Blood Composition and Function

Blood Composition

• Blood is a specialized connective tissue with cellular elements and plasma
• The cellular elements are erythrocytes, leucocytes, and platelets

Plasma

• Plasma is a translucent, yellowish fluid containing high or low molecular weight substances
• The composition is water (90%), proteins (7%), inorganic salts (1%), and organic compounds(2%)
• Organic compounds include amino acids, lipids, and vitamins
• The total blood volume in humans is about 5L(depends on body size)
• Blood undergoes coagulation or clot formation outside blood vessels
• Repairing damaged blood vessels and preventing blood loss are the functions of coagulation

Blood Cell Functions

• Erythrocytes and platelets function inside the blood vessels
• Leucocytes reside temporarily in blood vessels and then leave via capillary walls and venules
• Leucocytes enter either connective tissues or lymphoid tissues
• The ratio of erythrocytes to the total blood volume is about 44%, known as haematocrit
• Plasma comprises 55% of the total blood volume

Nervous Tissue

• Nervous system consists of all nervous tissue in the body
• CNS (brain and spinal cord) and PNS are the anatomical divisions
• CNS nervous tissue does not contain more connective tissue than what is found in meninges and large blood vessel walls The main cells in nervous tissue are neurons, and supporting cells (glia)

Neurons

• Neurons are the functional and structural units of nervous tissue
• These are terminally differentiated cells that are mitotically inactive, and cannot divide
• Neurons act as a site of integration/analysis of nerve impulses via conducting pathways
• Neurons have a large cell body (perikaryon) and nucleus surrounded by cytoplasm
• Neuronal cell bodies can be spherical, ovoid, or angular with variable diameters from 5 -150μm
• Neurons have a large nucleus with dispersed chromatin and a prominent nucleolus
• The cytoplasm contains abundant rER, basophilic granules made of polyribosomes (Nissl bodies), intermediate neurofilaments, microtubules, and diffuse Golgi
• Multivesicular bodies transport to organelles

Neuronal Processes

• The processes can be divided into dendrites and axons based on function and morphology
• Dendritic tree shape and orientation determines information amount and type
• Axon shape determines to which neurons the information may be passed
• Neuron location in CNS determines the major system the neuron belongs to

Dendrites

• Dendrites are drawn out extensions of the cell body
• Highly branched and tapering, dendrites either end in specialized sensory receptors (primary sensory neurons) or synapses
• They primarily receive stimuli, and information input, and generally convey impulse toward nerve cell body (afferent)
• Contain most cell organelles besides Golgi bodies

Axons

• The axon (nerve fibre) is a specialised extension of the neuron
• Cylindrical, usually single, arise from the axon hillock
• It terminates on other neurons or effector organs through branches ending in terminal boutons
• It conveys impulses away from the nerve cell body (efferent)
• Axons are marked through the absence of Nissl bodies beyond the hillock except in the motor end plate with striated muscle, prominent sER, and elongate mitochondria

Types of Neurons

• Neurons can be classified according to their function (motor, sensory, or integrative)
• Neurons can be classified according to how their axon relates to its dendrites
• Multipolar neurons are most common and motor; contain numerous dendrites projecting from cell body and are subdivided into:
• Purkinje cells of the cerebellum
• Stellate or polygonal cells of the anterior horn cells of spinal cord
• Pyramidal cells of the cerebral cortex
• Bipolar Neurons have a single dendrite arising opposite the origin of the axon; receptor neurons for sensation and present in retina/inner ear/olfactory mucosa
• Unipolar neurons are found in the spinal nucleus of trigeminal nerve.
• Pseudo-unipolar neurons are primary sensory neurons; with single dendrite and axon arising from a common fusion stem; present in spinal ganglia

Peripheral Nerves (PN)

• Peripheral nerves can be afferent with sensory fibers entering the spinal cord via dorsal roots, or efferent with motor fibers leaving the spinal cord via ventral roots.
• A nerve fiber consists of an axon and its nerve sheath
• Each axon in the peripheral nervous system is surrounded by a sheath of Schwann cells
• Schwann cells may surround axons for micrometers; unmyelinated nerve fibers may surround separate axons
• Axons are housed within infoldings of the Schwann cell cytoplasm and cell membrane forming mesaxon
• Myelinated nerve fibers contain Schwann cells that form a sheath around an axon with layers of cell membrane.
• Schwann cells form myelin sheath which insulates the axon, and improves its ability conduct and, thus, saltatory transmission of impulses
• Each Schwann cell forms a myelin segment so the cell nucleus is located approximately in the segment's middle
• The node of Ranvier is the place along that the axon where two myelin segments abut.

