Chapter 11-Part II Biology 2121 Neurophysiology

Neurophysiology Nerve impulses Afferent impulses Efferent impulses Nerve impulses and action potentials are the result of electrical changes in neurons (ion movement)

Membrane Potential Leaky channels Potential difference (–70 mV) across the membrane of a resting neuron Ionic (charge) differences are the consequence of: Na+ and K+ permeability difference across the membrane sodium-potassium pump Restoration Na leaks into, and K leaks out of the membrane Leaky channels

How is the Resting Membrane Potential Changed? Membrane charges - reversed Graded potential Action potential Events Polarized ---------- depolarization Hyperpolarization Repolarization

Changes in Membrane Potential From -70 mV to +30 mV

Channels and Changes in the Membrane Potential Chemically Gated Channels Voltage-Gated Channels When these gates are opened, ions move down their electrochemical gradient!

What are Graded Potentials? Short-lived Decrease in intensity with distance Their magnitude varies directly with the strength of the stimulus Sufficiently strong graded potentials can initiate action potentials Any membrane can become ‘depolarized’ But it cannot form an action potential

How are Graded Potentials Different from Action Potentials?

Action Potentials Only occur in ‘excitable membranes’ Muscle cells Neurons Axon Hillock Graded potentials may turn into action potentials Summation Temporal and Synaptic Comments Strength does not decrease Self propagating

Generation of an Action Potential

Propagation (fig. 11.13)

Thresholds Do all depolarization events cause an AP? All or Nothing Must reach threshold (-50 mV) or 15-20 mV from resting potential More Na+ channels open and increased depolarization occurs All or Nothing

Refractory Periods Ion channels - open Types Neuron cannot respond to another stimulus Na+ channels must close first! Resets normal polarized state Types Absolute Relative Absolute: Time from the opening of the Na+ activation gates until the closing of inactivation gates The absolute refractory period: Prevents the neuron from generating an action potential Ensures that each action potential is separate Enforces one-way transmission of nerve impulses Relative refractory period: The interval following the absolute refractory period

Pre-Synaptic and Post-Synaptic Activity Depolarization waves Moves towards the synapse Conduction speed depends on Diameter of axon Myelination Nerve Fiber Types Group A, B, C

Saltatory Conduction

Multiple Sclerosis Autoimmune Disease Scleroses Consequences Demyelination of the myelin sheath Scleroses Lesions are formed Disturbance of nerve transmissions Axon not damaged Consequences Relapse and remission Speech, incontinence, blindness, muscular weakness

When the Nerve Transmission Reaches the Synapse

Synapses Electrical Synapse – less common; coordinate neuron activity between neurons; ions move through gap junctions; most abundant in embryonic tissue; brain find stereotyped movements (hippocamus); glial cells of CNS Chemical Synapses – ions flow between channels; Neurotransmitters are released

Chemical Synapse

At the Synapse Synaptic Delay Signal Variation Summation – EPSPs EPSP NT binding ------------- depolarization (post-synaptic membrane) AP usually not occur at the membrane Only contain chemically gated channels (axons can – have voltage gated channels EPSPs are created --- trigger AP at the axon hillock Only lasts milliseconds ---- returns to polarized state IPSP Reduces post-synaptic membrane’s ability to produce AP Summation – EPSPs

Summation

Neural Integration Neurons of CNS – neuronal pools Integrate incoming information Discharge zone Facilitated zone

Types of Circuits 1. Diverging 2. Converging 3. Oscillating Common in sensory and motor systems 2. Converging Sensory and motor systems 3. Oscillating Rhythmic activities Sleep-wake; breathing, etc. 4. Parallel after – discharge only involved in complex, mental processing Serial and Parallel Processing

Neural Circuits

Developmental Aspects Surface Ectoderm Dorsal neural tube and neural crest (12.1) Neural tube – forms CNS Origin of the neurons Neuroepithelial cells – second month differentiate and proliferate Become neuroblasts