Habenular Nucleus

Development of the Rhombencephalon: Cerebellum and Ventricular System

The cerebellum develops from dorsal thickenings of the alar plates in the rostral metencephalon. These extensions of the alar plates are called the rhombic lips. With the combined effects of their growth and the deepening of the pontine flexure, they approach each other in the midline overlying the fourth ventricle. Fusion of the rhombic lips results in the formation of the cerebellar plate. The developing cerebellum becomes separated into cranial and caudal parts by the formation of a groove, termed the posterolateral fissure. The flocculus and the nodule develop into the flocculonodular lobes, which function in association with the vestibular apparatus. The flocculonodular nodes are the most phylogenetically ancient and primitive parts of the cerebellum.

The vermis is a narrow median swelling that connects the cerebellar hemispheres. The vermis and anterior part of the cerebellar hemispheres develop into the anterior lobe, which plays a role in the interpretation of sensory data from the limbs. The anterior lobe (also called the cranial lobe) grows more rapidly than the flocculonodular lobe and will eventually dominate cerebellar function. During development, the vermis and the cerebel-lar hemispheres fold extensively. By the end of the third month, a deep primary fissure has developed that divides the cranial part into an anterior lobe and a middle lobe. These subdivide further through the appearance of more transverse fissures. The thin gyri produced by this folding are termed folia, from their leaf-like appearance.

The posterior lobe is derived from the posterior part of the hemispheres and controls limb movement. Development begins at about the end of the sixth week, and continues to grow after birth. Nevertheless, the gross morphology at birth is the same as in the adult.

Histologically, the developing cerebellum comprises three layers, the neuro-epithelium, the mantle, and the marginal layer. The neuroepithelium of the rhombic lips proliferates initially, and forms the three layers. In the third month, the ventricular layer produces a further layer, the external germinal layer, and the original forms the inner germinal layer. This inner layer will differentiate into the cere-bellar nuclei, and will produce primitive Purkinje neuroblasts that will migrate into the cerebellar cortex. It will also produce Golgi neuroblasts, which will differentiate into Golgi cells.

The external layer will give rise, via basket, granule, and stellate neuroblasts, to basket, granule, and stellate cells. The granule cells and some of the other two types will eventually form the granular layer of the cortex.

The roof of the fourth ventricle, composed of a layer of vascular pia mater overlying the ependymal layer, is collectively called the tela choroidea. With growth of the pia mater, invaginations of the roof protrude into the ventricle to form the choroid plexus. These plexuses are formed in the third ventricle and the lateral ventricles and produce cerebrospinal fluid. Growth of the pia mater also causes three outpouchings of the tela choroidea, which rupture to form two foramina of Luschka. These foramina serve as a communication point between the central foramen of Magendie, subarachnoid space, and the ventricle.

mesencephalon nodule foramen of Luschka posterior medullary velum cerebellum at 16 weeks third ventricle flocculus_

cerebellum at term mesencephalon (midbrain)

rhombencephalon (hindbrain)

spinal cord central canal fourth ventricle aqueduct developing choroid plexus and ventricular system cerebellar hemisphere cerebellar hemisphere mesencephalon nodule foramen of Luschka posterior medullary velum

Pouch Luschka

roof plate of fourth ventricle foramen of Magendie roof plate of fourth ventricle foramen of Magendie

nodule third ventricle

telencephalon (hemisphere)

choroid plexus lateral ventricle telencephalon (hemisphere)

choroid plexus lateral ventricle

Development of the Mesencephalon

The mesencephalon is composed mainly of dense tracts of neurons passing from the forebrain to the spinal cord, but it also contains the nuclei of three cranial nerves. The mesencephalon contains nuclei of the trochlear nerve (IV), the oculomotor nerve (III), and the trigeminal nerve (V). The trigeminal nuclei and the trochlear nuclei arise from metencephalic alar and basal neuroblasts respectively but migrate into the midbrain. There are two nuclei of the oculomotor nerve, which are both derived from neuroblasts of the basal plate of the mesencephalon: the Edinger-West-phal nucleus and the somatic nucleus. The Edinger-Westphal nucleus controls accommodation and pupillary constriction through parasympathetic pathways while the somatic nucleus controls most of the extrinsic ocular muscles.

