The basic function from which others derive. It depends on a special chemical and electrical properties of the nerve cells and their long processes.
This property reflect two fundamental attributes of protoplasm:
The function of all parts of the body are integrated by the NERVOUS SYSTEM.
The communicative functions of neurons depend not only on their irritability and conductivity but also on their structure, integrative properties, and connections, as well as on chemical compounds that they synthesize and deliver to other neurons, to muscle, or to glands.
NEUROTRANSMITTERS – acts rapidly and locally to alter the activity of the target cells
NEUROMODULATORS – regulates, but generally do not directly effect nuerotransmission
NEUROMORPHONES – exert slow and wide spread influences usually through the extracellular fluid or blood.
Soma – cell body
Perikaryon – cytoplasm
Nissl Bodies – clumps of intensely chromophillic material
Dendrites are fairly short and confined to the vicinity of the soma. They receive impulses from other neurons via their synapses with axon terminals from different nerve cells.
Axon, specialized for conducting signals from one nerve cell to another, arise from the conical extension of the cell body called axon hillock. It is usually thinner and very much longer than the dendrites of the same cell.
Dendrites, Axon and
Neurons usually have multiple dendrites arising directly from the cell body, and these branches provide most of the surface for receiving signals from other neurons. However, the cell body and the initial segment of the axon may also receive afferent synapses. Where the dendrites emerge from the perikaryon they are relatively thick but they taper gradually along their length.
Dendrites of most neurons are fairly short and confined to the immediate vicinity of the soma. They bifurcate, typically at acute angles, into primary, secondary, tertiary, and higher orders of branches that form patterns ranging from simple to highly complex.
At the base, the fine structure of the dendrites is similar to that of the perikaryon. They may contain extensions of the Golgi apparatus, small Nissl bodies, mitochondria, agranular endoplasmic reticulum, microtubules, and neurofilaments.
Spinal motor and Betz cells are exceptional in having very large numbers of dendritic neurofilaments. Tubules of endoplasmic reticulum and free ribosome diminish, whereas mitochondria become more numerous and are concentrated in the finer ramifications of the dendrites.
Dendrites receive impulses from other neurons via their synapses with axon terminals. The number of such inputs may be very large. As stated earlier, the terminals on the dendritic tree of the large Purkinje cells may number in hundreds of thousands. The impulse carried over an axon is “all- or-one” but the integrative local changes in electric potential.
The axon arises from a conical extension of the cell body called the axon hillock. Occasionally, it may arise from the base of a principal dendrite. In certain very small local-circuit neurons (e.g., amacrine cells of the retina) there may be no axon, but this is quite uncommon. The axon is usually thinner and very much longer than the dendrites of the same cell.
The part of the axon between the hillock and the beginning of the myelin sheath is called the initial segment. Here, the plasmalemma is underlain by the thin layer of moderately electron-dense material unknown nature. A similar layer occurs at the nodes of Ranvier, but not elsewhere along the length of the myelinated axon.
The axon carries the response of the neuron, the nerve impulse, in the form of a propagated action potential. The axon hillock and initial segment give rise to this potential and constitute the so-called “spike trigger zone.” These two regions also provide a receptive zone for inhibitory afferents. Signal transmission at its ending and trophic relationships with the target neurons, muscles, and glands are others. Basic to these is the function of axonal transport, the movement of the substances up and down of the axon. Two forms of axonal transport exist: anterograde transport from the perikaryon to the axon terminal and retrograde transport in the opposite direction.
An important role of axon transport is the delivery to the axon ending of synaptic vesicles and enzymes involved in transmitter synthesis. Although neurotransmitters can also be transported along the axon, the quantity reaching the ending y this mechanism is probably small compared to the amount synthesized at the endings.
Axoplasmic transport is vital to nerve cell functions. It provides fro replacement of catabolized proteins in the axon and its endings, transport of enzymes for transmitters synthesis at the endings, movement of macromolecular recursors of cytoskeletal elements, and feedback from the periphery, contributing to regulation of the synthetic activity of the cell body.
