Module 3 - Lecture 4 to Lecture 8

Module 3: Lecture 4, Action Potential

Dendrites

Receive inputs from other neurons

Cell body

Integrates all the inputs it receives

Axon

• Sends output signals to the next
  neuron

• Generates and propagates action
  potentials (aka impulse, spike)
  → How signals are sent around the
  nervous system

Depolarisation

Inside of membrane becomes more positive

→ Na+ coming into neuron

Repolarisation

Return to resting membrane potential from being depolarised

Hyperpolarisation

Inside becomes more negative

→ K+ leaving the neuron

Action potential

A rapid, brief change in membrane potential

Factors effecting speed of action potential
propagation

1. Axon diameter: Thicker axons propagate axons faster

2. Temperature: Reactions occur faster at higher
   temperatures

3. Myelination: Insulates the membrane
   and speeds up propagation

Module 3: Lecture 5, Synaptic Transmission

Synapse

A junction between two nerve cells, consisting of a minute gap across which impulses pass by diffusion of a neurotransmitter

Process of Synapse

Pre-synapse → Synaptic cleft → Post-synapse

Pre-synaptic neuron

Impulse happens at end of axon

Triggers opening of Ca+ channels

Neurotransmitter molecules are released

Synaptic cleft

Neurotransmitter molecules are released

Bind with receptors in post-synaptic cleft

Postsynaptic neuron

After neurotransmitter molecules bind into receptors

Results in another action potential down a neuron

Chemically gated channels

Responsible for triggering graded/local potentials at a
synapse

Voltaged-gated ion channels

Responsible for triggering action potentials in an axon

Excitatory synapses (EPSPs)

Depolarise the postsynaptic membrane

→ Closer to action potential initiation
threshold

Inhibitory synapses (IPSPs)

Hyperpolarize the postsynaptic membrane

→ Further from action potential initiation threshold

Summation of postsynaptic potentials

Determines whether the cell will reach action potential threshold

Temporal summation

Two EPSPs from the same presynaptic neuron occur close in time to depolarise the membrane to threshold

Spatial summation

Two EPSPs from different presynaptic neurons occur close together in time, to depolarise the
membrane to threshold.

EPSPs and IPSPs cancellation

No net change in membrane potential

Synaptic transmission dependency

1. The type of cell it is terminating on

2. The type of neurotransmitter the neuron releases

3. The types of neurotransmitter receptors on the postsynaptic membrane

Module 3: Lecture 6, Cell Communication & Receptors

Cell communication

Local and Long distance signalling

Local Signalling

1. Paracrine signalling: One cell releases a signalling
   molecule that acts on nearby cells.

2. Synaptic signalling (synaptic
   transmission): Main communication between neurons.

Long Distance Signalling

Signalling molecule is released from a
cell into the blood stream (Hormone)

3 stages of cell communication

1. Reception

2. Transduction

3. Response

Reception

The signalling molecule binds to a specific receptor

Transduction

A signalling pathway is initiated in the cytoplasm

Response

A cellular response is activated e.g. to activate an enzyme

Four families of receptors

1. Ligand-gated ion channels
2. G protein coupled receptors
3. Tyrosine kinase receptors
4. Steroid receptors

Receptors involved in neurotransmission

1. Ligand-gated ion channels

2. G protein coupled receptors

Ligand-gated ion channels

• Fast neurotransmission

• “Direct” neurotransmission

G protein coupled receptors

• Slower neurotransmission

• “Indirect” neurotransmission

Common neurotransmitters

Adrenaline: Fight or Flight

Dopamine: Pleasure

Noradrenaline: Concentration

Serotonin: Mood

Gaba: Calming

Acetylcholine: Learning

Glutamate: Memory

Endorphins: Euphoria

Tyrosine kinase receptors

• Cell surface receptors

• Respond to hormones only

• Slower processes such as metabolism, growth

Steroid receptors

• Intracellular receptors

• Bind to steroid hormones and affect gene expression

Module 3: Lecture 7, Autonomic Nervous System

Motor (Efferent) Division

• Motor nerve fibers

• Conducts impulses from CNS to effectors (muscles and glands)

2 Types of Motor Division

1. Somatic/motor

2. Autonomic

Somatic/Motor Nervous System

• Voluntary movement

• Intervention and activation of skeletal muscle

Autonomic nervous system

• Involuntary movement

• Innervation of organs, glands,
  smooth muscle
  

2 Types of Autonomic nervous system

• Sympathetic nervous
system
• Parasympathetic nervous
system

Sympathetic nervous system

• “Fight or flight

• Prepares the body for action

Parasympathetic nervous system

• “Rest and digest”

• Relaxes the body

Three differences between parasympathetic and sympathetic

1. Sites or origin

2. Relative lengths of fibers

3. Location of ganglia

Neurotransmitters of the ANS

1. Acetylcholine (ACh) → Produced by Parasympathetic

2. Noradrenaline (Ne) → Produced by Sympathetic

Module 3: Lecture 8, Reflexes

Types of movement

• Reflex

• Rhythmic

• Voluntary

Reflex

Least complex, integrated at the spinal cord or brain stem

Rhythmic

Intermediate complexity, integrated in spinal cord with input from the brain

Voluntary

Most complex, integrated in cerebral cortex

Reflex

Rapid, automatic responses to stimuli

Classification of Neutral Reflex

1. Efferent division (Somatic/Automatic)

2. Integrating region (Spinal/Cranial)

3. Time (Innate/Learned)

4. Neurons (Monosynaptic/Polysynaptic)

Reflex Arc

1. Receptor

2. Sensory neuron

3. Integration centre

4. Motor neuron

5. Effector

Muscle stretch

Special sensory receptors called muscle spindles inside the muscle are activated.

Stretch Reflex

1. Stimulus: Muscle is stretched.

2. Receptor: Muscle spindle detects the stretch.

3. Afferent neuron (sensory): Sends signal from spindle to the spinal cord.

4. Synapse: Directly synapses (monosynaptic) with an alpha motor neuron in the spinal cord.

5. Efferent neuron (motor): Sends signal to the same muscle.

6. Response: Muscle contracts to resist further stretch.

➡ Fast, monosynaptic reflex.

Knee Jerk Reflex

1. Stimulus: Tap to the patellar tendon stretches the quadriceps muscle.

2. Receptor: Muscle spindle in quadriceps detects the stretch.

3. Afferent neuron: Sends signal to spinal cord (L2–L4).

4. Synapse: Monosynaptic connection with motor neuron.

5. Efferent neuron: Stimulates quadriceps to contract.

6. Response: Lower leg extends (kicks out).

➡ Simple, monosynaptic stretch reflex used in clinical testing.

Crossed Extensor Reflex

1. Stimulus: Painful stimulus (e.g., step on a nail).

2. Receptor: Pain receptors in foot activated.

3. Afferent neuron: Sends signal to spinal cord.

4. Synapse: Multiple interneurons involved (polysynaptic).

   • Ipsilateral side (same side): Activates motor neurons that contract flexors and inhibit extensors → leg withdraws.

   • Contralateral side (opposite leg): Activates extensors and inhibits flexors → leg extends to support the body.

5. Response: One leg flexes (withdrawal), other leg extends (support).

➡ Polysynaptic reflex involving coordination across both sides of the body.

Autonomic Nervous System Reflexes

• Somatic reflexes: one motor neuron

• Autonomic reflexes: 2 efferent neuron

Example of Autonomic NS reflexes

E.g. Need to urinate in response to a
full bladder stretching
E.g. Contraction of GI smooth muscle
in response to stretch from food
E.g. Changing blood pressure in
response to standing up
Marieb