Peripheral Nervous System

∞ generated and posted on 2018.09.02 ∞

The peripheral nervous system is involved in conduction of neuron-associated signals to and from the spinal cord and, to a lesser extent, directly to the brain as well.

The Peripheral Nervous System consists of motor neurons and sensory neurons which carry out these two functions, respectively, with motor neurons originating in the spinal cord of the central nervous system and projecting outward to the rest of the body and sensory neurons originating peripheral to the central nervous system and directing signals (action potentials) towards the central nervous system.

This page contains the following terms: Peripheral Nervous System, Somatic division, Sensory nerves, Motor nerves, Autonomic division, Sympathetic division, Parasympathetic division, Receptor, Sensory receptors, Nociceptors, Afferent pathway, Integrating center, Efferent pathway, Reflex, Reflex arc

Peripheral Nervous System

That aspect of vertebrate innervation that is found outside of the brain and spinal cord.
The peripheral nervous system can be differentiated into sensory (or afferent) neurons versus what can be described as effector, efferent, or motor neurons. The effector neurons can be differentiated into what is known as the somatic division versus instead the autonomic division.

Links to terms of possible interest: Autonomic division, Cardiac muscle, Central nervous system, Glands, Motor division, Parasympathetic division, Peripheral nervous system, Sensory division, Skeletal muscles, Smooth muscles, Somatic division, Sympathetic division

The above video provides a nice, straightforward introduction to the different divisions of the nervous system.

Somatic division

What otherwise is known as the voluntary, peripheral nervous system.
Also known as the somatic nervous system, the somatic division is that aspect of the peripheral nervous system that is under voluntary control; that is, which is subject to conscious manipulation. This somatic division of the peripheral nervous system is responsible for controlling skeletal muscles. Thus, you move and display other behaviors as a consequence of voluntary control or conscious control, and these behaviors you control via the actions of your somatic nervous system.

Note that the converse is not true, that is, not all skeletal muscles are controlled solely via conscious control. An important exception is that control can be mediated also by reflex arcs, which involves the non-voluntary integration of information deriving from sensory nerves and which results in an involuntary stimulation of skeletal muscles. Thus, for example, you quickly move your hand away from touching a hot stove and this movement, though involving skeletal muscles, does not also involve conscious control by the brain.

The somatic division of the peripheral nervous system, either as directed by conscious components of the brain or instead in combination with the action of reflex arcs, gives rise to your behavior as mediated via the action of skeletal muscles. Contrast autonomic nervous system.

The above video contrasts the autonomic nervous system with the somatic nervous system.

Sensory nerves

The means by which your body relays action potentials towards the central nervous system.
Also described as sensory neurons or afferent nerves (nerve fibers or neurons), sensory nerves gather up information from numerous sources that collectively can be described as senses. These consist of various neuron and other specialized structures that are able to convert specific stimuli, both internal to and external to the body, into action potentials.

The action potentials then propagate up sensory nerves towards the central nervous system where the information they provide – which is specific to the location and type of signal receptor receiving the original stimulus – is integrated and a response may consequently be formulated. This information may lead to action on the part of the somatic division of the peripheral nervous system or instead lead to the activation of reflex arcs.

Links to terms of possible interest: Nerve, Neuron, Motor nerve, Sensory nerve

Motor nerves

The pathway over which your body relays action potentials from the central nervous system to effector tissues such as skeletal muscles.
Motor nerves house the axons associated with motor neurons. Motor neurons are also known as efferent neurons and their role is to effect as effector neurons the contraction, for example, of skeletal muscles, that is, to cause skeletal muscles to become stimulated.

Motor neurons possess cell bodies that technically are found in the central nervous system but which have axons that are found outside of the central nervous system (that is, axons which instead are part of the peripheral nervous system) and it is these axons which are housed within motor nerves.

The motor neurons innervating skeletal muscles terminate in neuromuscular junctions, which are essentially synapses between motor neurons and muscles. The action potentials that are carried within motor nerves thus originate within the central nervous system, often within the spinal cord, and then propagate towards for example skeletal muscles.

