Week 2 - Synaptic Transmission

Synaptic Transmission, Autonomic Nervous System, and Cranial Nerves

Locate the different parts on the neurons

1. Dendrites

2. Cell body

3. Axon Hillock

4. Axon

5. Axon terminal

How are neurons classified?

1. By the number of processes that originate from the cell body

2. Functional classification

3. By the type of neurotransmitter used

Classification of neurons based on the number of processes from the soma

1. Unipolar

2. Pseudounipolar

3. Bipolar

4. Multipolar

1. Unipolar Neuron

• Not in humans

• Invertebrates

2. Pseudounipolar Neuron

• One projection that splits into 2 axons, no true dendrites

• Ex: Somatosensory neurons (dorsal root ganglion)

3. Bipolar Neuron

• 1 axon, 1 dendritic root

• Ex: eye

4. Multipolar

• 1 axon, many dendrites

Functional Classification of Neurons

1. Afferent

2. Efferent

3. Interneuronal

1. Afferent

• Convey information into the CNS (usually sensory)

2. Efferent

• Transmit information from CNS to peripheral structures (muscles and glands) (usually motor)

3. Interneuronal

• Send information between neurons

Classificiation: Neurotransmitter used

• ACh

• Dopamine

• Serotonin

• GABA

• Glutamate

Communication in the Nervous System

• Neurons generate electrical signals

  • Local Potentials

  • Action Potentials

Local Potentials

Transmit information over short distances

• Graded potentials

Action Potentials

Transmit information along an axon by repeated generation of a signal

• All or none

Neuroelectrophysiology

• Inside (intracellular) and outside (extracellular) the neuron there are charged ions (i.e., Na+, K+, Cl-)

• These ions move back and forth between the inside and outside of the neuron

Membrane Potential

• The difference in electrical charge (voltage) between the interior and the exterior of the neuron

Resting Membrane Potential

The membrane potential when not transmitting information

• There is an unequal distribution of ionic charge across the membrane

Extracellular Charge & Ions

Higher concentration of Sodium (Na+) and chloride (Cl-) than intracellular fluid (positively charged

Intracellular Charge & Ions

Higher concentration of potassium (K+) and anions (A-) (negatively charged)

Where is the electrical signal generated?

In the axon hillock

• They travel through the length of the axon to the axon terminal, from where they are transmitted to the next neuron

What are the stages of an Action Potential?

1. Resting membrane potential

2. Threshold

3. Depolarization

4. Repolarization

5. Hyperpolarization

6. Returning to resting membrane potential

At ________ _________ ___________ the membrane channels are closed and at -70mV

Resting Membrane Potential

Threshold & Depolarization

• Following a stimulus, ions move and cause change in membrane potential

• If threshold is reached (~55mV), many voltage-gated Na+ channels open and there is an influx of Na+ into the cell

• Neurons become more positively charged inside than the outside

• Peaks at about +35mV, Na+ channels close

Repolarization

• Na+ channels are closed

• K+ channels open, K+ exits the cell

• Causes a reversal of the membrane potential

Hyperpolarization

• Continued efflux of K+ causes the cell membrane to become even more negative than at rest

• At this time, it’s more difficult to initiate another AP (it’s in refractory)

Absolute refractory

Membrane unresponsive to stimuli

Relative refractory

Stimulus must be stronger than normal to elicit an AP

What prevents backwards propagation of an action potential?

Refractory Period

Returning to Resting Membrane Potential

• Membrane channels are all closed

• Na+K+ pump: actively moves Na+ out of the neuron and K+ back into the neuron

  • 3 Na+ out for 2 K+ into the cell per cycle

• Leak channels

Propagation of the Action Potential

• Once an AP has been generated, it spreads passively along the axon

• Spread of AP along axon is dependent on:

  • Passive properties of an axon

    • Diameter

    • Myelin

  • Active opening of ion channels (saltatory conduction)

How would a larger diameter of an axon affect the action potential traveling down it?

• The bigger the axon the faster

• The smaller the slower

How does myelin affect the action potential traveling down it?

Myelinated = faster (saltatory conduction)

Unmyelinated = slower

Nodes of Ranvier

Small unmyelined patches located on myelinated axons

• New AP is generated at these nodes

• Makes the AP faster

• Saltatory conduction: AP appears to “jump” from node to node

Saltatory conduction

• AP appears to “jump” from node to node

Synapse

• A point where interchange of information occurs between two neurons

Synaptic Terminal

• Contains vesicles, which contain neurotransmitters

Neurotransmitters

• Enable communication between neurons

• Present in the presynaptic terminal

• Released into the synaptic cleft

• Opens channels to allow ions in or out, possibly resulting in an action potential in the postsynaptic neuron

• Must be removed from the synaptic cleft

What are the ways in which a neurotransmitter can be removed from the synaptic cleft?