Neuroblasts derived from the alar plates give rise to structures such as the substantia nigra, the aqueductal gray matter, and the stratified nucleus of the inferior colliculus. Those from the basal plates give rise to structures such as the red nuclei, the mesencephalic nucleus of the trigeminal nerve (V), and the somatic motor nucleus of the oculomotor nerve (III). This area in the mesencephalon is known as the tegmentum. Ventral to this area, cerebral fibers descending to the spinal cord and the brain stem pass through the mesencephalon in the cerebral peduncles.

The proliferation and migration of alar neuroblasts into the roof of the midbrain form the inferior and superior colliculi. This area is known as the tectum. A longitudinal midline groove, the corpora bigemina, separates adjacent colliculi. The inferior colliculi receive information from the cochlea and mediate auditory reflexes while the superior colliculi receive information from the retina and mediate visual reflexes.

During development, the neural canal, which widened to form the fourth ventricle in the rhombencephalon, narrows throughout the mesencephalon to form the cerebral aqueduct (aqueduct of Sylvius). This meets the third ventricle ros-trally in the diencephalon. Abnormalities of the cerebral aqueduct can lead to a condition known as hydrocephalus. Hydro-cephalus describes enlargement of the ventricular system of the brain caused by unequal absorption and production of cerebrospinal fluid (CSF), and can be classified into communicating and non-communicating types. Noncommunicat-ing hydrocephalus occurs when there is an obstruction in the ventricular system, for example, in the cerebral aqueduct or in the foramina of Luschka or Magendie. Obstruction of the cerebral aqueduct results most frequently from fetal infection with Toxoplasma gondii or cytomegalovirus, but aqueductal stenosis can also be transmitted as an X-linked recessive trait. This causes dilatation of the lateral and third ventricles, whereas blockage of one or more of the foramina causes dilatation of all the ventricles. The ventricular dilatation causes expansion of the brain and may result in thinning of the calvaria bones, atrophy of the cerebral cortex and basal ganglia compression.

The Arnold-Chiari malformation describes the herniation of the cerebellum and the medulla through the foramen magnum. This interferes with and reduces the absorption of the CSF causing a communicating hydrocephalus, which results in distension of the whole ventricular system. The malformation is associated with spina bifida with myelomeningo-coele (see also p. 58).

Corpora Bigemina

corpora bigemina lamina quadrigemina basal plate lamina quadrigemina basal plate

Metencephalic Alar Plate

12 weeks inferior colliculus aqueductal gray matter mesencephalic nucleus of trigeminal nerve (V)

trochlear nucleus substantia nigra

12 weeks decussation of superior cerebral peduncle

16 weeks corpora bigemina

Harbenula Nucleus

stratified nucleus of inferior colliculus superior colliculi stratified nucleus of inferior colliculus visceral motor nucleus of oculomotor nerve (III) (Edinger-Westphal nucleus)

somatic motor nucleus of oculomotor nerve (III)

red nucleus cerebral peduncle yellow: alar plate origin brown: basal plate origin

The Diencephalon and Pituitary Gland

The diencephalon is the medial part of the prosencephalon. It differs from vesicles described previously in that the roof plate and the basal plates are not distinguishable. Important structures that develop in the diencephalon are the thalamus, a relay for sensory information, the hypothalamus, which controls endocrine and autonomic function, and the pituitary gland, which is involved in endocrine function. The cavity of the diencephalon forms the third ventricle.

By the sixth week of development, a groove is visible running across the center of the diencephalic walls (derived from the alar plates) which separates two swellings. The groove is the hypothalamic sulcus and the swellings are the thalamus and the hypothalamus. The mamillary body is a visible swelling on the ventral surface of the hypothalamus. By the seventh week of development the epithalamus is visible in the dorsal area of the diencephalon and is separated from the thalamus by the epithalamic sulcus. The thalami grow quickly, and in about 70% of brains, meet in the midline forming the interthalamic adhesion. The epithalamus later develops into the trigonum habenulae and the posterior and anterior commissures.

