Both nervous system and the endocrine system are considered integrative systems. This designation stems from the fact that both systems control and regulate biological functions and act at distance, receiving information from organs and tissues and sending effector commands (nervous impulses or hormones) to organs and tissues, thus integrating the body.
The structures that form the nervous system can be divided into the central nervous system (CNS) and the peripheral nervous system (PNS).
The organs of the CNS are the brain (cerebrum, brainstem and cerebellum) and spinal cord. The PNS is made of nerves and neural ganglia. In addition to these organs, the meninges (dura-mater, arachnoid and pia-mater) are also a part of the nervous system, since they cover and protect the encephalon and the spinal cord.
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The main cells of the nervous system are neurons. In addition to neurons, the nervous system is also made up of glial cells.
Glial cells and neurons are the cells that form the nervous system. Neurons are cells that have the function of receiving and transmitting neural impulses whereas glial cells (astrocytes, microgliacytes, ependymal cells and oligodendrocytes) are the cells that support, feed and insulate (electrically) the neurons. The Schwann cells that produce the myelin sheath of the peripheral nervous system can also be considered glial cells.
The three mains parts into which a neuron can be divided are: dendrites, the cell body and the axon.
Dendrites are projections of the plasma membrane that receive the neural impulse from other neurons. The cell body is where the nucleus and the main cellular organelles are located. The axon is the long membrane projection that transmits the neural impulse at a distance to other neurons, to muscle cells and to other effector cells.
The terminal portion of the axon is called the presynaptic membrane. Through this membrane, neurotransmitters are released into the synaptic junction.
Synapses are the structures that transmit a neural impulse between two neurons.
When the electric impulse arrives, the presynaptic membrane of the axon releases neurotransmitters that bind to the postsynaptic receptors of the dendrites of the next cell. The activated state of these receptors alters the permeability of the dendritic membrane and the electric depolarization moves along the plasma membrane of the neuron to its axon.
Most neurons are located within the brain and the spinal cord (the central nervous system) in places known as neural nuclei. Neural ganglia, or simply ganglia, are structures of the peripheral nervous system located beside the spine or near certain organs, in which neuron cell bodies are also located.
Neurons located at specific points can may have distant axonal terminations and can also receive impulses from the axons of distant neurons. An example of this are the inferior motor neurons in the spinal cord, since their axons can transmit information to the extremities of the lower limbs, triggering foot contractions.
There are three types of neurons: afferent neurons, efferent neurons and interneurons. Afferent neurons only transmit sensory information from the tissues to neural nuclei and ganglia (where they come into contact with interneurons or effector neurons). Efferent neurons transmit commands for tasks to be performed in several parts of the body. Interneurons, also known as association neurons or relay neurons, serve as a connection between the other two types of neurons.
Afference is the conduction of sensory impulses and efference is the conduction of effector impulses (impulses that command some action in the body).
Axons extend throughout the body inside nerves. Nerves are axon-containing structures which are home to a large number of axons and which are covered by connective tissue. Nerves connect neural nuclei and ganglia with tissues.
Nerves may contain only sensory axons (sensory nerves), only motor axons (motor neurons) or both types of axons (mixed nerves).
Ganglia (singular ganglion), or neural ganglia, are structures located outside the central nervous system (for example, beside the spine or near viscera) made of a concentration of neuron bodies.
Examples of neural ganglia are the ganglia that contain the cell bodies of sensory neurons in the dorsal roots of the spinal cord and the ganglia of the myenteric plexus, which are responsible for the peristaltic movements of the digestive tract.
In the central nervous system (CNS), concentrations of neuron bodies are called nuclei and not ganglia.
The peripheral nervous system is made up of the nerves and ganglia of the body.
The function of the myelin sheath is to improve the safety and speed of neural impulse transmission along the axon. The myelin sheath serves as an electrical insulator, preventing the dispersion of the impulse into other adjacent structures. Since the myelin sheath has gaps called Ranviers’ nodes along its length, the neural impulse “jumps” from one node to another, thus increasing the speed of the neural transmission.
Not all neurons have a myelin sheath. Axonal fibers may be myelinated or unmyelinated.
In the central nervous system (CNS), the myelin sheath is made of an apposition of oligodendrocyte membranes. Each oligodendrocyte may cover portions of the axons of several different neurons. In the peripheral nervous system (PNS), the myelin sheath is made of consecutive Schwann cell membranes covering segments of a single axon. The Ranviers’ nodes appear in the intercellular space between these cells.
