch (e.g. when we move our fingers over an object). They are primarily located in glabrous (hair-free) skin particularly fingertips and lips. Along with Merkel discs these receptors make it possible to detect tiny changes in the surface texture making it possible, for example, to read Braille. The difference is that the Merkel discs respond to sustained touch while the Meissner’s corpuscles react to rapid changes of pressure along the skin.
Also known as the lamellar corpuscles, these encapsulated nerve endings are mechanoreceptors that sit deep in subcutaneous layer of glabrous skin and respond to vibration and pressure. They are rapidly adapting (phasic) receptors that respond only to sudden changes. Their role in detecting vibration is thought to help them detect the surface texture in finer details than Meissner’s corpuscles. They consist of a single axon that loses its myelin as it is encapsulated by several layers of cytoplasm. Pressure applied to the corpuscles opens up pressure-sensitive sodium ion channels creating an action potential.
Bulbous corpuscles of Krause are encapsulated cutaneous thermoreceptors sensing cold temperature. They are found in the conjunctiva of the eye, in the mucous membrane of of the lips and the tongue, in the penis and the clitoris, and in the synovial membranes of some joints.
It’s useful to understand what each one of the cutaneous nerve endings looks like so here’s a quick illustration. Notice how the Merkel disc and the Peritrichial plexus (and obviously free nerve endings) are non-encapsulated.
It is easy to get confused about the nerve endings especially Merkel discs, Meissner’s corpuscles, Ruffini cylinders and Pacinian corpuscles. Let’s make it easier to memorise these by imaging you’re lifting a piece of fruit and summarising how the nerve endings react in a table:
|Location||Epidermis, just above dermis||Dermis, just below epidermis||Dermis||Deep in dermis|
|Skin||Any||Mostly glabrous||Any||Mostly glabrous|
|Responds to||Sustained light touch||Low frequency vibration||Indentation, stretching||Vibration|
|Mnemonics||Long serving Angela Merkel.||A lighting fast footballer Messi.||A sprain after a rough play.||Pacing really fast.|
Here are a few examples to better understand these nerve endings:
|Running hand over sandpaper, feeling its rough texture||Meissner’s corpuscles|
|Running hand over silk, feeling its smooth texture||Pacinian corpuscles|
|Adjusting grip to better hold chopsticks||Ruffini’s corpuscles|
|Feeling an insect crawling on the skin||Peritrichial endings|
|Feeling a sharp pain of a pin prick||Free nerve endings, Aδ fibres|
|Feel a dull pain of a stubbed toe||Free nerve endings, C fibres|
|Feeling a cold or a warm surface||Mostly free nerve ending|
Motor Nerve Endings
Motor nerve endings are supplied by efferent neurons. Somatic efferents are under voluntary control, that is you can consciously cause muscles to contract or relax. Visceral efferents, on the other hand, control our internal organs and glands.
These arise in the ventral gray horn of the spinal cord and motor nuclei of cranial nerves. Their axons terminate in motor end plates of skeletal muscles. In response to action potential they release acetylcholine which is a neurotransmitter that forces muscles to contract.
These are part of the autonomic nervous system and act on smooth muscles, cardiac muscles and secretory cells. Parasympathetic nervous system affects organs directly via cranial nerves and some pelvic nerves in the sacral region of the spinal cord.
Sympathetic system, on the other hand, acts via the sympathetic chain ganglia (mostly red column in the picture). Spinal nerve fibres (in this case usually called preganglionic neurons) enter the ganglia and then synapse inside with another neuron (postganglionic) which then carries the information to the viscera.
Proprioceptive Nerve Endings
Proprioception (proprius = one’s own) is the sense of the position, movement and acceleration of one’s own body and limbs. This is critical to being able to move in space. It allows us to walk in darkness without losing balance, learn to write or draw, drive a car, walk or dance. Kids with proprioceptive dysfunction don’t know the concept of “light” or “heavy”, slam doors, play with animals with too much force hurting them.
