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Chapter 3. Neurophysiology

Highter category: 【Brain Science】 Brain Science Index

1. The characteristics of the brain

2. Neuron

3. Glia

4. Resting membrane potential

5. Differential potential vs Active potential

6. The generation of the active potential

7. The conduction of the active potential

8. Chemical synapse

9. The histology of the cerebrum and the cerebellum

10. Learning and memory

1. The characteristics of the brain

⑴ Immersed in cerebrospinal fluid within the skull and protected by three layers of meninges (pia mater, arachnoid mater, dura mater).

⑵ The human brain is composed of approximately 100 to 200 billion neurons.

① Number of neurons by species:

○ C. elegans (302)

○ Fruit fly (~150,000)

○ Zebrafish (~5 million)

○ Mouse (~71 million)

○ Zebra finch (~131 million)

○ Octopus (~500 million)

○ Marmoset (~636 million)

○ Rhesus macaque (~6.4 billion)

○ Human (~86 billion)

○ Elephant (~257 billion)

⑶ Supported by glial cells, which are about 10 times more numerous than neurons.

⑷ Human brain weight: about 2% of body weight (1,200–1,400 g).

⑸ Human brain volume: approximately 1,400 mL; in great apes, around 400 mL.

⑹ Metabolism: consumes about 20% of total oxygen and 20% of total blood flow.

⑺ The brain uses energy at a rate of 10 to 60 watts: approximately 0.1 nanowatts per neuron.

⑻ The brain uses only glucose as an energy source; if glucose is not continuously supplied, the brain’s glucose stores are depleted within 10 minutes

⑼ Staining techniques: Golgi, Nissl, Weigert

Figure 1. Staining techniques of brain sections; Golgi, Nissl, Weigert from left to right

2. Neuron

⑴ Structure of a Neuron

① Length: as short as 2–3 mm, or as long as about 1 m

② Cell body (soma)

○ Contains nucleus, ribosomes, endoplasmic reticulum, Golgi apparatus, mitochondria, etc.

○ The nucleus is located in the center, and within the center of it is a distinct nucleolus.

○ Rich in ribosomes, so it is stained by basic dyes.

○ Nissl body

○ Stained rough endoplasmic reticulum ribosomes and granular endoplasmic reticulum, appearing in a leopard-spot pattern.

○ The dye used for Nissl staining is a basic dye.

Figure 2. A cross-section of the spinal ganglion

The big, red-looking ones are ganglion cells

③ Dendrite: nerve parts branching like tree limbs

○ Not covered with myelin.

○ Receive information from other cells (postsynaptic).

○ Dendritic spines: increase surface area.

○ Excitatory neurons synapse with the spines; inhibitory neurons synapse with the shaft.

Figure 3. The dendrites of pyramid cells

Excitatory synapses are marked red and inhibitory synapses are marked blue

④ Axon (also called nerve fiber)

○ Covered with myelin.

○ Myelinated nerve: nerve covered with myelin.

○ Unmyelinated nerve: nerve without myelin (e.g., squid axon).

○ Length: up to 1–1.5 m.

○ Transmits information to other cells (presynaptic).

○ Axon collateral: a branch of the axon.

⑤ Nerve terminal (also called bouton or synaptic terminal)

⑵ Formation of Neurons

① Approximately 86 billion neurons exist; synapses number around 10¹⁴–10¹⁵.

② Neuronal growth and connection to target cells begin during embryonic development and are strengthened through learning and training.

③ Only some neurons can regenerate after injury: if neuronal debris or the cell body is absent, regeneration is not possible.

⑶ Classification of Neurons

① By function

○ Afferent neuron (sensory neuron): delivers information to the central nervous system through the dorsal region of the spinal cord.

○ Efferent neuron (motor neuron): transmits information from the central nervous system through the ventral region of the spinal cord.

○ Interneuron: connects afferent and efferent neurons.

② By presence of axon

○ Type I: with axon; generates action potential; includes motor and sensory neurons.

○ Type II: without axon; does not generate action potential; includes interneurons.

③ By presence of myelin sheath (Schwann cells)

○ Myelinated nerve: motor and sensory neurons in vertebrates.

