Introduction to Structure and function of Computational the nervous system … · 2019. 9. 2. · Structure and function of the nervous system Introduction to Computational Neuroscience

Structure and function of the nervous system

Introduction to ComputationalNeuroscienceLecture 2: Structure and function of the NS

lunes, 5 de septiembre de 16

Applications

Cognitive

Models

Analyses

Basics

Lesson Title

1 Introduction

2 Structure and Function of the NS

3 Windows to the Brain

4 Data analysis

5 Single neuron models

6 Network models

7 Artificial neural networks

8 Artificial intelligence

9 Learning and memory

10 Perception

11 Attention & decision making

12 Brain-Computer interface

13 Neuroscience and society

14 Future and outlook

15 Projects presentations



Physiology

Anatomy

Clinical

Computational

Cognitive

Behavioral

Structure

Function

Cellular, Molecular, Developmental, Evolutionary,...

Informationprocessing


“One of the difficulties in understanding the brain is that it is like nothing so much as a lump of

porridge”

R.L. GregoryEye and the Brain: the psychology of seeing,

New York, 1966, McGraw-Hill


“What I cannot create, I don’t understand”

R. Feynman


Where mental faculties sit?

Ancient Egypt

Aristotle

Plato

Galen

Hippocrates

vs.


What are the building blocks?

1906 Nobel Prize in Medicine or Physiology


Learning objectives

• Cell types (neurons and glial cells)

• Methods of communication (synapses)

• Organization of the NS (the basic plan)


Introduction to the NS

Gross anatomy of the CNS

Neuronal signaling


Most animals have a nervous system thatallows responses to stimuli


Evolution of the NS


Evolution of the NS


Evolution of the NS


Division of the vertebrate NS

Central Nervous System (CNS)* Brain* Spinal cord

Peripheral Nervous System (PNS)Nerves outside the brain and spinal cord* Cranial nerves* Spinal nerves

2 The Human Brain

cephalon is hidden from view by the massive cerebral hemispheres. The brainstem is that part of the CNS, exclusive of the cerebellum, that lies between the cere-brum and the spinal cord.

The Principal Cellular Elements of the Nervous System Are Neurons and Glial Cells

Despite the large size and widespread distribution of the nervous system, it contains only two principal catego-ries of cells—nerve cells, or neurons, which are the

information-processing and signaling elements, and glial cells, which play a variety of supporting roles. Both neurons and glial cells are present in enormous numbers. There are around 100 billion* neurons in the human nervous system and perhaps 10 times that many glial cells.

Neurons Come in a Variety of Sizes and Shapes, but All Are Variations on the Same Theme

Neurons are in the business of conveying information. They do so by a combination of electrical and chemical signaling mechanisms: electrical signals are used to convey information rapidly from one part of a neuron to another, whereas chemical messengers are typically used to carry information between neurons. Hence there are anatomically specialized zones for collecting, integrating, conducting, and transmitting information (Fig. 1-3; Table 1-1). All neurons have a cell body (soma,or perikaryon)† that supports the metabolic and syn-thetic needs of the rest of the neuron. Most neurons have a series of branching, tapering processes called den-drites that receive information from other neurons via synaptic contacts (or synapses) and one long, cylindri-cal process called an axon that conducts information away from the cell body. The axon gives rise to a series of terminal branches that form synapses on other neurons. Hence neurons are anatomically and function-ally polarized, with electrical signals traveling in only one direction under ordinary physiological circum-stances. (The molecular underpinnings of this anatomi-cal and functional polarization are discussed in Chapters 7 to 9.)

Despite the basic similarity among all neurons, there is wide variability in the details of their shapes and sizes (Fig. 1-4). Certain aspects of somatic, dendritic, and axonal morphology give rise to a descriptive terminol-ogy for neurons. The vast majority of vertebrate neurons are multipolar, meaning that there are multiple den-dritic projections from the cell body and almost always an axon as well (Fig. 1-4A to E); in many cases the pattern of the dendritic processes is characteristic of that type of neuron. Some neurons are bipolar (Fig. 1-4F) or

CNS

PNS

Figure 1–1 Central and peripheral nervous systems. The central nervous system (CNS) is encased in the skull and vertebral canal. The peripheral nervous system (PNS) is attached to the CNS, but its nerve fi bers are distributed throughout the body. (Redrawn from Krstic RV:General histology of the mammal, Berlin, 1985, Springer-Verlag.)

