Question

Assertion: Nerve impulse can never be transmitted from dendrite or cell body of one neuron to the axon of the next neuron, across a synapse. Reason: This happens because of the synaptic delay at each synapse.

Short answer

The assertion is true, but the reason given is false. The nerve impulse does not progress from the dendrite or cell body of one neuron to the axon of the neighboring neuron. However, the reason provided does not accurately support the assertion. The synaptic delay is not responsible for the nerve impulse's inability to be transmitted as described in the assertion; instead, it is the physiology and working mechanism of neurons.

Step-by-step solution

Consider the assertion

The assertion states that the nerve impulse cannot be transferred from the dendrite or cell body of one neuron to the axon of the next neuron across a synapse, which is a small gap between two neurons. This is true, as the nerve impulse actually follows a specific pathway starting from the dendrites, moving to the cell body, and then finally to the axon of the same neuron. It is the duty of neurotransmitters to carry this message across the synapse to the dendrites of the next neuron, thus continuing the chain.

Consider the reason

The reason presented refers to the synaptic delay at each synapse. The synaptic delay is the brief period of time it takes for the neurotransmitter to be released from the presynaptic neuron (the neuron sending the signal), cross the synapse, and bind to the postsynaptic neuron (the neuron receiving the signal). This delay, however, does not mean that the nerve impulse cannot be transmitted across the synapse, rather it slightly slows down the transmission. It is a process that occurs usually in milliseconds and is a necessary part of the nerve impulse transmission.

Linking the assertion and the reason

From the elucidated information, it can be inferred that the reason provided is not supplementing the assertion comprehensively. The synaptic delay does not prevent the nerve impulse from a dendrite or cell body of one neuron reaching the axon of the next neuron, instead it's the structure of neurons and the nature of impulse propagation that causes this narrative.

Key concepts

Neurons

Neurons are the fundamental building blocks of the nervous system. They are specialized cells designed to transmit information throughout the body. Every neuron has three main components: the dendrites, cell body, and axon.

• Dendrites: They receive signals from other neurons and convey this information towards the cell body.
• Cell Body (Soma): It contains the nucleus and is responsible for maintaining the life of the neuron.
• Axon: This long, slender projection transmits the electrical signal away from the cell body to the axon terminals, where communication with other neurons occurs.

In a typical pathway, an impulse travels from the dendrites, passes through the cell body, and moves along the axon to reach another neuron's dendrites, continuing the chain of communication.

Synapse

The synapse is the microscopic gap or junction between two neurons. It's the site where nerve impulses are transmitted from one neuron to the next. The synapse functions as a switchboard operator, directing signals to their next destination. It comprises several parts:

• Presynaptic Neuron: This is the neuron sending the signal. The end of its axon, or terminal, releases neurotransmitters into the synaptic gap.
• Postsynaptic Neuron: This neuron receives the signal. It has receptors on its dendrites that recognize and bind with neurotransmitters.
• Synaptic Cleft: The tiny gap that separates the presynaptic neuron from the postsynaptic neuron.

The efficient functioning of synapses is crucial for the proper transmission of nerve impulses, making them essential in the communication network of neurons.

Neurotransmitters

Neurotransmitters are chemical messengers that play a key role in transmitting signals across the synapse. When a nerve impulse reaches the end of an axon, it triggers the release of these chemicals into the synaptic cleft. Neurotransmitters then cross the gap to bind with specific receptors on the postsynaptic neuron.

• This binding initiates a new nerve impulse in the receiving neuron, allowing the signal to continue along the chain.
• The presence of different types of neurotransmitters can either excite or inhibit postsynaptic neurons. For example, dopamine is associated with pleasure and reward, while serotonin helps regulate mood.

Without neurotransmitters, nerve impulse transmission would stall, causing breakdowns in communication between neurons.

Synaptic Delay

Synaptic delay is the brief pause that occurs during the transfer of a signal across a synapse. It's the time taken for neurotransmitters to be released, cross the synaptic cleft, and bind to receptors on the postsynaptic neuron. Despite being short, usually around a few milliseconds, this delay is significant in neural communication.

• It's caused by the time required for the complex biochemical processes of neurotransmitter release and binding.
• Synaptic delay doesn't prevent nerve impulses from transferring; it just slightly slows the transmission, playing a critical role in the timing and processing of signals within the nervous system.
• This delay is a natural and necessary aspect of synaptic transmission, ensuring that signals are accurately relayed and processed appropriately.

Understanding synaptic delay helps illustrate why nerve impulses are staggered as they navigate the elaborate network of neurons.