Chapter 12 - TERMS AND QUESTIONS*

If the resting membrane potential is -70mV and the threshold is -55mV, a membrane potential of -60mV will

make it easier to produce an action potential

Describe the steps involved in the generation of an action potential.

The membrane depolarizes to threshold. Next, voltage-gated sodium channels are activated, and the membrane rapidly depolarizes. These sodium channels are then inactivated, and potassium channels are activated. Finally, normal permeability returns. The voltage-gated sodium channels become activated once the repolarization is complete; the voltage-gated potassium channels begin closing as the transmembrane potential reaches the normal resting potential.

Distinguish between the somatic and autonomic nervous systems.

Somatic nervous system controls skeletal muscle contractions. Autonomic nervous system provides automatic regulation of smooth muscle, cardiac muscle, and glandular secretions at the subconscious level

What is the difference between temporal summation and spatial summation?

Temporal summation is the addition of stimuli that arrive are a single synapse in rapid succession. Spatial summation occurs when simultaneous stimuli at multiple synapses have a cumulative effect on the transmembrane potential.

Describe the events that occur during nerve impulse transmission at a typical cholinergic synapse.

The action potential arrives at the synaptic knob, depolarizing it *Extracellular calcium enters the synaptic knob, triggering the exocytosis of Ach *ACh binds to the postsynaptic membrane and depolarizes the next neuron in the chain *ACh is removed by AChE

State the all-or-nothing principle of action potentials.

The all-or-none principle of action potentials states that any depolarization event sufficient to reach threshold will cause an action potential of the same strength, regardless of the amount of stimulation above threshold.

How does a neuron become hyperpolarized?

Through inhibitory post-synaptic potential the neuron moves away from 0, causing it to be less likely for an action potential.

What is the functional difference among voltage-gated, chemically gated, and mechanically gated channels?

Voltage-gated channels open or close in response to changes in the transmembrane potential. Chemically gated channels open or close when they bind specific extracellular chemicals. Mechanically gated channels open or close in response to physical distortion of the membrane surface.

Distinguish between the absolute and relative refractory periods.

absolute refractory periods last 0.4-1.0msec; the smaller the axon diameter, the longer the duration. The relative refractory period begin when the sodium channels regain their normal resting conditions and continues until the transmembrane potential stabilizes

In the CNS, a neuron typically receives information from other neurons at its

dendrites

Describe the four types of neuroglia found in the CNS.

ependymal cells: have slender processes that branch extensively and make direct contact with neuroglia in the surrounding neural tissue *astrocytes: maintaining the blood-brain barrier, creates a 3-D framework for the CNS, repairing damaged neural tissue, guiding neuron development, controlling the interstitial environment *oligodendrocytes: possess slender cytoplasmic extensions, but the cell bodies are smaller and have fewer processes; play a role in structural organization by tying clusters of axons together, and these neuroglia improve the functional performance of neurons by wraping axons within a myelin sheath *microglia: have fine processes that allow them to move freely throughout neural tissue

Phagocytic cells in neural tissue of the CNS are

microglia

What three functional groups of neurons are found in the nervous system? What is the function of each type of neuron?

sensory neurons: transmit impulses form the PNS to the CNS *motor neurons: transmits impulses from the CNS to the peripheral effectors *interneurons: analyze sensory inputs and coordinate motor outputs

What is the functional difference among voltage regulated, chemically regulated, and mechanically regulated channel?

voltage regulated: they open or close in response to changes in the transmembrane potential *chemically regulated: open or close when they bind specific chemicals *mechanically regulated: open or close in response to physical distortion of the membrane surface

Muscle Sclerosis is a myelination disorder. How does this condition produce muscle paralysis and sensory losses?

-Myelin allows for saltatory propagation, speeding up the action potential. If there is a lack of myelin, it slows down leading to slow responses and eventually loss of coordination (paralysis).

Depolarization of a neuron plasma membrane will shift the membrane potential toward

0mV

What are the major components of the central nervous system? the peripheral nervous system?

CNS: brain, and spinal cord *PNS: all other nerve fibers, divided between the efferent division (which consists of the somatic nervous system and the autonomic nervous system) and the afferent division (which consists of receptors and sensory neurons)

What are the major components of the central nervous system? the peripheral nervous system?

CNS: brain, and spinal cord *PNS: all other nerve fibers, divided between the efferent division, which consists of the somatic nervous system and the autonomic nervous system; and the afferent division which consists of receptors and sensory neurons.

What is meant by saltatory propagation? How does it differ from continuous propagation?

In saltatory conduction, which occurs in myelinated axons, only the nodes along the axon can respond to a depolarizing stimulus. In continuous conduction, which occurs in unmyelinated axons, an action potential appears to move across the membrane surface in a series of tiny steps.

List and describe the three broad classes of sensory receptors.

Interoceptors: monitor the digestive, respiratory, cardiovascular, urinary, and reproductive systems and provide sensations of taste, deep pressure, and pain *Exteroceptors: provide information about the external environment in the form of touch, temperature, or pressure sensations and the more complex senses of sight, smell, ad hearing *Proprioceptors: monitor the position and movement of skeletal muscles and joints

What can't most neurons in the CNS be replaced when they are lost to injury or disease?

Neurons lack centrioles and therefore cannot divide and replace themselves.

