Definitions
From an anatomic point of view, one normally distinguishes between:
- the central nervous system (CNS), consisting of brain and spinal cord, and
- the peripheral nervous system (PNS) to which the nervous pathways outside the CNS are numbered and which makes sure afferent and efferent information flows between the periphery and the CNS.
In the sense of a coping aid for studying the nervous system this is divided into two interrelated units that interact with each other:
- The central nervous system (CNS), or neuraxis, which arises from the neural tube. It comprises the brain (cerebral hemispheres, diencephalon, brain stem, cerebellum) and the spinal cord and is the subject of this module.
- 1
- Telencephalon
- 2
- Diencephalon
- 3
- Mesencephalon
- 4
- Cerebellum
- 5
- Pons
- 4+5
- Metencephalon
- 6
- Myelencephalon (Bulbus encephali, Medulla oblongata)
- 7
- Spinal cord
- I
- Prosencephalon
- II
- Mesencephalon
- III
- Rhombencephalon
This diagram provides an overview of the descendents of the embryonic primordia (I, II, III) as they exist at the end of the fetal phase.
Descendents of the original brain mass in adults
Illustration
Macroscopy
Descendents of the secondary cerebral vesicles in adults
Illustration
Macroscopy
The peripheral nervous system (PNS) comprises an afferent or sensory branch as well as an efferent or motoric branch, both of which lie outside the CNS (**). Essentially, the PNS arises from the cells of the neural crests. The neural tube, though, delivers the somatomotoric fibers as well as the preganglionic fibers of the autonomous nervous systems (see below). In addition, the dura mater or pachymeninx and the connective tissue of the peripheral nerves develop from the mesoderm (in contrast, the leptomeninx, i.e., the arachnoïdea and pia mater, arise from the neural crest).
(**) The somatomotoric and preganglionic fibers of the autonomous nervous system form an exception in that, despite belonging to the PNS, the perikarya that belong to them are to be found in the spinal cord and brain stem and thus in the CNS!
The afferent sensory pathways comprise both somatic and also visceral neurons. These transmit the information that is collected by the receptors of the somatic (skin, muscles) and visceral organs (intestines, lungs, etc.) to the central nervous system. The cell bodies of the sensory neurons form the spinal ganglia and the paravertebral ganglia.
The efferent motoric pathways can be subdivided into:
- the somatomotoric pathway (SNS) that accompanies the cerebral nerves of the brain stem and the spinal nerves of the spinal cord.
- The visceromotoric pathway (VNS), which comprises all the efferents that regulate the internal milieu (homeostasis). They are responsible for the steering the cardiac muscles, the smooth muscles of the intestines and blood vessels as well as the glands. The preganglionic part accompany the cerebral nerves and the spinal nerves. The postganglionic part (the sympathetic) forms a plexus that continues periarterial or as fibers that accompany the larger peripheral nerves before they reach the target organ via a terminal branch.
The vegetative nervous system can be divided into two sections:
- the sympathetic and
- the parasympathetic
Generalizing, the parasympathetic is in the service of regeneration, the recuperation of the organism and the excitation of the digestive apparatus.
Responsible neurotransmitter: acetylcholin.
Correspondingly, the sympathetic cares for an elevated alertness and concentration of the organism and for preparing for physical and intellectual achievements (widening of the bronchia, elevation of the pulmonary frequency and cardiac activity, pupil dilation.
Responsible neurotransmitters: noradrenalin and adrenalin
= Sensitive zones that can be activated for additional information
the spinal cord within the spine
the brain stem consisting of the medulla oblongata, pons, cerebellum, mesencephalon as well as telencephalon and diencephalon
the afferent sensory pathways
efferent motoric pathways
The somatosensory pathways proceed from the receptors in the periphery (namely, in the skin), the viscerosensory pathways transmit information that is apprehended by receptors in the smooth muscles, the cardiac muscle and the glands.
The peripheral nervous system is treated in a separate module.
The most important structural elements and functional relationships
= Sensitive zones that can be activated for additional information
The cell bodies of the corresponding neurons are to be found in the regions of the organs that derive from sensory placodes, namely the olfactory placode (smell) and the auditory placode (hearing and balance).
Somatosensoric neurons in the head region (n. trigeminus V, n. facialis VII, n. glossopharyngeus IX, n. vagus X).
The cell bodies of the involved neurons are to be found in sensory ganglia of the associated nerves
The cell body sits in one of the 32 spinal ganglia that derive from the neural crest. The sensory fibers transmit signals from skin receptors (touch, temperature, pressure), pain receptors (nociception), muscle and tendon spindles to the central nervous system.
The conscious component of the visceral afferents also comprise intestinal pain. In this the phenomenon of projected pain can arise. It is true that this has its origin in the intestines but is nevertheless percieved by the central nervous system as somatic pain.
They transmit efferents via the motoric brain nerves (nn. oculomotorius III, trochlearis IV, abducens VI, accessorius XI and hypoglossus XII) as well as via the mixed branchial arch nerves (Nn. trigeminus V, facialis VII, glossopharyngeus IX and vagus X).
They transmit efferents via the 32 spinal nerves between C1 and the first coccygeal vertebra.
The perikarya of the preganglionic neurons lie in the side horn of the spinal cord segments T1 to L2. Following synaptic transission to the postganglionic neuron their nerves supply the smooth muscles, the cardiac muscle and the glands. They prepare the body for emergency situations and elevate muscle activity.
The perikarya of the preganglionic parasympathetic neurons of the head region lie in the corresponding core areas of the III, VII, IX and X brain nerves. The remaining cell bodies of the parasympathetic system are to be found in the side horn of the spinal cord segments S2 to S4. Their nerves are involved in forming the plexus pelvicus. The parasympathetic system serves physiologic processes such as digestion and breathing and is involved with regeneration and recuperation.