NERVE TISSUES

The nervous system enables the body to respond to continuous changes in its external & internal environmentIt controls and integrates the functional activities of the organs and organ systems

Anatomically, the nervous system is divided into:CENTRAL NERVOUS SYSTEM: consist of the brain and spinal cord, located in the cranial cavity and spinal canal respectivelyPERIPHERAL NERVOUS SYSTEM: consists of cranial, spinal, & peripheral nerves that conduct impulses from (efferent or motor nerves) and to (afferent or sensory nerves) the CNS, collections of nerve cell bodies outside the CNS called ganglia & specialized nerve endings

Functionally, the nervous system is divided into:SOMATIC NERVOUS SYSTEM: consists of the somatic parts of CNS & PNS.Provides sensory & motor innervation to all parts of the body except viscera, smooth muscle & glandsAUTONOMIC NERVOUS SYSTEM:Consists of the autonomic parts of the CNS & PNSProvides efferent involuntary motor innervation to smooth muscle, the conducting system of the heart & glandsAlso provides afferent sensory innervation from the viscera

Sympathetic systems are dominant when activity levels increase In a stressful situation the sympathetic division is dominant. These responses alert the body to a need for action. The stimulation of the adrenal glands to secrete adrenaline is a common emergency response. Other responses under sympathetic control are:constriction (narrowing) of the blood vessels of the stomach, the intestines, and the interior of body;inhibition of salivation;blood sugar release; anddilation (widening) of the pupils of the eyes.

Parasympathetic systems are dominant in relaxation In non stressful situations the parasympathetic division is in control. Parasympathetic responses act to conserve the resources of the body and to maintain its internal equilibrium. Some responses under parasympathetic control are: dilation (widening) of the blood vessels of the stomach, the intestines, and the interior of body;stimulation of salivation;inhibition of blood sugar release; andconstriction (narrowing) of the pupils of the eyes.

COMPOSITION OF NERVE TISSUENerve tissue consists of 2 principal type of cells, neurons & supporting cells

Neuron (or nerve cell) is the functional unit of the nervous systemSupporting cells are non-conducting cells that are intimate apposition to neurons

Supporting cells provide: Physical support (protection) for delicate neuronal processesElectrical insulation for nerve cell bodies and processesMetabolic exchange pathways between the vascular system and the neurons of the nervous system

The nervous system allows rapid response to external stimuliThe autonomic part of the nervous system regulates the function of internal organsSmooth muscleCardiac conducting cellsGlandular epithelium

The NeuronThe neuron is the basic structural and functional unit of the nervous systemThe human nervous system contains more than 10 billion neuronsSensory neurons: convey impulses from receptors to the CNS. Processes of these neurons are included in somatic afferent & visceral afferent nerve fibers

Somatic afferent fibers convey sensations of pain, temperature, touch & pressure from the body surface. Also conveys pain & proprioceptionVisceral afferent fibers transmit impulses of pain and other sensations from mucous membranes, glands, and blood vessels

Motor neurons convey impulses from the CNS or ganglia to effector cells. Processes of these neurons are included in somatic efferent & visceral efferent nerve fibersSomatic efferent neurons send voluntary impulses to skeletal musclesVisceral efferent neurons transmit involuntary impulses to smooth muscles, cardiac conducting cells (Purkinje fibers) and glands

Interneurons or intercalated neurons form communicating and integrating network between the sensory and motor neurons

The functional components of a neuron include the cell body, axon, dendrites and synaptic junctionsCell body (perikaryon) contains the nucleus & those organelles that maintain the cellAxon is the longest process extending from the cell, w/c transmits impulses away from the cell body to a specialized terminal (synapse) that makes contact w/ another neuron or effector cellDendrites are shorter process that transmit impulses from the periphery toward the cell body

Neurons are classified on the basis of the number of processes extending from the body:Multipolar neurons have 1 axon and 2 more dendritesBipolar neurons have 1 axon and 1 dendriteUnipolar (or pseudounipolar) have one process, the axon, w/c divides close to the cell body into 2 long processes. The vast majority of unipolar neurons are located in the dorsal root ganglia & cranial nerve ganglia

Motor neurons and interneurons are multipolarSensory neurons are unipolarTrue bipolar neurons are limited to the retina of the eye and the ganglia of the vestibulocochlear nerve (cranial nerve VIII) of the ear

Cell BodyThe cell body of a neuron has characteristics of a protein-producing cellNeurons do not undergo division, but some parts of the brain neural stem cells are present and are able to differentiate and replace damaged nerve cells

Dendrites and AxonsDendrites are receptor processes that receive stimuli from other neurons or from the external environmentAxons are effector processes that transmit stimuli to other neurons or effector cellsSome large axon terminals are capable of local protein synthesis, w/c maybe involved in processes

SynapsesAre specialized junctions between neurons that facilitate transmission of impulses from one (presynaptic) neuron to another (postsynaptic) neuron.Synapses may be classified morphologically as:Axodendritic: occurring bet. axons & dendritesAxosomatic: occurring bet. axons & cell bodyAxoaxonic: occurring bet. axons & axons

