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 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