CNS

The nervous system

It receives each minute millions of information from the different sensory organs &then integrates all these to determine responses to be made by the body.
The central nervous system contains more than 100 billion neurons.
Most activities of the nervous system are initiated by sensory stimuli exciting sensory receptors(visual receptors –eyes) (auditory receptors –ears),( tactile receptors -surface of the body), ect.
This sensory experience cause immediate reaction from the brain which is called motor functions of the nervous system.

Motor functions of the nervous system, like

(1) contraction of appropriate skeletal muscles throughout the body
(2) Contraction of smooth muscle in the internal organs
(3) secretion of active chemical substances by both exocrine and endocrine glands in many parts of the body.
The muscles and glands are called effectors.
Parallel to this axis autonomic nervous system the controlling smooth muscles, glands, and other internal bodily systems

Integrative Function of the Nervous System

More than 99 per cent of all sensory informationis discarded by the brain as unimportant.
Thus, if a person places a hand on a hot stove, the response is to lift the hand and other associated responses follow, such as moving the entire body away from the stove, and even shouting with pain.

Major Levels of CentralNervous System Function

Three major levels of the central nervous system have specific functional characteristics:
(1) The spinal cord level
(2) The lower brain or subcortical level
(3) The higher brain or cortical level.


Spinal Cord Level

We often think of the spinal cord as being only aconduit for signals from the periphery of the body to the brain, or in the opposite direction from the brain back to the body. This is far from the truth, neuronal circuits in the cord can cause:
(1) walking movements
(2) reflexes that withdraw portions of the body from painful objects
(3)reflexes that stiffen the legs to support the body against gravity
(4) reflexes that control local blood vessels, gastrointestinal movements, or urinary excretion.
In fact, the upper levels of the nervous system simply“commanding” the cord centers to perform their functions.

Lower Brain or Subcortical Level

Many of subconscious activities of the body are controlled in the lower areas of the brain
(in the medulla, pons, mesencephalon, hypothalamus, thalamus, cerebellum, and basal ganglia).
For instance, subconscious control of arterialpressure and respiration is achieved mainly in themedulla and pons. Feeding reflexes, such as salivationand licking of the lips in response to the taste of food,are controlled by areas in the medulla, pons, mesencephalon, amygdala, and hypothalamus.

Higher Brain or Cortical Level

The cortex never functions alone but always in association with lower centers of the nervous system.
Without the cerebral cortex, the functions of the lower brain centers are often imprecise.
The cerebral cortex is an extremely large memory storehouse and opens a world of stored information for use by the mind.

Central Nervous SystemSynapses

The synapse is the junction point from one neuron to the next.
Synapses determine the directions that the nervous signals will spread through the nervous system. Information is transmitted in the central nervous system mainly in the form of nerve action potentials, called “nerve impulses,”
There are two major types of synapses:
• the chemical synapse and
• the electrical synapse


chemical synapses
The majority of the synapses in the central nervous system are of this type
In these, the first neuron secretes a chemical substance called aneurotransmitter and this transmitter in turn acts on receptor proteins in the membrane of the next neuron to excite the neuron or inhibit it.

More than 40 important transmitter substances

acetylcholine, norepinephrine, epinephrine, histamine, gamma-aminobutyric acid(GABA), glycine, serotonin, and glutamate.
they always transmit the signals in one direction: that is, from the neuron that secretes the transmitter substance ( the presynaptic neuron) to the neuron on which the transmitter acts ( the postsynaptic neuron).
This is the principle of one-way conduction at chemical synapses

Electrical synapses

These characterized by direct open fluid channels that conduct electricity from one cell to the next.
Most of these consist of gap junctions(small protein tubular structures) that allow free movement of ions from the one cell to the next.
Often transmit signals in either direction.
Only a few examples of gap junctions have been found in the central nervous system.