The myelin sheath is rich in lipids but also contains proteins.
Multiple sclerosis is a severe disease caused by progressive destruction of the myelin sheath of the central nervous system. Guillain-Barré disease is due to the destruction of the myelin sheath in the peripheral nervous system caused by autoimmunity (attacks carried out by the immune system of the body). A genetic deficiency in the formation or preservation of the myelin sheath is an X-linked inheritance called adrenoleukodystrophy. The movie “Lorenzo’s Oil” featured a boy with this disease and his father's dramatic search for a treatment.
Meninges are the membranes that enclose and protect the central nervous system (CNS). Cerebrospinal fluid is the fluid that separates the three layers that form the meninges. It has the functions of nutrient transport, defense and the mechanical protection of the CNS.
Cerebrospinal fluid fills and protects cavities of the brain and the spinal cord.
The concept of brain, or encephalon, includes the cerebrum (mostly referred to as the hemispheres, but in reality, the concept also includes the thalamus and the hypothalamus), the brainstem (midbrain, pons and medulla) and the cerebellum. The brain and spinal cord form the central nervous system (CNS).
The cerebrum is divided into two cerebral hemispheres, the right and the left. Each hemisphere is composed of four cerebral lobes: the frontal lobe, the parietal lobe, the temporal lobe and the occipital lobe.
Each cerebral lobe contains gray matter and white matter. Gray matter is the outer portion and is made of neuron bodies; gray matter is also known as the cerebral cortex. White matter is the inner portion and is white because it is in the region where the axons of cortical neurons pass.
In the central nervous system, the cerebellum is the main controller of motor coordination and balance. (Do not confuse this with muscle command, which is performed by the cerebral hemispheres).
The cerebellum is the main structure in the brain that coordinates the movement and balance of the body. For this reason, it appears to be more developed in mammals that jump or fly (such as bats). The cerebellum is also very important for the flight of birds.
The neural regulation of breathing, blood pressure and other physiological parameters such as heartbeat, digestive secretions, peristaltic movements and transpiration is performed by the medulla.
The medulla, together with the pons and the midbrain, is part of the brainstem.
In the brain, conscious sensory information is received by neurons located in a special region called the postcentral gyrus (or sensory gyrus). Gyri are the convolutions of the cerebrum. Each of the two postcentral gyri are located in one of the parietal lobes of the cerebrum.
Voluntary motor activity (voluntary muscle movement) is commanded by neurons located in the precentral gyrus (or motor gyrus). Each of the two precentral gyri are located in one of the frontal lobes of the cerebrum.
The names post- and pre-central refer to the fact that the motor and sensory gyri are spaced apart in each cerebral hemisphere by the sulcus centralis, a fissure that separates the parietal and frontal lobes.
The spinal cord is the dorsal neural cord of vertebrates. It is the part of the central nervous system that continues into the trunk to facilitate the nervous integration of the whole body.
The spinal cord is made of groups of neurons located in its central portion forming gray matter, and axon fibers in its exterior portion forming white matter. Neural bundles connect to both lateral sides of spinal cord segments to form the dorsal and ventral spinal roots that join to form the spinal nerves. Dorsal spinal roots contain a ganglion with neurons that receive sensory information; ventral spinal roots contain motor fibers. Therefore, dorsal roots are sensory roots and ventral roots are motor roots.
According to researchers, some of the main regions of the nervous system associated with the phenomenon of memory are the hippocampus, located in the interior portion of the temporal lobes, and the frontal lobe cortex, both of which are part of the cerebral hemispheres.
The cerebral hemispheres contain neurons that centrally command and control muscle movements. These neurons are called superior motor neurons and are located in a special gyrus of both frontal lobes known as the motor gyrus (or precentral gyrus). These superior motor neurons send axons that transmit impulses to the inferior motor neurons of the spinal cord (for neck, trunk and limb movements) and to the motor nuclei of the cranial nerves (for face, eyes and mouth movements).
The fibers cross to the other side in specific areas of those axon paths. About 2/3 of the fibers that go down the spinal cord cross at the medullar level forming a structure known as the pyramidal decussation. The other (1/3) of fibers descend on the same side as their original cerebral hemisphere and cross only within the spinal cord at the level where their associated motor spinal root exits. The fibers that command the inferior motor neurons of the cranial nerves cross to the other side just before the connection with the nuclei of these nerves.