○ Unmyelinated nerve: neurons in invertebrates, interneurons in vertebrates.

⑷ Types of Neurons

① Unipolar neuron:

○ Has dendrite and axon extending in one direction.

○ Found in invertebrates.

② Pseudo-unipolar neuron:

○ A single axon branches into two, functioning separately as dendrite and axon.

○ Example: sensory ganglion cells of the spinal ganglion.

③ Bipolar neuron:

○ Has one dendrite and one axon extending from the cell body.

○ Example: retinal cells, olfactory cells.

④ Multipolar neuron:

○ Has one axon and multiple dendrites extending from the cell body.

○ Example: most neurons.

Figure 4. Types of neurons

⑸ Patch Clamp Method

① A technique used to study electrophysiology by attaching a patch to the membrane of a living cell.

3. Glia

⑴ There are  1 trillion cells in an adult body.

① The etymology of “glia” is “glue”.

② It plays a role like connective tissue.

③ Although all the roles aren’t discovered, the glia is regarded as the principal cells in the nerve system.

⑵ Astrocyte

① It is largest among glias.

② Class : Protoplasmic astrocyte, fibrous astrocyte

③ Location : It contacts the vascular tube in a direction and the neuron, the “pia meter”, the neural fiber in the opposite direction.

④ Function

○ It braces other neurons.

○ It contributes to the mass transfer between the neuron and the vascular tube.

○ It covers the nodes of Ranvier so that the axon isn’t exposed to the environment.

○ It maintains the microscopic environment by cleaning out the reaction products and reducing increased potassium ion concentration around the neuron.

○ It helps maintain BBB(blood brain barrier) in the vascular tube.

⑶ Oligodendrocyte : It creates the myelin of the neuron in CNS(central nerve system).

⑷ Schwann cell : It creates the myelin of the neuron in PNS(peripheral nerve system).

⑸ Microglia

① Phagocytosis : It is included in “mononuclear phagocytic system”.

○ Macrophage is also included in “mononuclear phagocytic system”.

② It plays immune function in the nerve system.

⑹ Ependymal cell : As epithelial cell, it covers inner surface of the central canal and ventricles.

4. Resting membrane potential

⑴ Membrane potential is the inner-membrane electrical potential relative to the outer-membrane electrical potential. (unit : ㎷)

⑵ The equilibrium states of restion membrane potential is -70 ㎷.

① The nonhomogenous distribution of ions around the cell membrane.

② The selective transparency of the cell membrane (Na⁺ < K⁺)

③ Na⁺/ K⁺ pump : It moves 3 molecules of Na⁺ outside the cell and moves 2 molecules of K⁺ inside the cell.

④ The attractive force by the negative-charged proteins in the cell

⑶ Nernst equation

5. The generation of the active potential

⑴ It follows the law of “all or none”.

⑵ Resting state

① Sodium channel : Activation gate is closed and inactivation gate is open at this step.

⑶ Depolarization

① Threshold potential : -40 ~ -50 ㎷, Depolarization should make the membrane potential exceed the threshold potential in order to generate the active potential.

② Sodium channel : Activation gate is open and inactivation gate is closed at this step.

③ Potassium channel : It is closed at this step.

⑷ Repolarization

① Sodium channel : Activation gate is open and inactivation gate is closed (right after the membrane potential hits 30 ㎷)

② Potassium channel : When the membrane potential hits 30 ㎷, repolarization begins by the leakage of the potassium ion.

③ Astrocyte begins to lower the concentration of the potassium ion.

⑸ Hyperpolarization

① Sodium channel : Activation gate is closed and inactivation gate is open at this step.

② Potassium channel : It is closing at this step.

③ By the mechanism of restion membrane potential, the membrane potential returns to -70 ㎷ slowly.

⑹ Refractory period

① The period from the potassium ion moves outside the axon and the membrane potential returns to -70 ㎷

② Absolute refractory period : In this period, it is impossible to regenerate the active potential no matter how big the stimulus is. It is related to the inactivation gate.

③ Relative refractory period : In this period, it is possible to regenerate the active potential. It is related to the activation gate.

