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42 C. Jaeger
peripheral nervous system, highlighting how information about the body and the
external environment is detected by specialized receptors and relayed to and from the
central nervous system (CNS).
4.2 Overview of the Peripheral Nervous System
The peripheral nervous system comprises the parts of the nervous system outside of
the brain and the spinal cord (Fig. 4.1). The PNS is composed of the somatic and
autonomic nervous systems. The somatic nervous system consists of the sensory
nervous system, which provides information about the surrounding environment,
and the motor system, which enacts voluntary and involuntary muscle function. The
autonomic nervous system governs bodil y functions that occur involuntarily and are
important for maintaining homeostasis. These include heart rate, breathing rate, and
digestion. The autonomic nervous system can be further divided into the sympathetic, parasympathetic, and enteric nervous systems. The cranial nerves are also
considered part of the PNS. The cranial nerves innervate the face, neck, and tongue.
There are 12 cranial nerves, with some classified as part of the somatic nervous
system and the rest classified as part of the autonomic nervous system (Table
4.1).
Fig. 4.1 Overview of the nervous system divisions. The nervous system is composed of the CNS
(purple) and the PNS (black). The PNS can be further divided into the somatic nervous system,
consisting of sensory input and motor outputs, and the autonomic nervous system, which is
composed of the sympathetic, parasympathetic, and enteric nervous systems
4 Basic Anatomy: Peripheral Nervous System 43
Table 4.1 Cranial nerves
CNS
Number Name Function
I Olfactory Smell Olfactory
II Optic Vision Thalamus Sensory
III Oculomotor Eye movement, pupil contraction Brainstem Motor
IV Trochlear Eye movement Brainstem Motor
V Trigeminal Sensation in the face, chewing Brainstem Both
VI Abducens Eye movement Brainstem Motor
VII Facial Contraction of facial muscles, taste,
VIII Vestibulochoclear Balance, hearing Brainstem Sensory
IX Glossopharyngeal Taste, swallowing, salivation, blood
X Vagus Speech, swallowing, regulation of
XI Spinal accessory Neck and shoulder movement Brainstem/
XII Hypoglossal Tongue movement, throat movement Brainstem Motor
salivation,
pressure
lungs, digestion
heart,
lacrimation
regulation
connection
bulb
Brainstem Both
Brainstem Both
Brainstem Both
spinal
cord
Type
Sensory
Motor
4.3 Basic Anatomical Unit of the PNS: Ganglia and Nerves
The different PNS domains share common anatomical components. Similar to the
CNS, the basic unit in the PNS is the neuron. Neurons make up the structures called
the ganglia and peripheral nerves. Within the PNS, collections of neuronal cell
bodies form ganglia. Their axons extend outward, traveling together in bundles
that make up peripheral nerves, eventually reaching target tissues in the periphery.
These bundles of axons are called fascicles and are surrounded by connective tissue
called the perineurium, which provides structural support. The blood vessels surrounding the fascicles nourish axons with oxygen. Several fascicles are further
bundled together by connective tissue to form a peripheral nerve. The signa l
transmitted along peripheral nerves needs to travel long distances to and from the
peripheries to the CNS. To facilitate this, Schwann cells form a myelin sheath around
axons in the PNS. Myelination speeds up electrical signal transmission along the
nerves, allowing faster communication.
There are
two categories of peripheral nerves: afferent and efferent. Afferent
nerves transmit signal from the peripheries to the brain. These nerves are also called
sensory nerves as they detect environmental or peripheral stimuli and relay specific
information about the stimul i to the brain. Efferent nerves relay motor commands
from the CNS to the muscles, organs, and glands in the body. These nerves are also
called motor nerves as they carry commands for voluntary and involuntary motor
action.
44 C. Jaeger
4.4 Anatomy of Somatic Nervous System
The somatic nervous system connects the central nervous system to the periphery
and performs voluntary movements. The somatic nervous system is also known as a
single-cell nervous system, as the motor and sensory neuron bodies synapse onto the
central nervous system directly.
4.4.1 Receptors
The sensory nerves have specialized receptors at the end of the nerve that respond to
different types of stimul i and enable the electrochemical conversion of the signal.