*It is hard to get a sense of how big such numbers really are, so analogies sometimes help. If you could count one neuron per second, and took no breaks for anything, it would take you more than 3000 years to count 100 billion neurons!

†Karyon is Greek for “nucleus,” and, strictly speaking, the peri-karyon is the cytoplasm surrounding the nucleus of a neuron. However, the term is commonly used to refer to the entire cell body.

The nervous system has central and peripheral parts:


Division of the vertebrate NS


CHAPTER 1 Introduction to the Nervous System 3

Figure 1–2 Three-dimensional reconstruction of the entire CNS, seen from the left side (A), from directly in front (C), and from halfway in between (B). The eyes are included with the reconstruction because, as described in Chapter 2, the retina develops as an outgrowth from the neural tube.

Dendrite

Axon

Synapse

Figure 1–3 Schematic view of a typical neuron, indicating synaptic inputs to its dendrites (although other sites are possible) and information fl ow down its axon, reaching synaptic endings on other neurons. Information fl ow is unidirectional due to molecular specializations of various parts of neurons, as described in Chapters 7 and 8. The pink segments covering the axon represent the myelin sheath that coats many axons (see Figs. 1-24 and 1-30), and the gap in the axon represents a missing extent that might be as long as a meter in the longest axons.

Diencephalon

Diencephalon

Brainstem

Brainstem

Cerebellum

CerebellumSpinal cord

Cerebralhemisphere

A B C

Division of the CNS

The brain itselfhas multiple subdivisions:

Cerebrum

Cerebellum

Brain stem


Division of the PNS

Sensory divisionPicks up sensory information and delivers it to the CNS

Motor divisionCarries information to muscles and glands

* Divisions of the Motor division* Somatic carries information to skeletal muscle* Autonomous carries information to smooth muscle, cardiac muscle, and glands


Function of the NSCNS and PNS must work in harmony to carry3 main functions



1 Receive sensory input Monitor changes occurring inside and outside the body (changes = stimuli)

* Sensory receptors gather information* Information is carried to the CNS



2 Perform integrationTo process and interpret sensory input and decideif action is needed

Sensory information is used to create* Sensations* Memory* Thoughts* Decisions



3 Generate motor output A response to integrated stimuli is given

* Decisions are acted upon* Impulses are carried to effectors (muscles or glands)


Cellular elements

2 cell categories in the nervous system:

Neurons (information processing, signaling elements, 100 billion)

Glial cells (supporting roles, 10 x neurons)


Neurons: compartments

Soma supports metabolicand synthetic needs

Dendrites receive information from other neurons via synapses

Axon conducts information away from cell body


Figure 1–2 Three-dimensional reconstruction of the entire CNS, seen from the left side (A), from directly in front (C), and from halfway in between (B). The eyes are included with the reconstruction because, as described in Chapter 2, the retina develops as an outgrowth from the neural tube.

Dendrite

Axon

Synapse

Figure 1–3 Schematic view of a typical neuron, indicating synaptic inputs to its dendrites (although other sites are possible) and information fl ow down its axon, reaching synaptic endings on other neurons. Information fl ow is unidirectional due to molecular specializations of various parts of neurons, as described in Chapters 7 and 8. The pink segments covering the axon represent the myelin sheath that coats many axons (see Figs. 1-24 and 1-30), and the gap in the axon represents a missing extent that might be as long as a meter in the longest axons.

Diencephalon

Diencephalon

Brainstem

Brainstem

Cerebellum

CerebellumSpinal cord

Cerebralhemisphere

A B C

Neurons have specialized zones for collecting, integrating, conducting, and transmitting information.