Note that because a single axon as carried within a motor nerve can terminate in multiple axon terminals, a single motor neuron may innervate multiple muscle fibers, though it is unlikely that a single axon will innervate all of the muscle fibers making up a single muscle since if the latter were the case then a single nerve impulse would necessarily stimulate all of the fibers making up a single muscle to simultaneously contract.

In addition to motor neurons being associated with the somatic division of the peripheral nervous system, that is, as innervating muscle cells, motor neurons include as well those of the autonomic division.

Autonomic division

What otherwise is known as the involuntary, peripheral nervous system.
The autonomic nervous system, also described as visceral or involuntary, combines with the somatic division to make up the efferent aspect of the peripheral nervous system. The autonomic division specifically is that aspect of the peripheral nervous system that is not under voluntary control. The autonomic nervous system instead is responsible for those aspects of the maintenance of homeostasis that do not involve or instead are in addition to behavioral responses to various internal conditions.

Basically, components of the autonomic division are all of those things that the body does to keep the body going that you are not aware of the body being up to, and which involve signaling from the brain via neurons, rather than directly via hormones, where those neurons are components of the peripheral nervous system.

These phenomena include such things as your heart's beating (and how fast), your breathing, your digestion of food, etc. The autonomic nervous system traditionally is divided into what are known as sympathetic division versus parasympathetic division.

Links to terms of possible interest: Acetylcholine, Autonomic division, Epinephrine, Fight or flight, Norepinephrine, Neurotransmitter, Nitric oxide, Parasympathetic division, Postganglionic neuron, Preganglionic neuron, Rest and digest, Spinal cord, Spine, Sympathetic division, Sympathetic ganglion

The above video is a surprisingly effective walk through the autonomic nervous system.

The above video is a little redundant but makes its points well.

Sympathetic division

Aspect of the peripheral nervous system that is responsible especially for involuntary responses to emergency situations.
The sympathetic division of the autonomic nervous system is responsible for involuntary responses to what are often referred to as "fight or flight" (versus "rest and digest"). That is, when your body needs to become "revved up" to escape especially something that is frightening, and/or that is simply stress inducing, it is the sympathetic nervous system, along with especially the release of the hormone epinephrine (that is, adrenalin) that instructs the body how to respond.

This includes up regulation of certain functions (e.g., breathing and heart rates) and down regulation of others (e.g., digestion and urination). These stress responses essentially counter those effected instead by the parasympathetic nervous system.

Links to terms of possible interest: Autonomic division, Cranial nerve, Ganglia, Parasympathetic division, Spinal cord, Spinal nerve, Sympathetic division, Sympathetic chain, Sympathetic trunk

The above video provides a nice introduction to the sympathetic nervous system.

Parasympathetic division

Aspect of the peripheral nervous system that is responsible especially for involuntary responses to non-emergency situations.
The parasympathetic division of the autonomic nervous system (effecting "rest and digest") is responsible for implementing involuntary responses to everything except what is involved in "fight or flight". These are all of the involuntary things that bodies need to do when they are not "revved up" to escape especially something that is frightening, and/or something that is simply stress inducing.

Basically, the body is responsible for doing three things: Obtaining and utilizing resources, protecting itself, and reproducing, and it is the parasympathetic division of the peripheral nervous system that provides nervous-system control of especially the first and last of these, that is, the routine (every day) acquisition of resources (and their utilization) along with much of what goes into generating the next generation (that is, producing offspring). Contrast with the sympathetic nervous system.

As is also the case for the sympathetic division, the parasympathetic nervous system is complemented by endocrinal (hormonal) control of various body functions as well. Thus, during the relatively normal intervals of our lives there are nerves and hormones that contribute to the maintenance of homeostasis for the sake resource acquisition, resource utilization, and otherwise normal body functioning (parasympathetic division and associated hormones) and during abnormal, fight or flight situations the body is able instead to modify its general activities towards more basal survival modes (sympathetic division and especially the hormone epinephrine/adrenaline).

The above video provides a nice, and at times quite hilarious introduction to the parasympathetic nervous system.


