12.3 Neurones and Synapse
The nervous system consists of millions of specialised nerve cells called neurones. Neurones transmit information throughout the body as electrical signals known as nerve impulses. To ensure coordinated responses, nerve impulses travel through distinct types of neurones—sensory neurones, relay neurones, and motor neurones—and cross specialised microscopic gaps known as synapses via chemical neurotransmitters.
Learning Objectives
- Identify the main parts of a generalised neurone and state the function of each part.
- Compare the structures, locations, and functions of sensory neurones, relay neurones, and motor neurones.
- Describe the structure of a synapse and explain the steps involved in the transmission of nerve impulses across a synapse.
- Explain the role of neurotransmitters and mitochondria in synaptic transmission.
Structure of a Synapse and Chemical Transmission
A synapse is a narrow microscopic gap between two neurones. Nerve impulses cross this gap chemically through neurotransmitters.
1. Release
An electrical impulse arrives at the synaptic knob, stimulating synaptic vesicles to release neurotransmitters into the synapse via exocytosis.
2. Diffusion
Neurotransmitters diffuse across the narrow synaptic gap and bind to specific receptor proteins on the dendrite membrane of the receiving neurone.
3. Binding & Impulse Initiation
The binding of neurotransmitters to specific receptor proteins triggers the initiation of a new electrical nerve impulse in the receiving neurone.
Short Explanation
What is a neurone?
A neurone is the functional unit of the human nervous system. It consists of dendrites that receive impulses, a cell body that coordinates cellular activities, and an axon that conducts impulses away to synaptic knobs.
Myelin sheath and nodes of Ranvier
To speed up impulse transmission, axons are wrapped in an insulating myelin sheath with periodic breaks called nodes of Ranvier, allowing impulses to jump rapidly from node to node.
Three-neurone pathway
Information travels through a three-neurone pathway: sensory neurones convey sensory signals from receptors to the CNS; relay neurones integrate and transmit signals within the CNS; and motor neurones carry commands from the CNS to effectors.
Synaptic transmission
Between adjacent neurones lies a gap called a synapse. Because electrical impulses cannot jump across this physical gap directly, the signal is converted into a chemical message.
Unidirectional flow
Neurotransmitters such as acetylcholine are released from vesicles in the synaptic knob, diffuse across the synaptic cleft, and bind to receptors on the receiving neurone to initiate a new electrical impulse, ensuring unidirectional flow.
Types of Neurones
Location: Found in the dorsal root of the spinal nerve; its cell body is located in the dorsal root ganglion.
Structure: Has long dendrons/dendrites that receive impulses from sensory receptors, a cell body situated along the side/middle of the axon in the ganglion, and an axon that carries impulses into the central nervous system.
Function: Carries nerve impulses from sensory organ receptors to the central nervous system (CNS).
Location: Found entirely within the central nervous system (brain and spinal cord); its cell body is present in clusters in the grey matter of the CNS.
Structure: Short, highly branched dendrites and axon.
Function: Connects the sensory neurone to the motor neurone; transmits nerve impulses from the sensory neurone to the CNS and within the CNS to the motor neurone.
Location: Found in the ventral root of the spinal nerve; its cell body is located in the grey matter of the spinal cord.
Structure: Features a large terminal cell body with multiple short dendrites and a long axon terminating at effectors.
Function: Receives nerve impulses from the relay neurone in the CNS and transmits them to effectors (muscles or glands) to produce an appropriate response.
Try to Answer First
Answer in your mind, then press “Check Answer”.
What is the function of the node of Ranvier on a myelinated neurone?
Where is the cell body of a sensory neurone located?
Why are there numerous mitochondria inside the synaptic knob?
Common Mistakes
- Confusing the location of sensory and motor neurone cell bodies: Students often incorrectly state that motor neurone cell bodies are in the ganglion, whereas sensory neurone cell bodies are in the dorsal root ganglion and motor neurone cell bodies are inside the grey matter of the spinal cord.
- Thinking nerve impulses travel across the synapse electrically: Nerve impulses travel electrically along the axon, but transmission across the synaptic gap is strictly chemical via neurotransmitters.
- Stating that dendrites carry impulses away from the cell body: Dendrites carry impulses towards the cell body, whereas axons carry impulses away from the cell body.
- Forgetting the directionality of synaptic transmission: Assuming impulses can travel both ways across a synapse; impulses can only travel from the presynaptic knob to the postsynaptic dendrite.
Concept Misunderstandings
The myelin sheath slows down the nerve impulse because it covers the axon.
The myelin sheath acts as an electrical insulator. Because the impulse cannot flow through the insulated myelin, it jumps from one uninsulated node of Ranvier to the next, significantly accelerating transmission.
Neurotransmitters enter into the receiving neurone to transmit the signal.
Neurotransmitters do not enter the cytoplasm of the receiving neurone; they bind to specific receptor proteins on the outer membrane surface of the dendrite, which triggers a new electrical impulse.
Sensory neurones connect directly to muscles.
Sensory neurones send impulses to the central nervous system (often via relay neurones); motor neurones are the ones that connect directly to effectors like muscles or glands.
Summary
- Neurone parts: Cell body (integrates signals), dendrites (receive impulses), axon (carries impulses away), myelin sheath (insulates, protects, nourishes), nodes of Ranvier (accelerates impulse speed), synaptic knob (secretes neurotransmitters).
- Sensory neurone: Dorsal root; cell body in dorsal root ganglion; transmits impulses from receptors to CNS.
- Relay neurone: Entirely in CNS grey matter; connects sensory neurone to motor neurone.
- Motor neurone: Ventral root; cell body in grey matter of spinal cord; transmits impulses from CNS to effectors (muscles/glands).
- Synaptic transmission: Electrical impulse reaches synaptic knob → synaptic vesicles release neurotransmitters → chemical diffusion across synapse → binding to specific receptor proteins on receiving dendrite → initiation of new electrical impulse.
- One-way transmission: Ensured by synapses because neurotransmitters are only stored in presynaptic vesicles and receptors are only present on postsynaptic membranes.
Short Activity
Objective: identify neurone parts, types of neurones, and the steps of synaptic transmission.
A. Objective Quiz
Which part of a neurone accelerates the flow of nerve impulses by allowing them to jump from one gap to the following gap?
What happens immediately after neurotransmitters diffuse across the synaptic gap?
B. Fill in the Blanks
The cell body of a sensory neurone is located in the dorsal root of the spinal nerve.
Synaptic knobs contain numerous to generate the energy required for the transmission of nerve impulses.
C. Matching / Drag and Drop
Match each type of neurone or synaptic structure to its correct function or feature. On a phone, tap a choice first, then tap the matching box. Tap the Answer Choices area to return a selected item.
C. Match the Structure / Neurone to Its Function / Feature
Write the correct structure or neurone next to each function or feature.
| Function / Feature | Structure / Neurone |
|---|---|
| 1. Carries nerve impulses from sensory receptors to the central nervous system | |
| 2. Connects the sensory neurone to the motor neurone within the central nervous system | |
| 3. Transmits impulses from the central nervous system to effectors such as muscles or glands | |
| 4. Insulating membrane that protects the axon and prevents electrical leakage |
Answer choices: Sensory neurone • Relay neurone • Motor neurone • Myelin sheath