Neurons: synaptic contacts

2 to 20 nm space (Cajal vs Golgi)

Computational advantages

Synapse: axonal end abupts on other neuron (dendrites). A few thousand per cell

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22 The Human Brain

Table 1–2 Components of the Peripheral Nervous System

Cell or Cell Part Type Location/Form

Neuronal cell bodies Sensory neurons Spinal and cranial nerve ganglia, some sensory epithelia

Autonomic ganglion cells Sympathetic, parasympathetic, enteric gangliaParts of neurons Axons of motor neurons, axons of autonomic neurons,

peripheral processes of sensory neuronsSpinal and cranial nerves

Glial cells Schwann cells Myelin sheaths, sheaths of unmyelinated axons, satellite cells

A BFigure 1–21 Synapses densely distributed over the surface of CNS neurons. A, Double immunofl uorescence micrograph of a dendrite of a hippo-campal neuron developing in tissue culture. The cell body (not seen in this fi eld of view) and dendrites were stained with a fl uorescent antibody directed against MAP2, a microtubule-associated protein restricted to the perikaryal-dendritic region of neurons (green fl uorescence). Axon terminals originating from other neurons not visible in this fi eld form a dense network of synaptic contact sites and were stained with a fl uorescent antibody directed against synaptotagmin, an integral membrane protein of synaptic vesicles. (Overlapping red and green fl uorescence, from sites where an axon terminal is superimposed on part of the dendrite, appears yellow.) B, Triple fl uorescence micrograph of CNS gray matter (deep cerebellar nuclei of a rat), stained for MAP2 as in A, showing neuronal cell bodies and dendrites (green fl uorescence). Axon terminals, which almost completely cover the cell bodies and dendrites, were stained with a fl uorescent antibody directed against synaptojanin, another protein concentrated in presynaptic terminals (red fl uorescence). A third dye (DAPI) was used to stain the nuclei of neurons and glial cells (blue fl uorescence). (A, courtesy Drs. Olaf Mundigl and Pietro De Camilli, Yale University School of Medicine. B, from the cover photograph accompanying McPherson PS et al: Nature 379:353, 1996.)

Electrical Chemical Electrical(neurotransmitters)



(Actual size)

A

B

C

D

E

F

G

Figure 1–4 Examples of multipolar (A to E), bipolar (F), and unipolar (G) neurons, all drawn to about the same scale to demonstrate the range of neuronal sizes and shapes. All were stained by the Golgi method (see Fig. 1-14A); dendrites are indicated by green arrows, axons by blue arrows. A, Purkinje cell from the cerebellar cortex. B, Granule cell from the cerebellar cortex. C, Projection neuron from the inferior olivary nucleus. D, Spinal cord motor neuron. E, Large pyramidal neuron from the cerebral cortex. F, Olfactory receptor neurons. G, Dorsal root ganglion cells (whose processes have axonal properties along almost their entire course). The tiny inset at the upper right shows the actual size of the pyramidal neuron. (Modifi ed from Ramón y Cajal S: Histologie du système nerveux de l’homme et des vertébrés, Paris, 1909, 1911, Maloine.)

Neurons come in a variety of sizes and shapes, but all are variations of the same theme

Cell bodies range from 5 to 100 micras in diameter

Most axons around 1 mm but some 1 m

Neurons: diversity


Sensory neurons take nerve impulses from sensory receptors to CNS

Neurons: functional types

Sensory receptors may be the end of a sensory neuron itself (a pain or touch receptor), or may be a specialized cell that forms a synapse with a sensory neuron


Interneurons occur entirely within the CNS

Neurons: functional

Convey nerve impulses between various parts of the CNS


Motor neurons carry nerve impulses from CNS to muscles or glands

Neurons: functional

Have many dendrites and a single axonCause muscle to contract or glands to secrete


Neuronal cell bodies and axons are largely segregated withinthe CNS

Neurons: segregation

Grey matter: cell bodies and dendrites (pinkish due to blood supply)

White matter: axons (myelin)


Figure 1–7 Horizontal slice of a whole human brain, approximately 6 mm thick, stained by a method that differentiates between gray and white matter. Pretreatment with phenol makes the white matter resis-tant to the blue copper sulfate stain, so white matter appears white and gray matter appears bright blue. (Prepared by Pamela Eller, University of Colorado Health Sciences Center.)

