Chapter 11 Objectives Anatomy A&P

Describe the mechanism of an IPSP.

An IPSP is received when an inhibitory presynaptic cell, connected to the dendrite, fires an action potential. The IPSP signal is propagated down the dendrite and is summed with other inputs at the axon hilllock. The IPSP decreases the neurons membrane potential and makes more unlikely for an action potential to occur.

Describe the mechanism of an EPSP

An excitatory postsynaptic potential (EPSP) occurs when sodium channels open in response to a stimulus. The electrochemical gradient drives sodium to rush into the cell. When sodium brings its positive charge into the cell, the cell's membrane potential becomes more positive, or depolarizes.

Explain the step by step process of the generation of a graded potential

A. Depolarization: A small patch of the membrane depolarizes. B. Depolarization spreads: Opposite charges attract each other. This creates local currents that depolarize adjacent membrane areas, spreading the wave of depolarization. C. Membrane potential decays with distance: Because current is lost through the "leaky" plasma membrane, the voltage declines with distance from the stimulus (the voltage is decremental) Consequently, graded potentials are short distance signals.

Astrocytes

Astrocytes are the most abundant CNS Neuroglia. The Astrocyte has many processes that attach to capillaries and neurons helping determine capillary permeability. They guide migration of young neurons and formation of synapses (junctions) between neurons. Good at mopping up leaked potassium ions and recapturing and recycling released neurotransmitters.

Describe the effect of stimulus intensity on a graded potential.

Graded potentials are called "graded" because their magnitude varies directly with stimulus strength. The stronger the stimulus, the more the voltage changes and the farther the current flows.

Describe the structure and function of ligand-gated channels.

Graded potentials are produced by stimuli opening a gated channel and are local potentials.

Differentiate between graded potentials and action potentials in terms of stimulus, longevity, and distance.

Graded potentials are usually incoming signals operating over short distances that have variable (graded) strength. Action potentials are long distance signals of axons that always have the same strength.

Explain how a converging circuit operates and give an example

Has many outputs and one input, it is a concentrating circuit. An example is different sensory stimuli can all elicit the same memory.

Explain how a diverging circuit operates and give an example.

Has one input and many outputs, it is an amplifying circuit. An example is a single neuron in the brain can activate 100 or more motor neurons in the spinal cord and thousands of skeletal muscle fibers.

Protein and membrane making machinery

Neuron cell bodies (not axons) have the organelles needed to synthesize proteins-rough endoplasmic reticulum (ER), free ribosomes and Golgi apparatus. The rough ER (chromatophilic substance-chromatophilic=color loving) or Nissl bodies, stains darkly with basic dyes.

Describe the distribution of ions (Na+, K+, Ca2+, and Cl-) and proteins across a membrane at rest.

Results from diffusion and active transports of ions. Mainly K+ Na+-K+ pump is important here. Na+ is high outside the cell and K+ is high inside the cell (3 Na+ out and 2 K+ in)

Spinal Nerves

Spinal nerves carry impulses to and from and the spinal cord.

Identify where graded potentials are formed compared to action potentials.

These are short lived, localized changes in membrane potential, usually in dendrites or the cell body.

Oligodendrocytes

Though they also branch, oligodendrocytes have fewer process than astrocytes. They line up along the thicker nerve fibers in the CNS and wrap their processes tightly around the fibers, producing an insulating coverage called a myelin sheath.

List and describe the three functions of the nervous system.

-Sensory input: The Nervous System uses its millions of sensory receptors to monitor changes occurring both inside and outside of the body. -Integration: The Nervous System processes and interprets sensory input and decides what should be done at each moment. -Motor output: The Nervous System activates effector (exiting) organs-muscles and glands-to cause a response.

Describe the step-by-step process by which a neural signal crosses a chemical synapse.

1. Action potential arrives at axon terminal. 2. Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal. 3. Ca2+ entry causes synaptic vessels to release neurotransmitter by exocytosis. 4. Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane. 5. Binding of neurotransmitter opens ion channels, resulting in graded potentials. 6. Neurotransmitter effects are terminated by reuptake through transport proteins, enzymatic degradation, or diffusion away from the synapse.

Identify and explain the structure of the 3 types of neurons based on structure.

