Nervous System: Structure, Function, and Signal Transmission
The Nervous System: Structure and Function
The nervous system, composed of nervous tissue, is responsible for regulating bodily functions through nerve impulses. In contrast, the endocrine system, consisting of endocrine glands, coordinates and regulates certain physiological functions through hormones transported by the blood. The nervous system’s response is rapid, short-lived, and very specific, whereas the hormonal response is slow, lasting, and of variable specificity.
The functions that require rapid and short-lived responses are directly regulated by the nervous system, while those requiring slow and continuous action are controlled by the endocrine system.
The nervous system receives, integrates, and transmits information from both the external and internal environment. It also coordinates and controls the organism’s responses to these stimuli. The nervous system is constituted of nervous tissue, formed by a cluster of cells called neurons, which are the basic functional units.
The nervous system regulates and coordinates organ function through nerve impulses; this is termed nervous coordination. It involves the following components:
- Receptors: Specialized parts of cells (forming the sense organs). They capture stimuli (both external and internal) and initiate the transmission of information via nerve impulses.
- Sensory nerve pathways: Transmission routes that conduct nerve impulses from receptors to modulators.
- Modulators: Organs that interpret impulses and develop precise orders.
- Motor nerve pathways: Carry orders from the modulators to the effectors.
- Effectors: Organs that receive impulses transmitted by the motor pathways and perform the imposed action.
Nerve Impulse Transmission
The transmission of signals to neurons is called a nerve impulse. This is due to electrical and chemical changes in the plasma membrane that separates the nerve cell from its extracellular environment. The nerve impulse travels along the plasma membrane of the neuron, and when it reaches the end of the axon, it can be transmitted to another neuron.
Resting and Action Potentials
The plasma membrane (MP) is polarized; its inner surface has a potential difference with respect to the outside. Inside, there is a dominance of negative charges, while outside, there is a dominance of Na+. This potential difference (-70 mV) is called the resting potential.
An alteration in the membrane’s permeability allows the entry of Na+ ions and reverses the polarity (positive inside, negative outside), causing depolarization. This is observed as a variation of the resting potential (-70mV to +30mV), known as the action potential. The electrical disturbances of depolarization spread to adjacent areas from the point where the stimulus was applied, propagating throughout the neuron. Enzymes in the membrane then extract Na+ and restore the initial state (repolarization).
Threshold of Excitability and All-or-Nothing Law
For a stimulus to be effective, it must have a minimum intensity called the threshold of excitability. Below this threshold, the impulse will not be initiated. This is known as the law of all or nothing: when a stimulus is strong enough to initiate an impulse, the impulse is conducted regardless of the stimulus’s nature or intensity. The speed of propagation depends only on the type of nerve fiber and its diameter.
After one impulse has started, another cannot immediately begin (refractory period). The neuron takes time to recover its polarity.