DRG

AG

From sensory receptors

To viscera

To skeletal muscle

Figure 1–8 Division of the CNS into gray matter and white matter, as typifi ed by the thoracic spinal cord in cross section. Gray matter contains interneurons, projection neurons, motor neurons, and endings of sensory fi bers and fi bers arriving from other parts of the CNS. White matter contains ascending and descending pathways. Neurons in the PNS are clustered in ganglia—some containing sensory neurons (dorsal root ganglion [DRG]), and some containing autonomic neurons (autonomic ganglion [AG]).

Neuronal Cell Bodies Synthesize Macromolecules

The neuronal cell body is the site of synthesis of nearly all the neuron’s enzymes, structural proteins, membrane components, and organelles, as well as some of its chemical messengers. Its structure (Fig. 1-9) refl ects this function. The nucleus is large and pale-staining, with most of its chromatin dispersed and available for tran-scription; it contains one or more prominent nucleoli, which are actively involved in the transcription of ribo-somal RNA. The cytoplasm contains abundant rough endoplasmic reticulum and free ribosomes for protein synthesis, together with stacks of Golgi cisternae for further processing and packaging of synthesized pro-teins. Many mitochondria are also present to meet the energy requirements of continuous, very active protein synthesis.

Ribosomes, whether studding the surface of the rough endoplasmic reticulum or free in the cytoplasm between the cisternae, are stained intensely by basic dyes, appear-ing by light microscopy as clumps called Nissl bodies or Nissl substance (Fig. 1-10). Nissl bodies are particularly prominent in large neurons, a consequence of the large total volume of cytoplasm contained in their processes,


Figure 1–7 Horizontal slice of a whole human brain, approximately 6 mm thick, stained by a method that differentiates between gray and white matter. Pretreatment with phenol makes the white matter resis-tant to the blue copper sulfate stain, so white matter appears white and gray matter appears bright blue. (Prepared by Pamela Eller, University of Colorado Health Sciences Center.)

DRG

AG

From sensory receptors

To viscera

To skeletal muscle

Figure 1–8 Division of the CNS into gray matter and white matter, as typifi ed by the thoracic spinal cord in cross section. Gray matter contains interneurons, projection neurons, motor neurons, and endings of sensory fi bers and fi bers arriving from other parts of the CNS. White matter contains ascending and descending pathways. Neurons in the PNS are clustered in ganglia—some containing sensory neurons (dorsal root ganglion [DRG]), and some containing autonomic neurons (autonomic ganglion [AG]).

Neuronal Cell Bodies Synthesize Macromolecules

The neuronal cell body is the site of synthesis of nearly all the neuron’s enzymes, structural proteins, membrane components, and organelles, as well as some of its chemical messengers. Its structure (Fig. 1-9) refl ects this function. The nucleus is large and pale-staining, with most of its chromatin dispersed and available for tran-scription; it contains one or more prominent nucleoli, which are actively involved in the transcription of ribo-somal RNA. The cytoplasm contains abundant rough endoplasmic reticulum and free ribosomes for protein synthesis, together with stacks of Golgi cisternae for further processing and packaging of synthesized pro-teins. Many mitochondria are also present to meet the energy requirements of continuous, very active protein synthesis.

Ribosomes, whether studding the surface of the rough endoplasmic reticulum or free in the cytoplasm between the cisternae, are stained intensely by basic dyes, appear-ing by light microscopy as clumps called Nissl bodies or Nissl substance (Fig. 1-10). Nissl bodies are particularly prominent in large neurons, a consequence of the large total volume of cytoplasm contained in their processes,


Glial cells (PNS)

Schwann cells:* Form myelin sheath in the PNS * Speed up axonal transmission

24 The Human Brain

AxonMyelin

Schwann cellnucleus

Schwann cell:nucleus cytoplasm

Unrolledinternode

c

c

c

Axon

Axon

Myelin

A

B

Figure 1–23 Myelin sheaths and nodes of Ranvier in peripheral nerve fi bers. A fi xed peripheral nerve was teased apart into individual nerve fi bers and stained with osmium (a lipophilic stain for membranes). The axon is the central pale area in each fi ber, and the myelin sheath stands out on both sides of each axon as a more densely stained area; a few nodes of Ranvier (arrowheads) are visible. The occasional diagonal clefts (arrows) that appear to cross the myelin sheaths are known as Schmidt-Lanterman incisures; they correspond to thin extensions of Schwann cell cytoplasm that spiral around with the myelinating membranes (see Fig. 1-24). (Courtesy Dr. Nathaniel T. McMullen, University of Arizona College of Medicine.)