The response to a sensory stimulus causes a change within a receptor cell or
transmembrane protein that changes the ion composition within the cell by opening
ion channels. This elicits an action potential within the sensory axon and relays the
action potential toward the central nervous system. The body has many specialized
types of sensory receptors that respond to very different environmental stimuli. A
detailed description of the composition and function of each receptor type is outside
the scope of this book. Here, we will focus on some of the sensory recept ors that are
involved in sensory modalities most prominently studied in conjunction with EEG.
A more in-depth overview of receptors is provided in Leclerc and Wray (
most
recognized senses, such as vision, audition, olfaction, and gustation, are called
specialized sensory modalities, which have a specialized organ that processes and
transmits the sensor y information to the brain; below we cover vision and audition.
In addition, we cover balance and general sensory modalities.
2023). The
4.4.1.1 Vision
The sense
the eye. These specialized receptors are called photoreceptors, which change their
membrane potential when stimulated by light energy or photons. The photoreceptors
regulate neurotransmitter release of the bipolar cells, which synapse onto the retinal
ganglion cell. The retinal ganglion cells converge into the optic nerve, which transmits the visual input as an electrical signal to the thalamus. There are two types of
photoreceptors classified as cones and rods. The cones are further divided into three
types of specialized photoreceptors that respond to different wavelengths in the light
spectrum: red, green, and blue. Their combined activity enables the perception of a
wide variety of colors. The rods are highly sensitive to light but lack color sensitivity. They are located on the periphery of the retina, transmit gray-scale light, and
assist with low-light vision (Openstax,
of vision is detected by specialized receptors located within the retina of
2022).
4 Basic Anatomy: Peripheral Nervous System 45
4.4.1.2 Audition
Another specialized sense is audition, which mediates the transduction of sound
wave
s into an electrical signal within the ear. The cartilaginous outer structure of the
ear, known as the auricle, funnels sound waves into the auditory canal. At the end of
the auditory canal, the tympanic membrane vibrates in response to incoming sound
waves. The vibrations from the tympanic membrane become amplified by three
small bones, known as ossicles, that are located in the inner ear. The enhanced
vibrations are transmitted to the oval window of the inner ear. Within the inner ear,
the cochlea is attached to the oval window. The cochlea contains a fluid-filled tube.
The pressure from the vibrating ossicles moves the fluid within the cochlea in a
wave-like pattern. The cochlear duct is surrounded by the basilar membrane that
contains hair cells, which have hair-like projections known as stereocilia. As the
fluid in the cochlear duct moves, it pushes the stereocilia to bend. As the stereocilia
bend, ion channels open, changing the ion composition of the hair cells. This causes
the hair cell to depolarize. This type of receptor is also known as a mechanoreceptor.
A mechanical change in the receptor cell causes the opening of ion channels.
Depolarization of the hair cells releases neurotransmitter, which stimulates the
cochlear nerve. The cochlear nerve projects to the brainstem, where the sense of
audition is then further transmitted to the auditory cortex.
4.4.1.3 Vestibular System and Balance
The ear
also contains the vestibular system, which helps with the sense of balance.
The mechanoreceptors responsible for balance are located within the vestibule of the
inner ear. The utricle and saccule sense head position. They are also composed of
specialized hair cells with stereocilia that extend into a viscous gel known as the
otolithic membrane. As the head tilts forward and back, the otolithic membrane
slides over the hair cells, causing the stereocilia to bend and some hair cells to either
depolarize or hyperpolarize. The position of the head is then determined by the brain,
depending on the composition of depolarized hair cells. Head rotation is sensed by
the semicircular canals, which consist of three ringlike structures oriented in different planes of the body. At the base of each semicircular canal, a structure known as
the ampulla contains hair cells. These hair cells extend into the gelatinous structure,
the cupula. As the head rotat es in a direction that is parallel to the plane of a given
semicircular canal, the fluid within the cupula causes the stereocilia on the hair cells
to bend, which again opens ion channels and causes depolarization of the hair cells.
The information is then transmitted to the vestibular ganglions and through the
vestibulocochlear nerve to the brain stem and cerebellum.
46 C. Jaeger
4.4.1.4 General Sensory Modalities
General sensory modalities have specialized receptor cells distributed throughout the
body.
