Human Nervous System
Explore how neurons transmit electrical signals, how the central and peripheral nervous systems coordinate your body, and how reflex arcs protect you from harm.
Neuron Structure
Neurons (nerve cells) are the functional units of the nervous system. They are specialised to transmit electrical impulses rapidly across the body. There are three main types: sensory neurons, motor neurons, and relay (inter) neurons.
Structure of a Motor Neuron
Cell Body
Contains nucleus and organelles
Dendrites
Receive signals from other neurons
Axon
Long fibre carrying impulse away from cell body
Myelin Sheath
Insulates axon; speeds up transmission
Axon Terminals
Release neurotransmitters at synapse
Synaptic Transmission
Electrical impulse arrives at the presynaptic terminal (axon terminal).
Vesicles release neurotransmitters (e.g. acetylcholine) into the synaptic cleft.
Neurotransmitters bind to receptors on the postsynaptic membrane, triggering a new impulse.
Central and Peripheral Nervous Systems
The nervous system is divided into the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS processes information, while the PNS connects the CNS to the rest of the body.
Central Nervous System (CNS)
- Brain: The control centre -- processes sensory input, initiates voluntary actions, stores memories, and regulates homeostasis.
- Spinal cord: A column of nerve tissue running through the vertebrae; relays signals between brain and PNS and coordinates reflexes.
Peripheral Nervous System (PNS)
- Somatic NS: Controls voluntary muscle movements via motor neurons; receives sensory information from receptors.
- Autonomic NS: Controls involuntary functions (heart rate, digestion). Divided into sympathetic (fight-or-flight) and parasympathetic (rest-and-digest) branches.
Key distinction: The somatic nervous system controls voluntary actions (e.g. picking up a pen), while the autonomic nervous system operates involuntarily (e.g. regulating heart rate during exercise).
Reflex Arcs
A reflex arc is a rapid, automatic neural pathway that produces an involuntary response to a stimulus. Reflexes protect the body from harm by bypassing the brain for faster response times.
Reflex Arc Pathway (e.g. touching a hot surface)
Stimulus
Hot surface contacts skin
Receptor
Thermoreceptors in skin detect heat
Sensory Neuron
Carries impulse to the spinal cord (CNS)
Relay Neuron (in spinal cord)
Connects sensory neuron to motor neuron
Motor Neuron
Carries impulse to the effector
Effector (muscle)
Hand muscles contract, pulling hand away
Why reflexes bypass the brain: The relay neuron in the spinal cord connects directly to the motor neuron, producing a response in as little as 0.01 seconds. The brain receives the pain signal slightly later -- which is why you feel the pain after you have already pulled your hand away.
Key Vocabulary
Neurotransmitter
A chemical messenger released at a synapse that transmits signals between neurons (e.g. acetylcholine, dopamine).
Myelin Sheath
A fatty insulating layer surrounding the axon that increases the speed of electrical impulse transmission via saltatory conduction.
Synapse
The junction between two neurons where information is transmitted chemically across a small gap (the synaptic cleft).
Reflex Arc
A rapid, involuntary neural pathway: stimulus → receptor → sensory neuron → relay neuron → motor neuron → effector.
Worked Examples
Describe the sequence of events when a person steps on a sharp object.
Step 1: Pain receptors (nociceptors) in the sole of the foot detect the sharp stimulus and generate an electrical impulse.
Step 2: The impulse travels along a sensory neuron to the spinal cord.
Step 3: In the spinal cord, a relay neuron passes the signal directly to a motor neuron.
Step 4: The motor neuron stimulates the effector (leg muscles), which contract to lift the foot away from the sharp object -- all before the brain consciously registers pain.
Explain how the myelin sheath increases the speed of nerve impulse transmission.
Step 1: The myelin sheath is a fatty insulating layer that wraps around the axon in segments, with gaps called Nodes of Ranvier.
Step 2: Electrical impulses cannot pass through the myelin, so the impulse "jumps" from one node to the next -- this is called saltatory conduction.
Step 3: This jumping mechanism is much faster than continuous conduction along an unmyelinated axon. Myelinated neurons can transmit impulses at speeds up to 120 m/s, compared to about 1 m/s in unmyelinated fibres.
Compare the sympathetic and parasympathetic divisions of the autonomic nervous system.
Sympathetic NS (fight-or-flight): Activated during stress or danger. Increases heart rate, dilates pupils, diverts blood to skeletal muscles, and inhibits digestion. Prepares the body for rapid action.
Parasympathetic NS (rest-and-digest): Dominant during relaxation. Decreases heart rate, constricts pupils, stimulates digestion, and promotes energy storage. The two divisions are antagonistic -- they have opposite effects to maintain homeostasis.
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
Which part of a neuron receives signals from other neurons?
Question 2
What is the role of neurotransmitters at a synapse?
Question 3
Which component of the nervous system is primarily responsible for reflex actions?
Question 4
The sympathetic nervous system would cause which of the following responses?
Question 5
What is the correct order of a reflex arc?
Key Concepts Summary
- ●Neurons transmit electrical impulses. Key parts: dendrites (receive), cell body, axon (transmit), axon terminals (release neurotransmitters).
- ●Synapses use chemical neurotransmitters to pass signals between neurons across the synaptic cleft.
- ●The CNS (brain and spinal cord) processes information; the PNS connects the CNS to the body.
- ●The autonomic NS has sympathetic (fight-or-flight) and parasympathetic (rest-and-digest) divisions that are antagonistic.
- ●Reflex arcs produce rapid involuntary responses by routing signals through the spinal cord, bypassing the brain.