The motor fibers that descend from the superior motor neurons to the inferior motor neurons of the spinal cord form the pyramidal tract. Injuries to this tract caused by spinal sections or by central or spinal tumors, for example, may lead to paraplegia and tetraplegia.
In some situations, the movement of skeletal striated muscles does not depend on commands from superior motor neurons, meaning that it is not triggered by volition.
Involuntary movements of those muscles may occur when sensory fibers that make direct or indirect contact with inferior motor neurons are unexpectedly stimulated in situations that suggest danger to the body. This happens, for example, in the patellar reflex, or knee jerk reflex, when a sudden percussion on the knee patella (kneecap) triggers an involuntary contraction of the quadriceps (the extension muscle of the thigh). Another example of the reflex arc occurs when someone steps on a sharp object: one leg retracts and the other, through the reflex arc, stretches to maintain the balance of the body.
In a reflex arc, first a sensory neuron located in the ganglion of a dorsal spinal root collects the stimulus information from the tissue. This sensory neuron makes a direct or indirect (through interneurons) connection with the inferior motor neurons of the spinal cord. These motor neurons then command the reflex reaction. Therefore, sensory neurons, interneurons and inferior motor neurons participate in the reflex arc.
The gray matter of the spinal cord mainly contains neuron bodies (inferior motor neurons, secondary sensory neurons and interneurons). The white matter is mainly made up of axons that connect neurons of the brain with spinal neurons.
The sensory fiber that first triggers the reflex arc connects with neurons of the reflex arc as well as with secondary sensory neurons of the spinal cord that transmit information on to other neurons of the brain. This is obvious, since the person that received the initial stimulus (for example, something hitting his/her kneecap) perceives it (meaning that the brain became conscious of the fact).
The reflex arc only depends on the integrity of the fibers at a single spinal level. In the reflex arc, the motor response to the stimulus is automatic and involuntary and does not depend upon the passage of information to the brain. Therefore, it happens even if the spinal cord is damaged at other levels.
The poliovirus is parasitic to and destroys spinal motor neurons, causing the paralysis of the muscles that depend on these neurons.
The efferences (reactions) of the nervous system can be classified into voluntary, when they are the result of volition, and involuntary, when they are not consciously controlled. Examples of reactions triggered by volition are the movements of limbs, the tongue and respiratory muscles. Examples of involuntary efferences are those that command peristaltic movements, the heartbeat and arterial wall muscles. Skeletal striated muscles are voluntarily contracted; whereas cardiac striated and smooth muscles are involuntarily contracted.
Functionally, the nervous system can be divided into the somatic nervous system and the visceral nervous system.
The somatic nervous system includes the central and peripheral structures that constitute the voluntary control of efferences. Central and peripheral structures that participate in the control of the vegetative (unconscious) functions of the body are included in the concept of the visceral nervous system.
The efferent portion of the visceral nervous system is called the autonomic nervous system.
The autonomic nervous system is divided into the sympathetic nervous system and the parasympathetic nervous system.
The sympathetic nervous system includes the nerves that extend from the ganglia of the neural chains lateral to the spine (near the spinal cord) and therefore are located at a distance from the tissues they innervate. The central and peripheral neurons associated with those neurons are also a part of the sympathetic nervous system.
The parasympathetic nervous system is made up of nerves and central or peripheral neurons related to the visceral ganglia, neural ganglia located near the tissues they innervate.
In general, the actions of the sympathetic and the parasympathetic nervous systems are antagonistic, meaning that when one stimulates something, the other inhibits it and vice versa. The organs, with few exceptions, receive efferences from these two systems and the antagonism between them serves to balance their effects. For example, the parasympathetic system stimulates salivation while the sympathetic system inhibits it; the parasympathetic system constricts pupils while the sympathetic system dilates them; the parasympathetic system contracts the bronchi while the sympathetic system relaxes them; and the parasympathetic system excites the genital organs while the parasympathetic system inhibits the excitation.
Considering invertebrates, it is possible to observe that evolution accompanies the increasing complexity of organisms with the convergence of nervous cells at special structures for controlling and commanding: the ganglia and the brain. In simple invertebrates, such as cnidarians, nervous cells are not concentrated rather they are found dispersed in the body. In platyhelminthes, the beginning of cephalization with an anterior ganglion concentrating neurons is already verified. In annelids and arthropods the existence of a cerebral ganglion is evident. In cephalopod molluscs, the cephalization is even greater and the brain controls the nervous system.
