Physiology 4th Week Summary (Lecture 3)

## Physiology 4th Week Summary ### Chapter 5 (part 1) In this chapter, we will talk about "action potential" (AP) which allows nerve cells "neurons" to transport its nerve signals through its axon until the nerve signals reach the axon terminal. - **Nucleus** - **Soma (cell body)** - **Dendrite**...

## Physiology 4th Week Summary ### Chapter 5 (part 1) In this chapter, we will talk about "action potential" (AP) which allows nerve cells "neurons" to transport its nerve signals through its axon until the nerve signals reach the axon terminal. - **Nucleus** - **Soma (cell body)** - **Dendrite** - **Myelin sheath** - **Axon** - **Axon terminal** **First, let's take a closer look at the neuron's plasma membrane:** - **Axon:** - **Na+** - **K+** - **Cl-** - **Protein** **mV** 0 -70 **Time (milliseconds)** 0 10 20 **OFF** **ON** - **ECF:** - **Na+ channel** - **K+ channel** - **Na+/K+ pump** - **K+ leak channel** - **ICF** **Firstly, you will notice that the inside of the cell is negatively charged while the outside is positively charged. Why?** First because Na+/K+ pump that pumps 3 Na+ outside the cell & 2 K+ inside the cell (so net movement is 1+ ion outside the cell) which makes the positive ions outside more than inside. Secondly, the leak channels (LC)..the Na+ LC allows small amount of Na+ to move down electrochemical gradient (from outside to inside) while K+ LC allows small amount of K+ to move down electrochemical gradient (from inside to outside). But, the K+ LC is way more permeable than the Na+ LC, which means more K+ is moving outside than Na+ moving inside, which also makes the positive ions more in the outside than inside. Also, the cell contains many intracellular proteins which are negatively charged. **Now the movement of ions across the membrane causes "membrane potential". So what's that?** **Membrane potential:** having charges (voltage) difference between the 2 sides of membrane (between inside & outside the cell), which drives the movement of ions according to permeability. Remember that everything likes to move from high energy state to low energy state (to be more stable)..or from high concentration to low concentration etc..that explains the pattern of ions movement. Let's assume that the membrane is ONLY permeable to Na+. While the Na+ is moving down electrochemical gradient, it will carry positive charges with it (as it's positive). So, the inside of the cell will have electro-positivity (positive charge) while the outside will have electro-negativity (negative charge) because other negative ions (anions) can't get inside or outside as the membrane is impermeable to them. Let's assume that the membrane is ONLY permeable to K+. While K+ is moving down electrochemical gradient, it will carry positive charges to the outside so the outside will have electro-positivity while the inside will have electro-negativity. So, when we have difference in charges that's "membrane potential" or sometimes we call it "diffusion potential" because it's caused by diffusion of ions. **NOTE:** the Na+ & K+ leak channels are ALWAYS open, they also allow very small amount of ions to pass through (K+ allows more K+ to pass through!), which means leak channels will cause charges difference but will NOT cause concentration difference (because it's a very small amount that's being transported. Also, Na+/K+ pump maintains the concentration difference by pumping 3 Na+ outside & 2 K+ inside). **Now let's assume that the membrane is impermeable to all ions. Then we will have no membrane potential. Also, if the membrane is freely permeable (allowing everything to pass through), we won't have membrane potential either!** (because ions will balance themselves between inside & outside of the cell *equilibrium*). That's why the membrane is SEMI permeable so it can maintain concentration & charges differences. **\*membrane potential always refers to electrical potential INSIDE the cell. Ex: if** ### Membrane Potential During an Action Potential +30 **Membrane Potential (mV)** 0 -55 -70 **Time** **Now let's summarize what occurs during AP:** 1. When the cell is at rest, membrane potential will be -70mV & all voltage gated channels will be closed. If a stimulus affects the cell, it will open some Na+ voltage gated channels (Na+ VGC), which will allow more Na+ (other than that of Na+ LC) to go into inside of the cell which will increase membrane potential a little bit (because more positive ions will be inside so the inside of cell is becoming a little bit more positive, so let's say this stimulus rises the potential from -70 to -65 for ex). A stronger stimulus will open more Na+ VGC, which means more + charges inside, which means higher membrane potential, however, as long as the stimulus doesn't reach "threshold" value. **NOT ALL of the Na+ VGC will be opened & even the opened Na+ VGC will close very soon bringing the membrane potential back to -70mV again as shown in the picture.** **NOTE:** When membrane potential is at -70mV this is called "polarized" state. **NOTE:** Increasing membrane potential is called "depolarization". 