Lesson 1.4: Control of Body Processes through Nervous System

• The fundamental process of neural transmission occurs in all types of neurons. This transmission, in order to be successful, requires the generation of nerve impulses.
• A nerve impulse is a signal transmitted along a nerve fiber. Neurons are excitable cells. They need to be stimulated to start the generation of signal transmission.
• Neurons are able to transmit signals through changes in the electrical potential of their membrane by the movement of ions across the membrane.
• An electrochemical gradient, or the difference in electric charges between the inner and outer parts of the neuron, governs the movement of these ions which results in an electrical impulse.
• The generation of nerve impulses can be divided into two phases: the resting membrane potential and action potential.
• Within the phases are the four major states or events that lead to the generation of an impulse: polarization, depolarization, propagation of impulse, and repolarization
• The generation of a nerve impulse starts with the first phase called the resting membrane potential, which means that the cell is not firing an impulse
• Resting Membrane Potential

  • This is established by the unequal distribution of sodium ions (Na+ ) outside of the cell and potassium ions (K+ ) inside the cell, making the outside of the cell more positively charged compared to the inside
• This state of the neuron is called polarization, which means that it is not carrying an electrical impulse
• The electrochemical gradient is established and maintained by the sodium-potassium pump
• When a neuron is stimulated by any stimulus, Na+ ion channels open and Na+ ions flow into the cell. This leads to a change in the potential across the membrane called depolarization.
• Action potentials are electrical impulses that maintain their amplitude and strength down the length of the axon
• The action potential travels down the axon when the depolarization of an area of the membrane causes adjacent Na+ ion channels to open. This is the propagation state.
• The influx of sodium ions results in membrane depolarization along the membrane.
• After a very short delay, K + ion channels open and K + ions flow out, restoring the positive charge outside and the negative charge inside of the neuron through the process called repolarization.
• For the neuron to fire again, the resting membrane potential needs to be reestablished.
• Sodium-potassium pump is used to move Na+ out and letting K + ions in, reestablishing the resting membrane potential
• The transmission of electrical impulses is very rapid and takes place in milliseconds. An individual neuron is capable of transmitting hundreds of action potentials (impulses) each second. This rapid transmission is reinforced by the myelin sheath wrapped around the neural processes (axons and dendrites)
• Neurons communicate with each other through the extracellular gaps, called synapses, between one neuron’s axon and another neuron’s dendrite.
• At the axon terminal, the electrical impulse generated through a series of events passes to another cell through these synapses
• The space between the presynaptic neuron and a postsynaptic cell is called the synaptic cleft
• The presynaptic neuron contains signal molecules called neurotransmitters that are packaged inside synaptic vesicles
• When the action potential reaches the end of a neuron, neurotransmitters are released from the neuron into the synaptic cleft.
• Neurotransmitters bind to the adjacent cell at receptor sites attached to ion channels. The channels open, allowing the movement of ions into or out of the effector cell, which alters its membrane potential, thereby transmitting the signal from the neuron to the effector cells
• If you can notice, the transmission of an impulse is an electrochemical event
• The transmission down the length of the neuron’s membrane is electrical, but the next neuron is stimulated by a neurotransmitter, which is a chemical.
• Reflexes are rapid, instantaneous, and involuntary responses of the body to stimuli that do not undergo integration in the brain. They are controlled by the somatic nervous system of the PNS. Reflexes occur over neural pathways called reflex arcs.
• Reflex arcs vary in terms of structure and components. Most reflex arcs have five main components, namely the receptors that receive the stimulus, the sensory neurons that send signals to the spinal cord, interneurons (association neurons) that serve as a middleman between the two neurons, the motor neurons that carry the impulse from the spinal cord, and the effector muscles that carry out the reflex response.