As in the mesencephalon, the roof plate in the diencephalon is composed of two layers, the ependymal layer and the vascular pia mater, which form the choroid plexus of the third ventricle. An epithelial thickening appears caudally in the roof plate, which invaginates in the seventh week. The structure formed is called the epiphysis, or the pineal gland.

The pituitary gland develops from two distinct ectodermal sources. The anterior lobe or adenohypophysis is formed from the roof of the stomodeum, which is the primitive oral cavity. The posterior lobe or neurohypophysis is formed from a downgrowth of the diencephalon.

Greenstein, Color Atlas of Neuroscience © 2000 Thieme

Rathke's pouch, from the stomodeum, develops during the fourth week and passes between the basisphenoid and presphenoid bones of the skull. The connection of the oral cavity to Rathke's pouch usually degenerates during the sixth week; persisting remnants of the pouch are known as pharyngeal hypophysis. Within the pituitary, Rathke's pouch derivatives are the anterior lobe, the pars intermedia, and the pars tuberalis, which grows around the infundibular stem. Craniopharyngiomas are remnants of Rathke's pouch that form tumors usually situated in or above the sella turcica. (The sella turcica is a depression in the body of the sphenoid bone in which the pituitary gland lies.) Symptoms develop before the age of 15 years and are similar to those resulting from anterior lobe tumors. The remainder of the pituitary develops from the infundibulum, an outpocketing of the third ventricle in the diencephalon. The median eminence, the infundibular stem, and the posterior lobe are formed from this structure.

Tumors of the pituitary gland can lead to hormonal imbalances due to the inappropriate secretion of trophic hormones such as luteinizing hormone, follicle-stimulating hormone, and adrenocorti-cotropic hormone (ACTH). These tumors can therefore cause sterility and symptoms of Cushing's disease, among others. When possible, the tumor, or the entire pituitary, is removed surgically. During development of the diencephalon, retinal fibers from the primitive optic cup run centrally. Just before they enter the brain, the optic fibers meet to form the optic chiasm.

epithalamus thalamus pineal gland posterior commissure region of geniculate bodies mamillary body.

hypothalamus, diencephalon at 10 weeks epithalamus thalamus pineal gland posterior commissure region of geniculate bodies mamillary body.

Hippocampal Commissure Embryo

cerebral hemisphere intraventricular foramen of Monro lamina terminalis corpus callosum anterior commissure olfactory bulb infundibulum infundibulum cerebral hemisphere intraventricular foramen of Monro lamina terminalis corpus callosum anterior commissure olfactory bulb

Primitive Mouth Oral Groove

Rathke's pouch stomodeum (primitive mouth cavity)

notochord infundibulum

Rathke's pouch stomodeum (primitive mouth cavity)

notochord cranial end of embryo at 36 days

Infundibular Stalk Brain Thalamus

36 days vRathke's pouch infundibular stem posterior lobe of pituitary (pars nervosa)"

pituitary gland development regressing stalk 16 weeks of Rathke's pouch optic chiasm parstuberalis pars intermedia anterior lobe of pituitary developing sphenoid bone

The Telencephalon

The telencephalon is the rostral part of the prosencephalon. This vesicle gives rise to the cerebral hemispheres, which contain most of the brain cortex. It also differentiates into the hippocampus, which is involved in memory, and the olfactory bulb, which is part of the olfactory system. The telencephalon also gives rise to the basal ganglia, which consist of several large gray matter structures that are embedded deeply in the white matter of the cerebrum. These include the caudate and lenticular nuclei (together known as the corpus striatum; see below). These nuclei play an important role in motor function. The lenticular nucleus consists of the putamen and the globus pallidus (see also p. 34). The telencephalon also differentiates into the commissures, which connect the two opposite sides of the brain.