1. Multipolar neurons (polar=end, pole) have 3 or more processes-one axon and the rest dendrites. They are the most common neuron type in humans, with more than 99% of neurons in this class. Multipolar neurons are the major neuron type in the CNS. 2. Bipolar neurons have two processes-an axon and a dendrite-that extend from opposite sides of the cell body. These rare neurons are found in some of the special sense organs such as the retina of they and in the olfactory mucosa. 3. Unipolar neurons have a single short process that emerges from the cell body and divides T-like into proximal and distal branches. Unipolar neurons are more accurately called pseudounipolar neurons because they originate as bipolar neurons.

Identify 3 special characteristics of neurons that are different from other cells

1. Neurons have extreme longevity. Given good nutrition, they can function optimally for a lifetime. 2. Neurons are amitotic. As neurons assume their roles as communicating links of the nervous system, they lose their ability to divide. Neurons cannot be replaced if destroyed. There ARE exceptions to this rule such as olfactory epithelium and some hippocampal regions of the brain can produce new neurons throughout life. 3. Neurons have exceptionally high metabolic rate and require continuous and abundant supplies of oxygen and glucose. They CANNOT survive more than a few moments without oxygen.

Describe the mechanism of G protein-linked receptors in chemical synapses.

1. Neurotransmitter (1st messenger) binds and activates receptor 2. Receptor activates G Protein 3. G Protein activates adenylate cyclase 4. Adenylate cyclase converts ATP to cAMP (2nd messenger) 5A. cAMP changes membrane permeability by opening or closing ion channels. 5B. cAMP activates enzymes 5C. cAMP activates specific genes

Explain the step-by-step process of the generation of an action potential.

1. Resting stage: No ions move through the voltage-gated channels. 2.Depolarization: is caused by Na+ flowing into the cell. 3. Repolarization: is caused by K+ flowing out of the cell. 4.Hyperpolarization: is caused by K+ continuing to leave the cell.

Identify and explain the structure of the 3 types of neurons based on function.

1. Sensory, or afferent, neurons transmit impulses from sensory receptors in the skin or internal organs toward or into the CNS. Except for certain neurons found in some special sense organs, virtually all sensory neurons are unipolar, and their cell bodies are located in sensory ganglia outside the CNS. 2. Motor, or efferent, neurons carry impulses away from the CNS to the effector organs (muscles and glands) of the body. Motor neurons are multipolar. Except for some neurons of the autonomic nervous system, their cell bodies are located in the CNS. 3. Interneurons, or association neurons, lie between motor and sensory neurons in neural pathways and shuttle signals through CNS pathways where integration occurs. Most interneurons are confined within the CNS. They make up 99% of the neurons in the body and almost all of them are multipolar.

Explain step-by-step process of the propagation of an action potential.

AP is generated by the influx of Na+ through a given area of the membrane. This influx establishes local currents that depolarize adjacent membrane areas in the forward direction (away from the origin of the nerve impulse), which opens voltage-gated channels and triggers an AP there. This process continues down the length of the axon so that an AP is self-propagating and continues along the axon at a constant velocity. Following depolarization, each segment of axon membrane repolarizes, restoring the resting membrane potential in that region. The repolarization wave chases the depolarization wave down the length of the axon.

Differentiate between the absolute refractory period and the relative refractory period.

Absolute refractory period is the period that begins with the opening of the Na+ channels and ends when the Na+ channels begin to reset their original resting state. Relative refractory period follows the absolute refractory period . During the relative refractory period, most Na+ channels have returned to their resting state, some K+ channels are still open, and repolarizing is occurring.

Differentiate between continuous conduction and saltatory conduction.

Action potential propagation in nonmyelinated axons occurs by continuous conduction because the voltage-gated channels in the membrane are immediately adjacent to each other. Saltatory conduction: When an AP is generated in a myelinated fiber, the local depolarizing current does not dissipate through the adjacent membrane regions, which are non-excitable.

Correlate the phases and events of an action potential with an action potential trace and the opening and closing of ion channels in the cell membrane.

An action potential has three phases: depolarization, overshoot, repolarization. There are two more states of the membrane potential related to the action potential. The first one is hypopolarization which precedes the depolarization, while the second one is hyperpolarization, which follows the repolarization.

Explain what a refractory period is.

An important characteristic of the action potential is the refractory period, which is the time after initiation of an action potential when it is impossible or more difficult to generate a second action potential.

Describe how IPSPs summate.

