Introduction to the central and peripheral nervous systems

Introduction to the central
and peripheral nervous systems
Classification and function of the nervous system. Brain and spinal cord. Types of nerves
~ 10 min

In this note, we are going to discuss the basic anatomy of the nervous system (systema nervosum).

Nervous system (systema nervosum)
Nervous system (systema nervosum)

This system provides communication among all other organ systems, monitors internal and external changes, and develops proper responses.

For ease of understanding, the nervous system is structurally subdivided into the central nervous system (or CNS) (systema nervosum centrale), which includes the brain (encephalon) and spinal cord (medulla spinalis), and the peripheral nervous system (or PNS) (systema nervosum periphericum), which includes nerves (nervi) and ganglia (ganglion) outside the CNS.

The peripheral nervous system is made up of two functional units: the somatic (that controls voluntary actions), and the autonomic (that controls the involuntary activities).

Nervous tissue

Both the central and peripheral parts of the nervous system consist of nervous tissue, which in turn is formed by neurons (or nerve cells) and neuroglia.

Neurons can receive, generate, and transmit electrical impulses, providing communication with other neurons as well as with effector cells, such as muscle cells.

Neurons consist of the cell body and two types of extensions, or processes, that arise from it.

Neuron body
Neuron body

Dendrites are short processes. Each neuron may have many such projections. They receive incoming signals from other neurons.

Dendrites
Dendrites

The second type, axons, are long. Each neuron has only one axon. It is used to transmit impulses to other neurons.

axon
axon

Some axons are covered with a layer of lipids and proteins that speed up the conduction of nerve impulses. This structure is called the myelin sheath.

Myelin sheath
Myelin sheath

The electrical impulse eventually reaches the distal parts of the axon, called the axon terminals.

Terminals
Terminals

They meet with the dendrite, the body, or with the axon of another neuron, and this contact site is called a synapse.

Synapse
Synapse

Depending on the location of a neuron within the nervous system, the number of its processes may vary:

  • Multipolar neurons have several dendrites and a single axon;
Multipolar neuron
Multipolar neuron
  • Bipolar neurons have one large dendrite and one axon;
Bipolar neuron
Bipolar neuron
  • Unipolar neurons contain only one process; according to many researchers, this type of neuron is only present during embryogenesis;
Unipolar neuron
Unipolar neuron
  • Pseudounipolar neurons have one process that arises from the body and immediately divides in a T-shaped manner. One part of this projection acts as an axon, and the other acts as a dendrite;
Pseudounipolar neuron
Pseudounipolar neuron
  • Axonless neurons contain many processes, but the axon is so short that it cannot be distinguished from dendrites.
Axonless neuron
Axonless neuron

Neuroglia, or glial cells, are the other cell type found in the nervous system. There are several types of neuroglia, they function as a maintenance staff who support and nourish neurons.

Neurons and neuroglia are discussed in more detail in the relevant topics.

Central nervous system

Alright, now let’s discuss the central nervous system, which consists of the brain and spinal cord.

The main role of the central nervous system is to coordinate and integrate nerve impulses that arrive from the periphery. This provides the functioning of our muscles and internal organs and allows us to think, learn, and experience emotions.

The brain (encephalon) consists of several large parts.

Brain (encephalon)
Brain (encephalon)
  • These include the cerebral hemispheres (hemispheria cerebri)
Cerebral hemispheres (hemispheria cerebri)
Cerebral hemispheres (hemispheria cerebri)
  • The diencephalon (diencephalon)
Diencephalon (diencephalon)
Diencephalon (diencephalon)
  • The cerebellum (cerebellum)
Cerebellum (cerebellum)
Cerebellum (cerebellum)
  • And the brainstem (truncus encephali)
Brainstem (truncus encephali)
Brainstem (truncus encephali)

The brainstem, in turn, includes:

  • The midbrain (mesencephalon)
Midbrain (mesencephalon)
Midbrain (mesencephalon)
  • the pons (pons),
Pons (pons)
Pons (pons)
  • and the medulla oblongata (medulla oblongata).
Medulla oblongata (medulla oblongata)
Medulla oblongata (medulla oblongata)

The spinal cord (medulla spinalis) divides into 31 segments.