The precursors of the cerebral hemispheres are the cerebral vesicles, which appear as lateral outgrowths of the telen-cephalon at the end of the fourth week of gestation. Subsequent gross development of the vesicles is bilaterally symmetrical. The cavity of the vesicle and the subsequent hemisphere forms the lateral ventricle and communicates with the third ventricle through the interventricu-lar foramen of Monro. The medial wall of the developing hemisphere runs along the diencephalon roof along the choroid fissure. In this area of the telencephalon, the hemisphere is composed only of ependy-mal cells and vascular pia mater and forms the choroid plexus of the lateral ventricle. The hemisphere grows rapidly and caudally, covering the mesencephalon and the rhombencephalon. The caudal part of the hemisphere grows in a ventral and rostral direction forming the temporal lobe whose cavity becomes the inferior horn with its own choroid plexus. A structure called the corpus striatum, a swelling in the floor of the hemisphere appearing in the sixth week, grows more slowly than the hemisphere and this results in the curvature of the hemispheres. The corpus stri-atum becomes divided by the internal capsule, which contains fibers running to and from the cerebral cortex, into the caudate nucleus and the lentiform nucleus. The head and body of the caudate nucleus lie in the floor of the lateral ventricle while its elongated tail lies in the roof of the inferior horn. The hippocampus develops from a thickening of the hemisphere just above the choroidal fissure.

Of the commissures that develop the four most prominent are found in the lamina terminalis, which is the midline portion of the telencephalon flanked by the cerebral vesicles on either side. The anterior and hippocampal commissures are the earliest to develop and they connect the olfactory bulbs and hippocampi respectively. The largest commissure is the corpus callosum, connecting the neocor-tices, whose growth extends beyond the lamina terminalis over the roof of the diencephalon. The optic chiasm, in the ventral area of the lamina, is a crossover point for fibers from the retina. The sulci and gyri, which are distinguishing furrows and elevations of the developed brain, are formed so that the massive growth of the cerebral cortex can be contained in the limited space of the cranium.

occipital lobe habenular commissure thalamus occipital lobe habenular commissure thalamus

Habenular Commissure

parietal lobe ofcortex infundibulum_ medial surface of forebrain of 10-week-old embryo infundibulum_ medial surface of forebrain of 10-week-old embryo parietal lobe ofcortex corpus callosum hippocampal commissure foramen of Monro lamina terminalis frontal lobe of cortex anterior commissure olfactory bulb optic chiasm lateral ventricle choroid plexus in lateral ventricle choroid fissure projection fibers of internal capsule third ventricle choroid fissure projection fibers of internal capsule third ventricle

Capsula Externa Cerebeli

lentiform nucleus hypothalamus choroid plexus in third ventricle thalamus caudate nucleus (part ofcorpus striatum)

lentiform nucleus hypothalamus transverse section at level shown above

The Neuron

The nervous system is built up of nerve cells, or neurons, and their supporting cells, or glia. Neurons are electrically excitable, capable of generating and propagating action potentials.

The neuron consists of a cell body, also called a perikaryon or soma, from which processes radiate. These processes include dendrites, which may branch to form dendritic 'trees', whose large surface area facilitates the reception of multiple signals from other neurons. They also include the main process, or axon, which conducts the nerve impulse from the soma to other cells. The axon, too, may give off branches, or collaterals, which extend the potential complexity of information transmission through the nervous system.

The neuronal cell body, when stained, shows a large nucleus with a prominent nucleolus. The cytoplasm contains numerous mitochondria, which are necessary for the generation of ATP, free ribo-somes and rough endoplasmic reticulum (rER), which synthesize proteins, and the Golgi apparatus, which modifies and packages newly synthesized proteins. The axon hillock is the part of the soma where the axon leaves it.

