Neuronal Communication¶

Part of Module 5: Communication, homeostasis and energy.

The nervous system provides the body's fastest communication pathway. Electrical signals called action potentials travel along neurones at up to 120 m s⁻¹, allowing near-instantaneous responses to stimuli. Understanding neuronal communication requires following the action potential from its generation at a receptor, along a sensory neurone, through synaptic connections, and out to an effector via a motor neurone — and then into the effector itself, which may be a muscle.

What You Need to Learn¶

Further detail: AS Biology A (H020) and A Level Biology A (H420).

The structure of neurones, how the resting potential and action potential are generated and conducted, how receptors transduce stimuli, how the nervous system is organised, the regions of the brain, reflex arcs, synaptic transmission and summation, neuromuscular junctions, and the structure and sliding filament contraction of skeletal muscle.

Neurone Structure¶

Neurones are specialised nerve cells vital for transmitting electrical impulses quickly throughout the body. All neurones share a common structural plan built around a cell body (soma) containing the nucleus, mitochondria, ribosomes, rough ER and Golgi apparatus — organelles crucial for the production of neurotransmitters. From the cell body extend two types of process:

• Dendrons and dendrites: short branches extending from the cell body, further dividing into highly branched dendrites to receive signals from other neurones and carry them towards the cell body
• Axon: a single long extension that carries the impulse away from the cell body to the next cell or to an effector

Axons of motor and sensory neurones may be over a metre long in large mammals. Many axons are surrounded by a myelin sheath formed by Schwann cells, which wrap their cell membrane repeatedly around the axon producing a thick, lipid-rich insulating layer. Schwann cells also remove debris via phagocytosis and aid axon regeneration. Between adjacent Schwann cells are short unmyelinated gaps called nodes of Ranvier.

Types of Neurone¶

The typical pathway of an impulse is: receptor → sensory neurone → relay neurone → motor neurone → effector.

The Resting Potential¶

Between impulses, the axon membrane is polarised — the inside of the cell is negatively charged relative to the outside. This voltage difference is the resting potential, approximately –70 mV.

The resting potential is maintained by three mechanisms:

1. Sodium-potassium pump (Na⁺/K⁺-ATPase): An active transport protein that uses ATP to move 3 Na⁺ out for every 2 K⁺ in, creating an electrochemical gradient: Na⁺ is more concentrated outside; K⁺ is more concentrated inside.

2. Potassium leak channels: The resting membrane has open K⁺ channels, allowing K⁺ to diffuse out down its concentration gradient. Net loss of positive ions makes the inside negative.

3. Sodium channel closure: Voltage-gated Na⁺ channels are closed at rest, preventing Na⁺ from re-entering. The extracellular space therefore accumulates more positive ions, maintaining the polarised state.

The stable –70 mV resting potential is the point at which the tendency of K⁺ to diffuse out (chemical gradient) is balanced by the electrical attraction pulling K⁺ back in (the negative interior), together with continuous Na⁺/K⁺ pump activity restoring ion concentrations.

The Action Potential¶

A stimulus depolarises the membrane: if the membrane potential rises above the threshold (approximately –55 mV), a self-propagating action potential is triggered. The generation of an action potential is an example of a positive feedback mechanism — the initial Na⁺ influx depolarises the membrane, which opens more Na⁺ channels, causing further influx and further depolarisation.

Stages¶

1. Depolarisation - Stimulus causes voltage-gated Na⁺ channels to open - Na⁺ diffuses rapidly into the axon down its electrochemical gradient - The inside becomes less negative, then positive, reaching approximately +30 mV

2. Repolarisation - Voltage-gated Na⁺ channels close - Voltage-gated K⁺ channels open (slightly delayed) - K⁺ diffuses out rapidly, making the inside negative again

3. Hyperpolarisation (undershoot) - K⁺ channels close slightly slowly, so K⁺ continues to leave briefly - Membrane potential overshoots below –70 mV (to approximately –80 mV)

4. Restoration of resting potential - Na⁺/K⁺ pump restores ion concentrations - Voltage returns to –70 mV

All-or-Nothing Principle¶

An action potential is all-or-nothing:

• Once the threshold potential (~–55 mV) is reached, an action potential is always triggered, regardless of stimulus strength
• If the threshold is not reached, no action potential is initiated — there is no partial response
• The size of the action potential does not vary with stimulus strength; instead, frequency of action potentials encodes stimulus intensity — a stronger stimulus produces more frequent action potentials

