NERVOUS TISSUE
Nervous tissue includes
• Neurons, or neurocytes – cells capable for
the generation and transduction of electric
impulses, which are responsible for the
receptive, integrative, and motor functions of
the nervous system
• Neuroglia – cells, which support and protect
neurons; include astrocytes, ependimocytes,
oligodendrogliocytes, and microgliocytes
THE NEURON
• The neuron or nerve cell is the structural and functional unit
of the nervous system
• All neurons have a cell body (pericaryon) and processes, the
axon and dendrites
• Dendrites are neuronal processes that receive stimuli from
other nerve cells or from the environment
• Axons are neuronal processes that transmit stimuli to other
neurons or to effector cells
• There is only one axon for each neuron
• Nissl bodies – which is rough endoplasmic reticulum - extend
into the dendrites but not into the axon
CLASSIFICATION OF NEURONS
Functional classification
• Sensory neurons convey impulses from receptors to the CNS
• Motor neurons convey impulses from the CNS or from ganglia
to effector cells
• Interneurons – form a communicating and integrating
network between the sensory and motor neurons
Morphological classification is based on the number of
processes:
• Multipolar neurons have one axon and two or more dendrites
• Bipolar neurons have one axon and one dendrite
• Unipolar (pseudounipolar) neurons have one process, that
divides close to the cell body into two long processes – axon
and dendrite
The various types of neurons
SYNAPSES
Neurons communicate with other neurons and with effector
cells by means of synapses – specialized junctions between
neurons that transmit impulses from one neuron to another
or to effector cells such as muscle or gland cells
Synapses between neurons may be classified as:
- axodendritic
- axosomatic
- axoaxonic
- dendrodendritic
At a typical synapse there is a:
• Presynaptic knob – the end of the neuron process from wich
neurotransmitter is released
• Synaptic cleft – 20-30 nm space
• Postsynaptic membrane, wich has receptor sites on the
plasma membrane
SUPPORTING CELLS OF NERVOUS TISSUE
Within the CNS, the supporting cells are designated neuroglia or
glial cells. The four types of glial cells are as follows:
• Oligodendrocytes – the myelin-forming cells of the CNS (in
PNS cells with similar functions are called Schwann cells)
• Astrocytes – the cells, that provide physical and metabolic
support for the neurons of the CNS, include
- protoplasmic astrocytes;
- fibrous astrocytes
• Ependimal cells – form the lining of the ventricles of the brain
and of the spinal canal
• Microglia – the phagocytic cells of the CNS
The various types of neuroglial cells
SCHWANN CELLS AND MYELIN SHEATH
Axons in the peripheral nervous system may be
described as myelinated or unmyelinated
• The myelinated axons are surrounded by a lipid-rich
layer called the myelin sheath
• The myelin sheath is composed of multiple layers of
Schwann cell plasma membrane wrapped
concentrically around the axon
• Node of Ranvier, devoid of myelin – is junctional
zone, where two adjacent Schwann cells meet
• Internodal segment – the myelin sheath between
two sequential nodes of Ranvier
Schematic representation of the myelin
structure in CNS and in PNS (inset)
Schematic of a myelinated nerve fiber, c.s.
Schematic of a nonmyelinated nerve fiber, c.s.
Reflex arcs – somatic and visceral
Nervous system subdivided anatomically
• Central nervous system (CNS) includes
– Brain (i.e. cerebrum, cerebellum, medulla
oblongata, pons Varolii etc.)
– Spinal cord
– Retina of the eye
• Peripheral nervous system (PNS) includes
– Peripheral and cranial nerves
– Ganglia (sensory and autonomic)
– Nerve endings (sensory and effectory – motor
and secretory)
Main anatomical divisions of the CNS
Terms attributed to the organs of CNS
• Grey matter (conglomerates of neurons and neuroglial
cells)
– Cortex and nuclei (basal ganglia) – in cerebrum and
cerebellum
– Horns (anterior, posterior and intermediate) –
within the spinal cord
• White matter (accumulation of myelinated nerve fibers
and neuroglial cells) – forming tracts in brain and
fasciculi within the spinal cord
• Cerebrospinal fluid (produced by choroid plexus)
• Meninges (dura mater, arachnoid, pia mater)
Cerebral cortex = 6 cellular layers
•
•
•
•
Molecular layer (horizontal cells)
External granular layer (stellate cells)
External pyramidal layer (pyramidal cells)
Internal granular layer (stellate and pyramidal
cells)
• Internal pyramidal (ganglionic) layer (large
pyramidal cells of Betz)
• Multiform layer (cells of Martinotti)
LM of a cerebral cortex
Cortex of cerebellum = 3 cellular layers
• Molecular layer (basket and stellate cells,
dendrites of Purkinje cells)
• Purkinje cell (ganglionic) layer (cell bodies of
Purkinje cells)
• Granular layer (small granule cells, cerebellar
glomeruli, and axons of the Purkinje cells)
LM of a cerebellar cortex
Purkinje cells of cerebellar cortex
Spinal cord = grey matter + white matter
• Grey matter
– Ventral horns
– Dorsal horns
– Lateral horns
• Central canal with cerebrospinal fluid
• White matter
– Anterior, posterior, and lateral tracts (fasciculi)
– Median fissure, dorsolateral sulcus, ventrolateral
sulcus
Spinal cord: schematic view and LM
Functional subdivisions of nervous
system
• Somatic
• Autonomic, which include
- sympathetic
- parasympathetic
- enteric subdivisions
Peripheral nerve
Myelinated nerve fibers, which are forming
• Fascicles (or bundles)
and
Connective tissue investments, forming
• Endoneurium
• Perineurium
• Epineurium
Schematic of a peripheral nerve
LM of peripheral nerve, c.s.
LM and EM of myelinated and unmyelinated
nerve fibers in peripheral nerve
Spinal ganglion
Parasympathetic ganglion