Page 399 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

VetBooks.ir 17

Nervous system

(systema nervosum)

The processes that take place within the body are con- The somatic system encompasses all components of
trolled by systems that perceive external stimuli and the nervous system, including the brain, spinal cord, affer-
coordinate internal responses. In addition to the regula- ent and efferent nerve pathways and ganglia. Hence it
tory functions performed by the endocrine and immune is also referred to as the cerebrospinal nervous system
systems, the nervous system is specialised for detecting, (Figure 17.1).
transmitting, processing, storing and – as appropriate – The autonomic nervous system is responsible for con-
responding to chemical and physical stimuli. The capacity trol and coordination of the vital functions of the internal
for excitability is particularly highly developed in nerve organs. This system also enables the organism to adapt
cells (neurons) (see Chapter 5, ‘Nervous tissue’). Chains to environmental conditions by controlling functions
of neurons are structurally and functionally linked in a such as respiration, circulation, metabolism and ther-
network that, together with the glial cells, constitutes the moregulation. The autonomic nervous system operates
nervous system. involuntarily, without conscious input from the organism.
In its simplest form, transmission of information Macroscopically, the nervous system is divided into two
between neurons involves the delivery of a signal by an components:
afferent neuron (neuron 1) to the spinal cord or brain. The
electrical signal is passed on via a synapse to an efferent neu- · central nervous system (CNS; spinal cord and brain),
ron (neuron 2) that initiates a response by an effector organ. · peripheral nervous system (PNS; peripheral nerves
Synaptic connection of an afferent to an efferent neuron and ganglia).
constitutes a simple reflex arc. Most neuronal pathways
are more complex systems, involving signal coordination Central nervous system (pars centralis,
and integration within the spinal cord and/or brain. This systema nervosum centrale)
is necessary for ensuring normal functional interaction of The structural elements of the central nervous system are
the organs of the body in the presence of numerous exog- comprised of neurons and glial cells (described in detail
enous and endogenous stimuli. in Chapter 5, ‘Nervous tissue’). These are accompanied by
blood vessels and connective tissue.
Divisions of the nervous system During ontogenesis of the nervous system, neuro-
In functional terms, the nervous system is divided into blasts and glioblasts differentiate from the neuroectoderm.
neuronal pathways that transmit information between the Within the neural tube, these cells migrate from the inner
organism and its environment, or that serve to regulate proliferative layer to the mantle layer where they form the
the function of internal organs. These pathways act in dorsolateral alar plates (sensory) and ventrolateral basal
concert and exert an influence on each other. They com- plates (motor). Thin floor- and roof-plates connect the alar
prise the: and basal plates on each side. Axons extend outward from
neurons in the mantle layer to form the marginal layer of
· somatic nervous system and the neural tube.
· autonomic nervous system. Differentiation of these components of the embryonic
neural tube gives rise to the central nervous system,
The somatic nervous system detects sensory and physico- including the nuclei and nerve tracts (see Veterinary
chemical stimuli in the animal’s environment and transmits Anatomy of Domestic Mammals: Textbook and Colour Atlas).
these to the central nervous system for integration and The mantle layer, containing large numbers of unmy-
processing. A nerve impulse then passes through efferent elinated nerve cell perikarya, forms the grey matter. This
nerve fibres to the muscles of the locomotor system and is composed of a dense meshwork of nerve cell bodies,
initiates an appropriate response. axons, dendrites and glial cells (astrocytes). Grey matter

Vet Histology.indb 381 16/07/2019 15:08