Refractory Period¶

Immediately after an action potential, the axon membrane enters a refractory period during which Na⁺ channels remain closed, preventing re-depolarisation:

• Absolute refractory period: Na⁺ channels are inactive and cannot open regardless of stimulus strength; no action potential is possible
• Relative refractory period: Na⁺ channels have recovered but K⁺ channels are still open (hyperpolarisation phase); a larger-than-normal stimulus is needed to reach threshold

The refractory period serves essential functions:

1. Ensures action potentials are discrete, separated events — impulses cannot merge or overlap
2. Limits the frequency at which impulses are transmitted
3. Ensures action potentials travel in one direction only — the region behind the impulse is in its refractory period and cannot be re-stimulated

Propagation of the Action Potential¶

The action potential moves along the axon as a wave of depolarisation:

1. Opening of Na⁺ channels causes local depolarisation, allowing positive ions to spread sideways
2. Adjacent voltage-gated Na⁺ channels open in response
3. Depolarisation of nearby membrane areas occurs
4. As each patch of membrane activates the next, an advancing wave is formed
5. The region behind the wave is in its refractory period and remains unresponsive
6. This ensures the wave moves in only one direction, preventing backward flow

Once triggered, the action potential self-propagates at constant size without decrement.

Saltatory Conduction¶

In myelinated axons, the myelin sheath insulates the membrane along internodal segments. Ion movement can only occur at the unmyelinated nodes of Ranvier. The action potential therefore 'jumps' from node to node — saltatory conduction (from Latin saltare, to jump).

Saltatory conduction is much faster than propagation in unmyelinated fibres, which must depolarise every point along the membrane:

Factors Affecting Transmission Speed¶

Three main factors affect the speed of action potential transmission:

Receptors as Transducers¶

Receptors are specialised cells that detect stimuli from the environment. They act as transducers, converting one form of energy (stimulus energy) into an electrical signal (nerve impulse). They detect only one particular type of stimulus.

How Receptor Cells Work¶

1. At rest, the receptor cell membrane maintains a resting potential (~–70 mV)
2. When a stimulus is detected, the membrane becomes more permeable, allowing ion flow
3. This alters the membrane voltage, creating a generator potential (graded depolarisation)
4. A larger stimulus produces a larger generator potential
5. If the generator potential reaches the threshold of the associated neurone, an action potential is triggered

Generator potentials are graded (their size varies with stimulus intensity), unlike action potentials. More action potentials indicate a stronger stimulus. If the stimulus is too weak, the threshold is not reached and no action potential is generated.

Pacinian Corpuscle¶

The Pacinian corpuscle is a mechanoreceptor in the skin that detects pressure and vibrations. Its structure illustrates the receptor-transducer concept clearly:

• An onion-shaped structure with concentric lamellae of connective tissue (with viscous gel between layers) surrounding a sensory neurone ending
• Pressure deforms the lamellae, pressing on the sensory neurone ending
• The neurone's membrane stretches, changing its shape
• This opens stretch-mediated sodium ion (Na⁺) channels, increasing Na⁺ permeability
• Na⁺ diffuses into the neurone, causing a generator potential
• If large enough, this triggers an action potential in the associated neurone

Organisation of the Nervous System¶

Structural Organisation¶

The mammalian nervous system divides into two main parts:

Central nervous system (CNS):

• Consists of the brain and spinal cord
• Serves as the primary command centre

Peripheral nervous system (PNS):

• All nerves connecting the CNS to the rest of the body
• Divided further into:
• Sensory nervous system — sensory neurones carrying impulses from receptors to the CNS
• Motor nervous system — motor neurones carrying impulses from CNS to effectors

Functional Organisation¶

Somatic (voluntary) nervous system: conscious control of voluntary muscle movements

Autonomic (involuntary) nervous system: subconscious control of involuntary activities (heartbeat, digestion, etc.)

Autonomic Nervous System Divisions¶

The autonomic nervous system splits into two divisions that typically have opposing effects:

Structure and Function of the Brain¶

The brain is contained within the skull and is the control centre of the CNS. It receives and processes sensory information about changes in the internal and external environment to produce a coordinated response.

