Lab quiz 6

Correctly identify the sensory (afferent) neuron, association neuron (interneuron), and motor (efferent) neuron in the figure on page 161 of your lab manual. a) Which of these neuron types is/are unipolar? b) Which is/are most likely multipolar?

a) Sensory neuron. b) Interneuron and motor neuron

What is a nerve?

A bundle of neuron fibers or processes that extends to and/or from the CNS and visceral organs or structures of the body periphery such as skeletal muscles, glands, and skin.

Describe how the Schwann cells form the myelin sheath and the neurilemma encasing the nerve processes. (You may want to diagram the process.)

Axons in the peripheral nervous system are myelinated by special supporting cells called Schwann cells, which wrap themselves tightly around the axon in jelly-roll fashion so that when the process is completed, a tight core of plasma membrane material called the myelin sheath encompasses the axon. The Schwann cell nucleus and the bulk of its cytoplasm end up just beneath the outermost portion of its plasma membrane. This part of the Schwann cell which is external to the myelin sheath, is referred to as the neurilemma.

How is one-way conduction at synapses ensured

Neurons have only one axon that carries impulses away from the nerve cell body toward the synapse.

State the location of each of the following connective tissue coverings: a.) endoneurium perineurium epineurium What is the value of the connective tissue wrappings found in a nerve Define mixed nerve:

Question 2 Answer: a. Surrounds each nerve fiber b. Surrounds a group of nerve fibers c. Surrounds the bundles of fibers called fascicles Question 3 Answer: The connective tissue wrappings help insulate the nerve. Question 4 Answer: Nerves carrying both sensory (afferent) and motor (efferent) fibers

The cellular unit of the nervous system is the neuron. What is the major function of this cell type?

The major function of the neuron is to transmit messages (nerve impulses) from one part of the body to another.

What anatomical characteristic determines whether a particular neuron is classified as unipolar, bipolar, or multipolar?

The number of processes attached to the cell body determines the structural class of a neuron.

The supporting cells, or neuroglia, have numerous functions. Name three.

The supporting cells act as phagocytes, protect and myelinate, and act as a selective barrier between the capillary blood supply and the neurons.

Despite its complexity, nervous tissue is made up of just two principal cell populations: neurons and supporting cells, also called neuroglia or glial cells. CNS made up of brain and spinal cord Neuroglia (Glial Cells) CNS-several cell types that serve the needs of the neurons by acting as phagocytes and by protecting and myelinating the delicate neurons. Also act as a selective barrier between the capillary blood supply and the neurons PNS-schwann sells, which insulate nerve fibers Neurons specialized to transmit messages (nerve impulses) from one part of the body to another cell body, from which slender processes extend. make up the gray matter of the nervous system. Ganglia clusters of neuron cell bodies outside the CNS Nuclei clusters of neuron cell bodies that are typically found in the CNS Tracts white matter formed from neuron processes running through the CNS Dendrites neuron processes that conduct electrical currents toward the cell body Axons (Nerve Fibers) neuron processes that carry impulses away from the nerve cell body Neurons have only one axon (which may branch into collaterals) A neuron is excited by other neurons when their axons release neurotransmitters close to its dendrites or cell body that bind to receptors and create an electrical current that travels down its axon. When an impulse reaches the axon terminals, some of the synaptic vesicles fuse with the plasma membrane and then rupture and release neurotransmitter into the synaptic cleft. The neurotransmitter then diffuses across the synaptic cleft to bind to membrane receptors on the next neuron, initiating the action potential. Specialized synapses in skeletal muscle are called neuromuscular junctions Axon Terminals Many small structures of an axon that store neurotransmitter chemicals in tiny vesicles Synaptic Cleft tiny gap that separates the axon terminal from the cell body or dendrites of the next neuron Myelinated Fibers long nerve fibers that are covered with a fatty material called myelin Schwann Cells special neuroglia that myelinated axons in the peripheral nervous system. Wrap themselves tightly around the axon in jelly-roll fashion so that when the process is completed a tight core of plasma membrane material called the myelin sheath encompasses the axon. Outer Collar of the Perinuclear Cytoplasm neurilemma. the part of the schwann cell external to the myelin sheath Myelin Sheath Gaps/Nodes of Ranvier gaps/indentations in the myelin sheath. The Schwann cell nucleus and the bulk of its cytoplasm end up just beneath the outermost portion of its plasma membrane. This part of the Schwann cell external to the myelin sheath is referred to as the neurilemma. Because the myelin sheath is formed by many individual Schwann cells, it has gaps or indentations called nodes of Ranvier Unipolar Neurons one very short process, which divides into peripheral and central processes, extends from the cell body. functionally, only the most distal portions of the peripheral process act as dendrites; the rest acts as an axon along with the central process. Nearly all neurons that conduct impulses toward the CNS are this type Bipolar Neurons have two processes-one axon and one dendrite-attached to cell body. quite rare; they typically serves as sensory receptor cells in some special sense organs Multipolar Neurons Many processes come from the cell body-they are all dendrites except for one axon. Most neurons in the brain and spinal cord and those whose axons carry impulses away from the CNS fall into this category Sensory/Afferent Neurons neurons carrying impulses from the sensory receptors in the internal organs to in the skin. Dendritic endings often bear specialized receptors that are stimulated by specific changes in their immediate environment. Cell bodies are always found in a ganglion outside the CNS. Typically unipolar Motor/Efferent Neurons neurons carrying activating impulses from the CNS to the viscera and/or body muscles and glands. Most often multipolar, cell bodies almost always located in CNS Interneurons/association neurons situated in pathways that connect sensory and motor neurons. cell bodies are always located within the CNS, multipolar Within a nerve, each fiber is surrounded by a delicate connective tissue sheath called an endoneurium, which insulates it from the other neuron processes adjacent to it. Groups of fibers are bound by a coarser connective tissue, called the perineurium, to form bundles of fibers called fascicles. Finally, all the fascicles are bound together by a tough, white, fibrous connective tissue sheath called the epineurium, forming the cordlike nerve Major physiological properties of neurons 1) ability to respond to stimuli and convert them into nerve impulses 2) ability to transmit the impulses to other neurons, muscles, or glands. Resting Membrane Potential a neuron in this state is polarized. the outside surface of the membrane is slightly more positively charged than the inside surface. Main intracellular cation is K+ and Na+ are found in greater concentration in extracellular fluids. Depolarization when the inside of the membrane becomes less negative and the outside becomes less positive. happens when he neuron is activated by a threshold stimulus and the membrane briefly becomes more permeable to sodium, which comes into the cell. Repolarization reestablishes the resting membrane potential. happens when membrane permeability changes, and K+ permeability increases; K+ rushes out of the cell so that the external membrane surface is again positive relative to the internal membrane Nerve Impulse propagation of the action potential in neurons Endoneurium delicate connective tissue sheath which insulates each fiber from the other neuron processes adjacent to it Perineurium a coarser connective tissue that binds groups of fibers to form bundles of fibers called fascicles epineurium a tough, white fibrous connective tissue sheath that binds together all the fascicles Mixed Nerves nerves carrying both sensory and motor fibers. all spinal nerves Sensory/Afferent Nerves nerves that carry only sensory processes and conduct impulses only toward the CNS a few of the cranial nerves Motor Nerves carry only motor fibers. the ventral roots of the spinal cord.

