Download THE NERVOUS SYSTEM

THE NERVOUS SYSTEM
EXCHANGE WITH THE ENVIRONMENT
Þ An animal’s size and shape are fundamental aspects of form that significantly affect the way the animal interacts
with its environment
Þ Animals must exchange nutrients, waste products, and gases with their environment, and this requirement can
impose an additional limitation on body plans
Þ Exchange occurs as substances dissolved in an aqueous solution move across the plasma membrane of each cell
o The rate of exchange is proportional to membrane surface area involved in exchange
o The amount of material that much be exchanged is proportional to the body volume
Þ In most animals, the evolutionary adaptations that enable sufficient exchange with the environment are
specialized surfaces that are extensively branched or folded
HIERARCHAL ORGANISATION OF BODY PLANS
Þ Cells are organised into tissues; groups of cells with a similar appearance and a common function
Þ Different types of tissues are further organised into functional units called organs
Þ An organ system is groups of organs working together, providing an additional level of organisation and
coordination
Þ There are four types of animal tissues: epithelial, connective, muscle, and nervous
NERVOUS TISSUE
Þ Nervous tissue function in the receipt, processing, and transmission of information
Þ It contains neurons, or nerve cells, which transmit nerve impulses and support glial cells
Þ A concentration of nervous tissue in many animals forms a brain, an information-processing centre
Þ Neurons are the basic units of the nervous system, they receive nerve impulses from other neurons via the cell
body and dendrites
Þ There are various types of glia, which help nourish, insulate, and replenish neurons, and in some cases can
modulate neuron function
COORDINATION AND CONTROL
Þ The two major systems for coordinating and controlling responses to stimuli are the endocrine and nervous
system
Þ In the endocrine system, signalling molecules are released into the bloodstream by endocrine cells and are
carried to all locations in the body
o The signalling molecules the endocrine system uses are called hormones, different hormones cause
distinct effects, and only cells that have receptors for that particular hormone respond
o The effects of hormones are often long-lasting, since hormones can remain in the bloodstream for
minutes or even hours
Þ In the nervous system, neurons transmit signals along dedicated routs connecting specific location in the body
o Signals in the nervous system are called nerve impulses, and they travel to specific target cells along
communication lines consisting mainly of axons
o These can act on other neurons, muscle cells, and on cells and glands that produce secretions
o The nervous system only conveys information by the particular pathway the signal takes
o These signals are extremely fast and the effects last only a fraction of a second
Þ The endocrine system is well adapted for coordinating gradual changes that affect the entire body, such as
growth, development, reproduction, metabolic, processes and digestion
Þ The nervous system is well suited for directing immediate and rapid responses to the environment, such as
reflexes and other rapid movements
NEURON STRUCTURE AND FUNCTION
Þ The ability of a neuron to receive and transmit information is based on a highly specialised cellular organisation
Þ The cell body contains most of a neuron’s organelles, and its nucleus
Þ A typical neuron will have numerous highly branched extensions called dendrites, together with the cell body
they function to receive signals from other neurons
Þ Neurons have a single axon, which is an extension which transmits signals to other cells
o The cone shaped base of an axon, called the axon hillock, is typically where signals that travel down the
axon are generated
o The axon divides into many branches at the other end
o The axon can be sheathed in myelin
Þ The branches of an axon transmit information to another cell at a junction called a synapse
o The part of each axon branch that forms this specialised junction is a synaptic terminal
Þ Neurotransmitters are the chemical messengers that pass information from the presynaptic neuron to the
postsynaptic neuron
Þ The neurons of vertebrates and most invertebrates require glial cells (or glia), which nourish neurons, insulate
the axons, and regulate the extracellular fluid surrounding neurons
o Glia can also function in replenishing certain groups of neurons and in transmitting information
o Glia outnumber neurons in the brain
INFORMATION PROCESSING
Þ Information processing by the nervous system occurs in three stages: sensory input, integration, and motor
output
Þ Sensory neurons transmit information about external stimuli such as light, tough, or smell, or internal
conditions such as blood pressure of muscle tension
Þ Neurons in the brain or ganglia integrate the sensory input, taking into account the immediate context and the
animal’s experience, the vast majority of neurons being interneurons in the brain, which form the local circuits
connecting neurons in the brain
Þ Motor neurons extend out of the processing centres and trigger output in the form of muscle of gland activity,
they transmit signals to muscle cells and cause them to contract
Þ The central nervous system (CNS) is where neurons carry out integration are organised
Þ The neurons that carry information into and out of the CNS constitute the peripheral nervous system (PNS)
Þ Nerves are bundled axons of neurons
RESTING POTENTIALS
Þ The attraction of opposite charges across the plasma membrane is a source of energy, and this charge difference,
or voltage, is called the membrane potential
Þ For a resting neuron (one not sending a signal), the membrane potential is called the resting potential
o This is typically between -60 and -80 mV (millivolts), it is negative and electrically polarised as there is
an unequal distribution of ions