Key Brain Regions¶

Hypothalamus Located just below the middle part of the brain. Functions:

• Homeostasis: senses changes in body temperature and initiates responses (sweating, shivering)
• Water balance: monitors blood plasma composition (water and glucose concentration)
• Hormonal regulation: regulates hormone secretion from the pituitary gland

Cerebrum The largest part of the brain, consisting of left and right cerebral hemispheres with an outer layer (cerebral cortex) with many folds. Functions:

• Processes sensory input (vision, hearing, etc.)
• Involved in learning, memory, and higher-level thinking
• Each sensory area receives information from specific receptor cells; association areas analyse and act on this information

Pituitary gland Located just below the hypothalamus. Functions:

• Produces, stores, and secretes hormones when triggered by the hypothalamus
• Its hormones prompt other glands (e.g. adrenal glands) to secrete their hormones
• Anterior pituitary: produces six hormones including FSH and growth hormones
• Posterior pituitary: stores and releases hypothalamic hormones including ADH

Medulla oblongata Located at the base of the brain, connecting it to the spinal cord. Functions:

• Involuntarily regulates breathing rate and heart rate
• Controls blood pressure and other autonomic functions (swallowing, peristalsis, coughing)

Cerebellum Located underneath the cerebrum. Functions:

• Coordinates and fine-tunes skeletal muscle contractions
• Maintains unconscious functions: posture, balance, involuntary muscular movement
• Receives information about balance and muscle/tendon tone, relaying it to motor control areas of the cortex

Reflex Arcs¶

A reflex arc is a neural pathway that causes an involuntary, rapid reaction to a stimulus. Reflex arcs are often composed of just three types of neurone, many of which are controlled by the spinal cord rather than the brain.

Stages of a Reflex Arc¶

1. Stimulus — triggers the reflex (e.g. heat from a hot object)
2. Receptor — specialised cells that detect the stimulus and generate nerve impulses
3. Sensory neurone — transmits impulses to relay neurones
4. Relay neurone — usually in the spinal cord, connects sensory neurones to motor neurones
5. Motor neurone — transfers impulses to effectors
6. Effector — muscle or gland that carries out the response
7. Response — the final action (e.g. withdrawing hand from hot object)

Types of Reflex¶

Spinal reflexes involve the spinal cord and not the brain:

• Withdrawal reflex — rapid removal of hand from a sharp or hot object
• Knee-jerk (patellar) reflex — leg kicks when tapped just below the kneecap; helps maintain posture

Cranial reflexes involve the brain and not the spinal cord:

• Blinking reflex — involuntary blinking when the cornea is stimulated, or in response to loud sounds or bright light

Importance of Reflex Arcs¶

Reflex arcs are:

• Involuntary — the brain can concentrate on complex processes
• Rapid — ensure a swift, protective response
• Protective — safeguard the body from potential injuries
• Innate — present from birth, requiring no learning

Synaptic Transmission¶

A synapse is the junction between two neurones (or between a motor neurone and a muscle — a neuromuscular junction). There is a narrow gap of approximately 20 nm between the presynaptic membrane and the postsynaptic membrane, called the synaptic cleft.

Structure of a Synapse¶

• Presynaptic neurone — releases neurotransmitters into the synapse
• Synaptic knob — the terminal end of the presynaptic neurone; contains mitochondria (for energy) and synaptic vesicles
• Synaptic vesicles — sacs containing neurotransmitter molecules, ready to be released
• Synaptic cleft — the ~20 nm gap between membranes
• Postsynaptic neurone — receives neurotransmitters and can generate new action potentials
• Neurotransmitter receptors — specific protein molecules on the postsynaptic membrane

Sequence of Events in Cholinergic Synaptic Transmission¶

Cholinergic synapses use acetylcholine (ACh) as their neurotransmitter:

1. An action potential arrives at the synaptic knob
2. Voltage-gated Ca²⁺ channels open; Ca²⁺ diffuses into the presynaptic knob from the extracellular fluid
3. Ca²⁺ causes synaptic vesicles to move towards and fuse with the presynaptic membrane by exocytosis
4. ACh is released into the synaptic cleft and rapidly diffuses across
5. ACh binds to ligand-gated receptor proteins on the postsynaptic membrane, causing them to change shape
6. Na⁺ channels open in the postsynaptic membrane; Na⁺ enters, depolarising the membrane
7. If depolarisation reaches threshold, a new action potential is generated in the postsynaptic neurone

