A&P1 Chapter 11 Short Answer

Explain the difference between and EPSP and an IPSP. What specifically determines whether an EPSP or IPSP will be generated at the post synaptic membrane?

EPSP: local depolarization of the postsynaptic membrane which brings the neuron closer to AP threshold. IPSP: local hyperpolarization of the postsynaptic membrane which drive the neuron away from AP threshold.

Since at any moment a neuron is likely to have thousands of neurons releasing neurotransmitters at its surface, how is neuronal activity (to fire or not to fire) determined?

ESPS's can summate to influence the activity of a postsynaptic neuron. They do this through temporal or spatial summation.

Since all Aps generated by a given nerve fiber have the same magnitude, how does the CNS " know" whether a stimulus is strong or weak?

Strong stimuli generate nerve impulses more often in a given time interval than do weak stimuli.

During a neurobiology lecture a professor repeatedly refers to group A and group B fibers, absolute refractory period, and myelin sheath gaps. Define these terms.

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Briefly describe the three stages of neuron development.

1. They proliferate. 2. Neuroblasts, potential neurons, become amitotic and migrate externally in their characteristic positions. 3. Neuroblasts sprout axons to connect with their functional targets and in so doing become neurons.

Describe the events that must occur to generate an AP. Relate the sequence of changes in permeability to changes in the ion channels, and explain why the AP is an all or non phenomenon.

As local currents depolarize the axon membrane, Na+ channels open. Na+ channels has two gates, a voltage sensitive activation gate and an inactivation gate. Depolarization opens and then inactivates sodium channels. This influx of positive charge depolarizes that local patch which opens more Na+ channels. When depolarization reaches a critical threshold (between -55 to -50mV), depolarization becomes self-generating. During repolarization, Na+ channels are inactivating and K+ channels open. During hyperpolarization, some K+ channels remain open and Na+ channels reset.

Explain both the anatomical and functional divisions of the nervous system. Include the subdivisions of each.

Central nervous system (brain and spinal cord); peripheral nervous system (cranial & spinal nerves, ganglia). Peripheral nervous system can be divided into sensory (afferent) and motor (efferent). Motor division can be divided into somatic & autonomic nervous system. The ANS can be divided into sympathetic and parasympathetic. (p. 390)

The effects of neurotransmitter binding are very brief. Explain.

Channel linked receptors mediate direct neurotransmitter action. It directly opens chemically gated ion channels. G Protein Linked Receptors are slower because the neurotransmitter acts indirectly to create a response.

What factors appear to guide the outgrowth of an axon and its ability to make the "correct" synaptic contacts?

Growth cone which is the growing tip of an axon. Extracellular and cell surface adhesion proteins provide anchor points for the growth cone. Neurotropins are chemicals that signal to the growth cone and directs it. Throughout growth n development, neurotrophic factors such as nerve growth factor (NGF) must be present to keep the neuroblast alive.

Contrast unipolar, bipolar, and multipolar neurons structurally. Indicate where each is most likely to be found.

Multipolar neurons have three or more processes, one axon the rest dendrites. They are most common with 99% of neurons in this class. It is the major neuron type in CNS. Bipolar neurons have two processes an axon and dendrite that extend from opposite sides of the cell body. They are found in special sense organs such as the retina. Unipolar neurons have a single short process that divides into proximal and distal branches. They are also called pseudounipolar neurons. They are mainly found in the ganglia of the PNS where they function as sensory neurons.

What is myelin? How does the myelination process differ in the CNS and PNS?

Myelin is a white, fatty, segmented sheath that covers the axon. Myelin protects and electrically insulates fibers. It also increases the transmission speed of nerve impulses. In the PNS, myelin sheaths are formed by Schwann cells. In the CNS, oligodendrocytes form the myelin sheath. Unlike a Schwann cell, which only forms one segment of a myelin sheath, an oligodendrocyte has multiple flat processes that can coil around as many as 60 axons at the same time. CNS myelin sheaths also lack outer collar of perinuclear cytoplasm. White matter in brain and spinal cord are myelinated.

Distinguish between serial and parallel processing.

Serial processing: the input travels along one pathway to a specific destination. Parallel processing: input travels along several different pathways to be integrated in different CNS regions.

Describe the composition and function of the cell body. How are axons and dendrites alike? In what ways do they differ?

The neuron cell body has a spherical nucleus with a nucleolus surrounded by cytoplasm. It is also called the perikaryon or soma. It is the major biosynthetic center of a neuron and contains usual organelles needed to synthesize proteins and other chemicals. Axons and dendrites are similar in that they have the same organelles except the axon lacks rough ER and Golgi apparatus which is typically used for protein synthesis and packaging. An axon depends on the cell body for proteins and membrane components and efficient transport mechanisms to distribute them. Axons quickly decay if cut or damaged. Each neuron has a single axon but has several dendrites. Dendrites convey incoming messages called graded potentials. Axons generate nerve impulses and transmits them.

What is the polarized membrane state? How is it maintained? Note the relative roles of both passive and active mechanisms.

The polarized membrane state is the resting membrane potential in which there is a potential difference in the resting neuron. Its value varies from -40 mV to -90 mV, on average -70mV. It is maintained through two factors: differences in the ionic composition of intracellular and extracellular fluids and the differences in permeability of the plasma membrane to those ions. This is through K+ loss through abundant leakage channels which establishes a negative membrane potential. Na+ entry through few leakage channels reduces the negative membrane potential slightly. Na+-K+ pumps maintain the concentration gradients which result in a resting membrane potential.