Nervous+System++Physiology

__ [in PROGRESS ] __ __**ACTION POTENTIALS**__ Excitability of neurons allows an action potential to be generated when it is adequately stimulated.
 * __ NEUROPHYSIOLOGY __** __ Mechanisms of Operation of the Nervous System __


 * __Types of channels in membranes:__**


 * **Chemically gated / ligand gated channels** open when the appropriate chemical binds to the membrane. In the case of neurons the chemicals are usually neurotransmitters.
 * **Voltage gated** channels opens and closes in response to membrane potential changes
 * **Mechanically gated** channels open in response to mechanical deformation of the receptor eg. touch and pressure receptors.


 * __Resting Membrane Potential__**

__**Measurement**__



__**Origin of the potential difference**__



Na-K pump ensures RMP remains constant



Depolarization - Potential differences becomes more positive / less negative Hyperpolarization - Potential difference becomes more negative than RMP, < -70mV) Graded potential - short-lived, local changes in MP or hyperpolarizations
 * __Changes in membrane potential....__**

These occur due to changes in cell environment, Cl - ..... and HCO 3 - may play a role


 * __ACTION POTENTIALS__**

__Steps in Generation and Propagation of Action Potentials__

1. Membrane at rest.... most channels closed

2. Depolarization phase .... when threshold potential is reached ... action increases due to Positive feedback

3. Repolarization phase ..... Decrease in Na+ permeability Increase in K+ permeability

4. Hyperpolarization ..... K+ continues to leave ... channels slow in closing

5. Restoration to resting potential


 * Propagation of Action Potential :**


 * Continuous, Self-propagating slower along unmyelinated fibres
 * Saltatory conduction - myelinated axons, very rapid
 * Saltatory conduction - myelinated axons, very rapid

Axon diameter and Degree of myelination influence speed of transmission.

Stimulus not strong enough (subthreshold) will not result in an action potential. It will not result in a weak action potential or just depolarize a short section and cause a weakened reaction....
 * __ ALL OR NONE RESPONSE __**
 * IT EITHER HAPPENS OR IT DOES NOT AT ALL. **


 * __ REFRACTORY PERIODS __**
 * Absolute Refractory Period** : While membrane is depolarized and being depolarized,

no stimulus will result in an action potential.
 * Relative Refractory Period** : When membrane is largely repolarized a strong enough stimulus


 * __ THE SYNAPSE __**

__ Types of synapses on the basis of structure __

An Axodendritic synapse :;: - Axon of Presynaptic neuron ,,, cleft ,,,, Dendrites of Postsynaptic neuron Dendodendritic Axoaxonic Axosomatic Dendrosomatic

__ On the basis of function: __
 * ELECTRICAL SYNAPSE** Less common = connexin in cellular junction

Cytoplasm of adjacent cells in contact

Ions move directly to next cell [concept of pseudo syncytium]


 * CHEMICAL SYNAPSE**

ACh — from CNS & PNS – partic at neuromuscular junctions. Dopamine, Norepinephrine , Epinephrine [catecholamines] serotonin, histamine [indolamines]
 * __Neurotransmitters__** :

1. Electrical impulse reach axonal terminal – Na+ channels open ,,,Voltage gated Ca++ channels open,,,These ions rush into the cell 2. Neurotransmitter released — exocytosis ; Ca++ moved to mitochondria or ejected by Ca++ pump 3. Neurotransmitter cross synaptic cleft – binds with receptors on postsynaptic neuron 4. Ion channels in postsynaptic neuron opens 5. Neurotransmitters may : Transfer is relatively slow across the synapse ∴ the more synapses, the longer it takes for action.
 * __ Information Transfer across Chemical Synapses __**
 * be degraded by enzymes
 * absorbed by astrocytes or return to presynaptic terminal
 * simply diffuse away from the site
 * Synaptic Potentiation** = the more terminals fire, the stronger the effect on postsynaptic neuron

__ POSTSYNAPTIC POTENTIALS __

Neurotransmitter causes depolarization of post synaptic neuron. Transmitted to axon hillock and propagated along axon .... Action
 * __ESPS = Excitatory Post Synaptic Potentials__**

Neurotransmitter results in hyperpolarization : no Na+ enter (channels remain closed) Cl– channels open and Cl– move into cell more negative inside ... no depolarization — no response from neuron
 * __ISPS = Inhibitory Post Synaptic Potentials__**


 * __ Summation __**

Many terminals make contact with same Post synaptic Neuron

(time bound)
 * __Temporal Summation__**

Many fire within short time to produce ‘stacked’ effect Many vesicles rapidly open up into synaptic cleft successively

(space bound)
 * __Spatial Summation__**

Large number of terminals make contact with same PostSN— not nec. from same PreSN Same effect as above.