Termination of the signal:

• ACh is broken down by acetylcholinesterase in the synaptic cleft → choline + ethanoic acid (acetate)
• This prevents continuous stimulation and allows the synapse to respond to the next impulse
• Choline and acetate are reabsorbed into the presynaptic knob by active transport and reassembled into ACh, which is packaged into vesicles ready for re-use
• Mitochondria in the synaptic knob provide ATP for ACh reformation

Why the Synapse Is Unidirectional¶

Vesicles containing neurotransmitter are present only on the presynaptic side. Receptors are present only on the postsynaptic side. Therefore, neurotransmitter can only flow in one direction — the signal cannot travel backwards.

Excitatory and Inhibitory Synapses¶

The same neurotransmitter can have different effects depending on the type of receptor on the postsynaptic membrane:

For example, acetylcholine is excitatory in the CNS and at neuromuscular junctions, but inhibitory at cardiac synapses.

Summation¶

A single excitatory synapse may not provide enough depolarisation to reach threshold. The postsynaptic neurone integrates signals through summation:

• Temporal summation: repeated impulses arriving at the same synapse in quick succession produce overlapping EPSPs that sum to reach threshold
• Spatial summation: EPSPs from several different synapses converging on the same postsynaptic cell sum together; inhibitory inputs can prevent firing by countering excitatory inputs

Summation means neurones act as signal processors, firing only when net excitatory input sufficiently exceeds inhibitory input.

Role of Synapses in Signal Integration¶

Synapses serve broader roles beyond simple relay:

• A single impulse from a presynaptic neurone can initiate impulses in multiple postsynaptic neurones (divergence)
• Impulses from several presynaptic neurones can combine into a single postsynaptic response (convergence)
• This enables the nervous system to filter noise, integrate information, and generate appropriate coordinated responses

Neuromuscular Junctions¶

A neuromuscular junction is where a motor neurone meets a skeletal muscle fibre. It is a specialised form of synapse that triggers muscle contraction.

Neuromuscular junctions are distributed along the muscle's length to ensure all fibres contract simultaneously for maximum power.

Motor Units¶

A motor unit consists of all muscle fibres supplied by a single motor neurone. The number of motor units stimulated determines the force of muscle contraction:

• Strong force → many motor units stimulated
• Weak force → few motor units stimulated

Sequence of Events at a Neuromuscular Junction¶

1. Action potential arrives at the end of the motor neurone
2. Voltage-gated Ca²⁺ channels open; Ca²⁺ enters the neurone ending
3. Acetylcholine (ACh) vesicles release their contents into the synaptic cleft
4. ACh diffuses across the cleft and binds to receptors on the sarcolemma (muscle fibre membrane)
5. Na⁺ channels open → depolarisation of the sarcolemma
6. Depolarisation spreads deep into the muscle fibre through T tubules (transverse tubules — extensions of the sarcolemma)
7. T tubules interact with the sarcoplasmic reticulum (specialised ER that stores Ca²⁺)
8. Ca²⁺ channels in the sarcoplasmic reticulum open, flooding Ca²⁺ into the sarcoplasm
9. This surge of Ca²⁺ triggers muscle contraction

Termination: Acetylcholinesterase breaks down ACh in the cleft → choline + ethanoic acid; choline returns to the synaptic knob for ACh resynthesis; mitochondria provide energy for this process.

Muscle Structure and Contraction¶

Types of Muscle¶

The body contains three types of muscle with distinct structural and functional characteristics:

Skeletal and cardiac muscle fibres show regular striations due to the arrangement of contractile proteins within myofibrils. Smooth muscle shows no striations under the microscope.