Figure 1–24 Schematic diagram of the formation of myelin in the PNS. A, A single Schwann cell forms an internode, unrolled from the axon it would nor-mally be wrapped around. The cell is fl attened into a two-membrane-thick sheet, with cytoplasm (c) remaining only as a thin rim around the periphery and as a few thin fi ngers extending between the membranes. B, Longitudinal section through the internode resulting from the Schwann cell in A spiraling around the axon. Most of the internode consists of tightly wrapped Schwann cell mem-branes. Some cytoplasm remains on the surface of the internode near the nucleus, as small pockets near the node, and as Schmidt-Lanterman incisures. (Redrawn from Krstic RV: Illustrated encyclopedia of human histology, Berlin, 1984, Springer-Verlag.)

Figure 1–28 CNS myelin sheaths, here in a transverse section of a rat’s optic nerve. Each axon contains microtubules (m) and neurofi laments (nf) and is bounded by a cell membrane (AxM). Processes of oligodendrocytes (Ol) wrap around each axon to form its myelin sheath. Tongues of oligo-dendrocyte cytoplasm at the inside and outside of the myelin sheath narrow until the inner surfaces of their membranes fuse, forming the dense line that spirals through the myelin (long arrows). The clefts between adjoining oligodendrocyte processes (short arrows) lead to the fainter zones between the dense lines. The actual diameter of each axon is about 0.5 µm. (From Peters A, Palay SL, Webster H deF: The fi ne structure of the nervous system: neurons and their supporting cells, ed 3, New York, 1991, Oxford University Press.)

nf

nf

AxM

AxM

m

Ol

Ol

Ol

28

Satellite cells:* Support clusters of neuron bodies (ganglia)

Glia = glue in Greek


Glial cells (CNS)

Microglia

Ependymal

Oligodendrocytes

Astrocytes

* Form myelin sheaths* Dispose of debris* Respond to injury

* Line ventricles* Secrete CSF

* Mop up excess ions* Connect neurons to blood vessels (Blood-brain barrier)* Scar tissue


Neurons vs. Glial cells

Glial cells (astrocytes) could be involved in processing

The vast majority of neurons do not divide. Why?

Glial cells divide

Most brain tumors are gliomas


Introduction to the Nervous System


Neuronal signaling


Gross anatomy

CHAPTER 3 Gross Anatomy and General Organization of the Central Nervous System 57

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g)

A B

Figure 3–3 A, Average brain weights of human males and females at different ages. Notice how the brain grows rapidly after birth, doubling in the fi rst year of life, before reaching its full size at about age 11 years. At all ages, male brains have a greater average weight than female brains. However, as indicated in B, adult female brains actually account for a greater percentage of body weight than do adult male brains. Brain growth is substantial in utero, and we are born with brains that are very large relative to body size. After the brain growth spurt of the fi rst 1 to 3 years of life, body growth takes over, and the brain weight–body weight ratio declines progressively until about age 17. (Plotted from data in Dekaban AS, Sadowsky D: Ann Neurol 4:345, 1978.)

Cat

Elephant

Human

KangarooLion

Rabbit

CoyoteOpossum

Chimpanzee

Rhesusmonkey

5 cm

Figure 3–4 Brains of a series of representative mammals, all reproduced at the same scale. Brain size is partly related to body size (e.g., cat versus lion, human versus elephant) and partly related to mental abilities (e.g., lion versus human). Not all parts of the brain change size in proportion to one another. For example, the olfactory bulbs of opossums and coyotes (blue arrows) are relatively large, those of monkeys and chimpanzees (green arrows) are proportionally much smaller, and those of humans are barely discernible at this magnifi cation. (From www.brainmuseum.org, courtesy Dr. Wally Welker; supported by NSF grant 0131028.)