The never impulse that travels along the axon is a wave of transient membrane depolarization. In a resting neuron, the inner aspect of the membrane is negative relative to its outer aspect, and is, thus, said to be polarized. The resting potential at the site of stimulation disappears. The movement of ions across the membrane constitutes a current that generates an electrical signal called the action potential. The net effect of these events in propagation of the nerve impulse from the spike trigger zone (axon hillock plus initial segment) to the axon terminal. When the axon terminal lies in skeletal muscle and is, therefore, a neuromuscular synapse, it releases its neurotransmitter; acetylcholine was the first transmitter of the PNS.
Synapse and Myelin Sheath
synapse The specialized region of contact where nurotransmitter is released from an axon to stimulate another cell is called a synapse. in the CNS such contacts are usually with the dendrites of other neruns(axodentritic synapses), but they may be with the cell body(axosomatic synapses)such contacts with the axon(axoaxonic synapses) are uncommon In the PNS the synaptic contact of motor nerves is usually with muscle or glandular epithelial cell.
Structure and Function of the Synapse
The function of the synapse is to transfer electric activity (information) from one cell to another. The transfer can be from nerve to nerve (neuro-neuro), or nerve to muscle (neuro-myo). The region between the pre- and postsynaptic membrane is very narrow, only 30-50 nm. It is called the synaptic cleft ( or synaptic gap ). Direct electric communication between pre- and postjunctional cells does not take place; instead, a chemical mediator is utilized. The sequence of events is as follows :
An action pulse reaches the terminal endings of the presynaptic cell .
A neurotransmitter is released, which diffuses across the synaptic gap to bind to receptors in specialized membranes of the postsynaptic cell .
The transmitter acts to open channels of one or several ion species, resulting in a change in the transmembrane potential. If depolarizing , it is an excitatory postsynaptic potential ) EPSP); if hyperpolarizing , an inhibitory postsynaptic potential ) IPSP (
Transmission scross synapses is brought about by secretion of very low concentrations of chemicals called neurotransmitters that move across the gap.as the nerve impulse travels down the fiber,it causes vesicles in the axon ending of a presynaptic neroun to release the chimical neurotransmitters.acommon neurotransmitter is acetylcholine and other neurotransmitters in the body are epinephrine or adernaline,norepinephrine,serotonin,dopamine and the endorphins.
At a synapse,the presynaptic and post synaptic membranes are parallel and seperated by synaptic cleft.
The sytoplasmof the nerve ending contains afew mitochondria and accasional tubules sER,but its most conspicuous constituents are numerous,small synaptic vesicles,20_40nm in diameter,clustered near the presynaptic membrane.
Presynaptic membrane is covered on its inner syrface with small conical densities of unknown chemical nature.this region bearing these debsities and the associatedsynaptic vesicles are referred to as the active zone of the synapse.
When the action potential traveling down the axon reaches its terminal,voltage_gated channels are opened,permitting Ca+ ions to enter.this triggersrelease of neurotransmitter into the synaptic celf by exocytosis of synaptic vesicles docked in the active zone,the neurotrasmitter bind to the specifc receptor in the postsynaptic membrane.
Excitatory or inhibitory
Its depending on whether the transmitter depolarize or hyperpolarizes the postsynaptic membrane.which of these effects occurs depends on the chimical nature of neurotransmitter and the type of receptors in the postsynaptic membrane.
Stracture and function of myellin
Myelin is a dielectric ( electrically insulating ) material that forms a layer, the myelin sheath , usually around only the axon of a neuron . It is essential for the proper functioning of the nervous system . Myelin is an outgrowth of a glial cell . Schwann cells supply the myelin for peripheral neurons, whereas oligodendrocytes , specifically of the interfascicular type, myelinate the axons of the central nervous system . Myelin is considered a defining characteristic of the (gnathostome) vertebrates , but it has also arisen by parallel evolution in some invertebrates.Myelin was discovered in 1854 by Rudolf Virchow
Composition of myelin Myelin made by different cell types varies in chemical composition and configuration, but performs the same insulating function. Myelinated axons are white in appearance, hence the "white matter" of the brain. Myelin is composed of about 80% lipid and about 20% protein .
