Chapter 17: Nervous Tissue

Neurology

this is the branch of medical science that deals with the normal functioning and disorders of the nervous system.

Neuronal Circuits: CONVERGING CIRCUIT

*Convergence - in this arrangement, several presynaptic neurons synapse with a single postsynaptic neuron. permits more effective stimulation of the postsynaptic neuron. - in this circuit, the postsynaptic neuron receives nerve impulses from several different sources. -Ex.) a single motor neuron that synapses w/ skeletal muscles at neuromuscular junctions receives input from several pathways that originate in different brain regions.

Neuronal Circuits: DIVERGING CIRCUIT

*Divergence - is an arrangement that permits one presynaptic neuron to influence several postsynaptic neurons at the same time. - in this type of circuit the nerve impulse from a single presynaptic neuron causes the stimulation of increasing numbers of cells along the circuit. - Ex.) a small # of neurons in the brain that govern a particular body movement stimulate a much larger number of neurons in the spinal cord.

Peripheral Nervous System (PNS)

- Anatomical Organization - is composed of all nervous structures outside the CNS, such as cranial nerves and their branches, spinal nerves and their branches, ganglia, enteric plexuses, and sensory receptors. -its structures link all parts of the body to the CNS.

Spinal Nerves

- Location: PNS - one of 31 pairs of nerves that originate on the spinal cord from posterior and anterior roots.

Somatic Nervous System (SNS)

- Of the PNS - consists of sensory neurons, called somatic sensory neurons, that convey information from sensory receptors in the skin, skeletal muscles, joints, and for the special senses (vision, hearing, taste, scent) to the CNS. - the somatic sensory pathways are involved in the input of info to the CNS for processing/integration. *Somatic Motor Neurons - convey info from the CNS to skeletal muscle, the somatic motor pathways are involved in output of info from CNS that results in muscle contraction.

Synaptic Vesicles

- are membrane enclosed sacs in a synaptic end bulb that store neurotransmitter's.

Neroglia of the CNS: OLIGODENDROCYTES

- are responsible for forming and maintaining the myelin sheath around CNS axons.

Parts of a Neuron: CELL BODY (Perikaryon)

- contains a nucleus surrounded by cytoplasm that includes typical cellular organelles such as lysosomes, mitochondira, and Golgi Complex. -the cytoskeleton includes Neurofibrils, composed of bundles of intermediate filaments that provide cell shape and support. -many neurons also contain Lipofuscin, which is a pigment that occurs as clumps of yellowish brown granules in the cytoplasm.

Neuronal Circuits: PARALLEL AFTER-DISCHARGE CIRCUIT

- in this circuit, a single presynaptic cell stimulates a group of neurons, each of which synapses w/ a common postsynaptic cell. -these circuits may be involved in precise activities such as mathematical calculations.

Neuronal Circuits: REVERBERATING CIRCUIT

- in this circuit, the incoming impulse stimulates the first neuron, which stimulates the 2nd, which stimulates the 3rd, and so on. Branches from later neurons synapse w/ earlier ones. -this arrangement sends impulses back through the circuit again & again. - Ex.) breathing, sleeping, waking up, coordinated muscular activities.

Functional Organization: MOTOR FUNCTION

- in this function, once sensory info is integrated, the nervous system may elicit an appropriate motor response. *Motor/Efferent Neurons - conduct impulses from the brain towards the spinal cord or out of the brain and spinal cord into cranial or spinal nerves to effector's that may be either muscles or glands.

Neuronal Circuits: SIMPLE SERIES CIRCUIT

- in this type of neuronal circuit, a presynaptic neuron stimulates a single postsynaptic neuron. the second neuron then stimulates another, and so on.

Neurotransmitter

- is a molecule released from a synaptic vesicle that excites or inhibits postsynaptic neurons, muscle fibers, or gland cells.

