Neurophysiology PT 1

The Nervous System

• master controlling and communicating system of body

• Cells communicate by electrical signals that are rapid and cause immediate responses

Sensory input

(Nervous system Function)

monitoring stimuli occurring inside and outside the body

Integration

(Nervous system Function)

interpretation of sensory input

Motor output

(Nervous system Function)

response to stimuli by activating effector organs

The two principal cell types of the nervous system are

• Neuroglial – cells that surround and support neurons

• Neurons – excitable cells that transmit electrical signals

Anatomy of Neurons

Cell body

Dendrites

Axon hillock

Axon

Myelin sheath

Nodes of Ranvier

Telodendrites (terminal branches)

Axon terminals

Cell body

contains nucleus and organelles

Dendrites

branching extensions

Receptive to neurotransmitters from pre-synaptic neurons and transmit graded potential towards cell body

Axon hillock

where cell body tapers into axon

site where action potential originates

**Axon**

single process extending from cell body

transmits action potential away from cell body

**Myelin sheath**

formed by schwann cells

wrapping around the axon

resulting in aligned layers of plasma membrane

**Nodes of Ranvier**

**gaps in myelin sheath**

**Telodendrites (terminal branches)**

**distant branches of axon**

**Axon terminals**

enlarged distal ends containing secretory vesicles filled with neurotransmitters

What is a synapses?

* **junctions between neurons**
* **Function as control or decision point that can be excitatory or inhibitory**
* **Occurs between axon terminals and a cell body, dendrite, axon hillock, muscle or gland**

Structure of Chemical Synapses

**Presynaptic neuron**

**Synaptic cleft**

**Postsynaptic neuron**

**Presynaptic neuron**

transmits impulse towards the synapse, axon terminal with vesicles containing neurotransmitters

**Synaptic cleft**

**fluid filled space between pre and post synaptic neuron**

**Postsynaptic neuron**

transmits impulse away from synapse, contains receptors for neurotransmitters

Types of Ion Channels found in Neurons

**Ligand-gated channels**

**Mechanically gated channels**

**Voltage-gated channels**

**Leaky channels**

**Ligand-gated channels**

chemically gated

open when neurotransmitters bind

Found on dendrites, cell bodies, and axon hillocks

**Mechanically gated channels**

**open in response to physical forces**

**Voltage-gated channels**

open or close in response to changes in membrane potential

Found along axon

**Leaky channels**

**always open**

**non-gated**

**found everywhere**

**Electricity**

• When opposite charges are separated, they contain potential energy and when they come together electrical energy is released

• In cells, the separation of charges by the plasma membrane is “membrane potential”

Principles of Electricity

**Voltage**

**Voltage**

• the measurement of potential energy created by charge separation

• measured in millivolts

• The voltage depends on the quantity of charge and the distance between the charges

Membrane Potentials

**Resting Membrane Potential-**potential difference across the membrane in a resting neuron

2 types of gradient within a resting membrane potential

**Chemical gradient-**higher concentration of Na+ in the extracellular fluid and a higher concentration of K+ in the intracellular fluid

**Electrical gradient-**The inside of the membrane is  negatively charged and the outside is slightly positive

Factors contributing to the resting membrane potential

• Membrane is 50 – 75X more permeable to K+ so K+ ions leak out faster than Na+ leak in

• Intracellular proteins - fixed anions inside the cell

Sodium-Potassium pump maintains the chemical and electrical gradient – 3 Na+ out for every 2 K+ in

What happens in Membrane Potentials

**Stimuli will trigger disruptions in RMP(resting membrane potential)**

* Triggers a **graded potential** – a localized change in membrane potential
* Short lived and dissipates as it travels

Changes in Membrane Potential

(what happens if stimulus is excitatory?)

• If the stimulus is excitatory it will cause depolarization of the membrane

• Depolarization – the membrane potential becomes less negative

  • When neurons are stimulated Na+ channels open and Na+ rushes into the cell down its electrochemical gradient

Graded Potentials

(What does the magnitude of stimulus depend on?)

• Magnitude of the stimulus depends on how many Na+ channels open

• This determines the distance that the graded potential will travel

**Amount of Na+ channels affected by the stimulus depends on**

(graded potential)

• Frequency of stimuli - summation

• Amplitude of stimuli - strength

• Strong graded potentials can initiate action potentials if the threshold potential is reached at the trigger zone (axon hillock

**Threshold potential**

**=-55mV**

The critical level of membrane potential must reach to open voltage-gated Na+ channels on the axon to produce an action potential

Action Potential

(What does stronger stimuli increase?)

brief reversal of the membrane potential

• neuron sends information down an axon, away from the cell body

• Wave of depolarization followed by repolarization

• Stronger stimuli increases the frequency of axon potential

**Repolarization**

the membrane returns to its resting membrane potential

• Voltage gated Na+ channels close

• Voltage gated K+ channels fully open and K+ efflux restores the resting membrane potential

• Membrane potential becomes more negative as K+ rushes out

**Hyperpolarization**

(What happens when the K+ permeability last longer?)

* **the inside of the membrane becomes more negative than the resting potential**
* **Voltage gated K+ channels are sluggish to close**
* **K+ permeability lasts longer and membrane potential dips below resting potential**

**Restoring the Resting Membrane Potential:**

• Repolarization restores the electrical gradient

• Na/K pump restores resting ionic concentrations

**Refractory Periods**

time required for a neuron to generate another action potential

**Absolute Refractory Period**

* **when another AP cannot be generated**
* **From the opening of the Na+ activation gates until the resetting of the activation gates**
* **Ensures that each action potential is separate**
* **Enforces one-way transmission of nerve impulses**
* **Think of flushing the toilet**

**Relative Refractory Period**

* the interval of time during which a second action potential can be initiated, but initiation will require a greater stimulus than before due to a raise in threshold. Refractory periods are caused by the inactivation gate of the Na+ channel.
* Na+ gates are reset
* K+ gates are still open
* Hyperpolarization is occurring

Factors Influencing Conduction Velocity

**Myelination of axon**

**Saltatory conduction**

**Diameter of the axon**

**Alcohol, sedatives, and anesthetics**

**Insufficient blood flow to neurons**

**Myelination of axon**

(Acts as….)

* **increases impulse rate**
* **Acts as insulator preventing charge leakage**

**Saltatory conduction**

voltage gated channels are concentrated nodes

electrical impulses jump from node to node instead of traveling down the entire axon

**Diameter of the axon**

the larger the diameter the quicker the impulse travels, less resistance to current flow so adjacent membranes depolarize quicker

**Alcohol, sedatives, and anesthetics**

(Is pain still present?)

* **slow or block nerve impulses by reducing permeability to Na+.** 
* **Pain is still present but the brain can’t detect it**

**Insufficient blood flow to neurons**

(Why does foot tingle?)

* **slows impulses, caused by cold or pressure**
* **Foot falls asleep, then tingles when neurons fire again**