Peripheral Nerves, Distribution of Neurons and Cerebral Cortex
Peripheral nerves :
the nerves of the PNS consist of varing numbers of myelinated and unmyelinated axons originating from neurons located in the brain, spinal cord, or ganglia.
They are enclosed in three layers of differing characteristics :
The outermost layer , the epineurium ,consist of dense irregular connective tissue. Ites cellular elements are fibroblasts, mast cells, and limited numbers of adipose cells
Thinner extensions of epineurium layer penetrate into the nerve as the perineurium, a thin sleeve of flattened cells surrounding small bundles of nerve axons
Within each bundle of nerve fibers there is an endoneurium consisting of a delicate network of reticular fibers surrounding each Schwann cell-axon complex
Afferent fibers _ which carry information from the surface or the interior of the body to the CNS
Afferent fibers from end-organs sensing heat, cold, touch, or pain, are called sensory nerves
Efferent fibers _ which carry nerve impulses from the CNS to tissues and organs in the periphery
Efferent nerves to muscles stimulating their contraction, are called motor nerves , and the tissues responding to motor nerves are reffered to as their effectors
Many of the peripheral nerves contain both sensory and motor fibers and are, thus, termed mixed nerves
Distribution of neurons :
Gray matter _ contains the cell bodies of neurons, their dendrites, and the terminations of axons arriving from other regions
White matter _ is largely devoid of neuronal cell bodies and consists mainly of myelinated axons, the cell bodies of which are in the gray matter or the dorsal root ganglia
Neuroglial cells and their processes are present in great numbers and variety in both gray and white matter, but they may be distinguished only with electron microscope
Cerebral cortex :
The cerebral cortex isan outer zone of gray matter over the hemispheres of the brain
It receives and analyzes sensory information from the body and responds by voluntary initiation of motor activity
It is also involved in learning and memory
Type of cells :
Pyramidal cell _
its soma is roughly triangular in section with a large vesicular nucleus and abundant Nissl bodies in the perikaryon
a long apical dendrite extends toward the surface of the brain, with many branches that are sites of axodendritic synapses
an axon arises from its base and descends through the deeper layers of the cortex
Stellate cell, or granule cell _
These are relatively small, with numerous highly branched dendrites radiating from the cell body and a single relatively short axon
Horizontal cell _
Largely confined to one layer of the cortex, are fusiform, with radiating dendrites and a short axon that divides near the cell body, with the branches running in opposite directions
Cerebellar Cortex, Spinal Cord and Introduction to Neuroglia
Receives informational input from the eye,ears,and stretch receptos in the muscles. It has an important role in coordination and in the maintenance of balace and normal posture.
Layers of cerebellar cortex
Molecular layer (outer layer) :contains relatively few small neurons and many unmyelinated nerve fibers.
Purkinje cell s ( middle layer) :consists of a single layer of Purkinje cells Granula cell layer :consists of closely packed,small cells with short dendrits and an axon that courses upwards to the molecular layer to form parallel fibers synapsing with the Purkinje cells.
Receives motor commands from the brain and relays them via spinal nerves Receive sensory input from the body and relayes this information back to the brain. Made up of inner core of gray mater and an outer core of white mater.
Made up of inner core of gray mater and an outer core of white mater. H-shaped gray mater containing nerve cell bodies and their dendrites.
White mater made up of ascending and descending myelinated fibers. The cell bodies of sensory neurons entering the cord are located in afusiform expansion of each dorsal root,called the dorsal root ganglion and divides into two myelinated branches:
A long peripheral fiber that courses to distant sensory ending.
A shorter central fiber that enters the spinal cord.