Spinal cord (medulla spinalis)
Spinal cord (medulla spinalis)

Starting superiorly there are

  • 8 cervical (pars cervicalis),
Cervical part (pars cervicalis)
Cervical part (pars cervicalis)
  • 12 thoracic (pars thoracica),
Thoracic part (pars thoracica)
Thoracic part (pars thoracica)
  • 5 lumbar (pars lumbalis),
Lumbar part (pars lumbalis)
Lumbar part (pars lumbalis)
  • 5 sacral (pars sacralis),
Sacral part (pars sacralis)
Sacral part (pars sacralis)
  • and 1 coccygeal (pars coccygea) segments.
Coccygeal part (pars coccygea)
Coccygeal part (pars coccygea)

Next, let’s discuss two different colored areas of the nervous tissue: gray matter and white matter. You are most likely already familiar with these terms.

Gray matter histologically is made up of the neuron cell bodies. In the brain, gray matter forms the cortex and subcortical nuclei. In the spinal cord it is shaped like a butterfly and has two major divisions: the anterior horn, and the posterior horn.

White matter histologically consists of the processes of neurons, and the already-mentioned myelin sheath gives it its white color. In the central nervous system, axon bundles united by a specific function are called tracts or pathways.

Peripheral nervous system

The components of the PNS are similar to the CNS, but the terminology used is slightly different. In the PNS, a cluster of neuron bodies is called a ganglion (ganglion), and axon bundles are called nerves (nervi).

There are 31 pairs of spinal nerves (nervi spinales) and 12 pairs of cranial nerves (nervi craniales).

Cranial nerves exit the brain and innervate various structures of the head and neck.

These nerves can be divided into sensory, motor, and mixed.

Spinal nerves arise directly from the spinal cord via two roots: the anterior root

Anterior root (radix anterior)
Anterior root (radix anterior)

and the posterior root.

Posterior root (radix posterior)
Posterior root (radix posterior)

The posterior roots contain sensory (or afferent) fibers (neurofibrae afferentes) that transmit sensory information from the periphery to the posterior horn of the spinal cord.

Afferent fibers coming from receptors to the spinal cord
Afferent fibers coming from receptors to the spinal cord

The anterior roots contain motor (or efferent) fibers (neurofibrae efferentes) that transmit motor information from the anterior horn of the spinal cord to the organs (for example, skeletal muscles)

Efferent fibers coming from the spinal cord
Efferent fibers coming from the spinal cord

Remember that afferent fibers arrive and efferent fibers exit the spinal cord!

Both roots of the spinal cord merge to form a mixed spinal nerve, which has both motor and sensory fibers.

Spinal nerve (n. spinalis)
Spinal nerve (n. spinalis)

Each mixed spinal nerve almost immediately divides into two branches: the anterior ramus (ramus anterior) and the posterior ramus (ramus posterior).

Anterior ramus (r. anterior)
Anterior ramus (r. anterior)

и заднюю ветвь (ramus posterior)

Posterior ramus (r. posterior)
Posterior ramus (r. posterior)

Since both the anterior and posterior rami emerge from the mixed spinal nerves, they carry both sensory and motor fibers.

Anterior rami, heading to different areas of our body, give rise to multiple nerves and nerve plexuses (plexus).

Cervical plexus (plexus cervicalis)
Cervical plexus (plexus cervicalis)

Each segment of the spinal cord innervates a particular area of the body.

If we are talking about a specific area of the skin, it is called a dermatome, and if we are talking about a skeletal muscle or some part of it, it is called a myotome.

There are dermatome maps which are used in the diagnosis of sensory processing disorders.

Dictionary

Introduction to the central and peripheral nervous systems

Nervous system
systema nervosum
Central nervous system
systema nervosum centrale
Brain
encephalon
Spinal cord
medulla spinalis
Peripheral nervous
systema nervosum
system
periphericum
Nerve
nervus
Ganglion
ganglion
Cerebral hemispheres
hemispheria cerebri
Diencephalon
diencephalon
Cerebellum
cerebellum
Brainstem
truncus encephali
Midbrain
mesencephalon
Pons
pons
Medulla oblongata
medulla oblongata
Cervical part
pars cervicalis
Thoracic part
pars thoracica
Lumbar part
pars lumbalis
Sacral part
pars sacralis
Coccygeal part
pars coccygea
Spinal nerves
nervi spinales
Cranial nerves
nervi craniales
Anterior root
radix anterior
Posterior root
radix posterior
Sensory fibers
neurofibrae afferentes
Motor fibers
neurofibrae efferentes
Anterior root
r. anterior
Posterior root
r. posterior
Cervical plexus
plexus cervicalis
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