Unit 2 Topic C: Biological Psychology — The Nervous System, Neurons, and Synaptic Transmission
Welcome to one of the most foundational topics in biological psychology! Have you ever wondered how your brain tells your hand to catch a ball, or how chemical messengers influence your mood, thoughts, and actions? In this chapter, we explore the body's internal communication network: the central nervous system (CNS), the specialised cells that carry information (neurons), and the chemical process of passing messages across gaps (synaptic transmission).
Note: Later chapters in Topic C will examine how specific brain structures, hormones, and genes relate to behaviours such as aggression; here, we focus strictly on the fundamental neural machinery.
---1. The Central Nervous System (CNS)
The human nervous system is divided into different branches, but the master control centre of the body is the Central Nervous System (CNS).
Key Components of the CNS
The CNS consists of two primary structures:
1. The Brain: The control hub responsible for coordinating bodily functions, processing sensory inputs, making decisions, and generating conscious experience and complex behaviours.
2. The Spinal Cord: A long column of neural tissue running down the spine. It connects the brain to the rest of the body, carrying outgoing motor commands and incoming sensory data. It also manages simple, rapid reflex actions without waiting for the brain's direction.
The Role of the CNS in Human Behaviour
The CNS operates through a continuous cycle of three stages:
• Receiving Information: The CNS collects sensory input from both the external environment and internal organs.
• Processing and Integrating: The brain analyses this incoming information, draws upon stored experiences or memories, and decides on an appropriate response.
• Coordinating Response: The CNS sends out instructions through electrical impulses down the spinal cord to muscles and organs to produce behaviour.
Key Takeaway: The CNS is made up of the brain and spinal cord; it acts as the central command system that receives sensory input, processes it, and generates behavioural responses.
---2. The Structure and Role of the Neuron
The CNS is composed of billions of specialised communication cells called neurons (nerve cells). Neurons are designed to transmit information throughout the nervous system via both electrical impulses and chemical signals.
Structure of a Typical Neuron
Every neuron has specialised anatomical features that allow it to process and relay information efficiently:
• Dendrites: Branch-like extensions at the receiving end of the neuron. Their job is to receive chemical signals from neighbouring neurons and convert them into small electrical impulses directed toward the cell body.
• Cell Body (Soma): The control centre of the neuron containing the nucleus, which houses the genetic material and maintains cell life and function.
• Axon Hillock: The junction between the cell body and the axon where incoming electrical signals are summed together. If the threshold is reached, an electrical impulse is initiated.
• Axon: A long, slender cable-like fibre extending from the cell body that carries the electrical impulse (the action potential) away from the soma towards other neurons or effectors.
• Myelin Sheath: A fatty, insulating layer surrounding many axons. It protects the axon and dramatically speeds up electrical impulse conduction.
• Nodes of Ranvier: Periodic gaps in the myelin sheath along the axon. The electrical impulse "jumps" from one node to the next, which significantly increases the speed of transmission.
• Axon Terminals (Terminal Buttons): The swollen endings of the axon branches that lie close to the dendrites of the next neuron. They contain synaptic vesicles filled with chemical messengers known as neurotransmitters.
Analogy: The Neuron as a Tree
Imagine a neuron as a tree: the dendrites are the tree branches gathering sunlight (signals); the trunk is the axon carrying that energy down; the bark is the myelin sheath protecting the trunk; and the roots are the axon terminals spreading out to deliver chemicals into the soil!
The Role of the Neuron in Information Transmission
The primary role of a neuron is to transmit signals rapidly across distances. Within a single neuron, this transmission is electrical (an action potential travels from dendrite/soma to the terminal button). Between neurons, however, communication changes to a chemical signal across the microscopic gap separating them.
Key Takeaway: Neurons receive messages through dendrites, process them in the cell body, send an electrical signal down the myelinated axon, and prepare to release chemicals from the terminal buttons.
---3. Neurotransmitters and Their Functions
When an electrical impulse reaches the end of an axon, it cannot jump across the physical gap to the next neuron. It must be converted into a chemical message. These chemical messengers are called neurotransmitters.
What is a Neurotransmitter?
A neurotransmitter is a chemical substance produced by a neuron, stored in synaptic vesicles, and released across the synaptic cleft to transmit, amplify, or modulate signals between a neuron and another cell.
Excitatory vs. Inhibitory Neurotransmitters
Neurotransmitters do not all have the same effect on the receiving neuron. They generally fall into two functional categories:
• Excitatory Neurotransmitters: These act like an "accelerator pedal". When they bind to receptor sites on the receiving neuron, they increase the likelihood that the post-synaptic neuron will fire an electrical impulse (depolarising the post-synaptic membrane).
• Inhibitory Neurotransmitters: These act like a "brake pedal". When they bind to post-synaptic receptors, they make the inside of the receiving neuron more negative (hyperpolarising it), decreasing the likelihood that it will trigger an electrical impulse.