Types of Peripheral Nerve Fibers

• Type A (myelinated) fibers are 4-20 μm in diameter and conduct impulses at (15 - 120 m/ second) and include motor fibers or sensory fibers
• Type B (myelinated) fibers are 1-4 μm in diameter and conduct impulses at (3 - 14 m/ second) and include preganglionic autonomic fibers
• Type C (unmyelinated) fibers are 0.2 - 1 μm thick and conduct impulses at (0.2 - 2 m/ second) and include autonomic and sensory fibers

Types of Nerve Fibers in CNS

• Naked fibers are unmyelinated nerve fibers also without neurolemmal sheath, and present in gray matter of CNS and at the beginning/termination of nerve fibers
• Myelinated nerve fibers with neurolemmal sheath like peripheral somatic nerves
• Myelinated nerve fibers without neurolemmal sheath as in the white matter and optic nerve fibres
• Unmyelinated nerve fibers with neurolemmal sheath like in sympathetic nerve fibers.

Connective Tissue Covering Nerve Fibers

• Peripheral nerves contain considerable connective tissue
• The epineurium is a thick layer of dense connective tissue surrounding entire nerve
• Nerve fibers are grouped into bundles surrounded by a layer of connective tissue called perineurium
• The perineurium is formed by flattened cells
• The endoneurium is loose connective tissue between individual nerve fibers, which contain fibrocytes, macrophages, and mast cells
• Nerves are richly supplied by intraneural blood vessels, arterial networks reach into the perineurium and give off capillaries to the endoneurium

Neuroglia or Glial Cells

• Neuroglia are non-neuronal supporting cells in CNS tissue
• CNS tissue contains several types of non-neuronal supporting cells, neuroglia
• It is estimated that for every neuron, there are 10 neuroglia
• Neuroglia only occupy about 50% of the total volume of nerve tissue due to small size
• Neurons cannot exist or develop without neuroglia

How Neuroglia Differ from Neurons

• Can not transmit nerve impulses as there are no action potentials
• Are able to divide and are the source of tumors of nervous system
• Do not form synapses
• Form the myelin sheaths of axons

Types of Neuroglia

• There are 4 basic types of neuroglia, based on morphological and functional features:
  • Astrocytes (or Astroglia)
  • Oligodendrocytes (or Oligodendroglia)
  • Microglia
  • Ependymal cells
• The astrocytes and oligodendroglia are large cells and are collectively known as Macroglia

Oligodendrocytes

• Also known as oligoglia
• Have fewer and shorter processes
• Form myelin sheath around axons in the CNS
• The functional homologue of peripheral Schwann cells
• Form parts of the myelin sheath around several axons unlike Schwann cells

Astrocytes

• Also known as Astroglia
• Star-shaped cells present only in the CNS
• Largest of the neuroglia with long processes often terminating in perivascular foot processes or pedicels on blood capillaries, contributing to the blood-brain-barrier
• Provide physical and metabolic support to CNS neurons
• Participate in maintaining extracellular fluid composition
• There are two categories of astrocytes include protoplasmic astrocytes and fibrous astrocytes
• Protoplasmic astrocytes are present in the gray matter of the brain and spinal cord and have relatively thick processes
• Fibrous astrocytes are present in the white matter of the CNS and have thinner processes than protoplasmic astrocytes

Microglia

• Small cells with complex shapes, microglial cells are of mesodermal origin
• Derived from cell line giving rise to monocytes i.e. macrophage precursors circulating in the blood stream
• Microglia differentiates into phagocytotic cells in the case of tissue damage

Ependymal Cells

• Ependyma is composed of neuroglia which line internal cavities, or ventricles of brain and spinal cord (central canal)
• Similar appearance to stratified columnar epithelium
• Bathed in cerebrospinal fluid (CSF)
• Modified ependymal cells of the choroid plexuses of the brain ventricles are the main source of the CSF

Ganglia

• Ganglia are aggregations of nerve cells outside the CNS
• Cranial nerve and dorsal root ganglia (spinal ganglia) are surrounded by a connective tissue capsule continuous with dorsal root epi- and perineurium
• Individual ganglion cells are surrounded by a layer of flattened satellite cells
• Neurons in cranial nerve and dorsal root ganglia are pseudounipolar with a T-shaped process
• The T arms are neurite branches connecting the ganglion cell with the CNS (central branch) and periphery (peripheral branch)
• Branches act as one actively conducting processes transmitting data from the periphery to the CNS
• The stem is connected to the perikaryon of the ganglion cell and is the only originating process
• Ganglion cells in dorsal root ganglia do not receive synapses, and their function is the trophic support of their neurites
• Autonomic ganglia contain synapses
• Ganglion cells in them are multipolar, and have dendrites but are not surrounded by satellite cells
• Receive synapses from first neuron of the two-neuron chain connecting of the autonomic nervous system
• Some autonomic ganglia in walls of the organs they innervate (in tramural ganglia e.g. GIT and bladder)

