5.3 Neuronal communication

neurones

specialised nerve cells that transmit electrical impulses throughout the body

cell body

contains the nucleus and other cytoplasmic organelles

dendrons

short branches extending from the cell body, which divide into dendrites to receive nerve impulses from other neurones

axon

a long nerve fibre responsible for carrying impulses away from the cell body to other neurones or effectors

schwann cells

membranes of cells that surround some neurones

functions of schwann cells

1. their membranes form myelin sheath

2. remove debris via phagocytosis

3. aid regeneration

myelin sheath

fatty layer that surrounds parts of the axon, acting as an insulator

• prevents the passage of ions into or out of the axon at the region it covers

saltatory conduction

impulses that are rapidly conducted by myelinated axons. The electrical impulse ‘jumps’ between the nodes of ranvier, increasing transmission speed

sensory neurone

Carries impulses from sensory receptors to the central nervous system

relay neurone

Carries impulses within the central nervous system, between other neurones

motor neurone

Carries impulses from the central nervous system to effectors

sensory receptors

specialised cells that detect stimuli from the environment

what do receptors act as?

• transducers - converts one form of energy into another

• in a receptor, a transducer converts the stimulus energy into a nerve impulse that is passed to the CNS

stages of receptor cell function

1. at rest, the receptor cell surface membrane has a voltage across it due to differences in ion concentration inside and outside the cell (resting potential)

2. when a stimulus is detected, the cell surface membrane becomes more permeable, allowing more ions to flow in and out

3. the membrane’s voltage is altered, creating a generator (or receptor) potential

4. a larger stimulus results in a bigger voltage, producing a larger generator potential

5. if the generator potential reaches a threshold level, it triggers an action potential,

action potential

an electrical signal sent along a neurone

pacinian corpuscles

mechanoreceptors that detect pressure and vibrations

• contain the ending of a sensory neuron wrapped in lamellae

what happens when pacinian corpscule is stimulated?

1. The lamellae deform, pressing on the sensory neurone ending.

2. This stretches the neurone's membrane, causing it to change shape.

3. This opens stretch-mediated sodium ion (Na⁺) channels in the membrane, increasing its permeability to Na⁺.

4. Na⁺ diffuses into the neurone, depolarising it and resulting in a generator potential.

5. If this signal reaches the threshold, an action potential is triggered.

1. Resting potential

• Neurone is not transmitting signals

• cell surface membrane maintains a state of polarisation - there’s a difference in voltage across the membrane

• around -70mV

how is the resting potential achieved?

• sodium-potassium pumps - active transporters that move three sodium ions (Na+) out of the neurone for every two potassium ions (K+) they move in

• potassium ion channels - they allow the diffusion of K+ out of the neurone, down its concentration gradient

• sodium ion channels - they are closed, preventing the movement of Na+ into the neurone

2. Stimulus

Voltage-gated Na+ channels open, so more Na+ flows into the axon making the inside less negative

3. Depolarisation

If the threshold potential of around -55 mV is reached, more Na+ channels open causing an influx of Na+

4. Repolarisation

At around +30 mV, Na+ channels close and K+ channels open, so K+ flows out of the axon and the membrane starts repolarising

5. Hyperpolarisation

An excess of K+ leaves the axon, dropping the potential below the -70 mV resting level.

6. Refractory period

Various ion pumps and channels work together to restore the membrane back to the resting potential

how is the generation of an action potential an example of positive feedback?

because the initial Na+ influx depolarises the axon membrane, which opens more Na+ channels. This means a greater influx of Na+, further depolarising the membrane

why can’t the neurone’s membrane generate another action potential?

because sodium ion (Na+) channels remain closed during repolarisation, preventing depolarisation

roles of refractory period

• Ensuring action potentials don't overlap.

• Limiting the frequency at which impulses are transmitted.

• Guaranteeing that impulses travel in only one direction.

How action potentials travel as waves of depolarisation

1. The opening of Na⁺ channels results in local depolarisation, allowing positive ions to spread sideways.

2. Adjacent voltage-gated Na⁺ channels open in response to this change.

3. This action leads to the depolarisation of nearby membrane areas.

4. As each patch of membrane activates the next, an advancing wave is formed.

5. Areas of the membrane that have just experienced depolarisation are in the refractory period and remain unresponsive while they repolarise (K⁺ exits the axon and Na⁺ channels are closed).