Which two types of neuroglia insulate neuron cell bodies and axons in the PNS from their surroundings?

satellite cells *Schwann cells

Describe the anatomy of a typical neuron.

A neuron starts off with a cell body, which contain a relatively large, round nucleus with a prominent nucleolus. The cytoplasm surrounding the nucleus constitutes the perikaryon, which contains neurofilaments and neurotubules. A variable number of slender, sensitive processes known as dendrites extend out from the cell body.

In multiple sclerosis, there is intermittent and progressive damage to the myelin sheath of peripheral nerves. This results in poor motor control of the affected area. Why does destruction of the myelin sheath affect motor control?

Action potentials travel faster along fibers that are myelinated than fibers that are nonmyelinated. Destruction of the myelin sheath increases the time it takes for motor neurons to communicate with their effector muscles. This delay in response results in varying degrees of uncoordinated muscle activity. The situation is very similar to that of a newborn, who cannot control its arms and legs very well because the myelin sheaths are still being laid down. Since not all motor neurons to the same muscle may be demyelinated to the same degree, there would be some fibers that are slow to respond while others are responding normally, producing contractions that are erratic and poorly

What is the difference between anterograde flow and retrograde flow?

Anterograde flow is the movement of materials from the cell body to the synaptic knobs. Retrograde flow is the movement of materials toward the cell body.

Distinguish between EPSPs and IPSPs.

EPSPs are a graded depolarization caused by the arrival of a neurotransmitter and the postsynaptic membrane *IPSPs are a graded hyperpolarization of the postsynaptic membrane

Describe in detail the steps involved in the generation and propagation of an action potential.

Generation: (1) Depolarization to Threshold- an area of excitable membrane must be depolarized to its threshold by local currents (2) Activation of Sodium Channels and Rapid Depolarization- the sodium activation gates open, and the plasma membrane becomes much more permeable to Na+, sodium ions rush to the cytoplasm, the inner membrane surface has changed, and contains more positive ions than negative ones (3) Inactivation of Sodium Channels and the Activation of Potassium Channels- the inactivation gates of the voltage-gated sodium channels begin closing, the cytosol along the interior of the membrane contains an excess of positive charges, the sudden loss of positive charges then shifts the transmembrane potential back toward resting levels and repolarization begins (4) The Return to Normal Permeability- the voltage-gated sodium channels remain inactivated until the membrane has repolarized to near threshold levels *Propagation: (1) As an action potential develops in the initial segment, the transmembrane potential depolarizes to +30mV (2) A local current depolarizes the adjacent portion of the membrane to threshold (3) An action potential develops at this location, and the initial segment enters the refactory period (4) A local current depolarizes the adjacent portion of the membrane to threshold, and the cycle is repeated

Harry has a kidney condition that causes changes in his body's electrolyte levels (concentration of ions in the extracellular fluid). As a result, he is exhibiting tachycardia, an abnormally fast heart rate. Which ion is involved, and how does a change in its concentration cause Harry's symptoms?

Harry's kidney condition is causing the retention of potassium ions. As a result, the K+ concentration of the extracellular fluid is higher than normal. Under these conditions, less potassium diffuses from heart muscle cells than normal, resulting in a resting potential that is less negative (more positive). This change in resting potential moves the transmembrane potential closer to threshold, so it is easier to stimulate the muscle. The ease of stimulation accounts for the increased number of contractions evident in the rapid heart rate.

Twenty neurons synapse with a single receptor neuron. 15 of the 20 neurons release neurotransmitters that produce EPSPs at the postsynaptic membrane, and the other 5 release neurotransmitters that produce IPSPs. Each time one of the neurons is stimulated, it releases enough neurotransmitter to produce a 2mV change in potential at the postsynaptic membrane. If the threshold of the postsynaptic neuron is 10mV, how many of the excitatory neurons must be stimulated to produce an action potential in the receptor neuron if all five inhibitory neurons are stimulated? (Assume that spatial summation occurs.)

To reach threshold, the postsynaptic membrane must receive enough neurotransmitter to produce an EPSP of +20mV (+10mV to reach threshold and +10mV to cancel the IPSPs prodiced by the five inhibitory neurons). Each neuron releases enough neurotransmitter to produce a change of +2mV, so at least 10 of the 15 excitatory neurons must be simulated to produce this effect by spatial summation.

What are the structural and functional differences among type A, B, and C fibers?

Type A fibers are myelinated and carry action potentials very quickly (120m/sec). Type B are also myelinated, but carry action potentials more slowly due to their smaller diameter. Type C fibers are extremely slow due to their small diameter and lack of myelination.

Receptors that bind acetylcholine at the postsynaptic membrane are

chemically gated channels

The neural cells responsible for the analysis of sensory inputs and coordination of motor outputs are

interneurons

What two major cell populations are found in the nervous system? What is the primary function of each?

neurons: used for the intercellular communication through changes in membrane potential and synaptic connections *neuroglia: separate and protect the neurons, provide a supportive framework for neural tissue, act as phagocytes, and help regulate the composition of the interstitial fluid

Which types of neuroglia insulate cell bodies and axons in the PNS from their surroundings?

satellite cells and Schwann cells

Regulation by the nervous system provides

swift, long-lasting responses to stimuli

The primary determinant of the resting membrane potential is

the membrane permeability to potassium