Figure 3–5 The relative sizes of the brain of a rhinoceros and the alleged brain of the author. Although the rhino’s body weight is about 30 times greater, its brain weight is likely to be only half as great. (Rhino, courtesy Albrecht Dürer. Author, courtesy Mr. and Mrs. Nolte. Suggested by an illus-tration in Cobb S: Arch Neurol 12:555, 1965.)

Humans have large brains relative to other animals (1.1 to 1.7 kg)

CHAPTER 3 Gross Anatomy and General Organization of the Central Nervous System 57

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Figure 3–3 A, Average brain weights of human males and females at different ages. Notice how the brain grows rapidly after birth, doubling in the fi rst year of life, before reaching its full size at about age 11 years. At all ages, male brains have a greater average weight than female brains. However, as indicated in B, adult female brains actually account for a greater percentage of body weight than do adult male brains. Brain growth is substantial in utero, and we are born with brains that are very large relative to body size. After the brain growth spurt of the fi rst 1 to 3 years of life, body growth takes over, and the brain weight–body weight ratio declines progressively until about age 17. (Plotted from data in Dekaban AS, Sadowsky D: Ann Neurol 4:345, 1978.)

Cat

Elephant

Human

KangarooLion

Rabbit

CoyoteOpossum

Chimpanzee

Rhesusmonkey

5 cm

Figure 3–4 Brains of a series of representative mammals, all reproduced at the same scale. Brain size is partly related to body size (e.g., cat versus lion, human versus elephant) and partly related to mental abilities (e.g., lion versus human). Not all parts of the brain change size in proportion to one another. For example, the olfactory bulbs of opossums and coyotes (blue arrows) are relatively large, those of monkeys and chimpanzees (green arrows) are proportionally much smaller, and those of humans are barely discernible at this magnifi cation. (From www.brainmuseum.org, courtesy Dr. Wally Welker; supported by NSF grant 0131028.)

Figure 3–5 The relative sizes of the brain of a rhinoceros and the alleged brain of the author. Although the rhino’s body weight is about 30 times greater, its brain weight is likely to be only half as great. (Rhino, courtesy Albrecht Dürer. Author, courtesy Mr. and Mrs. Nolte. Suggested by an illus-tration in Cobb S: Arch Neurol 12:555, 1965.)

More complex interconnections and selective expansions of cerebral cortex involved in higher functions

400 gr at birth1400 gr adult (due to increase in myelin and new connections)50-80k neurons die every day


72 The Human Brain

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7

Centralnervoussystem

Brain

Spinalcord

Cerebellum (1) (coordination)

Brainstem

Cerebralhemisphere

Diencephalon

Midbrain (2)

Pons (3)

Medulla (4)

Cerebralcortex

Basalganglia(movement control; related structures in brainstem)

Hippocampus (5), amygdala (6) (limbic structures; drives, emotions, memory)

Thalamus (7) (relay to cortex)

Hypothalamus (8) (control of autonomics)

Frontal lobe (9) (motor cortex)

Parietal lobe (10) (somatosensory cortex)

Occipital lobe (11) (visual cortex)

Temporal lobe (12) (auditory cortex)

Limbic lobe (13) (drives, emotions, memory)

Caudate nucleus (14)

Lenticular nucleus (putamen [15]and globus pallidus [16])

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Figure 3–25 Overview of the subdivisions of the CNS. The major structures listed here, as well as many related structures, are the subjects of subse-quent chapters.

Gross anatomyOverview of the subdivisions of the CNS


Cerebral hemispheres are folded and convoluted* Gyri: bumps* Sulci: grooves

Gross anatomy

Corpus callosum is a huge bundle of axons connecting the two hemispheres (severed in split-brain patients)

Ventricles filled with cerebro-spinal fluid that bath the brainand provide protection and chemical environment of neurons


Cerebral cortex: outermost 6 layered structure of the neuraltissue of human and other mammals (2-4 mm). Key role in high cognitive functions (memory, attention, language, ...)