Autonomous Nervous System (ANS)

- is of the PNS. - also has sensory and motor components. - Visceral sensory (afferent) and visceral motor (efferent) neurons. Autonomic motor neurons conduct nerve impulses from the central nervous system to smooth muscle, cardiac muscle, and glands. - So named because this part of the nervous system was thought to be self governing/spontaneous. - Motor Part of ANS consists of 2 branches: 1.) Sympathetic Division 2.) Parasympathetic Division

ENTERIC NERVOUS SYSTEM

- is the part of the nervous system that is embedded in the submucosa and muscularis of the gastrointestinal tract (GI). -secretes hormones such as Gastrin, CHolecystokinin, peptide, secritin etc. - called the "brain of the gut".

Autonomic Nervous System: PARASYMPATHETIC DIVISION

- one of the two subdivisions of of ANS, having cell bodies of perganglionic neurons in the brain stem and in lateral gray horn of the sacral portion of spinal cord. -primarily concerned with activities that conserve body energy.

Functional Organization

- the nervous system carries out a complex array of tasks. Allows us to sense various smells, produce speech, memory, also provides signals that control body movements, and regulates the operation of viscera. - Can be grouped into 3 basic functions: 1.) Sensory (input) 2.) Integrative (control) 3.) Motor (output)

Parts of a Neuron: AXON

- the usually single, long processes of a nerve cell that propagates a nerve impulse toward the axon terminals. - is a long, thin, cylindrical projection that often joins the cell body at the Axon Hillcock. *Axoplasm - is the cytoplasm of an axon. *Axolemma - is the plasma membrane that surrounds axoplasm. *Axon Terminals(Telodendria) - the axon and its collateral's end by dividing into these fine processes.

Neuronal Circuits

- these are billions of neurons organized into complicated networks, each of which is a functional group of neurons that processes a specific kind of information.

Synaptic End Bulbs

- these are bulb shaped structures at the distal end of an axon that contain synaptic vesicles.

Ren shaw Cells

- these are cells in the spinal cord.

Structural Classification of Neurons: UNIPOLAR NEURONS

- these are sensory neurons that begin in the embryo as bipolar neurons. - during development, axon and dendrite fuse into a single process that divides into two branches. both branches have the characteristic structure and function of an axon. - are long cylindrical processes that propagate action potentials. - always sensory neurons.

Neuroglia of the CNS: ASTROCYTES

- these are star shaped cells w/ many processes, and are largest and most numerous of the neuroglia. - their functions include: 1.) contain microfilaments to support neurons 2.) processes of these wrap around blood capillaries and secrete chemicals that help in the blood-brain barrier. 3.) secrete chemicals that regulate the growth, migration, and interconnections among neurons in the brain. 4.) help maintain appropriate chemical environment for the generation of nerve impulses. 5.) play role in learning and memory by influencing the formation of neural synapses.

Parts of a Neuron: DENDRITES

- these are the receiving or input portions of a neuron. -usually are short, tapering, and highly branched: the greater the branching, the greater the surface area of the neuron for receiving synaptic communication. -form a tree shaped array of processes extending from cell body.

Pyramidal Cells

- these cells are found in the cerebral cortex of the brain, which have a pyramid shape to their cell bodies.

Neurons/Nerve Cells

- these cells process Electrical Excitability, which is the ability to respond to a stimulus and convert it into a nerve impulse. *Nerve Impulse - is an electrical signal that travels along surface of the membrane of a neuron. - these provide most of the unique functions of the nervous system, such as sensing, thinking, remembering, controlling muscle activity, and regulating glandular secretions.

Neuroglia of the PNS: SATELLITE CELLS

- these flat cells surround the cell bodies of neurons of the PNS ganglia. - provide structural support, regulate exchange of materials b/w neuronal cell bodies and interstitial fluid.

Neuroglia of the CNS: EPENDYMAL CELL

- these neuroglia are cuboidal to columnar cells that possess microvilli and cilia. - they produce, monitor, and assist in the circulation of cerebrospinal fluid. - also form the blood-cerebrospinal fluid barrier.

Neuroglia of the PNS: SCHWANN CELLS

- these neuroglia cells encircle PNS axons. - they form the myelin sheath around axons, however, they only myelinate a single axon. - they also participate in axon regeneration.