The several types of supporting cells of the central nervous system are called the neuroglia.
There are four types of neuroglia:
astrocytes, oligodendrocytes, microglia, and ependymal cells
Astrocytes, Oligodendrocytes and Microglia
They perform many functions, including biochemical support of endothelial cells which form the blood-brain barrier, provision of nutrients to the nervous tissue, maintenance of extracellular ion balance , and a principal role in the repair and scarring process of the brain and spinal cord following traumatic injuries.
Their main function is the insulation of axons in the central nervous system (the brain and spinal cord) of higher vertebrates. (The same function is performed by Schwann cells in the peripheral nervous system). A single oligodendrocyte can extend its processes to 50 axons, wrapping around approximately 1 mm of myelin sheath around each axon; Schwann cells, on the other hand, can wrap around only 1 axon.
Microglia constitute 20% of the total glial cell population within the brain. Microglia are distributed in large non-overlapping regions throughout the brain and spinal cord. Microglia are constantly scavenging the CNS for damaged neurons, plaques, and infectious agents. The brain and spinal cord are considered "immune privileged" organs in that they are separated from the rest of the body by a series of endothelial cells known as the blood-brain barrier, which prevents most infections from reaching the vulnerable nervous tissue.
Ependymal Cells, Repair in Central Nervous System Introduction to Autonomic Neurons System
These are the epithelial cells that line the CSF( cerebrospinal fluid )-filled ventricles in the brain and the central canal of the spinal cord.
The cells are ciliated simple cuboidal epithelium.
Ependymal Cells form sheets of cells that line canals and spaces of the CNS. Take part in creating fluid and moving fluid.
is a clear bodily fluid that occupies the subarachnoid space and the ventricular system around and inside the brain and spinal cord . In essence, the brain "floats" in it.
Ependymal cells , also named ependymocytes are also thought to act as neural stem cells.
One study observed that ependymal cells from the lining of the lateral ventricle might be a source for cells which can be transplanted into the cochlea to reverse hearing loss.
Repair of CNS
Glial cells , sometimes called neuroglia or simply glia (Greek for "glue"), are non- neuronal cells that maintain homeostasis , form myelin , and provide support and protection for the brain's neurons . In the human brain , there is roughly one glia for every neuron with a ratio of about two neurons for every three glia in the cerebral gray matter .
In –vitro stem cells
Autonomic Nervous System
Part of the nervous system that was once thought to be functionally independent of the brain. The autonomic nervous system regulates key functions of the body including the activity of the heart muscle (see below), the smooth muscles (e.g., the muscles of the intestinal tract), and the glands.
The autonomic nervous system has two divisions:
(1) the sympathetic nervous system that accelerates the heart rate, constricts blood vessels, and raises blood pressure
(2) the parasympathetic nervous system slows the heart rate, increases intestinal and gland activity, and relaxes sphincter muscles.
Your reactions are automatic.
ANS, SYMPATHETIC, PARASYMPATHETIC
Autonomic Nervous System
Actions are largely independent.
Influenced to NO small degree by conscious processes of the brain.
2 motor neurons in series are involved. Located in:
-> Center in the brain stem or spinal gray matter.
-> Ganglion outside the CNS
Somatic Nervous System
Voluntary control of body movements via skeletal muscles, and with sensory reception of external stimuli
The Motor neuron acts directly on its effector organ
Autonomic Nervous System
Part of the Nervous System concerned with reception of sensory impulses
Heartbeat, smooth-muscle contraction, and secretion of exocrine glands are regulated automatically
Digestion, Respiration rate, Salivation, Perspiration, Diameter of the pupils, and Sexual arousal.
Somatic Nervous System
Part of the Nervous System concerned with the voluntary generation of motor responses
Touching, Hearing, and Sight
2 Division of Autonomic Nervous System
The Sympathetic Division
The Parasympathetic Division
Sympathetic (thoracolumbar) division
Includes a chain of interconnected ganglia on either side of the vertebral column
The cell bodies of the preganglionics neurons are situated in the gray matter of the thoracic and lumbar regions of the spinal cord.
