16.3: Functions of the Nervous System
The nervous system is responsible for coordinating and regulating the body's functions. It functions through three main processes: sensory, integrative, and motor processes. Sensory function involves the detection and transmission of information about internal and external stimuli from sensory receptors to the CNS. The CNS processes this information through an integrative function, where it interprets and makes decisions based on the incoming sensory information. Finally, the motor function involves the transmission of instructions from the CNS to the effector organs (muscles and glands) to produce a response.
Nervous tissue is composed of two types of cells: neurons and glial cells. Neurons are excitable cells responsible for transmitting nerve impulses, which are electrical signals that travel through the nervous system. These impulses are generated by the movement of ions across the neuron's membrane, which creates a difference in electrical charge called the resting potential. When the neuron is stimulated, this resting potential is altered, triggering the generation of an action potential. The action potential travels down the length of the neuron's axon and is propagated to the next neuron or effector organ through the release of neurotransmitters. Glial cells provide support and nourishment to neurons and help to maintain the extracellular environment necessary for proper nerve impulse transmission.
In an example where an athlete sees a ball and kicks it, light from the ball enters the eye and is focused on the retina. Photoreceptor cells in the retina (rods and cones) convert the light into electrical signals (graded potentials) that are transmitted through the optic nerve to the brain. The visual signals from the retina are sent to the visual cortex in the occipital lobe of the brain. Here, the signals are integrated with other sensory information and memories, allowing the brain to recognize the ball and decide to kick it. The motor cortex in the frontal lobe of the brain sends a signal down the motor neurons in the spinal cord to the muscles in the leg, where it causes the muscles to contract and kick the ball. In this example, sensation occurs when graded potentials are generated in the photoreceptor cells of the retina and travel through the optic nerve to the brain. In the brain, the signals are integrated, and an action potential is generated in the motor cortex, which travels down the spinal cord to the muscles, causing a response. At the synapses between neurons, the electrical signal is converted to a chemical signal via neurotransmitters that cross the synapse and trigger the generation of a new electrical signal in the next neuron.