2. If a stimulus reaches "threshold" value (stimulus here is called threshold stimulus), then that will open ALL Na+ VGC which means maximum amount of Na+ will move into inside the cell (still it will just affect membrane potential, not concentration difference because the amount is still considered very small) so membrane potential will increase until it reaches "overshoot" value which is around +30mV. 3. Once the potential hits the overshoot potential, the Na+ VGC will close (closing non-resting state) & at the same time K+ VGC will open allowing K+ to move into outside which will decrease positivity inside the cell which will decrease membrane potential until it reaches "hyperpolarization". **NOTE:** Decreasing membrane potential is called "repolarization". **NOTE:** When membrane potential reaches threshold value again (-55mV), the Na+ VGC will transform from closed non-resting state to closed resting state. 4. Hyperpolarization means that the membrane potential is even below the resting membrane potential. So, it's less than -70mV, hyperpolarization occurs because of K+ VGC & K+ LC (as more K+ move into outside via LC than Na+ into inside via LC. Plus the effect of K+ VGC, which means excess K+ ions leave the cell. This is because K+ channels are slow in opening/closing). But, how does it come back to resting membrane potential? That's by 2 things: 1. K+ & Na+ leak channels 2. Na+/K+ Pump These two things will bring membrane potential back to -70mV (resting) to allow another action potential to take place (all VGC become closed again). So yes!! All of this mechanism is called "action potential". So, action potential is just a change in membrane voltage to allow transport of nerve signals & so on. - Now, we have 2 types of periods here, shown in picture below: +30 **Membrane Potential (mV)** 0 -55 -70 **Time** **A** **B** **C** 1. **Absolute refractory period:** it's from A to B (from threshold potential where Na+ VGC get opened until they get closed "resting closed state"). In this period, the stimulus no matter how strong it is, it won't affect the the membrane potential so it won't generate new action potential. 2. **Relative refractory period:** it's from B to C (from threshold potential where Na+ VGC move from closed non-resting state to closed resting state). In this period, the stimulus can affect the membrane potential which means it can generate new action potential. However, it has to be strong enough (suprathreshold stimulus) so we have 3 types of stimulus: - **Subthreshold stimulus:** below threshold value (below -55mV) - **Threshold stimulus:** equal to threshold value (-55mV) - **Suprathreshold stimulus:** above threshold value (above -55mV) **NOTE:** subthreshold stimulus causes "local" voltage change as it won't affect other. ### 4-Sacral region: 5 segments ### 5-Coccygeal region: 1 segment So we have 31 segments in spinal cord which means we have 31 pairs of spinal nerves. So, in total we have 43 pairs of nerves in our body (12 cranial & 31 spinal). We won't learn parts of the brain in this chapter, but we will learn some parts of the spinal cord. So let's take a closer look at this cross section of spinal cord: - **Central canal** - **Dorsal root** - **Dorsal root ganglion** - **Posterior horn** - **Grey matter** - **White matter** - **Anterior horn cell (soma of efferent)** - **Neuron before ganglion > preganglionic neuron** - **Neuron after ganglion > postganglionic neuron** - **Lateral horn** - **Spinal nerve** - **Ventral root** - **Anterior horn motor (efferent) neuron** - **Receptor (in skin)** - **Effector organ (skeletal muscle)** - **Somatic nervous system (Reflex arc)** **First, we have the grey matter (H shaped structure) that contains horn cells** (horn cells are neurons) **& the white matter that's surrounding the grey matter.** We also have central canal (we won't talk about it here). Notice that the grey matter is divided into several horns. But let's get into the most important parts. **We have 2 roots here:** 1. **Dorsal root (lies posteriorly) that carries the "afferent" or "sensory" neurons** 2. **Ventral root (lies anteriorly) that carries the "efferent" or "motor" neurons** & both roots unite to form spinal nerve that divides into anterior & posterior rami. **Ok, but what's the difference between sensory & motor neurons?** Now the sensory neuron carries AP from receptor to CNS (to receive stimulus) while motor neuron carries AP from CNS to effector organ (to produce response). **\*Ganglion: its area that is full of synapses between preganglionic neurons & postganglionic neurons.** The neuron that lies before ganglion is called preganglionic neuron while the neuron that lies after ganglion is called postganglionic neuron. So, AP is regenerated in ganglia (plural of ganglion) via chemical synapses. **\*Effector organ is the organ that's connected to motor neuron. So, this organ is affected by the neuron that is connected to it.** **NOTE:** The horn cell + its axon + the muscle fibers (part of a muscle) it supplies is called motor unit while ALL horn cells (+ their axons) supplying the WHOLE muscle (all of its fibers) is called motor pool. **NOTE:** Cell bodies (soma) are located as follows: * Sensory neuron >> cell body is located in ganglia * Motor neuron >> preganglionic neuron: cell body is located in grey matter / postganglionic neuron: cell body is located in ganglia Ok, now both dorsal & ventral roots unite to form a spinal nerve (as it's coming from spinal cord). Now until here the structure is the same for all kinds of nervous system. But, now we will discuss differences between somatic & autonomic nervous systems: There is something called "reflex arc". It's formed of 5 things: 1. **Receptor:** It receives AP from external stimulus (converts different forms of energy into nerve impulses). 