Neurons contain a cytoskeleton consisting of neurofibrils, which determine the shape of the soma and the various processes extending from it, and which transport substances through the neuron. There are three main types of neurofibrils:

(i) Actin, or microfilaments, present in high concentrations as a meshwork beneath the membrane in the axon. Actin is an important protein in axon development, and causes the movement of the growth cone (see p. 385). Actin appears to be present in all cells which can move, including muscle. Actin is also important in maintaining the shape of the cell;

(ii) microtubules and microtubule-associated proteins (MAP) are narrow longitudinal tubes present in all neuronal processes. The tubes maintain shape, and also transport molecules such as neuro-transmitters from the soma to the axon terminals (anterograde transport), or from the terminals to the soma (retrograde transport). There are at least two types of axonal transport: (a) rapid, at about 400 mm per day, and (b) slow, at less than 1 mm per day;

(iii) neurofilaments, also called intermediary filaments, are the most abundant of the fibrillar elements in the neuron, and form the 'bones' of the cytoskeleton.

Axons conduct electrical impulses, the speed of conduction depending on fiber diameter (see also p. 78). Efficiency of conduction also depends on good insulation, which is achieved by the myelin sheath. The myelin sheath coats the axonal membrane, or axolemma. Myelin is composed mainly of lipids and proteins and, in the peripheral nervous system, is made by the Schwann cells. In the CNS, myelin is made by oligodendrocytes (see p. 76). Microscopic examination reveals that the myelin sheath is arranged in spiral lamellae round the axon, and that the myelin is actually part of the Schwann cell. The Schwann cells are covered by the connective tissue endoneurium, while several bundles of fibers are covered by the perineurium. Larger bundles may be covered by yet another layer, the epineurium. In general, in physiology, a bundle of fibers, for example nerve or muscle, is termed a fasciculus.

endoneurium_J

nerve fiber endoneurium_J

nerve fiber

nucleus of

Schwann cell node of Ranvier nucleus of

Schwann cell node of Ranvier node of_ Ranvier myelin sheath endoneuriui fasciculus dendrite mitochondrioi fasciculus perineurium , r nucleu:

nucleolus_

nerve trunk nerve trunk myelin sheath axoplasm.

myelin sheath axoplasm.

Endoneurium Neuron

soma of neurone

Colgi apparatus_--

axon hillod soma of neurone bipolar neurone synapses presynaptic presynaptic

— bouton axo-axonic axo-dendritic axoplasi synaptii vesicle synapse structure axoplasi synaptii vesicle

Axoaxonic Synapse Cross Section

bouton naptic cleft presynaptic membrane postsynaptic membrane bouton naptic cleft presynaptic membrane postsynaptic membrane

Neuronal Cell Types

The structure of a neuron may provide valuable information about its function. Not only is its anatomical location important, but also its size, origin and destination of the axon and dendrites. The connections made with it may provide clues about its function. The degree of branching, or arborization of neuronal processes, particularly the dendrites, gives some indication of the complexity of information processing. Small neurons with short axons, such as those found in the cerebral cortex and in the spinal cord, clearly service a local network, which integrates information. Neurons, such as the spinal mo-toneuron, may have long axons, which convey messages over long distances.

Early workers on the nervous system had physically to dissect out neurons, since many of the available cell-staining methods stained all neurons, which produced indecipherable images. The Golgi stain, which uses silver deposition was used to great effect by the Spanish neuro-anatomist Ramón y Cajal to demonstrate neurons. The stain for some reason selectively stains only certain neurons, and is deposited selectively in the nerve cells but not in myelin. Myelin can be stained using the Weigert method, in which myelin is visualized by treating it with potassium dichromate, which renders myelin dark blue with hematoxylin. The Nissl method preferentially stains neuronal nuclei. Neurons can now be visualized with im-munocytochemical and biochemical techniques, which may identify specific neurotransmitters, hormones, enzymes and other neurochemicals. Formaldehyde-induced fluorescence can be used to distinguish between different neurotransmit-ters; norepinephrine (noradrenaline) neurons fluoresce pale green under UV light, while serotonin (5-HT) neurons fluoresce yellow.

