Nervous System: Coordination, Impulses, and Synapses
At the synapse, a neurotransmitter is released into the synaptic cleft and diffuses to the postsynaptic membrane. It then binds to specific receptors, which determines a potential change of the membrane (postsynaptic potential). If this potential reaches the threshold of excitation, it is propagated through the entire postsynaptic neuron. Once the neurotransmitter has acted, it is inactivated enzymatically, and the stimulation disappears.
Types of Synapses
Chemical Synapses
The arrival of the electrical signal at the terminal buttons is converted into a chemical signal at the synapse and then becomes electrical again in the postsynaptic neuron.
Electrical Synapses
The synaptic cleft is narrower. The nerve impulse passes directly from the presynaptic to the postsynaptic neuron without the intervention of neurotransmitters.
Coordination Systems
Animals have two systems of regulation and coordination to maintain equilibrium and respond to environmental conditions:
- The hormonal system: Endocrine glands coordinate and regulate physiological functions through hormones transported by the blood.
- The nervous system: Nervous tissue regulates organ function through rapidly transmitted nerve impulses.
The nervous system response is fast, short, and very specific, while the hormonal system is slow, lasting, and has variable specificity. The nervous system regulates rapid and short-lived actions, while the endocrine system controls slow and continuous actions.
The Nervous System
The nervous system is responsible for receiving and transmitting information from the external and internal environment, as well as coordinating and controlling the body’s responses. It is composed of nerve tissue formed by neurons. Coordination is a crucial nerve function, involved in most physiological and behavioral activities in animals.
Structures
- Receptors: Sensory cells, often part of sensory organs.
- Sensory nerve pathways: Transmit nerve impulses from receptors to modulators.
- Modulators: Interpret impulses and develop precise orders.
- Motor nerve pathways: Carry orders from modulators to effectors.
- Effectors: Receive impulses from motor pathways and perform actions.
The Nerve Impulse
Nerve impulses are signals that travel through neurons. The nerve impulse moves across the neuron’s plasma membrane. When it reaches the end of the axon, it can be transmitted to another neuron. The plasma membrane is polarized, meaning its inner surface has a potential difference (resting potential) relative to the outside, with a predominance of negative charges inside.
If a stimulus is effective, it causes an alteration in the membrane’s permeability, allowing a large influx of Na+ ions. This process, called depolarization, results in a variation of the resting potential known as an action potential. Transport enzymes in the membrane then extract Na+ and recover the initial state (repolarization).
For a stimulus to be effective, it must have a minimum intensity, called the threshold of excitability. Once the impulse starts, its speed does not increase much even if the intensity increases; this is known as the law of all or nothing. After an impulse, there is a brief period (refractory period) during which another impulse cannot start. This period allows the neuron to regain its polarity.
Synapses
Neurons are independent and separated by a small space called the synaptic cleft. A synapse is the functional communication between two neurons. Key elements include:
- Presynaptic membrane: The axon terminal of the neuron sending the information, also called the terminal button.
- Postsynaptic area: The specialized part of another neuron receiving the information.
Transmission across the synapse is usually carried out by chemical substances called neurotransmitters. These can act as activators or inhibitors, depending on the postsynaptic neuron. Neurotransmitters are stored in synaptic vesicles within the terminal buttons. These buttons synapse with the cell bodies or dendrites of other neurons. The arrival of a nerve impulse at the terminal buttons causes the emptying of synaptic vesicles, releasing the neurotransmitter into the synaptic cleft.