Gross anatomy


Gross anatomyCNS contains systematic distorted maps


Introduction to the Nervous System


Neuronal signaling


Neuronal signaling

To achieve long distance (several cm), rapid communication (150 m/s), neurons have evolved special abilities for sending electrical signals (Action potentials)

Communication between neurons is achieved at synapsesby the process of neurotransmission

Within the neuron (conduction)

Between neurons (transmission)


Electrical properties

Neurons spend a lot of energy to keep different concentration of ions inside and outside (resting membrane potential -70 mV)

Lipid bilayer at the cell surface acts as a capacitor (able to store charges)


Electrical properties

Diffusion: when channels open ions tend to move to less crowded places

Ion channels open and close as a function of the membrane potential (voltage-gated channels)

Neuron membranes are filled with pores that enable the selective pass of ions (ion channels)


http://www.youtube.com/watch?v=7EyhsOewnH4

Conduction: Action




Conduction: Action potential

An action potential is conducted whenever an input of threshold intensity or above is applied to the initial part of an axon (each action potential has the same strength)



Speed up of transmission* unmyelinated 5 m/s* myelinated 150 m/s (toe - spine in < 7 ms)

Saltatory conduction: action potential jumps between nodes but needs to be regenerated


166 The Human Brain

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A BFigure 7–17 Abnormal voltage-gated Na+ chan-nels in a patient with periodic paralysis. Repeated patch-clamp recordings of the current fl owing through single channels of normal muscle mem-branes (A) and those of the patient (B) during depolarization from 120 to 40 mv indicate that the patient’s channels do not inactivate rapidly. Aver-ages of many such records were used to calculate the probability of channels being open (POPEN) over time. The continued, albeit reduced, probability of the patient’s channels being open corresponds to a small but constant inward Na+ current that depo-larizes the fi ber. (From Cannon SC: Trends Neurosci 19:3, 1996.)

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48 na 48 na87 naA B CFigure 7–18 Effects of decreased Cl− conductance on goat muscle fi bers. A, Injection (through a microelectrode) of 87 nanoamperes of current into an intercostal muscle fi ber from a normal goat elicits a single action potential. At the termination of the current pulse the membrane potential decays quickly to the original resting potential. B, Injection of little more than half as much current into a muscle fi ber from a myotonic goat elicits a train of action potentials. At the termination of the current pulse the membrane stays somewhat depolarized (due to extracellular K+ accumulation), and action potentials continue at a slower rate. C, Replacing the Cl− in the solution bathing a normal goat muscle fi ber with an impermeant cation (sulfate) causes it to behave like a myotonic fi ber. (From Adrian RH, Bryant SH: J Physiol 240:505, 1974.)

Figure 7–19 A puffer fi sh. (From Heck JG: Heck’s pictorial archive of nature and science, New York, 1851, Rudolph Garrigue.)


Puffer fish (fugu) contains a potent poison that blocks Na+ channels resulting in failure to generate action potentials


Neuronal signaling

To achieve long distance (several cm), rapid communication (150 m/s), neurons have evolved special abilities for sending electrical signals (Action potentials)

Communication between neurons is achieved at synapsesby the process of neurotransmission

Within the neuron (conduction)

Between neurons (transmission)


Synaptic transmission

Electrical synapses: gap junctions between cells that allow ions to flow from one neuron to another (also in cardiac cells)

Chemical synapses: most neurons communicate by means of neurotransmitters at chemical synapses. The receiving neuron responds with a graded potential that may or may not initiate an action potential

7.2 THE POSTSYNAPTIC RESPONSE 173

RRVP

Recycling

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Postsynapticreceptors

Transmitter

Actionpotential

[Ca ]2+

PSCReleasemachinery

Fig. 7.1 Schematic of achemical synapse. In thisexample, the presynaptic terminalconsists of a single active zonecontaining a RRVP which isreplenished from a single reservepool. A presynaptic actionpotential leads to calcium entrythrough voltage-gated calciumchannels which may result in avesicle in the RRVP fusing withthe presynaptic membrane andreleasing neurotransmitter intothe synaptic cleft.Neurotransmitter diffuses in thecleft and binds with postsynapticreceptors which then open,inducing a postsynaptic current(PSC).

which, on release, may activate a corresponding pool of postsynaptic recep-tors (Walmsley et al., 1998). The RRVP is replenished from a large reservepool. The reality is likely to be more complex than this, with vesicles in theRRVP possibly consisting of a number of subpools, each in different states ofreadiness (Thomson, 2000b). Recycling of vesicles may also involve a numberof distinguishable reserve pools (Thomson, 2000b; Rizzoli and Betz, 2005).