6. This ensures that the wave moves in one direction, preventing the backward flow of the nerve impulse.

factors that affect the speed of transmission of an action potential

• myelination - myelinated neurones transmit impulses faster than unmyelinated neurones

• Axon diameter - A larger axon diameter means there is less resistance to ion flow, so the wave of depolarisation travels faster along the axon

• temperature - Higher temperatures accelerate the diffusion of ions, leading to faster depolarisation and faster impulse transmission. However, temperatures above 40°C can cause proteins to denature

synapse

a junction where information is transferred from one neurone to another neurone or to an effector cell

roles of synapses

1. transmit information through the release neurotransmitter chemicals

2. a single impulse from the presynaptic neuron can initiate impulses in multiple postsynaptic neurons or effector cells

3. impulses from several presynaptic neurons can be combined into a single postsynaptic response

presynaptic neuron

releases neurotransmitters into the synapse

synaptic knob

the section at the end of the presynaptic neurone that contains the organelles needed for neurotransmitter production

synaptic vesicles

sacs within the synaptic knob that store NTs until they are released

postsynaptic neuron

receives the NTs and can generate new action potentials

neurotransmitter receptors

specific molecules on the postsynaptic membrane that bind with the neurotransmitters

excitatory NT

• causes depolarisation to occur

• triggers AP if threshold is reached

• e.g. acetylcholine

inhibitory NT

• causes hyperpolarisation

• prevents AP

summation

The process where multiple excitatory and inhibitory postsynaptic potentials are added together at the postsynaptic membrane to determine whether the neurone reaches threshold

spatial summation

• multiple presynaptic neurons converge on a single postsynaptic neuron

• The combined input of neurotransmitters can trigger postsynaptic firing.

• Inhibitory inputs have the potential to prevent this firing.

temporal summation

• one neuron fires repeatedly, leading to continuous NT release

• An increased amount of neurotransmitter makes it more likely to trigger postsynaptic firing

synaptic transmission

the process by which a nerve impulse is transmitted from one neurone to another across a synapse

steps in synaptic transmission

1. An action potential arrives at the presynaptic knob.

2. This causes voltage-gated calcium ion (Ca²⁺) channels to open and Ca²⁺ flows into the presynaptic knob.

3. This causes synaptic vesicles, which contain neurotransmitters, to move towards and fuse with the presynaptic membrane.

4. The vesicles release neurotransmitters into the synaptic cleft through exocytosis, and the neurotransmitters rapidly diffuse across the synaptic cleft.

5. the neurotransmitters bind to receptor proteins on the postsynaptic membrane, causing the receptors to change shape.

6. This opens sodium ion channels in the postsynaptic membrane, leading to the depolarisation of the postsynaptic membrane.

7. If this depolarisation reaches a threshold level, an action potential is triggered in the postsynaptic neurone.

Cholinergic synapses

specific types of synapses that use acetylcholine (ACh) as their neurotransmitter

What happens after ACh binds to the receptors and triggers a response?

1. ACh is broken down by the enzyme acetylcholinesterase into choline and ethanoic acid (acetate).

2. These breakdown products are then reabsorbed into the presynaptic knob via active transport.

3. They can then be recycled to synthesise more ACh.

4. Ach is transported into synaptic vesicles, ready for another action potential.

parts of the nervous system

• CNS

• peripheral nervous system

CNS

• consists of the brain and spinal cord

• serves as the primary command centre for the body

PNS

consists of all nerves that connects the CNS to the rest of the body

divisions of the PNS

• sensory NS - consists of sensory neurones that carry nerve impulses from the receptors to the CNS

• motor NS - consists of motor neurons that carry nerve impulses from the CNS to effectors like muscles and glands

somatic nervous system

controls voluntary muscle movements

autonomic nervous system

controls involuntary activities such as heartbeat

ANS divisions

• sympathetic - activates fight or flight, increases activity levels, uses adrenaline and noradrenaline

• parasympathetic - activate rest and digest, decreases activity levels, uses acetylcholine

function of the brain

receives and processes sensory info from receptor cells and the hormonal system about changes in the external and internal environment to produce a coordinated response

hypothalamus

• homeostasis - regulates body temperature

• monitors the concentration of water and glucose in blood

• regulates hormone secretion in the pituitary gland

cerebellum

• processes sensory input - crucial for processes like vision and hearing

• involved in learning, memory and higher-level thinking

pituitary gland

• it produces, stores, and secretes hormones when triggered by the hypothalamus.