An example of such a sensory modality is somatosensation. Somatosensation
is a general term for proprioception, interoception, and touch and consists of
different sensory receptors that respond to vibration, temperature, pressure, and
pain. Somatosensory recept ors can be located in the skin, muscles, and within
visceral organs. They also consist of free nerve endings that have dendrites directly
embedded in the tissue or encapsulated nerve endings, which are covered in connective tissue. Two types of free nerve ending receptor cells include thermoreceptors
and nociceptors, which transduce temperature and pain. Temperature receptors
detect temperature that differs from body temperature. Nociceptors respond to either
thermal, chemical, or mechanical stimuli. A common example of a chemical ligand
that initiates pain is capsaicin, which is found in spicy foods. Capsaicin binds to an
ion channel regulating temperature and induces a sensation of heat. There are several
encapsulated mechanoreceptors, stretch receptors, or receptors that detect vibration.
4.4.2 Somatic Sensory System
The sensory signal ascends from the receptors along the peripheral process of the
sensory nerve along the dorsal root to the cell body, which sits in the dorsal root
ganglion just outside of the spinal cord. Figure
pathways
CNS. The two pathways are classified as the dorsal column system and the
spinothalamic tract and relay different information from the environment. Proprioception and fine touch sensory input is primarily relayed along the dorsal column
system (Koop & Tadi,
extend
through the dorsal column and synapses onto a secondary neuron located in the
medulla. Temperature and nociception are primarily relayed along the spinothalamic
tract. In the spinothalamic pathway, the central process, which extends from the cell
body in the dorsal root ganglion, synapses contralaterally to a second neuron whose
cell body is located in the spinal cord grey matter and terminates in the thalamus. In
both sensory ascending pathways, the secondary neuron is considered part of the
CNS and synapses with the thalamus, which relays peripheral information to the
somatosensory cortex.
nerves. Voluntary movement is controlled by the motor cortex in the CNS along the
corticospinal tract . This pathway is also known as the primary descending motor
pathway. Upper motor neurons descend from the primary motor cortex through the
brainstem along the spinal cord and synapse with the lower motor neurons. The cell
bodies of lower motor neurons are located in the ventral root, and the axons from the
lower motor neurons project to peripheral skeletal muscles via the ventral horn. The
by which sensory information from the dorsal root ganglion travels to the
2025). In the dorsal column system, a second central process
s from the cell body in the dorsal root ganglion and ascends ipsilaterally
The efferent
counterpart of the afferent sensory pathways is the motor peripheral
4.2 highlights the two different
4 Basic Anatomy: Peripheral Nervous System 47
Fig. 4.2 Somatic nervous system. (a) Receptors from the peripheries convert a stimulus into an
electrical signal that propagates along the myelinated sensory neuron to the dorsal root of the
vertebrae. From there, the signal can be directly transmitted to the efferent motor neuron that
innervates the effector muscle. The signal from the afferent sensory neuron also synapses onto a
secondary sensory neuron in the spinal cord that transmits the signal to the brain. (b) Afferent
pathways consist of either the dorsal column or the spinothalamic tracts. Efferent pathways are
divided into the anterior and lateral corticospinal tracts
electrical signal from the motor neurons is then translated into mechanical muscle
contractions at the neurom uscular junction. The neuromuscular junction is in the
center of the muscle fiber, also referred to as the innervation zone. Electromyography
(EMG) can be used to measure the summation of electrical activity generated by
muscle fibers contracting in response to motor unit stimulation at a muscle of
interest. The placement of EMG electrodes is covered in more deta
il in Chap. 12
(“Hardware for Recording EEG and Peripheral Physiology”).
4.4.3 Reflex Arc in the Somatic Nervous System
While the somatic nervous system is under voluntary control of the CNS, the body
must be able to respond quickly to dangerous external stimuli. There is an adaptation
of the body, known as a reflex, to briefly circumvent the CNS and mediate an
automatic, involuntary response to certain stimuli. Reflexes are present in both the
somatic and autonomic nervous systems. Reflexes consist of the reflex arc, which is
the neural pathway that receives stimuli from a sensory receptor at a specific location
and initiates an effector to elicit a functional response at the original site of sensory
input, such as a muscle contraction. Reflex arcs can be monosynaptic, which
involves a sensory neuron synapsing directly with a motor neuron to elicit an
immediate response, such as the patellar knee-jerk response. Reflex arcs can also
48 C. Jaeger
be polysynaptic, in which the sensory neuron synapses with an interneuron within
the dorsal horn, which transmits the signal to the efferent motor neuron to elicit a
response (OpenStax, 2022).
