Introduction
Neurons and synapses are fundamental components of the nervous system, responsible for transmitting and processing information throughout the body. This study note will delve into the structure and function of neurons, the nature of nerve impulses, and the mechanisms of synaptic transmission, as outlined in the International Baccalaureate (IB) Biology syllabus, particularly focusing on topic 6.5.
The Nervous System
The human nervous system consists of two main parts:
- Central Nervous System (CNS): Comprising the brain and spinal cord.
- Peripheral Nervous System (PNS): Comprising all the nerves outside the CNS.
The nervous system allows us to perceive our environment, respond to stimuli, and regulate bodily functions. Information is transmitted through the nervous system in the form of electrical impulses that travel along neurons.
Neurons: Structure and Function
Neurons are specialized cells designed to transmit information. They have several key features:
- Axon: A long, slender projection that conducts electrical impulses away from the neuron's cell body.
- Cell Body: Contains the nucleus and other organelles necessary for cell function.
- Dendrites: Branch-like structures that receive impulses from other neurons.
The cell body and dendrites form the receptive region of the neuron, while the axon is responsible for transmitting impulses.
Types of Neurons
Neurons can be classified based on their function:
- Sensory Neurons: Transmit sensory information from receptors to the CNS.
- Motor Neurons: Transmit commands from the CNS to muscles and glands.
- Interneurons: Connect neurons within the CNS, facilitating communication between sensory and motor neurons.
Nerve Impulses
Nerve impulses are the means by which information is transmitted along neurons. These impulses are not continuous electrical currents but rather momentary reversals in the electrical potential difference across the neuron's membrane.
Resting Potential
In a resting neuron, the inside of the axon has a negative electrical potential compared to the outside. This resting potential is typically around -70 millivolts (mV). It is maintained by:
- Sodium-Potassium Pumps: Actively transport $Na^+$ out of the neuron and $K^+$ into the neuron at a ratio of 3:2.
- Differential Permeability: The membrane is more permeable to $K^+$ than $Na^+$, allowing $K^+$ to diffuse out more easily, contributing to the negative charge inside the neuron.
$$ \text{Resting potential} = -70 \text{ mV} $$
Common MistakeIt is a common misconception that the resting potential is due solely to the sodium-potassium pump. In reality, differential permeability and negatively charged proteins inside the neuron also play crucial roles.
Action Potential
When a neuron is stimulated, its membrane potential changes, leading to an action potential. This involves:
- Depolarization: Sodium channels open, allowing $Na^+$ to rush in, making the inside of the neuron more positive.
