The human brain and nervous system control sensing, movement, thinking, memory, emotion, and body regulation. This cheat sheet helps students organize the main parts of the nervous system and understand how nerve signals travel. It is useful for reviewing brain regions, neuron structure, reflex pathways, and communication between neurons.
These ideas are central to biology, anatomy, and health science courses.
Key Facts
- The central nervous system, or CNS, includes the brain and spinal cord, while the peripheral nervous system, or PNS, includes nerves outside the CNS.
- The somatic nervous system controls voluntary skeletal muscle movement, while the autonomic nervous system controls involuntary functions such as heart rate, digestion, and breathing rate.
- A typical neuron pathway is dendrites receive signals, the cell body processes signals, and the axon sends signals to other cells.
- The sodium-potassium pump helps maintain resting potential by moving 3 Na+ out of the neuron and 2 K+ into the neuron.
- A resting neuron has an electrical potential of about -70 mV, meaning the inside is more negative than the outside.
- An action potential begins when depolarization reaches threshold, usually about -55 mV, causing voltage-gated Na+ channels to open.
- A reflex arc follows the basic pathway receptor, sensory neuron, interneuron in the spinal cord, motor neuron, effector.
- At a chemical synapse, neurotransmitters are released from the presynaptic neuron, cross the synaptic cleft, and bind receptors on the postsynaptic cell.
Vocabulary
- Neuron
- A specialized nerve cell that receives, processes, and transmits electrical and chemical signals.
- Central Nervous System
- The division of the nervous system made of the brain and spinal cord.
- Peripheral Nervous System
- The division of the nervous system made of nerves that connect the CNS to the rest of the body.
- Action Potential
- A rapid change in a neuron's membrane potential that carries a nerve signal along the axon.
- Synapse
- The junction where one neuron communicates with another neuron, muscle cell, or gland cell.
- Neurotransmitter
- A chemical messenger released by a neuron to signal another cell across a synapse.
Common Mistakes to Avoid
- Confusing the CNS and PNS is wrong because the CNS is only the brain and spinal cord, while the PNS includes the nerves outside them.
- Thinking all nervous system actions are voluntary is wrong because the autonomic nervous system controls many involuntary processes such as digestion and heart rate.
- Reversing neuron signal direction is wrong because signals usually enter through dendrites, pass through the cell body, and travel away along the axon.
- Saying reflexes must be processed by the brain first is wrong because many reflex arcs are coordinated through the spinal cord for a faster response.
- Forgetting neurotransmitter receptors is wrong because neurotransmitters affect the next cell only if they bind to matching receptors on the postsynaptic membrane.
Practice Questions
- 1 A neuron has a resting potential of -70 mV and reaches threshold at -55 mV. By how many millivolts must the membrane potential increase to reach threshold?
- 2 If a sodium-potassium pump completes 8 cycles, how many Na+ ions are moved out of the neuron and how many K+ ions are moved into the neuron?
- 3 Put these reflex arc steps in the correct order: motor neuron, receptor, sensory neuron, effector, interneuron in the spinal cord.
- 4 Explain why a hand-withdrawal reflex can happen before a person consciously feels pain.
Understanding The Human Brain & Nervous System
Nerve cells work because their membranes keep certain charged particles unevenly distributed. This creates stored electrical energy, much like a tiny battery. When enough incoming signals arrive at one part of a neuron, channels in the membrane change shape.
Charged sodium particles rush inward. The voltage changes rapidly and triggers a travelling pulse. This pulse does not fade as it moves along the axon.
Afterward, other channels help restore the original conditions. There is a brief recovery period when the neuron cannot fire normally. This prevents a signal from travelling backward and keeps messages moving in one direction.
The speed of a nerve signal depends on the neuron. Some axons have a fatty covering called myelin. Myelin acts as insulation around an electrical wire.
It allows the signal to jump between small gaps in the covering, rather than activating every section of membrane. This is faster and uses less energy. Damage to myelin can disrupt movement, vision, or sensation.
At the end of an axon, the electrical signal is usually converted into a chemical message. The released chemical may make the next cell more likely to fire or less likely to fire. Brain activity depends on the balance between these exciting and inhibiting messages.
Reflexes show that a response does not always need a conscious decision from the brain. If a person touches something very hot, sensory information reaches the spinal cord quickly. Nearby circuits can activate muscles to pull the hand away before the brain has fully interpreted the pain.
The brain still receives the information soon afterward, which is why the person becomes aware of what happened. Reflexes protect the body, but they can be adjusted by the brain.
A trained athlete may learn to control a movement that would otherwise happen automatically. The autonomic system works in a similar rapid way to adjust heart rate, pupil size, sweating, and digestion as body conditions change.
Different brain regions cooperate instead of working as isolated boxes. The cerebrum handles many conscious tasks, including planning, language, sensory interpretation, and voluntary movement. The cerebellum compares intended movement with incoming sensory feedback, helping actions become smooth and accurate.
The brainstem manages vital functions that must continue during sleep, including breathing rhythm and blood pressure. Learning changes the strength of connections between neurons. Repeated practice can make a pathway easier to activate, while unused pathways may weaken.
When studying diagrams, follow the direction of information carefully. Notice whether a structure receives information, processes it, sends a response, or regulates a body condition. This prevents confusion between similar terms.