4.5 Anatomy of the Autonomic Nervous System
The autonomic nervous system (ANS) consists of nerves innervating glands, blood
vessels, and internal organs. It is also called the involuntary or visceral nervous
system. The ANS is divided into the parasympathetic and the sympathetic nervous
systems, which are anatomically and functionally distinct. Most tissues are innervated by both systems, as the two systems work together with opposing effects.
Figure 4.3 highlights the function of the parasympathetic and sympathetic nervous
Fig. 4.3 Autonomic nervous system. The autonomic nervous system consists of the sympathetic
and parasympathetic nervous systems that innervate different organs of the body. The efferent
pathways of the autonomic nervous system consist of two neurons: the preganglionic and the
postganglionic neurons. The preganglia synapse onto the postganglia, whose cell bodies are located
in the paravertebral sympathetic ganglion chain
4 Basic Anatomy: Peripheral Nervous System 49
systems and how they work in an opposing manner. The ANS is a disynaptic
pathway in which the efferent pathways consist of two neurons, a preganglionic
neuron and a postganglionic neuron, that transmit signals from the CNS to the
periphery. The synapse between the postganglionic neuron and effector tissue is
called the neuroeffector junction, which is organized into varicosities that release
neurotransmitter over a large surface area (McCorry,
2007).
4.5.1 Sympathetic Nervous System
The sympathetic nervous system prepares the body to react to stressors in its
environment by initiating a rapid physiological response, often termed the “fight
or flight” response. Visceral sensory receptor cells detect stimuli, which are relayed
along afferent nerve fibers through the sympathetic chain ganglia to the dorsal root
ganglion. The sensory nerves then synapse to secondary nerves in the dorsal horn
that relay information to the CNS or to efferent connections via reflex arcs.
The efferent preganglionic neurons originate from the CNS, specifically from the
thoracic and lumbar regions (McCorry, 2007). The efferent preganglionic sympathetic
connections are also referred to as the thoracolumbar system. Within the
thoracolumbar system, the preganglionic cell body is located in the lateral horn of
the vertebral column, where it projects to a postganglion located in a network of
sympathetic chain ganglia that runs parallel to the vertebral column. A preganglion
can project to several postganglionic fibers. Projections from the postganglion
neuron project to the effector tissue in the periphery. The preganglionic neurons of
the sympathetic nervous system release acetylcholine, and most postganglionic
neurons of the sympathetic nervous system secrete norepinephrine. The sympathetic
nervous system regulates heart rate, respiration, and pupil responses.
4.5.2 Parasympathetic Nervous System
The parasympathetic nervous system stimulates “rest and digest” functions of the
ANS and regulates visceral organs. The parasympathetic nervous system is organized similarly to the sympathetic nervous system, except that some of the afferent
and efferent pathways are categorized as cranial nerves and have a direct connection
with the brainstem. The efferent preganglionic nerve fibers from the cranial division
also arise from the brainstem and synapse with the postganglionic nerves near the
effector tissue. The other division is located in the sacral region (McCorry,
sensory receptors located in the visceral organs transmit electrical signals along
The
the sensory afferent neurons to the sacral dorsal root ganglia, where they synapse
onto a secondary neuron that relays information from the spinal cord to the brain.
The preganglionic neurons pass through the lateral horn of the sacral spinal cord.
The parasympathetic preganglia target the terminal ganglia, which are located near
or within the target tissue. The postganglia project a short distance to the target organ
2007).
50 C. Jaeger
or tissue. The primary neurotransmitter of the preganglionic and the postganglionic
neurons of the parasympathetic nervous system is acetylcholine.
4.5.3 Autonomic Reflex Example
Autonomic reflexes play a large role in maintaining homeostatic balance within an
organism. The ANS is largely regulated by autonomic reflexes. Sensory input from
glands and organs is relayed to the hypothalamus and brainstem, which are the
control centers for bodily processes such as heart rate, blood pressure, body temperature, glandular secretion, and digestion. An example of an autonomic reflex is
the chemoreceptor reflex, which regulates respiration rate based on the concentration
of oxygen and carbon dioxide in the blood. Specialized chemoreceptors in the aortic
arch near the heart and in the carotid artery in the neck monitor partial pressure of
gases in the blood, sensing fluctuations in oxygen and carbon dioxide levels. The
chemoreceptors can also detect changes in pH or acidity of the blood, which is
correlated to the amount of dissolved carbon dioxide and hydrogen ions in the blood.