In vertebrates, the nervous system is well-defined, with a brain and dorsal neural cord protected by rigid skeletal structures. In most invertebrates, the nervous system is predominantly ganglial, with ventral neural cords.
In vertebrates, the brain and the spinal cord are protected by membranes, the meninges, and by osseous structures, the skull and the spine, respectively. These protective structures are fundamental in maintaining the integrity of these important organs, which control the functioning of the body.
Neurons receive and transmit chemical stimuli through neurotransmitters released in the synapses. However, the impulse transmission is electrical along the neuron body. Therefore, neurons conduct electrical and chemical stimuli.
The two main ions that participate in electrical impulse transmission in neurons are the sodium cation (Na⁺) and the potassium cation (K⁺).
As in most cells, the region just outside the surface of the neuron plasma membrane has a positive electrical charge compared to the region just inside it, which is therefore negative.
The normal (resting) potential difference across the neuron membrane is about –70 mV (millivolts). This voltage is called the resting potential of the neuron.
When at rest, the plasma membrane of a neuron maintains an electric potential difference between its external and internal surfaces. This voltage is called resting potential. A resting potential around –70 mV indicates that the interior is more negative than the exterior (negative polarization). This condition is maintained by the transport of sodium and potassium ions across the plasma membrane.
The membrane is permeable to potassium ions but not to sodium ions. At rest, the positive potassium ions exit the cell in favor of the concentration gradient, since within the cell the potassium concentration is higher than in the extracellular space. However, the positive sodium ions cannot enter the cell. Positive potassium ions exit the cell and not enough compensatory positive ions enter the cell, causing the intracellular space to become more negative and making the cell remain polarized.
When the neuron receives a stimulus via the binding of neurotransmitters to specific receptors, sodium channels open and the permeability of the plasma membrane in the postsynaptic region is altered. Sodium ions then enter the cell, causing a decrease (less negative) in the potential difference of the membrane. If the reduction in the potential difference of the membrane reaches a level called the excitation threshold, or threshold potential, around –50 mV, the action potential is generated, meaning that the depolarization intensifies until reaching its maximum level. The depolarization current is then transmitted along the remaining length of the neuronal membrane.
If the excitation threshold is reached, voltage-dependent sodium channels in the membrane open, allowing more sodium ions to enter the cell in favor of the concentration gradient, and an approximate level of –35 mV of positive polarization of the membrane is achieved. The voltage-dependent sodium channels then close and more voltage-dependent potassium channels open. Potassium ions then exit the cell in favor of the concentration gradient and the potential difference of the membrane decreases. This process is called repolarization.
The action potential triggers the same electrical phenomenon in neighboring regions of the plasma membrane and the impulse is therefore transmitted from the dendrites to the terminal region of the axon.
The excitation threshold of a neuron is the depolarization level that must be caused by a stimulus to be transmitted as a neural impulse. This value is about –50 mV.
The transmission of a neural impulse along the neuronal membrane obeys an all-or-nothing rule: either it happens at maximum intensity or nothing happens. Only when the excitation threshold is reached does the depolarization continue, causing the membrane to reach its maximum possible positive polarization, about +35 mV. If the excitation threshold is not reached nothing happens.
The primary cause of neuronal depolarization is the binding of neurotransmitters released in the synapse (by the axon of the neuron that sent the signal) to specific receptors in the membrane of the neuron that is receiving the stimulus. The binding of neurotransmitters to those receptors is a reversible phenomenon that alters the membrane permeability of the region, since the binding causes sodium channels to open. When positive sodium ions enter the cell in favor of their concentration gradient, the voltage of the membrane increases, thus decreasing its negative polarization. If this depolarization reaches the excitation threshold (about –50 mV), the depolarization continues, the action potential is reached and the impulse is transmitted along the cell membrane.
Action potential is the maximum positive voltage level achieved by the neuron during the process of neuronal activation, around + 35 mV. The action potential triggers the depolarization of the neighboring regions of the plasma membrane and therefore the propagation of the impulse along the neuron.
The resting potential is the voltage of the membrane when the cell is not excited, about –70 mV.
The excitation threshold is the voltage level, about –50 mV, which the initial depolarization must reach for the action potential to be attained.
Repolarization is the return of the membrane potential from the action potential (+35 mV) to the resting potential (-70 mV).
When the membrane reaches its action potential, voltage-gated sodium channels close and voltage-gated potassium channels open. As a result, sodium stops entering the cell and potassium starts to exit it. Therefore, the repolarization is due to the exit of potassium cations from the cell.
