Chapter 3 - Nerve Cells, Neural Circuitry, and Behavior

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is a self-regulating mechanism.

a motor neuron may have excitatory connections with both a muscle and an inhibitory interneuron that itself forms a connection with the motor neuron.

When the inhibitory interneuron is excited by the motor neuron, the interneuron is able to limit the ability of the motor neuron to excite the muscle (Figure 3-7B)

thus reduce their probability of firing. The effect is to dampen activity within the stimulated pathway and prevent it from exceeding a certain critical level
trigger actionsums the depolarization generated by the receptor potential. An action potential is generated only if the receptor potential exceeds a certain voltage threshold. Once this threshold is surpassed, any further increase in amplitude of the receptor potential can only increase the frequency with which the action potentials are generated, because action potentials have a constant amplitude. The duration of the receptor potential determines the duration of the train of action potentials. Thus, the graded amplitude and duration of the receptor potential are translated into a frequency code in the action potentials generated at the trigger zone. All action potentials produced are propagated faithfully along the axon.Action PotentialAction potentials are all-or-none. Because all action potentials have a similar amplitude and duration, the frequency and duration of firing encodes the information carried by the signal.Output Signal (Transmitter release)When the action potential reaches the synaptic terminal, it initiates the release of a neurotransmitter, the chemical substance that serves as the output signal. The frequency of action potentials in the presynaptic cell determines how much neurotransmitter is released by the cell.synaptic potential.The ensuing flow of current briefly alters the membrane potential of the motor cell, a change calledLocal signals (passive)1. Receptor Potentials 2. Synaptic PotentialsSynaptic Potentialsthe synaptic potential is graded; its amplitude depends on how much transmitter is released. In the same cell, the synaptic potential can be either depolarizing or hyperpolarizing spreads passively. Thus, the change in potential will remain local unless the signal reaches beyond the axon's initial segment where it can give rise to an action potentialPropagated (Active) SignalsAction Potentialsaction potential SignalAmplitude - Large (70-110 mV) Duration - Brief ( 1 - 10 ms) Summation - ALL OR NONE Effect - depolarizing Type - ActiveSignal Type - Receptor PotentialsAmplitude - Small (0.1-10) mV Duration - Brief (5-100 ms) Summation Graded Effect - Hyper of Depolarizing Type - PassiveSignal Type- Synaptic PotentialAmplitude - Small (0.1-10) mV Duration - Brief to Long (5 ms - 20 min) Summation - (Graded) Effect- Hyperpolarizing or depolarizing Type - PassiveSequence of Signals That Produce a Reflex Action (Figure 3-10)A. Sensory Signals B. Motor Signals C. Muscle SignalsSequence A - Sensory SignalsThe stretching of a muscle produces a receptor potential in the specialized receptor (the muscle spindle). The amplitude of the receptor potential is proportional to the intensity of the stretch. This potential spreads passively to the integrative or trigger zone at the first node of Ranvier. If the receptor potential is sufficiently large, it triggers an action potential that then propagates actively and without change along the axon to the axon terminal. At specialized sites in the terminal, the action potential leads to the release of a chemical neurotransmitter, the output signal. The transmitter diffuses across the synaptic cleft between the axon terminal and a target motor neuron that innervates the stretched muscle; it then binds to receptor molecules on the external membrane of the motor neuronSequence B - Motor SignalsThis interaction initiates a synaptic potential that spreads passively to the trigger zone of the motor neuron's axon, where it initiates an action potential that propagates actively to the terminal of the motor neuron's axon. At the axon terminal, the action potential leads to release of a neurotransmitter near the muscle fiberSequence C - Muscle Signalsthe neurotransmitter binds receptors on the muscle fiber, generating a synaptic potential. The synaptic potential triggers an action potential in the muscle, which causes a contraction.Explain what happens during the Knee-Jerk Reflex...A sensory neuron activated by a muscle spindle at the extensor (quads) muscle makes an excitatory connection with an extensor motor neuron in the spinal cord that innervates this same muscle group. It also makes an excitatory connection with an interneuron, which in turn makes an inhibitory connection with a flexor motor neuron that innervates the antagonist (biceps femoris) muscle group. Conversely, an afferent fiber from the biceps (not shown) excites an interneuron that makes an inhibitory synapse on the extensor motor neuron