• the hormones it secretes prompt other glands, like the adrenal glands, to secrete their hormones.

main sections of the pituitary gland

• anterior PG - produced six hormones including FSH

• posterior PG - stores and releases hormones produced by the hypothalamus, including ADH

medulla oblongata

• involuntarily regulates breathing rate

• involuntarily regulates heart rate and blood pressure

• controls ANS functions like swallowing

cerebellum

• coordinates and fine-tunes skeletal muscles contractions

• maintains unconscious functions like posture and balance

reflex arc

a neural pathway that causes an involuntary and immediate reaction to a stimulus

stages of a reflex arc

1. a stimulus triggers the reflex

2. receptors detect the stimulus and generate nerve impulses

3. the sensory neuron transmits the nerve impulses to relay neurons

4. the relay neuron connects the sensory neurons to motor neurons

5. the motor neuron transfers nerve impulses from the relay neurons to the effectors

6. the effector receives the signal and carries out a response

features of reflex arcs

• involuntary - they allow the brain to concentrate on complex processes

• rapid - ensure a swift response

• protective - safeguard the body from injuries

• innate - eliminates the need to learn them

knee-jerk reflex

• spinal reflex that causes the leg to kick when it is tapped just below the kneecap and helps maintain posture and balance

blinking reflex

• a cranial reflex that triggers involuntary blinking of the eyelids when the cornea is stimulated, or in response to bright light

three types of muscle

skeletal, cardiac and smooth muscle

skeletal muscle

muscle attached to bones, which helps move parts of the body

cardiac muscle

muscle that is unique to the heart and functions to circulate blood

smooth muscle

• located in the walls of hollow organs like blood vessels and intestines

• functions to move substances through these organs

fibre structure of each muscle

• skeletal - tubular, striated

• cardiac - branched, striated

• smooth - spindle-shaped, non-striated

how many nuclei per fibre of each muscle?

• skeletal - multiple

• cardiac - single

• smooth - single

arrangement of each muscle

• skeletal - regular, parallel bundles of myofibrils

• cardiac - branching network of myofibrils

• smooth - unorganised, no myofibrils

type of stimulation required for each muscle

• skeletal: neurogenic - contracts when stimulated by motor neuron impulses

• cardiac: myogenic - contracts automatically without nervous input

• smooth: neurogenic and can also stretch in response to pressure

components of a muscle fibre

• sarcolemma - cell-surface membrane

• sarcoplasm - cytoplasm

• T tubules - extensions of the sarcolemma that transmit electrical signals, to ensure that the entire muscle receives the impulse to contract simultaneously

• sarcoplasmic reticulum - specialised ER that stores and releases calcium ions

• myofibrils - subcellular structures designed for contraction

• multiple nuclei - because several cells merge to form one muscle fibre

• mitochondria - release energy in the form of ATP for muscle contraction

myofibrils

core units of muscle fibres that contains organised bundles of protein filaments

sarcomeres

repeating units that make up myofibrils

main filaments in a sarcomere

• myosin filaments - thick filaments composed of long rod-shapes with bulbous heads that project to the side

• actin filaments - thin filaments composed of two strands twisted around each other

sections of a sarcomere

• A band - area with both myosin and overlapping actin filaments

• I band - area containing only light actin filaments

• Z-lines - mark the boundaries of each sarcomere unit

• M-line - central line of a sarcomere

• H-zone - area with only myosin filaments

what happens during muscle contraction?

• the sliding of actin and myosin filaments within sarcomeres

• sarcomeres shorten, causing in the pattern of I bands and A bands