Their axons exit the cord through the ventral roots, but soon leave them to enter one of the paravertebral ganglia – that are interconnected in a chain running parallel to the vertebral column near the junctions of the dorsal and ventral roots
Sympathetic (thoracolumbar) division
Each preganglionic neuron synapses with multiple postganglionic neurons in the ganglion of the same segment or in ganglia of neighboring segments.
The axons of the postganglionic neurons return to spinal nerves and are distributed in the peripheral nerves to blood vessels (vasomotor fibers), sweat glands (sudomotor fibers), hair follicles (pilomotor fibers), salivary glands, heart, and lungs
Sympathetic (thoracolumbar) division
Some preganglionic fibers pass through the paravertebral ganglia without synapsing and travel in the sphanic nerves to synapse on nerve cell bodies in the celiac ganglion and mesenteric ganglia in the abdominal cavity
Postganglionic fibers from these ganglia innervate the gastrointestinal tract, kidneys, pancreas, liver, bladder, and external genitalia. Thus, sympathetic trunks and their ganglia are the avenues of outflow of impulses from the spinal cord to the viscera.
The Principal neurotransmitter of the Sympathetic system is Norepinephrine
Parasympathetic (craniosacral) division
The cell bodies of the postganglionic neurons are located in the brain stem and in several segments of the spinal cord.
Their axons do not synapse in the paravertebral ganglia, but extend for long distances to sysnapse with postganglionic neurons in small ganglia near, or within, their visceral targets.
Parasympathetic (craniosacral) division
The preganglionic fibers of the cranial component of this division emerge from the CNS in the oculomotor, facial, glossopharyngeal, and vagus nerves, and the synapse with postganglionics neurons in the ciliary, pterygopalatine, submandibular, and otic ganglia.
Those of sacral component, derived from the second to the fourth sacral segments, leave via the ventral roots and sacral nerves, and synapse with postganglionic neurons in ganglia associated with the pelvic viscera.
Parasympathetic (craniosacral) division
In the enteric component that controls the activity of the gastrointestinal tract, pancreas, and the gall bladder, the neurons are located in complex networks of ganglia and interconnecting nerves in the walls of these organs.
The two major networks are the Myenteric plexus (Auerbach's plexus) , between the longitudinal and circular layers of smooth muscle in the gut, and the Submucosal plexus (Meissner's plexus) between the mucosa and the circular muscle layer.
The chemical mediator of the parasympathetic division of the autonomics nervous system is Acetylcholine.
3 layers of connective tissue covering the brain and the spinal cord.
The outer most is the dura mater, the innermost is the pia mater, and an intermediate layer between these is the arachnoid.
The dura mater consists of dense connective tissue adhering rather loose to the inner aspect of the skull.
These attachments help to support the spinal cord
The dura enclosing the spinal cord is separated from the surrounding periosteum by an epidural space, which contains very loose connective tissue, a plexus of thin walled veins, and some adipose cells.
The internal and external surfaces of the spinal dura are covered by simple squamous epithelium.
The inner epithelium is connected to the sides of the spinal cord by a series of slender denticulate ligaments.
Consists of an outer layer of closely apposed cells in contact with dura, and an inner portion made up of long arachnoid trabecular cells that traverse the subarachnoid space to connect the arachnoid to the underlying pia mater.
This space contains cerebrospinal fluid
The arachnoid is devoid of blood vessels.
layer of very loose connective tissue, covered on the side towards the arachnoid, by a thin single layer of squamous cells.
Fine elastic and collagenous fibers are interposed between this layer of squamous cells and the underlying neural tissue.
The pia mater contains many blood vessels.
Macrophages, small group of lymphocytes, and scattered mast cells, are found among the pia cells and along blood vessels.
The arachnoid and the pia matter are so closey associated that they are often considered to be a single layer, the pia-arachnoid.