Another form of spatial summation is that which involve an inhibitory postsynaptic potential (IPSP) which reaches the neuron at the same time as the multiple EPSPs and the sum of the IPSP and EPSP's (the summed potential) is subthreshold so no action potential is generated as the IPSP has diminished the EPSPs.

Explain how axon diameter affects the speed of conduction of an action potential along an axon.

As a rule, the larger the axon's diameter, the faster it conducts impulses. Larger axons conduct more rapidly because they offer less resistance to the flow of local currents, bringing adjacent areas of the membrane to threshold more quickly.

Explain how channel-linked receptors function when stimulated by a neurotransmitter.

As the ligand binds to one or more receptor subunits, the proteins change shape. This event opens the central channel and allows ions to pass. As a result, the membrane potential of the target cell changes.

Autonomic Nervous System

Autonomic Nervous System: Consists of visceral motor nerve fibers that regulate the activity of smooth muscles, cardiac muscles, and glands. Autonomic means a "law unto its self". Also known as the involuntary nervous system.

Explain how graded potentials can stimulate the axon hillock to generate an action potential.

Because the current dissipates quickly and decays (declines) with increasing distance from the site of initial depolarization, graded potentials can act as signals only over very short distances such as dendrites and the cell body. Nonetheless, they are essential in initiating action potentials, the long-distance signals of axons.

Explain why there is a synaptic delay with chemical synapses but not with electrical synapses.

Because the current from the presynaptic membrane dissipated in the fluid filled cleft, chemical synapses prevent a nerve impulse from being directly transmitted from one neuron to another. Instead, an impulse is transmitted via a chemical event that depends on the release, diffusion, and receptor binding of neurotransmitter molecules and results in unidirectional communication between neurons.

Tracts

Bundles of axons in the CNS are called tracts.

Nerves

Bundles of axons in the PNS are called nerves.

Explain why action potentials travel in only one direction.

But action potentials move in one direction. This is achieved because the sodium channels have a refractory period following activation, during which they cannot open again. This ensures that the action potential is propagated in a specific direction along the axon.

Describe the structural and functional relationship between the following components/divisions of the nervous system

CNS: The sensory division keeps the CNS constantly informed of events going on both inside and outside of the body. The PNS has two functional subdivisions. Somatic sensory fibers convey impulses from the skin, skeletal muscles, and joints. (SOMA = body) Visceral sensory fibers transmits impulses from the visceral organs (organs within the ventral body cavity) Sensory (AFFERENT) division: The sensory, also known as the afferent division, consists of nerve fibers (axons) that convey impulses to the CNS from sensory receptors located throughout the body. Motor (EFFERENT) division: Transmits impulses from the CNS to effector organs which are the muscles and glands. These impulses activates muscles to contract glands to secrete. They effect (bring about) a motor response.

List the parts of the central nervous system and describe its general function.

Central Nervous System (CNS) consists of the brain and spinal cord which occupy the dorsal body cavity. CNS is the integrating and control center of the NS. It interprets sensory input and dictates motor output based on reflexes, current conditions, and past experiences.

Describe the structure of channel-linked receptors

Channel-linked receptors are ligand-gated channels that mediate direct neurotransmitters action.

Differentiate between chemically/ligand-gated channels, voltage-gated channels, and mechanically-gated channels.

Chemically gated channels (ligand-gated channels) open when the appropriate chemical binds. (In this case a neurotransmitter) Voltage-gated channels open and close in response to changes in the membrane potential. Mechanically gated channels open in response to physical deformation of the receptor (as in sensory receptors for touch and pressure).

Differentiate between nuclei and ganglia

Cluster of cell bodies in the CNS are called nuclei, whereas those that lie along the nerves in the PNS are called ganglia.

Cranial Nerves

Cranial nerves carry impulses to and from the brain.

Dendrites including function, location, and structure.

Dendrites of motor neurons are short, tapering, diffusively branching extensions. They are the main receptive or input regions providing an enormous surface area for receiving signals from other neurons.

Depolarization

Depolarization is a decrease in membrane potential. The inside of the membrane becomes less negative (moves closer to zero) than the resting potential. Depolarization also includes events in which the membrane potential reverses and moves above zero to become positive. This increases the probability of producing nerve impulses.