Axon hillock keeps account of positives and negatives and the resultant force will be executed. If EPSP’s override and is beyond threshold potential === ACTION If IPSP’s override = NO Action

Neurotransmitters == slow action on target cell metabolism Other chemicals influence neuronal activity
 * __ Neuromodulation __**



Neuromodulation in Bladder control

Studies have shown that the S3 region of the spinal cord plays a key role in the neurologic control of bladder function. Electrical stimulation, using electrodes implanted in near S3, has been shown to be effective in treating urge incontinence and it's associated conditions. The SANS device is a less invasive method of gaining access to the S3 junction of the spine, so that patients would not have to endure invasive surgery. SANS stimulates the nerve from the ankle, rather than through an invasive spinal procedure.



__ Presynaptic Inhibition __

Reduced excitability of presynaptic neuron by 'interference' of another neuron thru axoaxonic synapsing. Less neurotransmitter released ... smaller EPSP's

__ HOW THE BRAIN FUNCTIONS __ __ The Brain and its Blood Supply __



A Cerebral Aneurysm occurs mostly in this area.

NB**** you CANNOT develop a cerebral aneurysm fromthinking too much...

__ The BLOOD-BRAIN BARRIER __ 1. ‘Impermeable’ capillaries - extremely selective : essential nutrients exit

2. Thick basal lamina of capillary

3. Astrocyte extensions ‘hugging’ the capillaries — complex functioning ensures nutritional supply and constant environment around neurons



__ FUNCTIONAL SYSTEMS IN THE BRAIN __ __ The Limbic System __

__ Components of the Limbic System __
 * septal nuclei (Clusters of cell bodies in the septum pellucidum)
 * cingulate gyrus
 * hippocampus and parahippocampal gyrus
 * dentate gyrus
 * part of the amygdala
 * hypothalamus
 * anterior nucleus of thalamus
 * dentate gyrus
 * part of the amygdala
 * hypothalamus
 * anterior nucleus of thalamus
 * hypothalamus
 * anterior nucleus of thalamus
 * anterior nucleus of thalamus

__ Functions of Limbic system __

Primitive “smell brain” ,,,,,The survival of the fittest brain ,,,,,,,,,The Self-preservation brain ,,,,,,,,The animal brain

Affective - feelings, emotions

Amygdala - fearful facial expressions, danger, responses to fear ; Plays role in memory

Hypothalamus- Plays a role in memory ,, Origin of psychosomatic illnesses

Cingulate gyrus - Gesticulation when frustrated and words don’t suffice [Road-Rage Signalling ??]

Association of odours with events.... good or bad, nostalgia, memories of specific events

Links lower and higher brain activities ... Interacts with prefrontal lobe (logic and reason)
 * * sometimes emotions override logic and reason
 * * sometimes reason and logic prevents us from showing emotion
 * * sometimes reason and logic prevents us from showing emotion
 * * sometimes reason and logic prevents us from showing emotion

[Foetal development – primary brain functioning — emotional maturity ]

__DiEncephalon__

Epithalamus

Thalamus = Central Switchboard, one on one conversations.. Group Chat ...conference calls, videoconferencing,

Hypothalamus: Cerebrum : Cerebellum

Pituitary Gland/Hypophysis Brain Stem
 * Neurohypophysis :
 * Adenohypophysis: Master endocrine gland [with endocrine system]
 * Midbrain
 * Pons (Varolii)
 * Medulla Oblongata

SO^O ! How DOES the brain work?
 * It is ENTIRELY up to YOU
 * HOW YOU want to use your brain,
 * how much of it you want to use and
 * what you want to use it for
 * how much of it you want to use and
 * what you want to use it for
 * what you want to use it for

HOW DOES THE SPINAL CHORD WORK ?