1. Reuptake

2. Absorption by glial cells

3. Enzymatic breakdown at cleft

Neuromuscular Junction

Where motor neurons synapse with muscle fibers

• Voluntary & involuntary muscle movement

• Fast-acting obligatory response

• Main neurotransmitter: Acetylcholine (Ach)

Etiology

Cause/origin of a disease or abnormal condition

Pathology

What is actually happening in the body

Prognosis

The prospect of recovery from disease

Multiple Sclerosis

Etiology

• Autoimmune

Pathology: Demyelination of central nervous system

• Causes scar tissue “sclerosis”

Signs and Symptoms

• Variable depending on location of demyelination in the body

  • Sensory: Impaired/atypical sensation

  • Motor:Weakness

  • Cranial Nerves: Impaired vision

  • Cognition: Infrequent or mild (because most of cognition is not myelinated)

Multiple Sclerosis: Graphs

• Characterized by periods of relapse and remission

• The different types of multiple sclerosis include: (see graph)

Multiple Sclerosis: Prognosis & Treatment

Prognosis: Variable

• Currently no cure

OT

• Maintain & improve function where possible

• Avoid high temperature & excessive exertion

• Adaptations for ADLs

• Energy conservation & stress reduction techniques

• Time management

• Home modification

• Lifestyle redesign

Guillain Barre Syndrome

Etiology

• Autoimmune

Pathology

• Demyelination of PNS

Signs/Symptoms

• Motor: Weakness, paresis/paralysis

• Sensor: atypical sensations, pain

• Cranial Nerves: Motor cranial nerves most affected

Guillain Barre Syndrome: Prognosis & Treament

Prognosis

• Progressively gets worse for 2-3 weeks then gradual improvement

-Medical intervention

-PT

-OT

• Strengthening and regaining functional mobility

• Resuming or adapting ADLs

• Energy concervation techniques

Myasthenia Gravis

Etiology

• Autoimmune (antibodies attack & destroy Ach receptors)

Signs & Symptoms

• Motor: Weakness increases with muscle use

• Sensory: not impacted

• Cranial Nerves: not impacted by skeletal muscles innervated by cranial nerves show fluctuating weakness

Prognosis

• With medical treatment >90% survival rate

Anatomical Divisions of the Nervous System

1. Central Nervous System (CNS)

• Brain

• Spinal Cord

2. Peripheral Nervous System (PNS)

• Cranial Nerves

• Spinal Nerves

Peripheral Nervous System (PNS)

1. Somatic Nervous System

• Spinal Nerves (31 pairs)

• Cranial Nerves (12 pairs)

2. Autonomic Nervous System

• Sympathetic Nervous System

• Parasympathetic Nervous System

Sympathetic Nervous System

• Fight or flight

• Stress

• Activated during exercise, excitement, emergencies

Parasympathetic Nervous System

• Rest and Digest

• Peace

• Energy conservation & storage

What are the sympathetic and parasympathetic systems trying to maintain in the body?

Homeostasis (balance)

• Dynamic balance between autonomic branches

Autonomic Dysreflexia

• Spinal cord injury above T6

  • Sympathetic neurons below te level of lesion are activated due to some noxious stimuli

  • With lesions above T6, the spinal cord is prevented from receiving signals from the brain that inhibit sympathetic activity

• Excessive sympathetic response

• The sudden spike in blood pressure can be life threatening

Cranial Nerves

• Within the peripheral nervous system, in the somatic nervous system

• Innervate face and neck

• Can include sensory, motor, or both

Lesion of the Oculomotor Nerve (CN 3)

Diplopia: Double vision

Ptosis: Drooping of the eyelid

Trigeminal Neuralgia (Tic Douloureux)

Pathology

• Compression of the trigeminal nerve (CN 5)

Symptoms

• Sensory

  • No loss, but severe sharp stabbing pain

    • Pain triggered by stimuli which are usually non-noxious (eating, talking, touch, etc)

    • Begin and end abruptly

  • Motor: Intact

Prognosis

• Variable

Bell’s Palsy

Etiology

• Viral infection or immune disorder

Pathology

• Lesion to the facial nerve

Symptoms

• Unimpaired

• Motor: Unilateral paresis/paralysis of the muscles supplied by the facial nerve

• Autonomic: Salivation and production of tears affected (severe cases)

Prognosis

• In the majority of people, the muscles recover within 2 months depending on the severity of damage

Dysphagia

• Difficulty swallowing

• Can cause aspiration (food going down the wrong tube)

• Continued aspiration can lead to pneumonia

CN

Trigeminal (V)

Facial (VII)

Glossopharyngeal (IX)

Vagus (X)

Hypoglossal (XII)

Dysarthia

• Speech disorder resulting in poor articulation (slurred speech)

• No difficulty in understanding spoken language or ability to read or write

CN

Trigeminal (V)

Facial (VII)

Glossopharyngeal (IX)

Vagus (X)

Hypoglossal (XII)

Accessory (XI)