A decrease in oxygen inhibits oxygen-sensitive potassium ion channels in glomus
cells (type I cells) located in the carotid and aortic bodies. The inhibition of
potassium channels causes a buildup of potassium and subsequently leads to cell
depolarization. The glomus cell depolarization leads to neurotransmitter releases by
opening voltage-gated calcium channels. The release of neurotransmitters activates
afferent nerve endings of either the glossopharyngeal nerve within the carotid body
or the vagus nerve within the aortic body. The peripheral afferent nerves relay the
signal to the brainstem. The brainstem then regulates efferent autonomic responses
that increase breathing rate to enhance oxygen intake and increase the heart rate to
enhance oxygen delivery. When carbon dioxide dissolves in the blood, it reacts with
water molecules and forms carbonic acid, which lowers the blood pH. Type I glomus
cells are also sensitive to a decrease in pH levels. When the pH level of the blood
drops, potassium channels are inhibited, and a similar signal transmission pathway
as described for oxygen levels is activated, increasing breathing rate (Iturriaga,
2021). The autonomic chemoreceptor reflex can also be indirectly measured by
ing respiration rate and heart rate changes, which can be measured with
record
peripheral sensors, as explained in Chap.
Periph
eral Physiology”).
12 (“Hardware for Recording EEG and
4.6 Cranial Nerves
The 12 cranial nerves consist of afferent and efferent connections whose nuclei
originate from the brainstem or cerebrum. The cranial nerves can be classified as
sensory, motor, or both and mostly innervate the head and neck area. Some of the
cranial nerves are part of the autonomic nervous system, and some cranial nerves are
part of the somatic nervous system.
4 Basic Anatomy: Peripheral Nervous System 51
Table I lists the twelve cranial nerves, their function, the type of peripheral nerves
they are classified as, and their connection within the CNS. The olfactory and optic
nerves are sensory nerves that allow smell and vision. The oculomotor, trochlear,
and abducens nerves are all responsible for controlling eye movements; however,
they innervate different extraocular muscles. The trigeminal nerve is responsible for
the sensation of the skin on the face. It also innervates the muscles required for
chewing food. The vestibulocochlear nerve transmits sensor y information to the
brain for hearing and balanc e. The glossopharyngeal nerve contr ols musc le movements in the mouth and upper throat. It also transmits sensory signals for the sense of
taste and regulates the production of saliva. The vagus nerve regulates homeostatic
function of the organs in the thoracic and upper abdominal cavities and regulates
breathing rate, heart rate, and digestion. The spinal accessory nerve is a motor nerve
responsible for controlling the muscles of the neck and upper shoulders. The last
cranial nerve, the hypoglossal nerve, controls muscles of the tongue and throat.
Most of the cranial nerves belong to the somatic nervous system. The vagus nerve
is
considered part of the autonomic nervous system, specifically the parasympathetic
division. Three cranial nerves perform functions that belong to both the soma tic and
the autonomic nervous systems. The oculomotor nerve fibers both innervate the eye,
considered an organ, and the muscle within the eye, the iris. The facial and
glossopharyngeal nerves innervate glands involved in salivation.
4.7 Function of the PNS and CNS as a Unit
A key example of the function of the PNS working together with the CNS is the
processing of pain. To demonstrate how the two nervous systems work together in
the con text of electrophysiological research, we present an example of
neurofeedback and pain processing and perception.
Figure 4.4 shows
changes activate nociceptors in the skin, which convert the stimulus into electrical
impulses that travel along sensory neurons to the dorsal horn of the spinal cord. The
sensory signal is then transmitted along a reflex arc to the interneurons within the
dorsal horn, which relays the signal to the motor neurons. At the neuromuscular
junction, the signal is then converted to muscular contraction and results in a
muscular response, such as pulling the arm away from the hot stimulus by causing
muscle contraction in the arm. Simultaneously, the pain signal is further transmitted
from primary to secondary sensory neurons that travel to the brainstem and thalamus, where pain is further transmitted and processed within the somatosensory
cortex.
The CNS
transmits the signal generated by the painful stimulus to the prefrontal cortex and
anterior cingulate cortex. These areas are involved in decision-making and emotional regulation, respectively, and allow for conscious perception of pain. If the
stimulus is perceived as tolerable, the CNS sends inhibitory signals via the
can also override the reflex response. The somatosensory cortex also
the physiological response to touching a hot surface. Thermal