Physiologic Anatomy of the Synapse

Neuron is composed of three major parts:
-the soma,
-a single axon, which extends from the soma into a peripheral nerve and
-the dendrites,
-synaptic knobs called presynaptic terminals


The membrane of the presynaptic terminal it contains large numbers of voltage-gated calcium channels. When an action potential depolarizes the presynaptic membrane, these calcium channels open and allow large numbers of calcium ions to flow into the terminal.
When the calcium ions enter the presynaptic terminal,
causes the release sites to open through the membrane, allowing the release of transmitter.

The presynaptic terminals are:

either excitatory
or inhibitory

The membrane of the post synaptic neuron contains large numbers of receptor proteins, of two types:

An ion channel that allows passage of specified types of ions through the membrane

. Therefore, a transmitter substance is called an excitatory transmitter and the resting membrane potential has increased in the positive direction is called the excitatory postsynaptic potential (or EPSP) it will elicit an action potential in the neuron.
Conversely, opening anion channels allows negative electrical charges to enter like chloride ions, the increase in negativity in resting membrane potential is called inhibitory post synaptic potential (IPSP), which inhibits the neuron and transmitter substances are called inhibitory transmitters

Whatever the type of stimulus that excites the receptor, its immediate effect is to changes the receptor membrane characteristics and allows ions to flow through membrane channels.
When the receptor potential rises above the threshold, action potentials in the nerve fiber attached to the receptor will occur.

Presynaptic Inhibition

Another type of inhibition often occurs at the presynaptic terminals before the signal ever reaches the synapse. Inhibition is caused by release of aninhibitory substance onto the outsides of the presynaptic nerve fibrils the inhibitorytransmitter substance is GABA (gammaaminobutyricacid). This has a specific effect of opening anion channels.


Summation
Some times excitation of a single presynaptic terminal on the surface of a neuron not excites the neuron.
The reason for this is that insufficient transmitter substance is released by a single terminal we need to increase the transmitter substance to reach the threshold for excitation this is done by:
1-Spatial Summation
many neuron are stimulated at the same time.
2-Temporal Summation
Successive discharges from a single neuron, if they occur rapidly enough, can add to one another (summate).

Fatigue

When areas of the nervous system become over excited, the development of fatigue is aprotective mechanism against excess neuronal activity.
The mechanism of fatigue is mainly exhaustion of the stores of transmitter substance in the presynaptic terminals

Effect of Acidosis or Alkalosis on Synaptic Transmission.

Alkalosis(a rise in arterial blood pH to 8.0) greatly increases neuronal excitability. For instance, causes cerebral epileptic seizures.
Conversely, acidosis (a fall in pH below 7.0)greatly depresses neuronal activity; usually causes acomatose state.
For instance, in very severe diabetic or uremic acidosis, coma is developed.

Effect of Hypoxia on Synaptic Transmission

Cessation of oxygen for only a few seconds can cause complete inexcitability of some neurons. This is observed when the brain’s blood flow is temporarily interrupted, because within 3 to 7 seconds, the person becomes unconscious.



Effect of Drugs on Synaptic Transmission
Many drugs are known to increase the excitability of neurons, and others are known to decrease excitability. For instance,caffeine, theophylline, and theobromine, which arefound in coffee, tea, and cocoa, respectively, all increase neuronal excitability by reducing the threshold for excitation of neurons.
Most anesthetics increase the neuronal membrane threshold for excitation and thereby decrease synaptic transmission.

Types of Sensory Receptors and the SensoryStimuli

Classification according to their way of stimulation .
There are five basic types of sensory receptors:
(1) mechanoreceptors,which detect mechanical compression orstretching of the receptor
(2) thermoreceptors, which detect changes intemperature, some receptors detecting cold and others warmth;
(3) nociceptors(pain receptors), which detect damage occurring in the tissues, whether physicalor chemical damage;
(4) electromagnetic receptors,which detect lighton the retina of the eye
(5) chemoreceptors, which detect taste in themouth, smell in the nose, oxygen level in the arterial blood, osmolality of thebody fluids, carbon dioxide concentration.