Neuronal axons can be traced using degeneration techniques. Destruction of the soma of the neuron results in gradual axonal degeneration and, by following the path of degeneration of the axon, its course and ultimate connections may be ascertained. The origin of an axon or its destination may be discovered using retrograde or anterograde axonal transport respectively, in which a substance that can be visualized (for example radioactive amino acid, or the enzyme horseradish peroxidase) is applied to the soma or axon terminal, respectively. With the advent of electron microscopy, it became possible to visualize the subcellular organelles and observe the detailed structures of myelin, and of the synaptic contacts between cells.

Design of neuronal arrangement is best exemplified by that of the cerebellum (see also p. 21). The cerebellum, which is concerned with the regulation of movement and which is the largest part of the hind-brain, has specific neuronal cell types, namely basket, stellate, Golgi, and granule cells (see opposite), and these are all in direct or indirect contact with the Purkinje cell. The different cell types lie in different layers, and send their processes to other layers. Purkinje fibers are excited by, for example, afferent inputs from (excitatory) granule cells or mossy and climbing fibers, and inhibited by the basket, stellate granule and Golgi cells, which are cerebellar interneurons, whose processes do not leave the cerebellum. The Purkinje axons are efferents, which leave the cerebellum, and project to intracerebellar nuclei or relay stations, which in turn send projections to the cerebral cortex and brain stem which modify muscular activity.

Intracerebellar Nuclei

bipolar cell from dog retina

Pyramidal Neurons Cerebral Cortex

mammalian spinal motoneuron pyramidal cell from mouse cortex stellate cell stellate cell

Stellate Stem Cell

section of cerebellum

Golgi cell section of cerebellum

Golgi cell

Neuroglia (Glia)

Neuroglia occur in both the central and peripheral nervous systems. They surround and invest virtually every exposed surface that is not occupied by blood vessels and neurons. There are far more glial cells than neurons. They may support neurons, form myelin sheaths around them, and may provide nutritional, ionic, and mechanical support. They may regulate neuronal shape and synaptic connectivity. Neuroglia have been classified into two major classes, namely microglia and mac-roglia, which are in turn subclassified as astrocytes (astroglia), oligodendrocytes (oligodendroglia) and Schwann cells.

Astrocytes are CNS stellate (star-shaped) cells with many processes. On the basis of the length of the processes they are subclassified as protoplasmic or fibril-lary, respectively. These processes spread out to make 'feet' along the surfaces of the capillaries and around the surfaces of neurons except where the boutons make contact with other neurons. Astrocytes may provide a structural 'scaffolding' for developing nerve cells. Astrocytes form continuous sheets, called limiting membranes, and contribute to the blood-brain barrier between CNS and other tissues.

Astrocytes may exchange substances between capillaries and neurons. They may regulate extracellular concentrations of K+, which is particularly important in the brain, where the extracellular space is relatively small. K+ may increase rapidly due to neuronal excitation. Astrocytes may also mediate the removal and breakdown of neurotransmitters.

Oligodendrocytes have relatively few processes. They are central nervous glia which form the myelin sheaths around axons and are analogous to peripheral Schwann cells. They are often found close to the cell bodies of central neurons and for this reason are sometimes referred to as satellite cells. During development, the

Greenstein, Color Atlas of Neuroscience © 2000 Thieme oligodendrocyte sends out processes; when a process encounters an axon, it wraps itself round the axon to form the myelin sheath. Thus, one oligodendrocyte may provide myelin sheaths for many axons, whereas in the peripheral nervous system, one Schwann cell is dedicated to one neuron. The node of Ranvier is formed by myelin-free regions between two neighboring glial cells. Most sodium ion channels occur at the nodes of Ranvier. There is virtually no current flow across myelin sheaths. Action potentials are generated at the nodes and seem to jump from one node to the next, so-called saltatory conduction. Therefore the oligoden-drocyte plays a vital role in the propagation of impulses through its structural association with the neuron. Astrocytes appear to have limited phagocytic activity, and appear to form a sort of scar tissue at sites of CNS injury.