Þ Inputs from other neurons or specific stimuli can cause changes in the neuron’s membrane potential
Formation of the Resting Potential
Þ Potassium ions (K+) and sodium ions (Na+) play an essential role in the formation of the resting potential
o K+ is a principal intracellular cation, while Na+ is a principal extracellular cation
Þ These ions each have a concentration gradient across the plasma membrane of a neurons
o In most neurons, the concentration K+ is higher inside the cell, while the concentration of Na+ is higher
outside the cell
+
Þ The Na and K+ gradient is maintained by the sodium-potassium pump, which uses the energy of ATP
hydrolysis to actively transport Na+ out of the cell and K+ into the cell
o The sodium-potassium pump transports three Na+ out of the cell for every two K+ that transport into it
o This generates a net export of a positive charge
Þ An ion channel consists of pores formed by clusters of specialised proteins that span the membrane
o These allow ions to diffuse back and forth across the membrane
o As ions diffuse through channels, they carry units of electrical charge with them
Þ Any resulting net movement of positive or negative charge will generate a membrane potential, or voltage across
the membrane
Þ The concentration gradients of ions across the plasma membrane represent a chemical form of potential energy
that can be harnessed for cellular processes
Ions channels that convert this chemical potential energy to electrical potential energy can do so
because they have a selective permeability, allowing only certain ions to pass (e.g. a potassium channel
allows K+ to diffuse freely across the membrane, but not other ions)
Þ The diffusion of K+ through potassium channels that are always open (which can be called leak channels) is
critical for establishing the resting potential
o The K+ concentration is 140mM inside the cell, but only 5mM outside, thus the chemical concentration
gradient favours a net outflow of K+
Þ A resting neuron has many open potassium channels, but very few open sodium channels
Þ Since Na+ and other ions can’t readily cross the membrane, K+ outflow leads to a net negative charge inside the
cell, and this build up of negative charge within the neuron is the major source of the membrane potential
o This negative charge is stopped once the excess of negative charges inside the cell exert an attractive
force that opposes the flow of additional positively charged potassium ions out of the cell
o The separation of charge (voltage) will then result in an electrical gradient that counterbalances the
chemical concentration gradient of K+
o
Modelling the Resting Potential
Þ The net flow of K+ will continue until the chemical and electrical forces are in balance
Þ Once the neuron reaches equilibrium, the electrical gradient will exactly balance the chemical gradient, so that
no further net diffusion of K+ occurs across the membrane
Þ The magnitude of the membrane voltage at equilibrium for a particular ion is called that ion’s equilibrium
potential
Þ The concentration gradient of Na+ has an opposite effect to that of K+, instead it diffuses into the cell making it
less negative
Gated Ion Channels
Þ These open of close in response to one of three kinds of stimuli
Þ Stretch-gated ion channels
o Open when the membranes deformed
Þ Chemically-gated ion channels respond to a chemical stimulus
o Found in synapses
Þ Voltage-gated ion channels respond to a change in membrane potential
o Found in axons (and dendrites)
ACTION POTENTIALS
Hyperpolarization and Depolarization
Þ When gated ion channels are stimulated to open, ions flow across the membrane, changing the membrane
potential
Þ An increase in the magnitude of the membrane potential is called a hyperpolarization, which makes the inside
of the membrane more negative
o In a resting neuron, hyperpolarization results from any stimulus that increases the outflow of positive
ions or the inflow of negative ions
o Opening some other types of ion channels can have the opposite effect, and instead make the inside of
the membrane less negative
Þ A reduction in the magnitude of the membrane potential is a depolarisation, which involves gated sodium
channels
o If a stimulus causes gated sodium channels to open, the membrane’s permeability to Na+ increases, and
Na+ diffuses into the cell along its concentration gradient
Graded Potentials and Action Potentials
Þ A graded potential is a simple shift in the membrane potential in response to hyperpolarization or
depolarization, and it has a magnitude that varies with the strength of the stimulus
o A larger stimulus causes a greater change in the membrane potential
o Theses induce a small electrical current that leaks out of the neuron as it follows along he membrane
Þ An action potential occurs if a depolarization shifts the membrane potential sufficiently, resulting in a massive
change in the membrane voltage
Action potentials occur whenever a depolarization increases the membrane voltage to a particular value,
called the threshold, for many mammalian neurons this being -55mV
o Action potentials have a constant magnitude and can regenerate in adjacent regions of the membrane
o Action potentials can arise as some of the ion channels in neurons are voltage-gated ion channels,
opening or closing when the membrane potential passes a particular level
o If depolarization opens voltage-gated sodium channels, the resulting flow of Na+ into the neuron results
in further depolarization, causing more sodium channels to open and an even greater flow of current –
this is the result of a process of positive feedback
o Action potentials are an all-or-nothing response to stimuli, they either occur fully or not at all
Generation of Action Potentials
Þ Depolarization will open both types of channels, but they respond independently and sequentially
Þ Sodium channels open first, initiating the action potential
o Sodium channels become inactivated as the action potential proceeds; a loop of the channel protein