2. **Center:** It's the region that exhibits the motor neurons in spinal cord. 3. **Afferent:** It's the sensory neurons that lie in the dorsal root (or cranial nerve if it's afferent of cranial nerve) & it carries nerve impulses from receptor to CNS. 4. **Efferent:** It's the motor neurons that lie in the ventral root & it carries nerve impulses from CNS to effector organ. 5. **Effector (or effector organ):** It's the organ that's connected to motor neurons. Therefore, it's affected by the neurons that are connected to it. **NOTE:** Neuron is the anatomic unit of nervous system. Reflex arc is the structural unit of nervous system & reflex action is the functional unit of nervous system. **Reflex action:** It's involuntary action (response) caused by a stimulus. The reflex actions are performed by reflex arcs in response to stimuli. The stimulus is recognized by sensory receptors (located in skin for the somatic nervous system / located in viscus for the autonomic nervous system), then the sensory receptor sends APs to the brain via afferent (sensory) neurons, then the brain responds by sending APs to the effector organ via efferent (motor) neurons. These motor neurons will act on molecular receptors found within the effector organ to cause an action. Also, some reflexes are entirely integrated within the spinal cord without depending on the brain. Ex: skeletal muscle reflexes that are shown in pictures below. ## Reflex Arc **Sensory receptors** **Stimulus** **PAIN** **Nerve signals** **Stimulus** **RESPONSE** **Sensory neuron** **Dorsal root ganglion** **Relay neuron** **Ventral root** **Motor neuron** **Effector Organ** **White matter** **Grey matter** **Sensation relayed to the brain** **Spinal cord** ## Knee Jerk Reflex **Quadriceps muscle** **Cell body of sensory neuron in dorsal root ganglion** **Gray matter** **White matter** **Hamstring muscle** **Spinal cord (cross section)** **Sensory neuron** **Motor neuron** **Interneuron** **OK, let's discuss the differences between somatic & autonomic nervous systems in terms of reflex arc.** **\*Somatic nervous system** - **Receptor: located in the skin** - **Center: located in anterior horn so it's called anterior horn cell** - **Afferent: consists of 1 pseudo-unipolar sensory neuron** - **Efferent: consists of 1 motor neuron (so no ganglion here)** - **Effector: voluntary organ (ex: skeletal muscles)** **\*Autonomic nervous system** - **Receptor: located in viscus (deep inside the internal organs)** - **Center: located in lateral horn so it's called lateral horn cell** - **Afferent: consists of 1 pseudo-unipolar sensory neuron** - **Efferent: consists of 2 motor neurons that are connected by "ganglion"** - **Effector: involuntary organ (ex: smooth muscles that are found in urinary bladder, stomach, blood vessels & other organs..cardiac muscles & glands)** **NOTE:** The pictures below dearly show the difference between autonomic & somatic nervous system. Notice this difference: (autonomic nervous system shown is parasympathetic) * **Central canal** * **Dorsal root** * **Dorsal root ganglion** * **Posterior horn** * **Grey matter** * **White matter** * **Anterior horn cell (soma of efferent)** * **Neuron before ganglion > preganglionic neuron** * **Neuron after ganglion > postganglionic neuron** * **Lateral horn** * **Spinal nerve** * **Ventral root** * **Anterior horn motor (efferent) neuron** * **Receptor (in skin)** * **Effector organ (skeletal muscle)** * **Somatic nervous system (Reflex arc)** **Dorsal horn - Sensory receptors** **Venter - motor neuron to skm** **Lateral horn - motor neuron to Vicus** **\*Astrocytes >> they are responsible for nutrition, protection & support of adjacent neurons. So, they are a highly branched cells..& they are found in CNS** **\*Oligodendrocytes >> they are responsible for myelination of axons (nerve fibers) found within CNS** **\*Schwann cells >> they are responsible for myelination of axons found within PNS** **\*Microglial cells >> they act as macrophages within CNS** **\*Ependymal cells >> they line cavities found within CNS. They help in production of CSF (cerebrospinal fluid) as they help in forming choroid plexus** **\*Satellite cells >> they are found in dorsal root ganglia & autonomic ganglia. They have a regulatory function.& they are part of PNS** Now, we said earlier that we have 2 types of receptors: molecular receptors & sensory receptors. In this system, we will focus on sensory receptors! Sensory receptors alongside neuronal pools were discussed in detail in 7th week summary. So please refer to that :) **Ok let's discuss briefly the main components of CNS & PNS** **Structure of CNS & PNS** - **Cerebral hemisphere** - **Longitudinal cerebral fissure** - **Diencephalon** - **Mesencephalon** - **Pons** - **Medulla oblongata** - **Cerebrum** - **Cerebellum** - **Brainstem**