Microglia are the phagocytes of the nervous system and may be derived from the bone marrow. They express a common leukocytic antigen, CD45, which to date has only been described on cells derived from bone marrow. Microglia enter the brain during development, become uniformly distributed through it, and may form 5-20% of all CNS glial cells. During injury to the CNS, monocytes and macrophages migrate into the brain. Ependymal cells are cylindrical glial cells that line the ventricles.

Fusion The Pia Mater And The Ependyma
arrangement of astrocytes, pia, ependyma, and blood vessels of brain

Electrical Properties of Nerves I

Nerves transmit information as electrical signals. The nerve consists of an axon core, which is covered with the axon membrane, and a lipid bilayer, which allows the passage of ions across its surface through integral membrane ion channels. The lipid bilayer acts as an insulator between the intracellular and extracellular solutions of conducting ions, and therefore serves as a good capacitor, i.e. a reservoir of stored charge, which can be dissipated through the ion channels. These ion fluxes constitute the electrical current.

The nerve offers resistance to current, both longitudinally along the nerve axo-plasm (Rl), and transversely, across the membrane (RM), and can be thought of a set of units offering both transverse and longitudinal resistance. If the nerve were a passive conductor (e.g. copper wire), then the application of a voltage (Vo) would cause a current to flow along the nerve, and the voltage at increasing distance would fall off exponentially due to resistance, and to leakage of current across the axon membrane. Diameter influences this phenomenon. Doubling the diameter of the nerve halves RM and reduces RL to one quarter of its original value.

The rate of decay of V with distance along the cable is exponential, and depends on the ratio RM/RL. The rate of dissipation of the voltage, and its value at a given distance (x) after application can be calculated from the decay curve obtained. V is given by Voe-x/X, where e is a constant of value 2.718, and X is the space constant, defined as the distance from the point of application of Vo such that Vo has fallen to X/e of its original value. In fact, l is equal to V(Rm/Rl). From the above, it is readily apparent that the more the membrane leaks current, the smaller will be RM, and the shorter the space constant.

In reality, however, an applied voltage that is above a threshold value is not dissi pated by resistance and leakage, but generates an action potential whose amplitude, shape and rate of travel along the nerve are all at a rate independent of the original applied voltage. The independence of the properties of the action potential from those of the original stimulus constitutes what is called the all-or-none rule.

The mechanism whereby the nerve is able to initiate and propagate the action potential depends on the setting up of a local circuit around the area of the original applied stimulus. The first response is the passive spread of current (see above), which in turn sets up small local depolarizations in the nerve. These in turn increase the permeability of the membrane by opening ion channels, and the movement of the ions causes a larger depolarization. This depolarization sets up yet more local current flow and the action potential is generated, causing depolarization of the next part of the nerve. Movement of ions across the axon membrane is affected by the concentration gradients of the ions, and the voltage differences. Membranes with more ion channels will be more conductive to the ions, and the resistance of the membrane will be inversely proportional to the numbers of ion channels per unit area. The role of the ion channels in the propagation of the action potential and the nature of those ion movements are dealt with next.

Generation Action Potential

Electrical Properties of Nerve II: Generation of the Membrane Potential

The membrane potential is an electrical potential difference across the membrane. The ionic concentration is different on each side and the membrane is selectively permeable to certain ion species, which results in an ionic imbalance across the membrane.

The transmembrane potential is zero when concentrations of negatively and positively-charged ions are equal on each side of the membrane. When the K+ channels open, K+ ions, which occur in higher concentrations in the cytoplasm, diffuse down their concentration gradient through the membrane. However, negatively-charged counterbalancing anions, such as proteins, other organic anions and chloride, cannot cross through the K+-selective ion channels. This imbalance of charge creates a negative electrical potential on the cytoplasmic side. This negative potential drives K+ ions back across the membrane into the cytoplasm. When K+ influx and efflux are in equilibrium, then the net movement of K+ ions becomes zero, even though the K+ channels are open. The resulting electrical potential is called the equilibrium potential for potassium (EK). The value of EK for a particular ion can be calculated using the Nernst equation:

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  • gabriel mustafa
    What is generation of action potential?
    8 years ago

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