moves, blocking ion flow through the opening
o The channels will remain inactivated until after the membrane returns to the resting potential and the
channels close
Þ Potassium channels open more slowly than sodium channels, but remain open and functional until the end of
the action potential
Þ The process of an action potential goes as follows:
1) Most voltage-gated sodium channels are closed when the membrane of the axon is at resting potential,
some potassium channels are open, but most voltage-gated potassium channels are closed
2) When a stimulus depolarizes the membrane, some gated sodium channels open, allowing more Na+ to
diffuse into the cell. The Na+ inflow will cause further depolarization, which opens still more gated
sodium channels, allowing even more Na+ to diffuse into the cell
3) Once the threshold is crossed, the positive-feedback cycle rapidly brings the membrane potential close
to ENa, this stage being called the rising phase
4) Two things prevent the membrane potential actually reaching ENa; voltage-gated sodium channels
inactive soon after opening, halting Na+ inflow, and that most voltage-gated potassium channels open,
causing a rapid outflow of K+, both these events quickly bringing the membrane potential back toward
EK, this phase being the falling phase
5) The final phase of the action potential, called the undershoot, the membrane’s permeability to K+ is
higher than at rest, so the membrane potential is closer to EK than it is at the resting potential. The
gated potassium channels eventually close, bringing the membrane potential to resting potential
Þ The refractory period is the “downtime” when a second action potential cannot be initiated
Conduction of Action Potentials
Þ At the axon hillock, Na+ inflow during the rising phase creates an electrical current that depolarizes the
neighbouring region of the axon membrane
o Immediately behind the traveling zone of depolarization caused by Na+ inflow is a zone of repolarization
caused by K+ outflow
o In the repolarized zone, the sodium channels remain inactivated
Þ The depolarization is large enough to reach threshold, causing an action potential in the neighbouring region,
the process being repeated many times along the length of the axon
Þ Since an action potential is an all-or-none event, the magnitude and duration of the action potential are the
same at each position along the axon
Þ The net result is the movement of a nerve impulse from the cell body to the synaptic terminals
Þ The refractory period ensures the impulse is unidirectional
o
Evolutionary Adaptations of Axon Structures
Þ Axons have gotten wider, the width of an axon matters because resistance to electrical current flow is inversely
proportional to the cross-sectional area of a conductor
Þ The myelin sheath is an electrical insulation that surrounds vertebrate axons, causing the depolarizing current
associated with an action potential to travel father along the axon interior
o Myelin sheaths are produced by two types of glia: oligodendrocytes in the CNS, and Schwann cells in
the PNS
Þ In myelinated axons, voltage-gated sodium channels are restricted to gaps in the myelin sheath called nodes of
Ranvier
o The extracellular fluid is in contact with the axon membrane only at the nodes
Þ Action potentials propagate more rapidly in myelinated axons because the time-consuming process of opening
and closing of ion channels occurs at only a limited number of positions along the axon, called salutatory
conduction
NEURON COMMUNICATION AT SYNAPSES
Þ The majority of synapses are chemical synapses, which involve the release of a chemical neurotransmitter by the
presynaptic neuron
Þ At each terminal, the presynaptic neuron synthesizes the neurotransmitter and packages it in multiple
membrane-enclosed compartments called synaptic vesicles
Þ The arrival of an action potential at a synaptic terminal depolarizes the plasma membrane, opening voltage-gated
channels that allow Ca2+ to diffuse into the terminal
Þ The resulting rise in Ca2+ concentration in the terminal causes some of the synaptic vesicles to fuse with the
terminal membrane, releasing the neurotransmitter
Þ Once released, the neurotransmitter diffuses across the synaptic cleft, the gap that separates the presynaptic
neuron from the postsynaptic cell
Þ Information transfer is much more readily modified at chemical synapses than at electrical synapses
Chemical Synapses
1) Incoming action potential depolarizes synaptic terminal plasma membrane
2) Ca2+ gates opened
3) Elevated Ca2+ concentration causes synaptic vesicles to fuse with presynaptic membrane
4) Neurotransmitter released into synaptic cleft
5) Neurotransmitter binds to gated ion channels, causes the channels to open and for there to be an influx of Na+
and K+, a postsynaptic membrane potential being generated
6) The neurotransmitter is released from the gated ion channels, the channels close, the neurotransmitter is taken
up by synaptic vesicles
Þ An excitatory synapse involves the depolarisation of the postsynaptic membrane, it being an excitatory
postsynaptic potential (EPSP)
Þ The inhibitory synapse involves the hyperpolarization of postsynaptic membrane, an inhibitory postsynaptic
potential (IPSP) being involved
Summation of Graded Potentials
Þ Many synapses on a single neuron
Þ Single EPSP is too small to cross the threshold
Þ Integration (summation) of many EPSP and IPSP across dendrites and cell body
Þ Axon hillock is decision point
NEUROTRANSMITTERS
Þ Different neurotransmitters can have different effects on different cell types (excitatory or inhibitory) depending
on the receptor
Acetylcholine
Þ A common neurotransmitter in both invertebrates and vertebrates
Þ Vital for nervous system functions that include muscle stimulation, memory formation, and learning
Þ It is excitatory to skeletal muscle, and the main excitatory
Þ It is inhibitory to the vertebrate heart
Þ There are two major classes of acetylcholine receptor