Physio Ch. 7 Nervous System

Types of sense

• Somatic

• Special

• Viseral

Afferent Pathway

• Pathway into the CNS

Efferent Pathway

• Pathway into the CNS

• Subdivided into somatic and autonomic systems

Somatic system

• Under voluntary control

• Controls skeletal muscle

Autonomic system

• Under involuntary control

• Controls smooth and cardiac muscles

• Subdivided into sympathetic and parasympathetic systems

Sympathetic system

• Subdivision of the autonomic system

• Controls cardiac and smooth muscle

Parasympathetic system

• Subdivision of the autonomic system

• Controls enteric nervous sytem

Electron and chemical pathway signaling

• Step 1: Local potential in dendrite

• Step 2: Initiation of action potential in axon hillock

• Step 3: Action potential release into axon terminal

• Step 4: Signal is carried to another cell

Excitability

• A cell being able to produce and electrical signal by changing membrane potential

- Threshold to send an action potential varies depending on stage of process

Pumps affects

• Na and K pumps: Located throughout cell, establishes and maintains resting membrane potential

- Uses Na and K concentration gradients to do so

• Cl2+ pump: Located in the axon terminals, maintains low Cl concentration in cell

Leaky Channel

• Non-gated channel, always open

• Maintains resting membrane potential (RMP)

Gated channel

• Subdivided

- Ligand

- Voltage

- Mechanical

Ligand

• Opens when a ligand binds to specific receptor, chemical stimuli

• Mostly located in dendrites and soma

- Responds to neurotransmitters

Voltage

• Opens when membrane potential changes

• Located throughout the cell

- Na+ / K+ in the axon and axon hillock

- CL2+ dense in axon terminals

Mechanical

• Opens and closes with physical stimuli

• Located in sensory receptors

Resting membrane potential (RMP)

• Typically about -70 mV

- Referred to as “polarized”

• Influenced by ion concentrations

K affect on RMP

• If only based on K concentration the charge would be -94mV

• Larger impact on overall RMP than Na because cell is 25 times more permeable to K than NA

- More K leaks out of the cell than Na leaks in

Na affect on RMP

• If only based on K concentration the charge would be +60mV

• Lesser impact on overall RMP than K because cell is 25 times less permeable to Na than K

- More K leaks out of the cell than Na leaks in

Permeability changes in cell

• Rest: Permeable to both

- 25 times more permeable to K than Na

Depolarization

• Membrane potential becomes less negative

• Approaches the action potential threshold, +30mV

- Increases Na permeability to do this

Depolarization

• Membrane potential becomes more negative

• Falls back under the action potential threshold, +30mV and approaches the RMP of -70mV

- Decreases Na permeability and increases K permeability to do this

Hyper polarization

• Membrane potential becomes more negative

• Falls far under the RMP, -70mV

- Caused by leftover increases of K permeability from the depolarization step

• Lasts about 5 to 15 mili seconds

Gradient potential

• Used for localized signaling, sets off action potential if strong enough

• Can vary in grade depending on

- Number of channels open

- Strength / length of stimuli

• Subject to Decremental

Decremental

• The decreasing of strength in a gradient potential

- Only allows gradient potential to travel a few mm

Temporal summation

• The addition of different gradient potentials from multiple stimuli at a particular site

Spatial summation

• The addition of different gradient potentials from multiple stimuli at different sites

Action Potential

• Generated in axon hillock / trigger zone once threshold is reached

• Travels long distances through axons

• Irreversible once triggered

• Doesn’t vary in intensity

- Sends different messages through a coding papers

Na Channels

• 3 possible configurations

- Closed but capable of opening (Resting position): inactivation gate is open, activation gate closed

- Open: both gates open, -55mV

- Closed and incapable of opening: Inactivation gate closes 1 msec after initial

stimulus, closed until cell is repolarized to resting state, +30mV

K Channels

• 2 possible configurations

- Closed: Inactive but able to be activated, +30mV

- Open: Activated

Refectory period

• Cells dislike to re-fire after they have just fired

• Caused by hyper polarization pushing the membrane potential lower than the standard -70mV

- More work will be needed to trigger another action potential at -55mv

• Function:

- Stops back flow of signals

- Forces signal to continue down the axon

Conduction of Nerve Impulses

• Speed depends on diameter and myelination

- Myelinated: Faster

- Large diameter: Faster

Militated neuron

• Hay militated sheath around axon

• Signal travels through nodes of Ranvier

Unmyelinated neuron

• Naked axon

• Signal traves continusley down axon