Each type of receptor is highly sensitive to one type of stimulus

Thus, the rods and cones of the eyes are highly responsive to light but are almost completely nonresponsive to normal ranges of heat, cold, pressure.
The osmoreceptors of the supraoptic nuclei in the hypothalamus detect minute changes in the osmolality of the body fluids but have never been known to respond to sound.
Other Classifications of receptors according to position

labeled line principle:

The specificity of nerve fibers for transmitting only one modality of sensation(pain or touch or pressure or others) to a specific point in the central nervous system.
For instance, if a pain fibers stimulated, the person perceives pain regardless of what type of stimulus excites the fiber (can be electricity, overheating, damage to the tissue cells). Likewise, if a touch fiber is stimulated, the person perceives touch because touch fibers lead to specific touch areas in the brain.


Adaptation of Receptors
Another characteristic of all sensory receptors is that they adapt either partially or completely to any constant stimulus after a period of time.
That is, when acontinuous sensory stimulus is applied, the receptor responds at a high impulse rate at first and then at aprogressively slower rate until finally the rate of actionpotentials decreases to very few or often to none at all.
Mechanoreceptors adapt completely.
The mechanism of receptor adaptation is different for each type of receptor, adaptation of mechanoreceptors, part of the adaptation results from readjustments in the structure of the receptor itself, and part from an electrical type of accommodation in the nerve fiber.
In the eye, the rods and cones adapt by changing the concentrations of their light-sensitive chemicals

phasic receptors: sudden pressure applied to the tissue excites this receptor for a few milliseconds, and then its excitation is over even though the pressure continues. But later, it transmits a signal again when the pressure is released. Example, the pacinian corpuscle is exceedingly important in apprising the nervous system of rapid tissue deformations, but it is useless for transmitting information about constant conditions in the body.

Nonadaptingreceptors(The “Tonic” Receptors).

Slowly adapting receptors continue to transmit impulses to the brain as long as the stimulus is present.
Therefore, they keep the brain constantly appraised of the status of the body
For instance, impulses from the muscle spindles and Golgi tendon apparatuses allow the nervous system to know the status of muscle contraction.
Other slowly adapting receptors include receptors of the macula in the vestibular apparatus, pain receptors, baroreceptors and chemoreceptors.

General Classification of Nerve Fibers according to their velocities

The fibers are divided into types A and C,
Type A fibers are further subdivided into alpha , beta, gamma and delta (a, b, g, and d )fibers.
Type A fibers are the typical large and medium-sized myelinated fibers of spinal nerves.
Type C fibers are the small unmyelinated nerve fibers that conduct impulses at low velocities.
The C fibers constitute more than one half of the sensory fibers in most peripheral nerves as well as all the postganglionic autonomic fibers.



Divergence of Signals Passing Through Neuronal Pools
Often it is important for weak signals entering a neuronalpool to excite far greater numbers of nerve fibersleaving the pool. This phenomenon is called divergence.
Convergence of Signals
Convergence means signals from multiple
input suniting to excite a single neuron.

Neuronal Circuit with Both Excitatory andInhibitory Output Signals

Some times an incoming signal to a neuronal pool causes an output excitatory signal going in one direction and at the same time an inhibitory signal going elsewhere.

For instance, at the same time that an excitatory signal is transmitted by one set of neurons in the spinal cord to cause forward movement of a leg, an inhibitory signal is transmitted through a separate set of neurons to inhibit the muscles on the back of the leg so that they will not oppose the forward movement.
This type of circuit is characteristic for controlling all antagonistic pairs of muscles, and it is called the reciprocal inhibition circuit.

Reverberatory (Oscillatory) Circuit

Such circuits are caused by positive feedback within the neuronal circuit that feeds back tore-excite the input of the same circuit. Consequently, once stimulated, the circuit may discharge repetitively for a long time.

Rhythmical Signal Output

Many neuronal circuits emit rhythmical output signals—for instance, a rhythmical respiratory signal originates in the respiratory centers of the medulla and pons.
This respiratory rhythmical signal continues throughout life.