Neural Physiology

Anatomical organization

• One system– Two subdivisions

• CNS• Peripheral

Anatomical organization

• Central (CNS)– Brain– Spinal cord– Function

• Integration of information• Generation of memory• Control of various systems• Mental activity

Anatomical organization

• Peripheral– Sensory receptors

• Detection of various sensations

– Nerves• Conduits between sensory

receptors and CNS

– Ganglia• Cluster of nerve cells located

outside of the spinal cord

– Plexus• Network of neurons and

axons located outside of the CNS

Anatomical organization

• Nerves– Cranial

• 12 pairs

– Spinal• 31 pairs

• Afferent and efferent nerves– Afferent/sensory

• Transmission of signals from sensory receptors to the CNS

• Entry route– Dorsal horn of the spinal cord

– Efferent/motor• Transmission of information from the CNS to the organs

– Motor neurons

• Transmission routre– Ventral horn of the spinal cord

Autonomic nervous system (ANS)

• Two divisions– Sympathetic– Parasympathetic

• Enteric (not exactly a division of the ANS)– Regulation of the GI tract function• Plexuses

Neural tissue

• Neural cell– Generation and reception of neural signal• Action potential

• Glial cells– Support and protection

Types of neurons

• Multipolar– Many dendrites– Single axon

• Bipolar– Single dendrite– Single axon

• Unipolar– No dendrite– Single axon

Glial cells within the CNS

• Functions– Fluid secretion and composition maintenance

• CSF by ependymal cells• Brain fluid by astrocytes

– Formation of barrier• Blood-brain barrier by astrocytes

– Response to inflammation• Reactive astrocytes• Microglia

– Insulation of axon• Myelin sheath by origodendrocytes

Glial cells in the PNS

• Schwann cells– Myelin sheath production

• Satellite cells– Support and protection of the cell body

Myelination

• Myelin sheath– Insulation of axon– Increased transmission speed• Saltatory conduction from one node of Ranvier to the

next

– Greater the thickness of sheath, faster the conduction

Nerve tissue organization

• White matter– Bundles of myelinated axons

• Neural tracts• Propagation of action potential from one area to the next

– Neural cells and dendrites• Gray matter– Bundle of unmyelinated axons– Neural cells and dendrites– Integration and relaying of signals

Transmission of neural signal

• Action potential– Exactly the same principle as the AP in the

muscular system• Propagation– Across the entire length of the axon• One direction

• Generation of action potential– Unmyelinated• Same as muscle

– Myelinated• At node of Ranvier

– Concentrated ion channels

Saltatory conduction

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Classification of nerve fiber

• Basis– Functional specialization• Rapid transmission (motor neurons) = type A• Homeostasis/ANS = type B and type C (unmyelinated)

– Diameter of axon– Myelination

• Rate of action potential conduction– Myelination • Faster conduction• AP generated only at node of Ranvier

• Effects of axon diameter– Larger the diameter, faster the conduction• Greater surface area for Na channels

Synapse

• Junction between two cells– Communication• Nerve cells• Nerve cell and effector organs

– Presynaptic• Send signals

– Postsynaptic• Receive signals

• Types– Electrical• Rare (retina)• Conduct signals via ion flow from one cell to the next

– Connexon

– Chemical• Common• Conduct signals via neurotransmitter

Steps of neural transmission

1. Arrival of action potential to the presynaptic terminal

a. Increased flow of Ca ions

2. Release of neurotransmittera. Increased intracellular Ca level

3. Binding of neurotransmitters to the Na channelsa. Depolarization of postsynaptic membrane

4. Generation of local action potential5. Propagation of action potential

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Neurotransmitters

• Classes– Acetylchorine– Biogenic amines– Amino acids– Purines– Neuropeptides– Gases

• Neurotransmitter action– Specific receptor for the specific neurotransmitter– Response• Stimulatory (AP generation)• Inhibitory (hyperpolarization)• Depends on types of receptors present on the cell

Regulation of neurotransmitter level

• Rapid removal– Acetylchorinesterase• Removal of acetylchorine

– Monoamine oxidase• Removal of norepinephrine

• Neuromodulator– Influence generation of postsynaptic action

potential• Presynaptic influence• Postsynaptic influence

– Affects type of neurotransmitter being released

Excitatory vs. Inhibitory postsynaptic potential

• Response of postsynaptic terminal to a neurotransmitter– Depolarization• Excitatory postsynaptic potential• Excitatory presynaptic neuron

– Hyperpolarization• Inhibitory postsynaptic potential

– Increased K or Cl permeability

• Inhibitory presynaptic neuron

Presynaptic facilitation vs. inhibition

• Anoaxonic synapses– Affects release of neurotransmitter– Facilitation• Increased release

– Inhibition• Decreased release

Summation

• Generation of action potential– One presynaptic AP is not enough • Summation

• Temporal– Multiple AP from single presynaptic terminal• Facilitation for AP generation by the subsequent Aps

• Spatial– AP from multiple axons

• Generation of action potential – Excitatory vs. inhibitory stimulation• Depends on summation of stimulation by these fibers

• Importance of synapses within the CNS– Integration of information• Sensory input must be strong enough to generate AP• If not, ignored

– Large amount of information entering the CNS• Discarded

Neural pathway and circuit

• Convergent– Activation/inhibition of neural activity by combining

various information• Divergent– Simultaneous control of different system by a single

neural input• Circulatory– Repeated stimulation/inhibition of the same pathway

by a single neural input• Afterdischarge