Nervous System

Nervous System

Nervous System Overview: Lines of Communication Neurons are nerve cells that transfer information within the body

Neurons use two types of signals to communicate: electrical signals (long-distance) and chemical signals (short-distance)

Interpreting signals in the nervous system involves sorting a complex set of paths and connections

Processing of information takes place in simple clusters of neurons called ganglia or a more complex organization of neurons called a brain Evolution of the Nervous System

All animals except sponges have some type of nervous system Cnidarians radial symmetry

simplest animals with nervous system Neurons controlling contraction and expansion of gastrovascular cavity arranged in diffuse nerve nets Organization of Nervous Systems

Nervous systems of more complex animals contain nerve nets as well as nerves Nerves: bundles of fiber like extensions of

neurons Sea Stars: have a nerve net in each arm connected to a central nerve ring Cephalization

Clustering of neurons in a brain near the anterior end in animals with elongated bilaterally symmetrical bodies Allows for greater complexity of the

nervous system and more complex behavior Organization of the Nervous System CENTRAL NERVOUS SYSTEM

the part of the nervous system that coordinates the activity of all parts of the bodies of bilaterial animal PERIPHERAL NERVOUS

SYSTEM all multicellular animals

except sponges and radially symmetric animals such as jellyfish. It contains the majority of the nervous system and

consists of the brain and the spinal cord, as well as the retina The main function of the

PNS is to connect the CNS to the limbs and organs. Unlike the CNS, the PNS is not protected by bone or by the blood-brain barrier, leaving it exposed to toxins and mechanical injuries.

The peripheral nervous system is divided into the somatic nervous system and the autonomic nervous system 3 Stages of Information Processing

Sensors detect external stimuli and internal conditions and transmit information along sensory neurons

Sensory information is sent to the brain or ganglia, where interneurons integrate the information

Motor output leaves the brain or ganglia via motor neurons, which trigger muscle or gland activity Neuron Structure & Function

Most of a neurons organelles are in the cell body Most neurons have dendrites, highly branched extensions that receive signals from other neurons

The axon is typically a much longer extension that transmits signals to other cells at synapses

The cone-shaped base of an axon is called the axon hillock Synapse

Near its end, each axon usually divides into several branches each of which ends in a synaptic terminal where neurotransmitters are released The site of communication between one synaptic terminal and another is called

synapse presynaptic cell : transmitting neuron Postsynaptic cell: receiving neuron The Synaptic Terminal of one axon passes information across the synapse in the form of chemical

messengers called chemical messengers called neurotransmitters A synapse is a junction between an axon & another cell 3 Main Types of Neurons

Information Passage Information is transmitted from a presynaptic cell (neuron) to a post synaptic cell (neuron, muscle, or gland)

Supporting Cells Glia: supporting cells necessary for

the structural integrity of the nervous system and for normal functioning of neurons Several Types of Glial Cells Astrocytes Radial glia Oligodendrocytes

Schwann cells Supporting Cells Astrocytes

Radial glia Form tracks along which Provide structural support for neurons and regulate newly formed neurons

extracellular concentrations migrate from Both act as stem cells giving rise to the neural

of ions and neurotransmitters tubecells (the structure that During development facilitate new neurons

and glial gives rise to the CNS) formation of tight junctions between cells that line the capillaries in the brain and spinal cord resulting in the blood brain barrier which

restricts the passage of most substances in the CNS Supporting Cells Oligodendrocytes

In CNS Form the myelin sheaths around the axons of many vertebrate neurons Schwann cells

In PNS Form the myelin sheaths around the axons of many vertebrate neurons

on pumps & ion channels establish the resting potential of a neuron Every cell has a voltage (difference in electrical charge) across its plasma

membrane called a membrane potential The resting potential is the membrane potential of a neuron not sending signals

Changes in membrane potential act as signals, transmitting and processing information Formation of Resting

Potential In a mammalian neuron at resting potential, the concentration of K+ is highest inside the cell, while the concentration of Na+ is highest outside In neurons the membrane

the cell potential is between-60 & -80 milliVolts (mV) when the cell is Sodium-potassium pumps not transmitting signals of ATP

use the energy The minus sign indicates to maintain these K+ and Na+ gradients across that the inside of the

cell is the plasma membrane negative relative to the outside These concentration gradients represent

chemical potential energy Resting Potential Arises from ionic gradients maintained by the semi-permeability

of the plasma membrane and oneway channels This is the basis of nearly ALL electrical signals in the nervous system: The membrane potential can

change from its resting value when the membranes permeability to a Action Potentials are Signals Conducted by Axons Graded Potentials:

Hyperpolarization If a cell has gated ion channels, its membrane potential may change in

response to stimuli that open or close those channels Hyperpolarization: an increase in the magnitude of membrane potential (the inside of the membrane becomes more negative) Due to opening/closing of K+channels

Gated Ion Channels Gated ion channels: open or close in response to 1 of 3 kinds of stimuli Stretch gated ion channels: found in cells that

sense stretch and open then the membrane is deformed Ligand gated ion channels: found at synapses and open/close in response to specific chemical (like neurotransmitters) binding the channel Voltage gated ion channels: found in axons (and

sometimes in dendrites and cell bodies) respond to changes in membrane potential Equilibrium Potential

In a biological membrane, the reversal potential (also known as the Nernst potential) of an ion is the membrane potential at which there is no net (overall) flow of ions from one side of the membrane to

the other. Expressed as E ion Example ENa or EK Production of Action Potentials

If a depolarization shifts the membrane potential sufficiently, it results in a massive change in membrane voltage called an action potential Action potentials have a constant magnitude, are

all-or-none, and transmit signals over long distances They arise because some ion channels are voltage-gated, opening or closing when the

membrane potential passes a certain level Action Potentials All or nothing phenemenon

Once triggered, it has a magnitude independent of the stimulus that triggered it

Action potentials of most neurons usually very brief, 1-2milliseconds Brief APs allow neurons to produce them more frequently Neurons encode information in their action potential frequency

Generation of Action Potentials: A Closer Look An action potential can be considered as a series of stages

At resting potential 1.Most voltage-gated sodium (Na+) channels are closed; most of the voltage-gated potassium (K+)

channels are also closed Role of Voltage Gated Ion Channel in Production of an Action Potential Na+ has 2 gates, an activation gate and an

inactivation gate, both of which must be open for Na+ to diffuse across the membrane 2: depolarization of membrane rapidly opens activation gates and slowly closes inactivation

gates for Na+ 3.During the rising phase, the threshold is crossed, and the membrane potential increases

Role of Voltage Gated Ion Channel in Production of an Action Potential Once threshold is crossed, positive feedback quickly brings the membrane potential close to the

equilibrium potential of Na+ during the rising phase (the more Na+ ions, the more Na+ gates open to let in more Na+ ions)

Role of Voltage Gated Ion Channel in Production of an Action Potential 2 events prevent membrane potential from reaching ENa

inactivation gates close halting Na+ influx and activation gates on most K+ channels open, bringing the membrane potential back down during the falling phase When an action potential is generated .

4. Upon depolarization the K+ gate slowly opens

This is the falling phase voltage-gated Na+ channels become inactivated; voltage-gated K+ channels open, and K+ flows out of the cell

Refractory Period 5. During the undershoot, membrane permeability to K+ is at first higher than at rest, then voltagegated K+ channels close and resting potential is restored

During the refractory period after an action potential, a second action potential cannot be initiated

The refractory period is a result of a temporary inactivation of the Na+ channels Conduction of Action Potentials

For an AP to function as a long distance signal, it must travel without diminishing from the cell body to the synaptic terminals

It does so by regenerating itself along the axon Conduction of Action Potentials

At the site where an AP is initiated, usually an axon hillock, Na+ influx depolarizes the neighboring region of an

axon membrane The depolarization in the neighboring region is large enough to reach threshold, causing an AP to be reinitiated there This process is repeated many times along the length of the axon

Evolutionary Adaptation of Axon Structure The speed of an action potential increases with the axons diameter

In vertebrates, axons are insulated by a myelin sheath, which causes an action potentials speed to increase

Myelin sheaths are made by glia oligodendrocytes in the CNS and Schwann cells in the PNS Myelin Sheath

In a myelinated axon, voltage gated Na+ and K+ channels are

concentrated at gaps in the myelin sheath called nodes of Ranvier Extracellular fluid is in contact with axon only at nodes As a result, APs are not generated in between the nodes

Action potentials in myelinated axons jump between the nodes of Ranvier in a process called saltatory conduction

An Important Safety Mechanism Immediately behind the zone of depolarization due to Na+ influx, is a zone of repolarization due to K+ eflux

Because Na+ inactivation gates are closed behind the repolarized zone, the inward current necessary to depolarize the axon membrane ahead of the action potential can not produce another AP behind it

APs move in 1 direction Neurons communicate with other cells at synapses At electrical synapses, the electrical

current flows from one neuron to another At chemical synapses, a chemical neurotransmitter carries information across the gap junction

Most synapses are chemical synapses OKSo what happens when an AP reaches the end of an axon?