Summation: Deciding Whether to Fire
At any given moment, a single neuron may receive signals from thousands of other neurons — some excitatory and some inhibitory. The receiving neuron calculates the net balance in a process known as summation:
• If the excitatory influences outweigh the inhibitory influences and pass a specific threshold, the neuron fires an action potential.
• If inhibitory influences dominate, no action potential is triggered.
Key Takeaway: Neurotransmitters are chemical messengers that can either excite (stimulate) or inhibit (calm) the receiving neuron, determining whether the message continues down the neural pathway.
---4. Synaptic Transmission: Step-by-Step
The process by which neurons communicate with one another using neurotransmitters across a junction is known as synaptic transmission.
The Synapse: Three Key Parts
The synapse consists of three elements:
1. Pre-synaptic Neuron: The sending neuron (ending at the axon terminal).
2. Synaptic Cleft: The microscopic fluid-filled gap between the two neurons.
3. Post-synaptic Neuron: The receiving neuron (starting at the dendrite or cell body).
Step-by-Step Mechanism of Synaptic Transmission
Don't worry if this seems complex at first; simply follow the sequence step-by-step from sender to receiver:
Step 1: Arrival of the Action Potential
An electrical impulse (action potential) travels down the axon of the pre-synaptic neuron until it reaches the axon terminal (terminal button).
Step 2: Vesicle Movement and Fusion
The electrical impulse stimulates small membrane-bound sacs called synaptic vesicles, which contain neurotransmitters, to move toward the pre-synaptic membrane.
Step 3: Exocytosis (Release)
The vesicles fuse with the pre-synaptic membrane and burst open, releasing neurotransmitter molecules directly into the synaptic cleft.
Step 4: Diffusion
The neurotransmitter molecules diffuse (move from an area of high concentration to low concentration) across the microscopic fluid of the synaptic cleft.
Step 5: Receptor Binding (Lock and Key)
On the surface of the post-synaptic membrane are specialised protein structures called receptors. The neurotransmitters bind to matching receptors.
This operates like a "lock and key" model: only neurotransmitters with a specific molecular shape will fit into their corresponding receptor sites.
Step 6: Post-synaptic Effect
The binding of neurotransmitters to receptor sites alters the electrical charge of the post-synaptic neuron, creating either an excitatory or inhibitory post-synaptic effect.
Step 7: Clearing the Synapse
Once the message is delivered, neurotransmitters must be cleared from the synaptic cleft so the synapse can reset for future signals. This happens through two primary mechanisms:
• Reuptake: Transporter proteins reabsorb neurotransmitters back into the pre-synaptic neuron, where they are repackaged into vesicles or recycled.
• Enzymatic Breakdown: Specific enzymes present in the synaptic cleft break down remaining neurotransmitter molecules into inactive components.
Why Transmission is Unidirectional
Exam Tip: You may be asked why synaptic transmission only travels in one direction. The answer lies in the structural setup of the synapse:
• Synaptic vesicles containing neurotransmitters are found only in the pre-synaptic terminal.
• Specific receptor sites are located only on the post-synaptic membrane.
Therefore, signals can only ever flow from pre-synaptic to post-synaptic neurons.
Key Takeaway: Synaptic transmission involves an electrical impulse prompting the release of neurotransmitters from pre-synaptic vesicles, which diffuse across the synaptic cleft, bind like keys in locks to post-synaptic receptors, and are subsequently removed via reuptake or enzymatic breakdown.
---5. Quick Summary Table for Revision
Use this table to review the essential components covered in this chapter:
• Central Nervous System (CNS): Brain and spinal cord; responsible for processing and coordinating human behaviour.
• Dendrite: Receives chemical signals from neighbouring neurons.
• Axon: Carries electrical impulses away from the cell body.
• Myelin Sheath: Insulates the axon to speed up transmission of electrical impulses.
• Terminal Buttons: House vesicles containing neurotransmitters.
• Synaptic Cleft: The microscopic fluid gap between pre- and post-synaptic neurons.
• Excitatory Neurotransmitter: Increases the likelihood that the post-synaptic neuron will fire.
• Inhibitory Neurotransmitter: Decreases the likelihood that the post-synaptic neuron will fire.
• Reuptake: The reabsorption of neurotransmitters back into the pre-synaptic neuron.
Common Student Mistakes to Avoid
• Mistake 1: Confusing electrical and chemical transmission. Remember: transmission within a neuron is electrical (action potential along the axon); transmission between neurons is chemical (neurotransmitters across the synapse).
• Mistake 2: Thinking neurotransmitters enter the post-synaptic neuron. They do not enter; they simply bind to external receptors, triggering electrical changes on the post-synaptic membrane before being released back into the cleft.
• Mistake 3: Forgetting why transmission is one-way. Always mention where the vesicles are (pre-synaptic) versus where the receptors are (post-synaptic).