It is important to distinguish conscious proprioception communicated via the medial lemniscus pathway to the cerebrum and non-conscious proprioception communicated via dorsal spinocerebellar and ventral spinocerebellar tracts to the cerebellum. Non-conscious proprioception enables us to walk and keep our balance without thinking about it, it makes us tilt our head one way when the body tilt the other in order to keep the our gaze level with the horizon.
There’s a number of proprioceptive sensors located in muscles, joints and tendons.
Neuromuscular spindles are the proprioceptive receptors in skeletal muscles that detect changes in the length of the muscle. They run among and parallel to extrafusal (standard skeletal) muscles. They are innervated both by motor and sensory fibres.
Within the capsule of the muscle spindle there are nuclear bag fibres, nuclear chain fibres and afferent nerve fibres. Primary (annulospiral) sensory fibres twist around the middle of each nuclear bag and nuclear chain fibres. Secondary (flower spray) sensory endings are attached to the ends of the nuclear bag and nuclear chain fibres.
When the muscle spindle is stretched together with the muscle it is embedded in, the sensory fibers detect the change in the shape of the nuclear bag and nuclear chain fibres. Action potential is conducted to the spinal cord where axon synapses with alpha motor neurons innervating the main mass of the muscle forcing the muscle to contract. This is a reflex arc, or more specifically, a stretch reflex. This reflex keeps the muscle flexes at the same level without having to consciously flex it.
Motor fibers, specifically Aγ type fibres innervate intrafusal muscles. As gamma neurons become active, this causes intrafusal muscle to contract and stretch the annulospiral ring. This makes the ring more sensitive to any further changes in length due to contracting or stretching of the extrafusal muscles. Conversely, less activity from gamma neurons makes the muscle spindles less sensitive to the changes in the muscle tone.
Golgi Tendon Organ
This is a proprioceptive organ that senses changes in muscle tension and is located at the point of insertion of the muscle into the tendons. It is a collagen filled capsule innervated by a single Aα (Ib) nerve fibre. Stretching causes it to fire impulses to the spinal cord. As it is constantly monitoring the tension it provides protection against damage resulting from the excessive muscular contractions.
Around the capsual of synovial joints there are small corpuscles similar to Pacinian and Ruffini’s endings. They respond to the sensation of movement. Organics similar to the Golgi tendon organ protect ligaments from damage by mediating an inhibitory reflex when an excessive muscle contraction is used. Free nerve endings respond to potentially damaging mechanical stress.
Peripheral nerves are arranged in bundles called fascicles. The entire nerve is surrounded by connective tissue called epineurium (epi = above) which also fills the space between the fascicles. This tissue can be stretched as necessary to allow for the movement of various parts of the body.
Each fascicle is surrounded by another layer called perineurium (peri = round, about) which consists of several layers of flattened cells.
Individual nerve fibres are enclosed in another layer of connective tissue called endoneurium (endo = within).
Finally, individual axons are ensheathed in myelin (except for C type fibres) and the layer of Schwann cells. The latter is called neurolemma or the sheath of Schwann.
The myelin sheath electrically insulates the axon to greatly speed up the conduction velocity. It is however interrupted every 0.1 to 1 mm. These interruptions are called the nodes of Ranvier allowing the membrane of the axon to contact the extracellular fluid in order to effect the action potential along the nerve fibre. Between the nodes of Ranvier the electrical signal jumps almost instantaneously, this is called saltatory conduction.
Unmyelinated axons and their sheath of Schwann are called a Remak fibre. They conduct impulses via the propagation of the action potential without the acceleration of the myelin sheath and have therefore slower rates of conduction.
These article were created and published in order to help students study neuroscience. I do not guarantee the accuracy, correctness or currency of the information in my posts. Most importantly, none of my articles are intended to be used as medical advice. I often use the following sources:
- John A. Kiernan, Nagalingam Rajakumar. 2014. Barr's The Human Nervous System: An Anatomical Viewpoint.
- Mark F. Bear, Barrw W. Connors, Michael A. Paradiso. 2016. Neuroscience: Exploring The Brain.
- Wikipedia and other Internet sources.