Neuron Transmission - Nerve - Lecture 175

Neuron Transmission - Nerve - Lecture 175

1. Describe the general organisation and functions of the nervous system 

CNS - Brian and Spinal Cord

PNS - interface between CNS and environment

  - Sensory fibres - carry sensory information from receptor organs (skin, viscera, proprioceptors)

  - Motor fibres - response to sensory input

    - Somatic Motor 

    - Autonomic nerves

       - sympathetic

       - parasympathetic

2. Describe the cellular composition of the nervous system and functions 

Neurons - communication network:

• bipolar
• unipolar
• pseudopolar
• multipolar

Neuroglia - support network:

• Astrocytes - nourish, support
• Oligodendrocytes - increase speed of impulse 
• Microglia - phagocytes
• Ependymal cells - epithelium lining CNS

Innervation:

Sensory 

- skin and proprioception

- viscera

Motor

- muscles

- visceral organs - sympathetic and parasympathetic

Spinal nerves are always mixed: sensory and motor nerves

Cranial nerves can be mixed (V), purely sensory (I, II) or purely motor (XII)

Nerve fibre types: 

Mixed: 

 - sensory in (lie dorsally)

 - motor out (lie ventrally)

Sensory - Pseudounipolar 

 - soma outside spinal cord

 - impedance of sensation if soma inside spinal cord

Dorsal root ganglion = spinal ganglion

Autonomic Nervous System

Parasympathetic:

• ACh is neurotransmitter for both pre and post ganglionic receptors
• from Cranial and Sacral regions
• Long preganglionic neruon
• Short preganglionic neuron

Sympathetic: 

• ACh is neurotransmitter for preganglionic receptor
• NE (noradrenaline epinephrine) is neurotransmitter for postganglionic receptor
• from Thoracic lumbar region
• Short preganglionic neuron
• Long preganglionic neuron

Spinal Nerves: 

• GSM - General Somatic Motor - motor to skeletal muscles
• GVM - General Visceral Motor - motor to heart muscle, smooth muscles, glands 

• GSS - General Somatic Sensory - sensations of touch, pain, temperature, proprioception
• GVS - General Visceral Sensory - viscera sensory (heart intestine)

Cranial Nerves:

• SVM - Special Visceral Motor - Branchial motor - Motor to skeletal muscles that develop in brachial arches of embryo
• SSS - Special Somatic Sensory - special sense from organs developed in ectoderm of embryo (e.g. vision, hearing)
• SVS - Special Viscera Sensory - special senses from organs developed in association of GIT (e.g. smell, taste) 

3. Describe the general structure of a nerve cell 

• Cell body - Soma - contains nucleus and the other organelles
• Dendrites - cellular extensions containing microtubules and neurofilaments
• Axon - arise from soma (or dendrite) in a specialised region called the axon hillock
• Schwann cell - comprise the myelin sheaths (some nerves)
• Node of Ranvier - points between the Schwann cells - no myelin
• Axonal terminal - pre synapse

Nerve membrane structure:

• Phospholipid bilayer
• Protein molecules
• Receptor proteins - bind neurotransmitters
• Channel proteins - form pore for ion movement
• Transport proteins - bind and transfer ions (K⁺ and Na⁺)

4. Explain how action potentials are generated and propagated

Neuronal connections: 

Axodendritic - axon meets dendrite
Axosomatic - axon meets soma cell
Dendrodentric - dendrite meets dendrite
Axoaxonal - axon meets axon

Nerve conductance:
Selective permeability to proteins and ions

• Resting membrane
• Readily permeable to K⁺ ions
• Slightly permeable to Na⁺ ions
• Impermeable to large number of positively charged proteins and anions
• Due to large number of anions in cell, K⁺ is continuously drawn into cell by electromotive force (ion gradient). This is the resting membrane potential 
• Sodium-Potassium pump
• Use cellular energy
• move Na⁺ out of cell
• move K⁺ into cell
• Due to selective membrane permeability and sodium-potassium pump, the charged particle distribution is not uniform
• Therefore Resting Membrane potential:
• Na⁺ concentration increases outside of the cell
• K⁺ concentration and anions increase inside the cell
• Therefore there exists a potential across the membrane 

Action potential

Depolarisation:

• Stimulus causes membrane permeability to Na⁺ to increase causing reduction in membrane potential

Threshold:

• Critical voltage reached and voltage sensitive Na+ and K+ channels undergo conformational change
• Permeability to Na+ abruptly increases but K+ increases slowly

Repolarisation: 

• Na+ gates close at +35mV and K+ gates fully open
• K+ moves out and transmembrane potential becomes negative (-75mV)

AP Propagation

Dependant on: 

• myelination - insulates fibres reducing ion leaks
• node of Ranvier - saltatory effect - increases propagation of AP
• diameter of fibre - the larger the diameter, the greater the velocity