Structure of Skeletal Muscle¶

Skeletal muscles comprise numerous bundles of long, cylindrical muscle fibres. Individual cells fuse during development to form these fibres (avoiding weakness at cell junctions and increasing overall strength). Key components:

• Sarcolemma — the cell-surface membrane of a muscle fibre
• Sarcoplasm — the cytoplasm of a muscle fibre
• T tubules (transverse tubules) — extensions of the sarcolemma that transmit electrical signals, ensuring the entire muscle receives the impulse to contract simultaneously
• Sarcoplasmic reticulum — specialised ER responsible for storing and releasing Ca²⁺
• Myofibrils — subcellular structures designed for contraction, containing ordered protein filaments
• Multiple nuclei — because several cells merge to form one muscle fibre
• Mitochondria — release ATP for muscle contraction

Sarcomere Structure¶

Myofibrils are made of repeating units called sarcomeres, each containing two types of protein filament:

• Myosin filaments (thick) — long rod-shaped proteins with globular heads projecting to the side; heads have a hinge enabling movement, a binding site for actin, and an ATP binding site
• Actin filaments (thin) — two strands twisted around each other, with myosin-binding sites blocked by two regulatory proteins: tropomyosin (blocks the sites at rest) and troponin (holds tropomyosin in place)

The arrangement of these filaments creates a distinctive banding pattern visible under the microscope:

The Sliding Filament Theory¶

Muscle contraction occurs when actin filaments are pulled closer towards the M-line, shortening the sarcomere. The A bands remain constant in length; the I bands and H-zones shorten as overlap increases.

Steps in the sliding filament mechanism:

1. Ca²⁺ (released from sarcoplasmic reticulum) binds to troponin, altering its shape
2. This moves tropomyosin away from the actin-binding sites, exposing them
3. Myosin heads attach to exposed binding sites on actin, forming actin-myosin cross-bridges
4. The myosin head performs a power stroke, pulling the actin filament towards the M-line and releasing ADP
5. An ATP molecule binds to the myosin head, causing it to detach from actin
6. Ca²⁺ activates myosin's ATPase activity, hydrolysing ATP → ADP + Pᵢ, releasing energy that resets the myosin head to its upright position
7. The myosin head reattaches to a new actin site further along the filament
8. The cycle repeats, pulling actin progressively along

This simultaneous contraction of many sarcomeres in a myofibril leads to shortening of the entire muscle.

Energy Sources for Muscle Contraction¶

Common Confusions¶

• Resting potential and action potential: the resting potential is the stable potential of about −70 mV maintained by the sodium-potassium pump and leaky potassium channels. The action potential is a temporary reversal.
• Size and frequency: a bigger stimulus does not produce a bigger action potential. It produces more frequent ones.
• Refractory period: it ensures impulses travel in one direction and are discrete, because sodium channels cannot reopen immediately.
• Unidirectional synapses: neurotransmitter is released from the presynaptic neurone only, and receptors are only on the postsynaptic membrane.
• Muscle contraction: filaments do not shorten. They slide past each other, so the sarcomere shortens.

Check Yourself¶

1. Describe the changes in ion movement that produce the depolarisation and repolarisation phases of an action potential.
2. Explain how the refractory period makes sure that action potentials are discrete and travel in one direction.
3. An impulse travels 0.25 m in 2.5 ms. Calculate the speed and explain how myelination increases it.
4. Explain how a weak and a strong stimulus are distinguished by the nervous system.
5. Describe the sequence of events at a cholinergic synapse, from the arrival of an action potential.
6. Explain how the sarcomere shortens during contraction, referring to the A band, I band and H zone.

Key Terms¶

• Neurone: a specialised cell that transmits electrical impulses through the nervous system.
• Myelin sheath: lipid-rich insulating layer around many axons, formed by Schwann cells.
• Resting potential: the membrane potential of a neurone when it is not transmitting an impulse, usually about -70 mV.
• Threshold: the membrane potential that must be reached to trigger an action potential.
• Depolarisation: change in membrane potential in which the inside of the neurone becomes less negative.
• Repolarisation: return of the membrane potential towards its resting value after depolarisation.
• Action potential: rapid, temporary reversal of membrane potential that travels along an axon.
• Refractory period: short recovery period after an action potential.
• Saltatory conduction: transmission in a myelinated neurone where the action potential appears to jump between nodes of Ranvier.
• Synapse: a junction across which signals are transmitted chemically.
• Neurotransmitter: chemical messenger released from a synaptic vesicle into the synaptic cleft.
• Summation: combining of excitatory and inhibitory inputs to determine whether threshold is reached.
• Neuromuscular junction: specialised synapse between a motor neurone and a skeletal muscle fibre.

Connected Pages¶

• 5.1.1 Communication and homeostasis
• 5.1.4 Hormonal communication
• 5.1.5 Plant and animal responses
• 5.2.2 Respiration
• Module 5: Communication, homeostasis and energy