Differentiate between chemical and electrical synapses in terms of structure and speed of transmission

Electrical synapses are much less common than chemical synapses. These neurons are electrically coupled, and transmission across these synapses is very rapid. Depending on the nature of the synapse, communication may be unidirectional or bidirectional. These are found in the eyes and hippocampus (region of brain for emotions and memory) Chemical synapses are the most common type of synapse. They are specialized to allow the release and reception of chemical messengers known as transmitters. Slower than electrical synapses.

Ependymal Cell

Ependymal cells line cerebrospinal fluid-filled CNS cavities. The cells line brain or spinal cord tissue and have cilia that face fluid filled cavities. They range in shape from squamous to columnar, and many are ciliated. The beating of their cilia helps to circulate the cerebrospinal fluid that cushions the brain and spinal cord.

Explain how a graded potential is generated.

Graded potentials are triggered by some change (a stimulus) in the neuron's environment that opens gated ion channels. A receptor potential or a generator potential is produced when a sensory receptor is excited by its stimulus (light, pressure, chemicals) A postsynaptic potential is produced when the stimulus is a neurotransmitter released by another neuron. Here, the neurotransmitter is released into a fluid filled gap called a synapse and influences the neuron beyond the synapse.

Explain how graded potentials are different than action potentials.

Graded potentials are variable-strength signals that can be conveyed over small distances, whereas action potentials are massive depolarizations that can be transferred over long distances.

Explain how myelinated and unmyelinated fibers relate to white matter and gray matter.

Gray matter refers to the tissue which is made up of unmyelinated fibers whereas white matter refers to areas of myelinated fibers.

Hyperpolarization

Hyperpolarization is an increase in membrane potential making it more negative. (moves farther away from 0) This reduces the probability of producing nerve impulses.

Explain what integration means in the context of the nervous system

Integration is stimuli that is received by sensory input and communicated to the nervous system where that information is processed.

Explain the role of the sodium-potassium pump in an action potential.

It acts to transport sodium and potassium ions across the cell membrane in a ratio of 3 sodium ions out for every 2 potassium ions brought in.

Explain the all-or-none response if neurons.

It either happens completely or it doesn't happen at all. If the number of sodium ions (Na+) entering the cell are too low to achieve threshold, no AP will occur.

Describe the structure of a G protein-linked receptor system.

Looks like a puzzle piece

Identify a myelin sheath and describe its function.

Many nerve fibers, particularly those that are long or large in diameter, are covered with a whitish, fatty (protein-lipoid), segmented myelin sheath. Myelin protects and electrically insulates fibers, and it increases the transmission speed of nerve impulses.

Cytoskeletal elements

Microtubules and neurofibrils, which are bundles of intermediate filaments, maintain cell shape and integrity.

Myelinated fibers vs nonmyelinated fibers

Myelinated fibers ( axons bearing a myelin sheath) conduct nerve impulses rapidly, whereas nonmyelinated fibers conduct impulses more slowly.

Explain the process of neurotransmitter release in chemical synapses

Neurotransmitters are released from synaptic vesicles in presynaptic neurons in response to neural activity, diffuse across the synaptic cleft, and bind specific receptors in order to bring about changes in postsynaptic neurons.

List the parts of the peripheral nervous system and describe its general function

Peripheral Nervous System (PNS) is part of the nervous system outside of the CNS. The PNS consists mainly of nerves (bundles and axons) that extend from the brain, spinal cord, and ganglia (collections of neuron cell bodies).

Pigment inclusions

Pigments sometimes found inside neuron cell bodies include black melanin, a red iron-containing pigment and a golden-brown pigment called lipofuscin.

Identify the presynaptic and postsynaptic components of a synapse and explain the function of each.

Presynaptic is the neuron conducting impulses toward the synapse and the postsynaptic neuron is the neuron transmitting the electrical signal away from the synapse. At a given synapse, the presynaptic neuron sends the information and the postsynaptic neuron receives the information. Most neurons function as both presynaptic and postsynaptic.

Identify the components of a reflex and explain how it works.

Reflexes are rapid, automatic responses to stimuli, in which a particular stimulus always causes the same response.

Explain how a resting membrane potential of -70 mV is achieved and maintained.

Resting membrane potentials are maintained by two different types of ion channels: the sodium-potassium pump and the sodium and potassium leak channels. (3 Na+ out and 2 K+ in)

Explain how action potentials are generated at the axon hillock.

