Nervous System

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Nervous System is made up of the

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brain and spinal cord and the nerves

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The nervous system can be divided in various ways

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structure, direction of information flow, or control of effectors

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CNS

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structural and functional centers of the entire nervous system. Consists of the brain and spinal cord. The CNS integrates incoming pieces of sensory information evaluates the information and initiates an outgoing response

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PNS

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consists of the nerve tissues that lie in the periphery or outer regions of the nervous system. Nerves that originate from the brain are called cranial nerves and spinal cord nerves are spinal nerves

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Afferent division

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of the nervous system consists of all the incoming sensory or afferent pathways. Afferent (carry toward)

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Efferent division

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of the nervous system consists of all the outgoing motor or efferent pathways. Efferent (carry away)

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Some pathways of the somatic nervous system (SNS)

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carry information to the somatic effectors, which are skeletal muscles

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Somatic Sensory Division

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The SNS also includes the afferent pathways making up the Somatic Sensory Division that provides feedback from the somatic effectors

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Pathways of the autonomic nervous system (ANS) carry information to the autonomic or visceral, effectors which are

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smooth muscles, cardiac muscles, glands adipose tissue and other involuntary tissue

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The efferent pathways of the (ANS) can be divided into the

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sympathic division and the parasympathic divisions

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Sympathic

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made up of pathways that exit the middle portions of the spinal cord and is involved in preparing the body to deal with immediate threats to the internal environment. It produces the fight-or-flight

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Parasympathic

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exit at the brain and lower portions of the spinal cord and coordinate the body’s normal resting activities. Sometimes called the rest-and-repair

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Two main types of cells compose the nervous system

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neurons and glia

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Neurons

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are excitable cells that conduct the impulses that make possible all nervous system functions. They form the wiring of the nervous information circuits

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Glia

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or glial cells do not usually conduct information themselves but support the function of neurons in various ways. Glia means glue

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5 major types of glia

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4 are located in the CNS – Astocytes, Microglia, Ependymal cells & Oligodendrocytes. The 5th is located in the PNS – Schwann Cells/satellite cells

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Astocytes

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star shaped glia are the largest and most numerous type of glia. Their long delicate “points” extend through brain tissue attaching to both neurons and the tiny blood capillaries of the brain call “stars of the nervous system”. They feed the neuron by picking up glucose from the blood converting it to lactic acid and passing it along to the neurons to which they connected

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Because webs of Astocytes form tight sheaths around the brains blood capillaries they help form the

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Blood-Brain Barriers (BBB)

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BBB

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is a double barriers made up of Astocytes “feet” and the endothelial cells that make up the walls of capillaries

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Small molecules

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oxygen, carbon dioxide, water, alcohol diffuse rapidly through the barriers to reach brain neurons and other glia

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Microglia is

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small usually stationary cells found in the CNS. In inflamed or degenerating brain tissue it enlarges move about and carry phagocytosis

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Ependymal cells

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are glia that resembles epithelial cells, forming thin sheets that line fluid-filled cavities in the brain and spinal cord. Some take part in producing fluid that fills these spaces. Others have cilia that help keep the fluid circulating within the cavities.

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Oligodendrocytes

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smaller than Astocytes and have fewer processes means cells with few branches. They help hold nerve fibers together and also serve another and more importantly their function is to produce the fatty Myelin Sheath around nerve fibers on the CNS.

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Schwann cells

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are found in the PNS. Here they serve as the functional equivalent of the Oligodendrocytes. Supporting nerve fibers and sometimes forming a myelin sheath around them.

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The Myelin Sheath

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is formed by layers of Schwann cells membrane containing the white fatty substance Myelin microscopic gaps in the sheath between Schwann cells are called NODES OF RANVIER or simple myelin sheath gaps

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Schwann cells

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nucleus and cytoplasm are squeezed to the perimeter to form the neurilemma which is essential to the regeneration of injured nerve fibers

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Schwann cells – are known

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as neurolemmocytes

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The myelin sheath along with the neurilemma is called

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neuronal sheath

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Nerve fibers with many Schwann cells forming thick myelin sheaths are called

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myelinated fibers or white fibers

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When several nerve fibers are held by a single Schwann cell that does not wrap around them to form a thick myelin sheath the fibers are called

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unmyelinated fibers or gray fibers.

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because dendrites and axon are threadlike extensions form neurons cell body they are often called

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nerve fibers

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The cell body or peribaryon – contains

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nucleus, cytoplasm and various organelles found in other cells, for example mitochondria and a goli apparatus

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Some protein molecules in these vesicles (ER-Goli Apparatus) are needed for the transmission of nerve signals from neuron to another. Such proteins are called

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neurotransmitters

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Dendrites

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receive stimuli and conduct electrical signals toward the cell body and or axon of the neuron

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Axon

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of the neuron is a single process that usually extends from a tapered portion of the cell body call the Axon Hillock. The axon is sometimes covered by a fatty layer called myelin sheath conducts nerve impulses away from the cell body of the neuron.

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The distal tips of the axon form branches called

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telodendria that each terminate in the synaptic knob. Each synaptic knob contains mitochondria and numerous vesicles

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Nerve fibers are both

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axon and dendrites

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The axon hillock acts as a summation zone

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by adding together all the nerve impulses arriving from the cell body and dendrites.

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axon is the conduction zone because

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it conditions the nerve impulse from the axon hillock all the way to the end of the neuron

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Telodendria and synaptic knob is the

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output zone

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Gaps in the myelin sheath

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NODES OF RANVIER

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Axonal transport

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process of transporting vesicles small organelles and other structures along pathways inside the axon of a neuron

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3 types of neurons

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classified according to the direction in which they conduct impulses: afferent neurons, efferent neurons & interneuron’s

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Afferent neurons

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transmit nerve impulses to the spinal cord or brain

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Efferent neurons

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transmit nerve impulses away from the brain spinal cord to or toward the muscle glands

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Reflex Arc

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signal conduction route to and from the CNS—The most common form is the 3 neuron are. It consists of an afferent neuron, an interneuron, and an efferent neuron

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Afferent or sensory neurons concuct signals to the

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CNS from sensory receptors in the PNS

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Efferent or motor neurons conduct signals from

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the CNS to effectors

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2 – neuron arc is simplest form consists of

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afferent and efferent neurons

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Synapse

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nerve signals transmitted from one neuron to another