Entity that when interacting with a second, especially extracellular entity gives rise to a well-defined and anticipated change in the physiological state particularly of the first entity.
I've written this definition broadly to include individual proteins that can serve as receptors (i.e., receptor proteins), individual cells that can serve as receptors, or instead multicellular complexes that can serve as receptors. In any case, something that is found outside of the cell or cells – that is or are associated with or makes up these receptors – is what is detected by the receptor. The result can be a signal transduction pathway, that is, movement of the signal indicating that reception has occurred from outside of the receiving cell to inside of the receiving cell.

This is kind of like a text message coming from one person that leads to an actual verbal communication with another individual. Here there is a change in the form of the signal as well as its location, from electronic and over the airwaves to immediately local and involving sound waves, versus signal reception outside of a cell leading to biochemical pathways/reactions that take place inside of a cell. Alternatively, reception of a signal may be converted into an action potential that then propagates towards the central nervous system, i.e., as is the case of sensory receptors.

The above video is getting a bit ahead of ourselves, but discusses the differences between sensation and perception; note particularly that the role of receptors is only a fraction of the overall process of taking in information from the environment.

Sensory receptors

Specialized cells or complexes of cells that in animals are capable of detecting environmental or internal signals and converting those signals into action potentials.
Sensory receptors correspond to our various senses, including ones that we are not conscious of (the latter particularly those monitoring much of the internal goings on within our bodies). Anything that we perceive or which otherwise results in an action potential that moves towards the central nervous system is generated by some form of sensory receptor

These receptors differ in terms of their location in or on the body, in terms of what specific types of signals they are capable of detecting, and also in terms of their density in whatever tissues they are present in. The latter determines the resolution with which our brains are capable of perceiving the signal, such as where specifically on our bodies we are being touched.

The brain in the course of maturation learns how to integrate these signals into what literally is its world view, figuring out how to most effectively interact with the world around it, such as the understanding of what sensations are associated with what body parts. We literally are not born with that knowledge but instead must train our brains to effectively do this—much of what we perceive as our being alive involves our interaction with the outside world through numerous sensory receptors.

Links to terms of possible interest: Dermis, Epidermis, Hypodermis, Meissner's corpuscle, Nociceptor, Pacinian corpuscle, Skin, Thermoreceptor

Touch receptors

Skin- and hair-associated neurons that detect the presence of immediately local entities found outside of the body.
Touch is perceived by what are known as mechanoreceptors, which come in a number of varieties. The sensations we perceive as touch range from those associated with pressure to vibration to movement of hairs.

Generally what is being perceived are changes in the relative location of collagen fibers, particularly as found within capsule-shaped entities. The capsules are connected to receptor proteins found within the associated neuron and it is the movement of the capsule relative to the associated neurons that results in depolarization and subsequent propagation of an action potential.

If these endings from individual neurons cover only a small area of skin, then our ability to perceive exactly where we are being touched will be fairly precise (e.g., as on our lips, or the tips of our fingers). Alternatively, if the area covered by the endings associated with an individual sensory neuron – what is known as their receptive field – instead are diffuse, then our ability to resolve exactly where we are being touched will be much lower (such as on our backs).

What we feel and where we feel it, in terms of touch, in other words depends on various characteristics of the sensory neurons responsible for conveying that information to our brains.

Links to terms of possible interest: Bulbous corpuscles, Dermis, Epidermis, Free nerve endings, Hair follicle receptor, Mechanoreceptors, Merkel disks, Pacinian corpuscles, Ruffini organs


Specialized neurons that convey the sensation of pain to the central nervous system.
Nociceptors, also referred to simply as pain receptors, are a variety of sensory receptors found both internally and close to the surface of the body that recognize the occurrence or instead potential occurrence of tissue damage such as due to mechanical stress, excessive temperatures, or exposure to tissue-damaging chemicals. Because it is the brain that perceives pain, however, the perception of pain can in fact be unrelated to such conditions.

Because of the way that our internal body is innervated relative to the surface of our bodies – i.e., sensory neurons whose signals are received in similar regions of the brain tend to be perceived as being co-localized at their other ends such that a skin-associated nociceptor is perceived by the same areas of the brain as a more deeply located nociceptor – along with our greater familiarity with sensation associated with the surface of our bodies versus more deeply within tissues, we tend to consciously perceive pain as though it were occurring closer to the body surface than it may otherwise be originating. This concept is described as referred pain, and it represents a means by which our brain in essence converts less familiar perception into something which is more familiar (something our brains are constantly doing in many other ways besides in terms of the perception of pain).