Resting state where no ions move through the voltage-gated channels, depolarization is caused by Na+ flowing into the cell allowing Na+ entry, repolarization is where Na+ channels are inactivating, K+ channels open allowing K+ to exit, hyperpolarization is some K+ channels remain the same and Na+ channels rest.

Describe the state of voltage-gated sodium channels and potassium channels (activations and inactivation gates) during rest, depolarization, repolarization, and hyperpolarization.

Resting state: All voltage-gated Na+ and K+ channels are closed. Depolarization: Voltage-gated Na+ channels are open. Repolarization: Na+ channels are inactivating, and voltage-gated K+ channels open. Hyperpolarization: Some K+ channels remain open, and Na+ channels reset.

Differentiate between sensory input and motor output.

Sensory input is gathering information and motor output is using the information that was gathered.

Explain how a parallel after-discharge circuit operates and give an example.

Signal stimulates neurons arranged in parallel arrays that eventually converge on a single output cell, impulses reach output cell at different times, causing a burst of impulses called an after-discharge. An example is it may be involved in exacting mental processes such as mathematical calculations.

Explain how a reverberating circuit operates and give an example.

Signal travels through a chain of neurons, each feeding back to previous neurons, it is an oscillating circuit, and controls rhythmic activity. An example is this is involved in breathing, sleep-wake cycle, and repetitive motor activities such as walking.

Microglial Cells

Small, ovoid with relatively long "thorny" processes. Defensive cells in the CNS. Their processes touch nearby neurons, monitoring their health, and when they sense that certain neurons are injured or in other trouble, the microglial cells migrate toward them.

Motor Division has 2 main parts. Somatic and Autonomic

Somatic nervous system: Composed of somatic nerve fibers that conduct impulses from the CNS to skeletal muscles. It is often referred to as the voluntary nervous system because it allows us to consciously control our skeletal muscles.

Explain how stimulus intensity is communicated via action potentials.

Strong stimuli generate nerve impulses more often in a given time interval than do weak stimuli. Stimulus intensity is coded for by the number of impulses per second -that is, by the frequency of action potentials- rather than by increases in the strength of the individual APs.

Schwann cells (neurolemmocytes)

Surround all nerve fibers in the PNS and form myelin sheaths around the thicker nerve fibers. In this way, they are functionally similar to oligodendrocytes. Schwann cells are vital to regeneration of damaged peripheral nerves.

Satellite Cells

Surround neuron cell bodies located in the peripheral nervous system (PNS), and are thought to have many of the same functions in the PNS as astrocytes do in the CNS. (Their name comes from a fancied resemblance to the moons (satellites) around a planet.

Describe when the absolute refractory period occurs and why it does so.

The absolute refractory period is a period of time where it is impossible for the cell to send more action potentials. This is due to the gating mechanism on the voltage gated sodium channels. After a period of time of being open, the voltage gated sodium channels slam shut and are inactivated.

Axons including function, location, and structure.

The axon arises from a cone-shaped area of the cell body called the axon hillock. A NEURON NEVER HAS MORE THAN 1 AXON. The initial segment of the axon narrows to form a slender process that is uniform in diameter for the rest of its length.

Identify the chemical and electrical (voltage) gradients of a given electrochemical gradient.

The electrochemical gradient has two components. 1. The concentration gradient. Ions move along chemical concentration gradients from an area of their higher concentration to an area of lower concentration. 2. The electrical gradient. Ions move toward an area of opposite electrical charge. Often the two gradients oppose each other, each trying to drive ions in the opposite direction. Whichever gradient is strongest "wins" and drives the net flow of ions in its direction. Voltage (V)=current (I) x resistance (R)

List and state the general function of the various components of the neuron cell body

The general function is to conduct messages in the form of nerve impulses from one part of the body to another. The neuron cell body consists of a spherical nucleus (with a conspicuous nucleolus) surrounded by cytoplasm. The perikaryon (peri=around, kary=nucleus) or soma (body) ranges in diameter from 5 to 140 um. In most neurons, the plasma membrane of the cell body acts as a receptive region that receives information from other neurons. The cell body is the major biosynthetic center and metabolic center of a neuron.

Parasympathetic Division

The parasympathetic division is part of the ANS. Conserves energy and promotes housekeeping functions during rest. (Oversees digestion, elimination, and glandular function; the resting and digesting subdivision)

Explain how myelination affects the speed of conduction of an action potential along an axon.