2 types of synapses can be at the end of an axons terminals Electrical synapses Less common Gap junctions

Allows for electrical current to flow from cell to cell Chemical synapses More common Involve the release of neurotransmitters by the presynaptic neuron

Synapses The presynaptic neuron synthesizes and packages the neurotransmitter in synaptic vesicles located in the synaptic terminal

The action potential causes the release of the neurotransmitter

The neurotransmitter diffuses across the synaptic cleft and is received by the postsynaptic cell Generation of Postsynaptic Potentials

Direct synaptic transmission involves binding of neurotransmitters to ligand-gated ion channels in the postsynaptic cell

Neurotransmitter binding causes ion channels to open, generating a postsynaptic potential Postsynaptic potentials fall into two categories

Excitatory postsynaptic potentials (EPSPs) are depolarizations that bring the membrane potential toward threshold

Inhibitory postsynaptic potentials (IPSPs) are hyperpolarizations that move the membrane potential farther from threshold Chemical Synapses

Presynaptic neurons synthesize neurotransmitters and package them in synaptic vesicles which are stored in the neurons terminals

Hundreds of synaptic vessicles may interact with a cell body or dendrites of a postsynaptic neuron

Synaptic Cleft A narrow cleft/ gap that can separates the presynaptic neuron from the post synaptic

cell The effect of neurotransmitters on the post synaptic cell may be either Direct: neurotrasmitter binds ligand gated ion channels allowing specific ions to diffuse across a plasma membrane changing membrane potential

Indirect: neurotransmitter binds receptors not associated with ion channels Direct Chemical Synapses

When an AP depolarizes the plasma membrane of the axon terminals, it opens voltage gated Ca2+ channels triggering an influx of Ca2+ The elevated Ca2+ in the terminals causes synaptic vessicles to fuse with

the presynaptic membrane , releasing neurotransmitters to the synaptic cleft Direct Chemical Synapses

The neurotransmitter binds the receptor portion of a ligand gated ion channel in the post synaptic membrane, opening the channels of the membrane and allowing an influx of it particular ion (Na+ or K+)

Eventually the neurotransmitter is released from the receptor and Taken back up by the presynaptic cell Degraded Otherwise diffuses out of the synaptic cleft Summation of postsynaptic

potentials in direct synaptic transmission Unlike APs which are all or none,

postsynaptic potentials are graded Magnitude of postsynaptic potential depends on Amount of neurotransmitter released Frequency of postsynaptic AP 1 AP causing a presynaptic cell to release neurotransmitter is probably not enough to elicit a

response out of the postsynaptic cell Multiple APs in a presynaptic cell are required to initiate a new AP in a post synaptic cell Temporal Summation Summation of postsynaptic potentials in direct synaptic

transmission TEMPORAL SUMMATION When EPSPs occur in rapid succession at a single synapse, the 2nd

EPSP may begin before the postsynaptic cells membrane potential has reached resting potential and so the 2 EPSPs add together

SPATIAL SUMMATION EPSPs produced nearly simultaneously by different synapses on the same postsynaptic

neuron can have the same additive effect EPSPs & IPSPs

The interplay between the multiple EPSPs and IPSPs is the essence of integration in the nervous system The axon hillock is the neurons integrating center , the region where the membrane potential at any instant

represents the summed effects of EPSPs and IPSPs Whenever the membrane potential at the axion hillock reaches threshold, an AP is generated Indirect Synaptic Transmission

In indirect synaptic transmission, a

neurotransmitter binds to a receptor that is not part of an ion channel Activates signal transduction pathways involving second messengers in the postsynaptic cell Have slower onsets than direct transmission, but longer lasting

effects Neurotransmitters There are more than 100 neurotransmitters, belonging to five

groups: Acetylcholine biogenic amines amino acids Neuropeptides and gases

A single neurotransmitter may have more than a dozen different receptors Neurotransmitters

Neurotransmitters are chemicals which relay, amplify, and modulate signals between a neurons and other cells Neurotransmitters are packaged in

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