Links to terms of possible interest: Dermis, Epidermis, Ganglion, Neuron, Nociceptor, Pain receptor, Skin, Spinal cord, Spinothalamic pathway, Nociceptor

This is a replacement of a video of nociception that unfortunately has gone missing on YouTube. It's a bit intense as YouTube videos go, but it's what we have for now…

Afferent pathway

Neurons involved in conveying the reception of a sensory stimulus towards the central nervous system.
"Afferent" means "to carry to" but (if you are sufficiently into the language) you can think of it as the pathway that "affects" the functioning of an integrating center (contrast efferent pathways). That is, afferent pathways carry signals from sensory receptors towards whatever perceives those signals (consciously or unconsciously), which basically is some control center associated either with the brain or the spinal cord or both. In other words: StimulusReceptorAfferent pathwayIntegrating center

Links to terms of possible interest: Afferent neuron, Brain, Efferent neuron, Gray matter, Interneuron, Sensory receptor, Spinal cord, White matter

Integrating center

Neurons involved in converting the reception of a sensory stimulus into a response to that stimulus.
In vertebrate animals the integrating center is either the brain or the spinal cord. For somatic reflexes it is the spinal cord that integrates the information, and does so using relatively simple pathways going from, in order, afferent pathways to efferent pathways.

In behaviors that are not reflexes, the integrating center instead is the brain, which typically will involve far more neurons to effect a response to a stimulus (as carried to the brain via afferent pathways). The response, as carried away from the brain via efferent pathways, in addition will require more time than a somatic reflex.

It is important to keep in mind that the spinal cord and brain can serve as integrating centers for information garnered via the same afferent pathways, with the spinal cord's action, and associated reflex simply occurring sooner due to a combination of proximity to sensory receptors (afferent pathways enter the spinal cord prior to entering the brain) and the involvement of fewer neurons separating afferent pathways from efferent pathways. Basically, it takes more time to become consciously aware of a perception and then act upon that perception than it takes for the body to simply "react" reflexively to a stimulus.

Links to terms of possible interest: Afferent pathway, Effector, Effector pathway, Integrating center, Negative feedback, Receptor, Set point

Efferent pathway

Nerves involved in effecting a response to a stimulus.
"Efferent" means "to carry away from" but (if you are sufficiently into the language) you can think of it as the pathway that the functioning of an integrating center "effects" (contrast, that is, afferent pathway). Efferent pathways carry signals from control centers (brain or spinal cord or both) towards whatever will be acting in response to those signals. In other words: StimulusReceptorAfferent pathwayIntegrating centerEfferent pathway → Whatever the efferent nerves are stimulating.

Thus, for example, in a classic withdrawal reflex the stimulus of pain is received by pain receptors found in our fingers and this results in the propagation of an action potential towards our spinal cord via afferent nerves. This information is then received by the spinal cord which effects a response by initiating the propagation of an action potential towards the appropriate muscles which then contract, causing the withdrawal of the affected fingers away from the source of pain.

Links to terms of possible interest: Afferent pathway, ANS, Cardiac muscle, CNS, Efferent pathway, Glands, Involuntary, PNS, Skeletal muscle, Smooth muscle, Somatic division, Visceral division, Voluntary,

Links to terms of possible interest: Afferent pathway, Afferent sensory information, Autonomic efferent nuclei, Central nervous system, Dorsal horn, Dorsal root ganglion, Efferent signals, Efferent pathway, Ganglion, Gray matter, Lateral horn, Somatic motor nuclei, Somatic sensory nuclei, Spinal cord, Ventral horn, Ventral root, Visceral sensory nuclei,


Rapid, involuntary responses to stimuli that do not involve integration at the level of the brain.
As these processes bypass the brain, they are both involuntary and do not require conscious thought. They are perceived by the brain, however, both in terms of the sensory input stemming from the original stimulus and in terms of the sensory input stemming from the response. Thus, while involuntary, you nonetheless can be aware of both the action and why it occurred.