6. The conduction of the active potential

⑴ Regenerative conduction : It generates the active potential by depolarizing the adjacent position.

⑵ Because the position in which the active potential already passes is in refractory period, the active potential conducts in one way.

⑶ Saltatory conduction

① Myelination

○ Myelin is slowly formed for a year after birth.

○ Oligodendrocyte : It makes encompassing myelin around the neuron by extending its protuberance.

○ Schwann cell : It makes encompassing myelin by rolling itself up around the neuron.

○ Myelin is made from the double-layer membrane and the main component of myelin is lipid which is also abundant in the membrane.

○ The boundaries of the oligodendrocytes or Schwann cells is also the boundaries of myelin and this boundary is called “node of Ranvier”.

② Myelination is insulation. Thus the active potential is generated at nodes of Ranvier and the conduction speeds considerably improved.

○ The period to generate the active potential is the period to diffuse the sodium ion and the potassium ion locally.

○ Not to generate the active potential all axon long, most position of axon should be insulated.

③ Saltatory conduction : It means that the active potential jumps to the next node of Ranvier at all steps.

⑷ The bigger the diameter of the nerve is, the faster the conduction is.

7. Chemical synapse

⑴ The principle of the chemical synapse

① Neurotransmitter

⑵ The synthesis and storage of neurotransmitter : They are stored in synaptic vesicles.

⑶ The release of neurotransmitter 

① Ca²⁺ promotes the fusion of the synaptic vesicle and the presynaptic membrane.

② Neurotransmitters are poured into the “synaptic cleft”.

⑷ EPSP/IPSP

① EPSP(excitatory postsynaptic potential)

② IPSP(inhibitory postsynaptic potential)

⑸ Synaptic integration

① Spatial summation, temporal summation

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② Shunting inhibition

⑹ The kind of neurotransmitter

① Ach

○ The life cycle of Ach

○ nAchR(Nicotinic Acetylcholine Receptor)

○ mAchR(Muscarinic Acetylcholine Receptor)

② GABA

③ Glu (excitatory)

④ Gly (inhibitory)

○ Africa native’s poisoned needle : Inhibition of Gly receptors → over-excitation of the circuit of the spinal cord → paralysis

⑤ NE

○ Influence to the heart beat

⑥ P substance

8. The histology of the cerebrum and the cerebellum

⑴ In the nerve system, the cell bodies are clustered functionally.

Example : Basal ganglion, thalamus, various nuclei of nerve, gray matter, ganglion, etc.

⑵ The histology of the cerebrum

① The cerebrum has the gray matter at its cortex, the white matter at its medulla and can be segmented with 6 layers.

② Molecular layer

○ It is the outer layer of the cerebral cortex.

○ There are few neurons.

○ There are neural fibers parallel to the cerebral cortex.

③ External granular layer : There are granule cells, glias, etc.

○ Granular layer cells receives information from the outside.

④ External pyramidal layer : There are pyramidal cells.

○ Pyramidal layer cells transmits information to the outside.

⑤ Internal granular layer : Small granule cells are located densely.

⑥ Internal pyramidal layer : There are the largest pyramidal cell at this layer.

⑦ Multiform layer : There are various forms of cells.

⑶ The histology of the cerebellum

① The cerebellum has the gray matter at its cortex, the white matter at its medulla and can be segmented with 3 layers.

② Molecular layer    

○ It is the outer layer of the cerebellar cortex.

○ Cells are distributed sparsely.

③ Purkinje cell layer : Very big purkinje cells are aligned between the molecular layer and the granular layer.

④ Granular layer : Neurons are distributed densely.

9. Learning and memory

⑴ LTP(long-term potentiation) : It takes place at the hippocampus.

① Molecular mechanism(see below)

② The change of synapse

⑵ LTD(long-term depression) : It takes place at the cerebellum.

① Molecular mechanism

② The change of synapse

③ The learning and memory in the cerebellum

○ The cerebellum is one-tenth of the cerebrum in size but is equal in the number of neurons.

○ Input : PF(parallel fiber), CF(climbing fiber); It represents address.

○ Output : PC(purkinje cell); There is the change of synaptic strength.

Input: 2018.09.17 23:53