The nervous system is responsible for carrying messages throughout the body. It is made up of specialized cells called neurons that can quickly send signals over long distances. Neurons are organized into circuits and networks that allow the brain and spinal cord to communicate with the rest of the body.
How do neurons send signals?
Neurons have a cell body containing the nucleus and organelles. Branching out from the cell body are extensions called dendrites and a single long fiber called the axon. Dendrites receive signals from other neurons, while the axon transmits signals to other cells.
When a neuron is stimulated, it generates an electrical signal that travels down the axon. This electrical signal is called an action potential. It is created by the movement of charged atoms (ions) across the neuron’s cell membrane. At the end of the axon are terminal buttons that connect to other cells. When an action potential reaches the terminal buttons, it triggers the release of chemical messenger molecules called neurotransmitters.
How do neurotransmitters work?
The neurotransmitters diffuse across the small gap between the terminal button of the transmitting neuron and the receiving cell. The receiving cell has receptor proteins that the neurotransmitters can bind to, just like a lock and key. This binding causes changes in the receiving cell, either exciting it or inhibiting it from generating its own electrical signal.
There are many different types of neurotransmitters, including:
The effects of a neurotransmitter depend on the type of receptor it binds to. For example, binding of acetylcholine to nicotinic receptors excites cells, while binding to muscarinic receptors inhibits cells. The complex interplay between different neurotransmitters shapes the signaling in the nervous system.
How are neurons organized in the nervous system?
The nervous system contains billions of neurons, organized and interconnected into specialized circuits and pathways:
- Central nervous system – The brain and spinal cord. Receives sensory information, processes it, and sends instructions out to the body.
- Peripheral nervous system – Nerves that connect the central nervous system to the rest of the body. Has two divisions:
- Somatic nervous system – Carries signals from sensory receptors to the CNS and motor commands from the CNS to muscles and glands.
- Autonomic nervous system – Regulates involuntary functions like heart rate, breathing, digestion, etc. Has sympathetic and parasympathetic divisions with opposing effects.
Sensory neurons carry signals from sensory receptors that detect stimuli like light, sound, touch, pain, and temperature to the brain. For example:
- Photoreceptors in the retina detect light and connect to neurons that carry visual information to the brain.
- Auditory hair cells in the ear detect sound vibrations and connect to neurons that carry auditory information to the brain.
- Touch receptors in skin connect to neurons that carry tactile information to the spinal cord and brain.
Motor neurons carry signals from the brain and spinal cord to effector cells like muscles and glands, causing them to take action. For example:
- Lower motor neurons connect the spinal cord to skeletal muscles to control movement.
- Upper motor neurons connect the brain to lower motor neurons in the spine.
- Cranial nerve motor neurons connect the brainstem to face and neck muscles.
- Autonomic motor neurons control smooth muscles, cardiac muscle, and glands.
Interneurons connect sensory and motor neurons within the brain and spinal cord, integrating information and influencing the output signals. This allows for more complex information processing and control of behaviors.
How fast do nerve signals travel?
Nerve signals travel very rapidly along neurons due to the myelin sheath. Myelin is a fatty insulating layer wrapped around the axons of many neurons. It prevents the electrical signal from dissipating out of the axon, allowing it to travel faster. Here are some nerve conduction velocities:
|0.5 – 10 m/s
|Small myelinated neurons
|10 – 100 m/s
|Large myelinated neurons
|70 – 120 m/s
This speed allows signals to travel from your toes to your brain in less than 100 milliseconds! Rapid conduction velocity is important for many time-sensitive reflexes and responses.
How do nerve signals cross between neurons?
The small gaps between neurons are called chemical synapses. At most chemical synapses, the signal must be converted from electrical to chemical and back to electrical again to cross between cells:
- Electrical signal (action potential) reaches the presynaptic terminal button and causes neurotransmitter release.
- Neurotransmitter diffuses across synapse and binds to receptors on postsynaptic neuron.
- Binding causes excitatory or inhibitory postsynaptic potential by opening or closing ion channels.
- Summation of these potentials triggers a new action potential in the postsynaptic neuron.
However, at electrical synapses, the signal passes directly between neurons through gap junction channels, allowing faster transmission. Electrical synapses are found in escape circuits, cardiac muscle, and other systems requiring speed.
How are nerve cell connections strengthened?
When two neurons communicate frequently, the strength of the synaptic connection between them is increased. This process underlies learning and memory formation in the brain. Strengthening of connections between neurons is called synaptic plasticity. Two major mechanisms are:
- Long-term potentiation (LTP) – Persistent increase in signal transmission between two neurons following repeated stimulation. Caused by changes in neurotransmitter release, receptor number, and other factors.
- Synaptic remodeling – Physical changes in the structure of the synapse, including increased size, number of receptors, and number of synaptic terminals.
Research shows that the more active a synapse is, the stronger it becomes. Nerve connections that are not used regularly will weaken over time. This ability of neural connections to change underlies the brain’s amazing capacity to adapt, learn new skills, and form memories.
In summary, the nervous system uses specialized neurons organized into circuits and pathways to rapidly transmit electrochemical signals between different parts of the body. Action potentials travel along axons and cross synapses through neurotransmitters to communicate sensory input, motor commands, and integrated information. Myelination allows fast conduction, and synaptic plasticity through mechanisms like LTP allows learning. The complex interconnections between billions of neurons creates a remarkably adaptable communication network that allows our brains to perform amazing computational feats!