Neuronal Synapses

• Neural activity and its control require the expression of genes
• The keys to understanding a neuron function are its processes shape, the chemicals (neurotransmitters), and how it may react to neurotransmitters released by other neurons
• Synapses are morphologically specialized contacts between a bouton formed by one neuron (presynaptic neuron) and the cell surface of another neuron (postsynaptic neuron
• Synaptic vesicles contain neurotransmitters
• Synaptic vesicles accumulate close to the contact site between the bouton and the postsynaptic neuron
• Neurotransmitters are released from synaptic vesicles into the synaptic cleft
• The synaptic cleft mediates the transfer of information from pre- to postsynaptic neuron
• Neurotransmitters either excite or inhibit the postsynaptic neuron
• L-glutamate is the prominent excitatory transmitter in the CNS
• GABA (gamma-amino butyric acid) is the prominent inhibitory transmitter in the CNS
• Other "main" neurotransmitters are dopamine, serotonin, acetylcholine, noradrenaline, and glycine
• Only one main transmitter is used per each neuron from start to end.
• One or more of the "minor" transmitters like cholecystokinin, endogenous opioids, somatostatin, and substance P may be used together with a main transmitter.
• The molecular machinery to mediate the events at excitatory synapses differ from that at inhibitory synapses.
• Functional differences accompany the morphological appearances of the synapses.

Receptors

• A multitude of receptors sensitive to one particular neurotransmitter exists
• Different receptors have response properties and properties
• They may allow different ion flux over the neurons' plasma membrane, or they may address postsynaptic neurons' second messenger systems
• The reaction of the neuron to neurotransmitters on its plasma membrane at the synapses is determined by types of receptors expressed by the neuron.

The Central Nervous System (CNS)

• The CNS consists of the cerebrum, cerebellum, and spinal cord
• Connective tissue is relatively sparse
• Brain tissue is soft and gil-like
• Cerebrum, cerebellum, and spinal cord feature white (white matter) and gray (gray matter) regions with differing distribution of myelin
• White matter is primarily myelinated axons with myelin-producing oligodendrocytes, and without neuronal cell bodies
• All neural cell bodies, dendrites, initial unmyelinated axon portions, and glial cells
• Gray matter is dominant surface feature of the cerebrum and cerebellum, and forms the cerebral and cerebellar cortex
• White matter is present in more central regions

Cerebrum

• The cerebrum is one of the largest portions of the brain, and divided into right and left hemispheres via a longitudinal fissure
• Conspicuous surface features are many gyrus folds greatly increasing cortex surface area
• Gyri grooves are called sulci, and central sulcus runs in the lateral cerebrum surface from superior to inferior
• Aggregates of neuronal cell bodies forming islands of gray matter embedded in white matter called nuclei
• The cerebral cortex' gray matter has six layers of cells with differentiated shape in different parts

Cerebral Cortex: Six Layers

• Molecular or plexiform layer :- outer layer of cerebrum, which contains nerve fibers and small astrocytes, also called Cajal's cells.
• Outer granular layer :- contain small pyramidal cells
• Outer pyramidal layer :- middle pyramidal cells in outer part with larger cells in basal part
• Inner granular layer :- main receiving regions of the cerebrum cortex
• Inner pyramidal layer :- contain the largest pyramidal cells
• Polymorphic or multiform layer : last layer of cortex near the white mater, with different shapes of nerve cells

White Matter

• The white matter of the brain between the cortex and nuclei is the cerebral medulla, which is the general term meaning the center of the structure
• Cerebral medulla is nerve tracts that connect cerebral cortex to the CNS
• These tracts fall into three main categories:
  • Association fibers which connect areas of the cerebral cortex within same hemisphere
  • Commissural fibers connecting one cerebral hemisphere to the other.
  • Projection fibers located between the cerebrum and other parts of the brain and spinal cord

Cerebellum

• Little brain is in communication with other CNS regions through three tracts
  • Superior, middle and inferior cerebellar peduncles
• A gray cortex and nuclei with white medulla is in between
• Cerebellar cortex has three layers
  • outer molecular layer
  • central layer is composed of large Purkinje cells
  • inner granule layer
• Purkinje cells have a conspicuous cell body and highly developed dendrites forming a fan, occupying the molecular layer
• The granule layer formed of small neurons, which are compactly disposed compared to cell-dense molecular layer.

Intermediate Region of Cerebellum Medulla

• Consists of different types of fibers:
  • Axons of Purkinje cells
  • Climbing fibers
  • Mossy fibers