The repolarization causes the potential difference to temporarily increase under –70 mV, below the resting potential, in a phenomenon known as hyperpolarization.
A neural impulse is transmitted along the neuronal membrane through the depolarization of consecutive neighboring regions. When a region on the internal surface of the membrane is depolarized, it becomes more positive in relation to the neighboring internal region. As a result, positive electrical charges (ions) move towards this more negative region and voltage-gated sodium channels are activated and opened. The action potential then linearly moves along the membrane until reaching the presynaptic region of the axon.
The structure through which a neural impulse passes from one cell to another is the synapse. The synapse is composed of the presynaptic membrane in the terminal portion of the axon of the transmitter cell, the synaptic cleft (or synaptic space) and the postsynaptic membrane in the dendrite of the receptor cell.
The propagation of the action potential along the axon reaches the region immediately anterior to the presynaptic membrane, causing its permeability to calcium ions to change and causing these ions to enter the cell. In the presynaptic area of the axon, there are a large amount of neurotransmitter-filled vesicles that, by means of exocytosis activated by the calcium influx, release the neurotransmitters into the synaptic cleft. The neurotransmitters then bind to specific receptors of the postsynaptic membrane. (The binding of neurotransmitters to their receptors is reversible, that is, the neurotransmitters are not consumed during the process.) With the binding of the neurotransmitters to the postsynaptic receptors, the permeability of the postsynaptic membrane is altered and the depolarization that will lead to the first action potential of the postsynaptic cell begins.
The following are important neurotransmitters: adrenaline (epinephrine), noradrenaline (norepinephrine), acetylcholine, dopamine, serotonin, histamine, gaba (gamma aminobutyric acid), glycine, aspartate and nitric oxide.
Since the binding of neurotransmitters to postsynaptic receptors is reversible, after these neurochemicals carry out their role, they must be eliminated from the synaptic cleft. Neurotransmitters then bind to specific proteins that carry them back to the axon they came from in a process called neurotransmitter re-uptake. They can also be destroyed by specific enzymes, such as acetylcholinesterase, an enzyme that destroys acetylcholine. In addition, they can simply diffuse out of the synaptic cleft.
Fluoxetine is a substance that inhibits the reuptake of serotonin, a neurotransmitter that acts mainly in the central nervous system. By inhibiting the reuptake of the neurotransmitter, the drug increases its availability in the synaptic cleft, thus improving neuronal transmission.
The neuromuscular synapse is the structure through which a neural impulse passes from the axon of a motor neuron to a muscle cell. This structure is also known as the neuromuscular junction, or motor end plate.
Like with the nervous synapse, the axonal terminal membrane releases the neurotransmitter acetylcholine into the cleft between the two cells. Acetylcholine binds to specific receptors of the muscle membrane, dependent sodium channels then open and the depolarization of the muscle membrane begins. The impulse is then transmitted to the sarcoplasmic reticulum, which releases calcium ions into the sarcomeres of the myofibrils, thus triggering the contraction.
Information about the conditions of external and internal environments, such as temperature, pressure, touch, spatial position, pH, metabolite levels (oxygen, carbon dioxide, etc.), light, sounds, etc., are collected by specific neural structures (different types for each type of information) called sensory receptors. Sensory receptors are distributed throughout tissues according to their specific roles. The receptors obtain information and transmit it through their own axons or through dendrites of neurons connected to them. The information reaches the central nervous system, which interprets it and uses it to control and regulate the body.
Sensory receptors are structures specialized in the acquisition of information, such as temperature, mechanical pressure, pH, chemical environment and luminosity, which transmit them to the central nervous system. Sensory receptors may be specialized cells, such as the photoreceptors of the retina, or specialized interstitial structures, such as the vibration receptors of the skin. In this last case, they transmit information to the dendrites of the sensory neurons connected to them. There also exist sensory receptors that are specialized terminations of neuronal dendrites (such as olfactory receptors).
Sensory receptors are classified according to the stimuli they obtain: mechanoreceptors are stimulated by pressure (touch or sound); chemoreceptors respond to chemical stimuli (olfactory, taste, pH, metabolite concentration, etc.); thermoreceptors are sensitive to temperature changes; photoreceptors are stimulated by light; nocireceptors send pain information; and proprioceptors are sensitive to the spatial position of muscles and joints (they generate information for the balance of the body).
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