A model of such a synapse could itself be very complex. The first stepin creating a synapse model is identifying the scientific question we wish toaddress. This will affect the level of detail that needs to be included. Verydifferent models will be used if our aim is to investigate the dynamics of aneural network involving thousands of synapses compared to exploring theinfluence of transmitter diffusion on the time course of a miniature exci-tatory postsynaptic current (EPSC). In this chapter, we outline the widerange of mathematical descriptions that can be used to model both chemicaland electrical synapses. We start with the simplest models that capture theessence of the postsynaptic electrical response, before including graduallyincreasing levels of detail.

The abbreviation IPSC,standing for inhibitorypostsynaptic current, is alsoused.

7.2 The postsynaptic responseThe aim of a synapse model is to describe accurately the postsynaptic res-ponse generated by the arrival of an action potential at a presynaptic termi-nal. We assume that the response of interest is electrical, but it could equallybe chemical, such as an influx of calcium or the triggering of a second-messenger cascade. For an electrical response, the fundamental quantity to bemodelled is the time course of the postsynaptic receptor conductance. Thiscan be captured by simple phenomenological waveforms, or by more com-plex kinetic schemes that are analogous to the models of membrane-boundion channels discussed in Chapter 5.

7.2.1 Simple conductance waveformsThe electrical current that results from the release of a unit amount of neu-rotransmitter at time ts is, for t ≥ ts:

Isyn(t ) = gsyn(t )(V (t )− Esyn), (7.1)

where the effect of transmitter binding to and opening postsynaptic recep-tors is a conductance change, gsyn(t ), in the postsynaptic membrane. V (t ) is


Synaptic transmission1 Action potential arrives to the axon terminal (pre-synaptic neuron) stimulates the release of packets of neurotransmitters into the synaptic cleft

2 Neurotransmitters diffuse across the synaptic cleft

3 Neurotransmitters bind to receptors at the post-synaptic neuron causing ion channels to open



http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120107/anim0015.swf::Chemical%20Synapse







Depending on which receptor is activated at the post-synaptic neuron the electrical response can be excitatory or inhibitory

EPSP IPSP


Neural integration refers to the conduction and addition of all PSP produced by various excitatory and inhibitory synapses. It determines if an action potential is generated. Two types:

integration decides ap likelihood


1 Temporal summation


Neural integration refers to the conduction and addition of all PSP produced by various excitatory and inhibitory synapses. It determines if an action potential is generated. Two types:

integration decides ap likelihood


2 Spatial summation



Drugs, diseases, and toxins interfere with synaptic neurotransmission (alcohol, nicotine, marihuana, antidepressants, botox,...)

Synaptic strength can be enhanced or depressed by changing neurotransmitter release or the density of receptors (synaptic plasticity)

too much botox...lunes, 5 de septiembre de 16

Summary

• NS evolved to provide a fast and coordinated response to stimuli.

• 2 types of cells: neurons (information processing) and glial cells (supporting role).

• Neurons have specialized compartments to receive (dendrites), integrate (soma), conduct (axon), and transmit (synapses) impulses.

• NS is divided in CNS (brain and spinal cord) and PSN (cranial and spinal nerves), with further subdivisions involved in specialized processing.


To know more

http://cnx.org/content/m47519/latest/?collection=col11569/latest

Chapters 1 and 3The human brain: an introduction of its functional

anatomy, John Nolte, Mosby, 2002






Applications

Cognitive

Models

Analyses

Basics

Lesson Title

1 Introduction

2 Structure and Function of the NS

3 Windows to the Brain

4 Data analysis

5 Single neuron models

6 Network models

7 Artificial neural networks

8 Artificial intelligence

9 Learning and memory

10 Perception

11 Attention & decision making

12 Brain-Computer interface

13 Neuroscience and society

14 Future and outlook




Documents

Introduction to Structure and function of Computational the nervous system … · 2019. 9. 2. · Structure and function of the nervous system Introduction to Computational Neuroscience