SynapsesMay also be classified as chemical or electricalChemical synapses in w/c conduction of impulses is achieved by the release of chemical substances (neurotransmitters) from the presynaptic neuronElectrical synapses w/c are common in invertebrates, contain gap junctions that permit movement of ions between cells and consequently permit the direct spread of electrical current from one cell to another

A typical chemical synapse contains a presynaptic knob, synaptic cleft & postsynaptic membranePresynaptic knob: the end of the neuron process from w/c neurotransmitters are releasedsynaptic vesicles are presynaptic components N-ethylmaleimide-sensitive factor (NSF) is a specific ATP-binding protein found on the membrane of synaptic vesicles functions for formation, targeting, and fusion of these vesicles w/ the presynaptic membrane

A typical chemical synapse contains a presynaptic knob, synaptic cleft & postsynaptic membraneSynaptic cleftIs a 20 30 nm space that separates the presynaptic neuron from the postsynaptic neuron or the target cell, w/c the neurotransmitter must crossPostsynaptic membraneContains receptor sites w/ w/c the neurotransmitter interacts

Parkinsons diseaseA slowly progressive neurologic disorder caused by the loss of dopamine (DA)-secreting cells in the substantia nigra and basal ganglia of the brainDA is a neurotransmitter responsible for synaptic transmission in the nerve pathways coordinating smooth & focused activity of skeletal muscles

Parkinsons diseaseLoss of DA secreting cells is associated w/ a classic pattern of symptoms:Resting tremor in limb, especially of the hand when in a relax position; tremors increases on stress & is often more severe on one side of the bodyRigidity or increased tone (stiffness) in all musclesSlowness of movement (bradykinesia) and inability to initiate movement (akinesia)Lack of spontaneous movementsLoss of postural reflexes, w/c leads to poor balance & abnormal walkingSlurred speech, slowness of thought, and small, cramped handwriting

Nerve fibersAre continuations of cytoplasmic processesMedullated or myelinated fibers is surrounded by a layer of myelin sheathNon-medullated nerve fibers are sympathetic nerve fibers and fibers in the sympathetic ganglia

GangliaAre groups of nerve cell bodies found outside the central nervous systemSympathetic ganglia occur along the sympathetic trunks within the walls of visceral organs and glandsCraniospinal ganglia are located in the dorsal roots of spinal nerves and in the paths of some cranial nerves

Nerve EndingsSensory endingsTactile (Meissners) corpuscle: touch corpuscle.Occurs frequently in the dermal papilla of thick skin, (palms, soles, tips of digits). Pacinian corpuscles: large, elliptical, lamellated structures usually found in the subcutaneous connective tissue, near joints, in mesenteries, and interstitial tissues of the pancreasMuscle spindle:elongated spindle structures w/c are found in muscle cells

Nerve endingsMotor nerve endingsThe ultimate branches of motor nerves terminating on the surface of muscle fibers

Spinal cordIs a flattened cylindrical structure that is directly continous w/in the brainDivided into 31 segments: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygenal; wherein each segment is connected to a pair of spinal nervesIn cross section, it appears as a butterfly shaped grayish tan inner substance, the gray matter surrounding the central canal, and a whitish peripheral substance, the white matter.

The cell bodies of motor neurons that innervate striated muscle are located in the ventral horn of the gray matterThe cell bodies of sensory neurons are located in ganglia that lie on dorsal root of the spinal nerve

Afferent (sensory) receptorsAfferent receptors are specialized structures located at the distal tips of the peripheral processes of sensory neuronsExteroreceptors: reacts to stimuli from external environmentEnteroreceptors: react to stimuli from within the bodyProprioceptors: also react to stimuli from within the body, providing sensation of body position and muscle tone & movement

Connective tissue components of a peripheral nerveEndoneurium: includes loose connective tissue surrounding each individual nerve fiberPerineurium: includes specialized connective tissue surrounding each nerve fascicleEpineurium: includes dense regular connective tissue that surrounds a peripheral nerve and fills the spaces between nerve fascicles

Connective tissue of CNS3 sequential connective tissue membranes, the meninges, cover the brain and spinal cord:Dura mater: outermost layer; relatively a thick sheet of dense connective tissueArachnoid layer: lies beneath the duraPia mater: is a delicate layer resting directly on the surface of the brain and spinal cord

Response of neurons to injuryDegeneration the portion of a nerve fiber distal to a site of injury degenerates because of interrupted axonal transport.The cell body of an injured nerve swells, its nucleus moves peripherally and there is loss of Nissl substance (chromatolysis)Scar formationIn the PNS, connective tissue & Schwann cells form a scar tissue in the gap between the ends of a severed or crushed nerve

Response of neurons to injuryRegenerationIn the PNS, Schwann cells divide and develop cellular bands that bridge a newly formed scarIf physical contact is reestablished between a motor neuron and its muscle, function is usually reestablished