第二信号系统（second signal system)

A、classic conditioned reflex

B、operant conditioned reflex

C、first signal system

D、second signal system

A、degradation of the second messenger

B、desensitization of receptors

C、deactivation of a signal transduction protein

D、all of the above

A、具有first signal system和second signal system

B、对环境变化具有更大的适应性

C、具有建立conditioned reflex的能力

D、具有对具体信号形成conditioned reflex的能力

The purpose of the second mention of "hormones" in line 4 is to point out

A．chemical signals that are common among insects

B．specific responses of various species to chemical signals

C．similarities between two chemical substances

D．how insects produce different chemical substances

A、Nervous system and digestive system

B、Endocrine system and respiratory System

C、Nervous system and endocrine system

D、Central nervous system and peripheral nervous system

E、The first messenger system and the second messenger system

A、Nervous system and endocrine system

B、Nervous system and digestive system

C、Endocrine system and respiratory System

D、Central nervous system and peripheral nervous system

E、The first messenger system and the second messenger system

A、Keep in mind at all times what you are searching for.

B、Do not read every word.

C、Learn to spot dates, statistics and other numbers.

D、Look for signal words such as first, second, next, etc.

E、Look for repeated key words or structure.

A．Many？extracellular？signals？acting？via？GPCRs？affect？the？activity？of？the？enzyme？adenylyl？cyclase？and？thus？alter？the？intracellular？concentration？of？the？second？messenger？molecule？cyclic？AMP.

B．Most？commonly,？the？activated？G？protein？α？subunit？switches？on？the？adenylyl？cyclase,？causing？a？dramatic？and？sudden？increase？in？the？synthesis？of？cyclic？AMP？from？ATP.

C．To？help？terminate？the？signal,？a？second？enzyme,？called？cyclic？AMP？phosphodiesterase,？rapidly？converts？cyclic？AMP？to？ordinary？AMP.？Cyclic AMP phosphodiesterase is continuously active inside the cell. Because it eliminates cyclic AMP so quickly, the cytosolic concentration of this second messenger can change rapidly in response to extracellular signals, rising or falling tenfold in a matter of seconds.

D．Cyclic AMP is water-soluble,so it can, in some cases, carry the signal throughout the cell, traveling from the site on the membrane where it is synthesized to interact with proteins located in the cytosol, in the nucleus, or on other organelles. One？way？that？caffeine？acts？as？a？stimulant？is？by？inhibiting？this？phosphodiesterase？in？the？nervous？system,？blocking？cyclic？AMP？degradation？and？thereby？keeping？the？concentration？of？this？second？messenger？high.

The resolution of ultra high definition television (UHDTV) is 3840×2160 and the fresh rate is 60 Hz. For each pixel, the quantization levels of different color samplings (R, G, B) are all set to 4096. Then, the number of bits per second required to transmit for this video signals is ________, and the minimum bandwidth required to transmit this signal with raised-cosine spectrum (the roll-off factor α=0.5) is ________Hz.

A、16.7G, 25G

B、16.7G, 12.5G

C、278M, 417M

D、278M, 208.5M

ng section, the preamplifier control section, and the power amplifier section. In very elaborate hi fi systems these three sections actually may be purchased as separate units, for increased flexibility and control or as a means of assembling the system in stages using the "building block" approach.

The Tuner Section

The tuner section may be equipped to receive only FM signals or both AM and FM signals. Since it is possible to broadcast stereo (two channel) sound via FM, most hi fi receivers have circuitry for decoding the complex composite received signal into separate "left and right" channel signals.

The front end of the tuner section selects the particular signal desired from the hundreds of radio signals present in the atmosphere. As you tune the dial, resonant or tuned circuits are changed so as to be responsive to a single frequency. In the case of FM signals, tuning is adjustable from 88 MHz (millions of alternations/second of the radio wave) to 108 MHz. Since each FM station occupies a space of 0.2 MHz, that means a theoretical maximum of 100 stations is possible in one geographical area. Actually fewer stations are assigned in any one area, so that one station' s signal will not cause audible interference with an adjacent station. In the case of AM, stations broadcast at frequencies ranging from 540 kHz to 1605 kHz (1 kHz = 1000 alternations per second) and selection of desired stations is similar to the technique used in FM.

The signal is amplified by the front end and mixed with a locally generated signal in the receiver to produce a different frequency known as the IF signal (Intermediate Frequency). In the case of AM, the IF frequency is generally 455 kHz. In the case of FM radio, the IF frequency is 10.7 MHz. The process is known as super heterodyne and since the local signal and received signal are always a fixed amount apart, the succeeding IF amplifiers need not be variably tuned but can be designed simply to amplify 455 kHz (for AM) or 10.7 MHz (for FM).

Difference Between FM and AM

In AM radio the amplitude of the radio wave is varied in accordance with the audio information to be broadcast. Hence the name Amplitude Modulation. In FM it is the radio frequency that is varied based on audio signals. It is this feature that makes FM relatively noise free since noise or static is an amplitude phenomenon. If noise accompanies the FM signals the noise can be sliced off by circuits called limiters without impairing the frequency-changing nature of the FM signal.

While the circuits themselves differ, the amplified IF signals are then applied to a detector circuit which strips off the original audio information. In FM, the detector is called a radio detector. The output of either the AM detector or the FM detector is an audio signal suitable for application to and further processing by the "preamplifier control" section o{ the receiver.

In the case o{ stereo FM the recovered audio signal must be further processed or "unscrambled" to recover the separate "left" and "right" audio channels. This is accomplished by the multiplex decoder circuits.

In an FM radio, the Intermediate Frequency is usually ______.

A．10.7 MHz

B．455 kHz

C．540 kHz

D．1605 kHz