Makindo Medical Notes

Anatomy and Structure of Neurons

Neurons are the fundamental units of the nervous system, responsible for transmitting electrical and chemical signals throughout the body. They are specialized cells designed to communicate information between different parts of the body and the brain. A typical neuron has three main parts: the cell body, dendrites, and axon.

Parts of a Neuron

• Cell Body (Soma):
  • The cell body contains the nucleus and other organelles that are essential for the neuron's metabolic activities, such as mitochondria, endoplasmic reticulum, and ribosomes.
  • The nucleus holds the cell's genetic material (DNA) and is involved in gene expression, which controls the function and maintenance of the neuron.
• Dendrites:
  • Dendrites are branched, tree-like structures that extend from the cell body and receive signals from other neurons. They conduct electrical messages toward the cell body in the form of graded potentials.
  • Dendrites increase the surface area available for synaptic connections, enabling neurons to communicate with multiple other neurons.
• Axon:
  • The axon is a long, slender projection that transmits electrical impulses (action potentials) away from the cell body to other neurons, muscles, or glands.
  • Axons can vary in length, from a few millimeters to more than a meter (in motor neurons connecting the spinal cord to muscles).
  • At the end of the axon, there are terminal branches (axon terminals) that form synapses with other neurons or target cells.

Images

Specialized Structures of Neurons

• Myelin Sheath:
  • The myelin sheath is a fatty, insulating layer that wraps around the axon of some neurons, produced by Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system.
  • Myelin increases the speed of action potential conduction through a process called saltatory conduction, where the impulse jumps between gaps in the sheath called Nodes of Ranvier.
• Nodes of Ranvier:
  • These are small gaps between the myelin sheath segments along the axon where ion exchange occurs during the transmission of an action potential.
  • They help speed up the transmission of the electrical signal by allowing it to jump from node to node.
• Axon Terminals:
  • The axon terminals are the endpoints of the axon, where the neuron communicates with target cells through synapses. The terminals contain synaptic vesicles filled with neurotransmitters.
  • When an action potential reaches the axon terminal, it triggers the release of neurotransmitters into the synaptic cleft, allowing the signal to be transmitted to the next neuron or effector cell.

Types of Neurons

Neurons are classified into three major types based on their function:

• Sensory Neurons:
  • Also known as afferent neurons, sensory neurons carry information from sensory receptors (such as those in the skin, eyes, or nose) to the central nervous system (brain and spinal cord).
  • They detect stimuli such as light, sound, heat, and pressure and convert them into electrical signals for processing by the brain.
• Motor Neurons:
  • Also known as efferent neurons, motor neurons transmit signals from the central nervous system to muscles or glands to trigger movement or secretion.
  • They are involved in voluntary and involuntary motor control, such as moving limbs or regulating the heart rate.
• Interneurons:
  • Interneurons, also called association neurons, connect sensory and motor neurons within the central nervous system. They act as intermediaries, processing information and creating complex reflexes or responses.
  • Interneurons are responsible for higher functions such as decision-making, cognition, and reflex arcs.

Functions of Neurons

• Signal Transmission: Neurons transmit electrical impulses (action potentials) and chemical signals (neurotransmitters) to other neurons, muscles, or glands. This allows communication across different regions of the body, facilitating movement, sensation, and cognitive functions.
• Synaptic Transmission: Neurons communicate at synapses, where an action potential reaching the axon terminal triggers the release of neurotransmitters. These chemical messengers cross the synaptic cleft and bind to receptors on the post-synaptic cell, initiating a response.
• Integration of Signals: Neurons can receive multiple inputs from other neurons via their dendrites. They integrate these signals, summing them at the axon hillock to determine whether an action potential will be generated.
• Plasticity: Neurons exhibit plasticity, meaning they can adapt and reorganize in response to experience, learning, or injury. This is especially evident in processes such as learning, memory formation, and recovery from neural damage.

Neurotransmitters

Neurotransmitters are chemical messengers released by neurons at synapses. Different neurotransmitters have varying effects on target cells.

• Excitatory Neurotransmitters: Increase the likelihood of generating an action potential in the target neuron. Examples include glutamate and acetylcholine.
• Inhibitory Neurotransmitters: Decrease the likelihood of an action potential by hyperpolarizing the target neuron. Examples include gamma-aminobutyric acid (GABA) and glycine.
• Modulatory Neurotransmitters: These neurotransmitters modulate the effects of other neurotransmitters and can have more prolonged effects on the neuron. Examples include dopamine, serotonin, and norepinephrine.

Nerve Impulse (Action Potential)

• Resting Potential: The resting potential of a neuron is the electrical charge across its membrane when it is not actively transmitting a signal, typically around -70 mV.
• Depolarization: When a neuron is stimulated, sodium channels open, allowing sodium ions to enter the cell, causing the membrane potential to become more positive (depolarization).
• Repolarization: After the action potential is transmitted, potassium channels open, allowing potassium ions to leave the cell, returning the membrane potential to its resting state (repolarization).
• Refractory Period: After an action potential, the neuron briefly becomes unresponsive to further stimulation, ensuring the signal only travels in one direction.