The presence of a myelin sheath dramatically increases the speed of propagation. The conduction velocity increases with the degree of myelination-lightly myelinated fibers conduct more slowly than heavily myelinated fibers.

Explain how action potentials are propagation of action potential.

The propagation of a nerve impulse is a more accurate term to use , because the AP is regenerated anew by the voltage-gated channels at each membrane patch, and every subsequent AP is identical to the one that was originally generated. Without voltage-gated channels, propagation cannot occur.

Describe when the relative refractory period occurs and why it does so.

The relative refractory period is the interval of time during which a second action potential can be initiated, but initiation will require a greater stimulus than before. Refractory periods are caused by the inactivation gate of the Na+ channel.

Sympathetic Division

The sympathetic division is part of the ANS. Mobilizes body systems during activity (Prepares the body to cope with some stressor; the fight, fright, or flight subdivision).

Explain how serial processing works and given an example.

The whole system works in a predictable all-or-nothing manner. One neuron stimulates the next, and so on, eventually causing a specific, anticipated response. The most clear cut example of this are spinal reflexes. Straight-through sensory pathways from receptors to the brain are also examples (reflexes)

Peripheral Nerves

These peripheral nerves serve as communication lines that link all of the parts of the body to the CNS.

Describe the effect of distance on a graded potential.

They can be either depolarizations or hyperpolarization. These changes cause current flows that decrease in magnitude with distance.

Differentiate between excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs).

Unlike axons which have voltage-gated channels that make a AP possible, postsynaptic membranes generally do not generate AP's. The dramatic polarity reversal seen in axons never occurs in membranes containing only chemically gated channels because the opposite movements of K+ and Na+ prevent excessive positive charge from accumulating inside the cell. For this reason, instead of APs, depolarizing graded potentials called excitatory postsynaptic potential occur. (EPSP) If K+ channels open, K+ moves out of the cell. If Cl- channels open, Cl- moves in. In either case, the charge on the inner face of the membrane becomes more negative. As the membrane potential increases and is driven farther from the axon's threshold, the postsynaptic neuron becomes less and less likely to "fire", and larger depolarizing currents are required to induce an AP. Hyperpolarizing changes in potential are called inhibitory postsynaptic potentials (IPSPs)

Explain/apply the relationship between voltage, current, and resistance in an electrical current and apply to action potential in a neuron.

Voltage is the measure of potential energy generated by separated electrical charges, is measured in either volts (V) or millivolts (1mV=0.001V) Voltage is always measured between two points called the potential difference or simply between the points. The greater the difference in charge between 2 points, the higher the voltage. Current is the flow of electrical charge from one point to another and can be used to do work (ex. power a flashlight). The amount of charge that moves between the 2 points depends on 2 factors; voltage and resistance. Resistance is the hindrance to charge flow provided by substances through which the current must pass. Substances with high electrical resistance are insulators, and those with low resistance are conductors.

Describe the structure and function of voltage-gated channels

Voltage-gated channels open and close in response to changes in the membrane potential. They are initially activated by local currents (graded potentials) that spread toward the axon along the dendritic and cell body membranes.

Explain what happens when the axon membrane reaches threshold of (-55mV)

When depolarization reaches a critical level called threshold, depolarization becomes self-generating, urged on by positive feedback.

Explain what an electrochemical gradient is and identify the direction of a given electrochemical gradient.

When gated ion channels open, ions diffuse quickly across the membrane. The direction an ion moves (into or out of the cell) is determined by the electrochemical gradient.

Explain why neurons in the discharge zone are more likely to fire than those in the facilitated zone of a neuronal pool.

When the incoming fiber is excited, it will excite some postsynaptic neurons and facilitate others by bringing them closer to threshold. Neurons most likely to generate impulses are those closely associated with the incoming fiber, because they receive the bulk of the synaptic contacts(these are in the discharge zone) Neurons farther from the center are not usually excited to threshold, but they are facilitated and can easily be brought to threshold by stimuli from another source. (facilitated zone)

Describe how EPSPs summate

While activation of either one of the excitatory synapses alone produces a subthreshold EPSP, activation of both excitatory synapses at about the same time causes the two EPSPs to sum together.

Explain why the resting membrane potential is -70 mV and not 0 mV.

You must have a potential before you can discharge a signal. It also helps prevent neurons from endlessly firing - once a signal is sent it takes time for the ion exchange to restore enough potential for another firing. This is known as the refractory period of the neuron.