C2.2.5 Synapses as junctions between neurons and between neurons and effector cells Notes

Synapses Are Junctions Between Neurons and Effector Cells

1. Synapses are junctions where neurons connect with other neurons or Effector cells (e.g., muscle fibers or gland cells).
2. These connections allow neurons to transmit signals, enabling the nervous system to coordinate complex activities like movement, sensation, and thought.

Synaptic Transmission Converts Electrical Signals into Chemical Communication

The process of synaptic transmission involves several steps:

1. Arrival of the Nerve Impulse: An action potential travels down the axon of the presynaptic neuron to the synaptic terminal.
2. Calcium Ion Influx: The depolarization of the synaptic terminal opens voltage-gated calcium channels, allowing calcium ions ($Ca^{2+}$) to flow into the neuron.
3. Neurotransmitter Release: The influx of calcium ions triggers vesicles containing neurotransmitters to move to the presynaptic membrane and release their contents into the synaptic cleft via exocytosis.
4. Diffusion Across the Synaptic Cleft: Neurotransmitters diffuse across the synaptic cleft and bind to receptors on the postsynaptic membrane.
5. Response in the Postsynaptic Cell: Binding of neurotransmitters to receptors causes ion channels to open, leading to changes in the membrane potential of the postsynaptic cell. This can generate an excitatory or inhibitory response, depending on the type of neurotransmitter and receptor involved.
6. Termination of the Signal: Neurotransmitters are quickly broken down by enzymes or reabsorbed by the presynaptic neuron to prevent continuous stimulation of the postsynaptic cell.

Why Signals Travel in One Direction

1. Synapses are designed to ensure unidirectional signal transmission.
2. This is because:
   1. Neurotransmitters are stored in vesicles in the presynaptic neuron and released into the synaptic cleft.
   2. Receptors for neurotransmitters are located on the postsynaptic membrane.
3. This arrangement prevents signals from traveling backward.

Types of Synapses

Synapses can form between different types of cells:

1. Neuron-to-Neuron Synapses: These are common in the brain and spinal cord, where neurons communicate with each other to process information.
2. Neuron-to-Effector Synapses: These occur between neurons and muscles (neuromuscular junctions) or glands, enabling the nervous system to control movement and secretion.

The Importance of Synapses in the Nervous System

Synapses play a critical role in:

1. Information Processing: Synapses allow neurons to integrate multiple signals and make decisions about how to respond.
2. Learning and Memory: Changes in synaptic strength underlie the brain’s ability to learn and store information.
3. Coordination of Responses: Synapses ensure that signals are transmitted accurately and efficiently, enabling the body to respond to stimuli in a coordinated manner.