Nerve Cells
Neurons, or nerve cells, are the fundamental units of the nervous system, responsible for transmitting electrical and chemical signals throughout the body.
Sensory/Afferent Neurons
Sensory neurons carry information from sensory receptors towards the central nervous system (CNS). They detect external stimuli, such as light, sound, or temperature, and internal stimuli like changes in blood pressure, transmitting this information to the brain and spinal cord for processing.
Interneurons
Interneurons, found exclusively within the CNS, act as the connectors between sensory and motor neurons. They process information received from sensory neurons and determine the appropriate response. Interneurons play an important role in reflex actions and higher brain functions, such as decision-making and memory.
Motor/Efferent neurons
Motor neurons transmit signals from the CNS to effectors such as muscles or glands, initiating a response. They control voluntary movements, like lifting an arm, and involuntary actions, such as the contraction of the heart.
Structure of neurons
Nerve cells have specialised structures that allow them to efficiently transmit signals:
- Dendrites: Dendrites are branch-like extensions from the soma (cell body) that receive signals from other neurons or sensory receptors. They act as the primary site for signal reception, converting external chemical signals into electrical impulses that travel towards the soma.
- Soma (Cell Body): The soma contains the nucleus and other organelles responsible for maintaining cell function. It integrates incoming signals from the dendrites and generates an action potential if the stimulus is strong enough.
- Axon: The axon is a long, slender projection that conducts electrical impulses away from the soma towards other neurons or effectors. It can vary in length, with some axons extending over a metre in the human body.
- Myelin Sheath: The myelin sheath is a fatty layer that insulates the axon, produced by specialised cells like Schwann cells in the peripheral nervous system and oligodendrocytes in the CNS. This insulation increases the speed of signal transmission by allowing electrical impulses to jump between gaps in the myelin.
- Nodes of Ranvier: These are small gaps in the myelin sheath along the axon. The presence of nodes allows for saltatory conduction, where the action potential jumps from node to node, significantly increasing the speed of nerve impulse transmission.
- Axon Terminal: The axon terminal, located at the end of the axon, is the site where electrical signals are converted into chemical signals. It contains synaptic vesicles filled with neurotransmitters that are released into the synapse when an action potential arrives.
- Synapse: The synapse is the junction between two neurons or between a neuron and an effector. It consists of the axon terminal, the synaptic cleft, and the membrane of the receiving cell. Neurotransmitters released from the axon terminal diffuse across the synaptic cleft, binding to receptors on the postsynaptic cell and triggering a response.