Neuroglia Of The PNS

- these neuroglia completely surround axons and cell bodies. - the two type of glial cells in PNS are: 1.) Schwann Cells 2.) Satellite Cells

Nueroglia of the CNS: MICROGLIA

- these neuroglia originate in red bone marrow and migrate into the CNS as it develops. - function as phagocytes - they remove cellular debris formed during normal development of the nervous system and phagocytize microbes and damaged nervous tissue.

NEUROGLIA

- these support, nourish, and protect neurons, and maintain the interstitial fluid that bathes them. They also continue to divide throughout an individuals lifetime. - they constitute about half of the volume of CNS. - in the CNS they include: *Astrocytes *Oligodendrocytes *Microglia *Ependymal Cells -in the PNS they include: * Schwann Cells *Satellite Cells

Structural Classification of Neurons: BIPOLAR NEURONS

- these type of neurons have on main dendrite and one axon. - are found in the retina of the eye, in the inner ear, and in the olfactory area of the brain.

Structural Classification of Neurons: MULTIPOLAR NEURONS

- these type of neurons usually have several dendrites and one axon. - most neurons in the brain and spinal cord are of this type, as well as all motor (efferent) neurons.

Postsynaptic Cell

- this is a nerve cell or effector that carries a nerve impulse away from the synapse. - it is the cell that receives the signal.

Nervous Tissue Histology

- this is a vascular tissue comprised of two types of cells: *Neurons *Neuroglia - these cells combine in a variety of way in different regions of the nervous

Parts of a Neuron: NERVE FIBER

- this is any neuronal process (extension) that emerges from the cell body of a neuron. -most neurons have two kinds of processes, multiple Dendrites and a single Axon.

Presynaptic Neuron

- this is nerve cell that carries a nerve impulse toward a synapse. - it is the cell that sends a signal.

Functional Organization: INTEGRATIVE FUNCTION

- this nervous system function processes sensory information by analyzing and storing some of it and by making decisions for appropriate responses. * Perception - this function is the conscious awareness of sensory stimuli and occurs in the brain. *Interneurons - these neurons participate in integration, interconnect with other neurons, with axons that extend only a short distance and contact nearby neurons in the brain or spinal cord. Make up the majority of neurons in the body.

Cental Nervous System (CNS)

-Anatomical Organization - is composed of the brain and spinal cord. -processes many different kinds of incoming sensory information. Is the source of thoughts, emotions, memories. Most nerve impulses that stimulate other systems originate here. *BRAIN - is enclosed and protected by the skull in the cranial cavity and contains about 100billion nuerons. *SPINAL CORD - enclosed, a mass of nervous tissue located in vertebral canal from which 31 pairs of spinal nerves originate.

Nerve

-Location: PNS - is a bundle of hundreds to thousands of axon's (nerve fibers) plus associated connective tissue and blood vessels that lies outside the brain and spinal cord. - each of these follows a defined path and serves a specific region of the body.

Cranial Nerves

-located in the PNS. - numbered 1-12, and emerge from the base of the brain. -these supply sensory sensory and motor neurons to the head, neck, part of the trunk, and viscera of the thorax and abdomen. - each is designed by a roman numeral and a name.

Autonomic Nervous System: SYMPATHETIC DIVISION

-one of the two subdivisions of the autonomic nervous system, having cell bodies of preganglionic neurons in the lateral gray columns of the thoracic segment and first few lumbar segments in spinal cord. - primarily concerned with processes involving expenditure of energy. -helps support so-called "fight or flight" responses. - is the major regulator of smooth muscle of the cardiovascular system.

Functional Organization: SENSORY FUNCTION

-sensory receptors detect internal stimuli, such as an increase in blood pressure, and external stimuli, such as a raindrop landing on your arm. * Sensory/Afferent Neurons - these neurons carry this sensory info to the brain and spinal cord through cranial and spinal nerves.

Purkinje Cells

-these cells are found in the cerebellum.

Nervous System

-this is one of the smallest and yet most complex of the 11 body systems. -Is a highly organized network of billions of neurons, and neuroglia. - Includes the brain, cranial nerves and their branches, ganglia, enteric plexuses, and sensory receptors.