The supporting cells in the CNS (central nervous system: brain and spinal cord) include several cell types that serve the needs of the neurons by acting as phagocytes and by protecting and myelinating the delicate neurons. In addition, they act as a selective barrier between the capillary blood supply and the neurons. The most important neuroglia in the peripheral nervous system (PNS: neural structures outside the CNS) are Schwann cells. Schwann cells insulate nerve fibers. The cytoplasm is riddled with neurofibrils, elements of the neuron, which have a support and intracellular transport function, and an elaborate rough endoplasmic reticulum called Nissl bodies or chromatophilic substance. In a resting neuron, there is an excess of positive ions outside the cell, with Na+ as the main extracellular fluid ion and K+ as the predominant intracellular ion. The plasma membrane has a low permeability to Na+. (b) Depolarization—local reversal of the resting membrane potential. Application of a stimulus changes the membrane permeability, and Na+ ions are allowed to diffuse rapidly into the cell. The interior face of the membrane becomes less negative (moves toward positive). (c) Generation of the action potential or nerve impulse. If the stimulus is strong enough, the depolarization wave spreads rapidly along the entire length of the membrane. (d) Repolarization—reestablishment of the resting membrane potential. The negative charge on the internal plasma membrane surface and the positive charge on its external surface are reestablished by diffusion of K+ ions out of the cell, proceeding in the same direction as in depolarization. (e) The original ionic concentrations of the resting state are restored by the Na+-K+ pump. (f) A tracing of an action potential.

1. reversal of the resting potential owing to an influx of sodium ions = depolarization 2. period during which potassium ions are diffusing out of the neuron = repolarization 3. transmission of the depolarization wave along the neuronal membrane = action potential 4. mechanism that restores the resting membrane voltage and intracellular ionic concentrations = sodium-potassium pump

Would a substance that decreases membrane permeability to sodium increase or decrease the probability of generating a nerve impulse? Question 1 Answer: It would decrease the probability 2. Why don't the terms depolarization and action potential mean the same thing? (Hint: Under what conditions will a local depolarization not lead to the action potential? Question 2 Answer: If the stimulus is of less than threshold intensity, depolarization is limited to a small area of the membrane, and no action potential is generated