These are not just responses to emergences (e.g., the withdrawal reflex) but also play key roles in a number of complex processes. The latter includes walking, where simple corrections to one's gate do not occur consciously and do not even involve the brain, but nonetheless do occur to a large extent over the course of effecting this movement.

The actual innervation involves what are known as a reflex arcs, and reflex arcs in vertebrate animals consist of a combination of an afferent pathway, an integrating center found within the spinal cord, that is, rather than found within the brain, and then a corresponding efferent pathway.

Links to terms of possible interest: Afferent neuron, Brain, Efferent neuron, Excitatory effector pathway, Hamstring, Inhibitory effector pathway, Interneuron, Muscle spindle, Patellar reflex, Patellar tendon, Stimulus, Quadriceps

The above video shows a knee-jerk reflex as seen in a dog!

Reflexive response to falling – grab on for your life!

What is known as the Moro response as demonstrated by a physician.

Withdrawal from a tickle as demonstrated by an adult.

Infant withdrawal reflex, though apparently here not to any particular stimulation.

The above video considers the scary biology of the startle reflex, contrasting it, e.g., with the visual sensation, though focusing more on what we find scary rather than the biology of the reflex.

Reflex arc

Neural pathway involving an afferent pathway that is followed by integration other than within the brain which in turn is followed by an efferent pathway and resulting response.
Thus, a signal is passed up to the spinal cord from sensory receptors via an afferent pathway, passes through neurons within the spinal cord that carry the afferent pathway signal directly to the efferent pathway, which then effects a response. These neural pathways are "hardwired" into our bodies and therefore are both not learned and otherwise tend to be generally shared except among those possessing neurological pathologies.

Indeed, testing for reflex responses, and therefore the functioning of reflex arcs, is a standard component of general medical examinations, e.g., the knee-jerk reflex, but also Moro reflex (i.e., the "startle reaction") as seen in infants (and which also represents a so-called infant, infantile, newborn, or primitive reflex, i.e., one that normally is found in infants but not normally in adults).

Links to terms of possible interest: Anterior horn, Anterior root, Axon, Cell body, Central canal, Effector, Ganglion, Gray matter, Interneuron, Motor neuron, Posterior horn, Reflex arc, Sensory neuron, Sensory receptor, Spinal cord, Ventral root, White matter,

The above video is well illustrated and to the point serves as an introduction to reflex arcs.

The above video focusses mainly on reflex arcs.

The above video provides an overview of different types of especially mechanical nerve damage to the peripheral nervous system.

Multiple sclerosis

Disease associated with inflammatory damage to myelin sheaths located in the central nervous system.
Though technically a disease of the central nervous system, multiple sclerosis, or MS, can affect numerous systems seen external to the central nervous system such as difficulty in walking. This illustrates both the importance of myelin towards normal neural functioning and the importance of the central nervous system towards normal functioning of the rest of the body.

Links to terms of possible interest: Degenerative disease, Multiple sclerosis, Myelin, Nervous system, Symptoms

Short overview of what multiple sclerosis consists of.


Virus-caused gastrointestinal disease that in relatively rare cases can lead to the death of a subset of motor neurons.
Known more formally as poliomyelitis or instead infantile paralysis – referring to the relatively rare impact of the polio virus on neuronspolio more commonly is a gastrointestinal disease though also can involve the respiratory tract. The paralytic disease is associated particularly with virus entrance into the blood in association with immune system insufficiencies in the sufferer.

Polio nonetheless is an acute, meaning relatively short-lived infection that also typically is relatively mild in terms of its effect on the body. It is paralytic poliomyelitis, however, that we vaccinate against, though both the paralytic and non-paralytic forms of the disease are caused by the same viruses.

Links to terms of possible interest: Anterior horn, Gray matter, Motor neuron, Paralytic poliomyelitis

A brief overview of poliomyelitis which provides a good sense of why the disease was feared.

The above is a historical film heralding the introduction of the Salk polio vaccine, in 1955. It was followed in 1962 with the Sabin polio vaccine.