BSCI 1511 Exam 2: Nervous systems

Stages of information processing

sensory input, integration, motor output, learning and memory
- these processes involve changes in the electrical potential (voltage) across the plasma membrane that arise from the regulated movement of ions across the membrane

Sensory input

receives information - sensing the external environment (e.g. light) or internal conditions (e.g. blood pressure)

Integration

processes information - processing input in context

Motor output

Transmits information directing a physiological or behavioral response (e.g. activation of muscle gland)

Learning memory

provides a mechanism for using experience to modify the response

Neuron

basic unit of the nervous system

How does a neuron transmit information?

- neurons use pulses of electrical current to receive, transmit and regulate the flow of information over long distances
- to transfer information from one cell to another, neurons often rely on chemical signals that act over very short distances
- interpreting nerve impulses involves sorting paths and connections, carried out in complex animals by groups of neurons organized into a brain or ganglia

How the neuron works

- the cell body, along with the dendrites, receive signals from other neurons
- signal travels to the axon hillock, and then via the axon
- each branched end of an axon contains synaptic terminals that transmit information to another cell at junctions called synapses
- chemical messengers called neurotransmitters pass information from the transmitting neuron (called the presynaptic cell) to the receiving cell (called the postsynaptic cell)

Sensory neurons (afferent neurons)

neurons that carry incoming information from the sensory receptors to the brain and spinal cord

Interneurons

neurons within the brain and spinal cord that communicate internally and intervene between the sensory inputs and motor outputs, forming circuits within the brain

Motor neurons (efferent neurons)

neurons that carry outgoing information from the brain and spinal cord to the muscles and glands

Central Nervous System (CNS)

brain and spinal cord
- neurons that carry out integration

Peripheral Nervous System (PNS)

the sensory and motor neurons that connect the central nervous system (CNS) to the rest of the body, carry information into and out of the CNS

Nerves

bundled axons that form neural "cables" connecting the central nervous system with muscles, glands, and sense organs

Glial cells

neuron's supporting cells
- nourish neurons
- insulate axons
- immune protection
- regulate the extracellular fluid surrounding neurons
- sometimes replenish neurons and transmit information
- facilitates nervous system

Ependymal cells

line the ventricles of the brain

Astrocytes

facilitate information transfer; participate in the formation of the blood-brain barrier; neuronal stem cells

Oligodendrocytes

myelinate axons in the CNS

Microglia

immune cells of CNS

Schwann cells

myelinate axons in the PNS

Resting potential

- ions are unequally distributed between the interior of cells and the surrounding fluid
- the inside of the cell is negatively charged relative to the outside of the cell due to the large amount of positively charged ions inside the cell leaving
- the attraction of opposite charges across the plasma membrane is a source of potential energy called the membrane potential (charge difference/voltage)
- input from other neurons or a specific stimulus causes changes in the neuron's membrane potential, and this acts as a signal that transmits information

ion channels

- selective: for specific ions
-move ions along their concentration gradient and have selective permeability (does not need energy)

Leak channel

- always open
- slow

Ligand-gated channel

- requires a signal
- binding of a ligand to a receptor associated with the channel

Voltage-gated channel

voltage changes across the plasma membrane

Mechanically gated channel

- require a signal
- mechanical deformations of the channel (by stretch, pressure, etc.)

Movement of sodium and potassium

- key to the transmission of information
- ions are unevenly distributed across the plasma membrane
- Potassium is higher inside the cell while Sodium is higher outside the cell
- ATP-dependent sodium-potassium pumps maintain the gradients of Potassium and Sodium ions
- pumps 3 sodium ions out for every 2 potassium in
- in a resting cell, potassium leak channels allow some potassium to re-exit the cell, but there are very few sodium leak channels
- potassium flows out because of the chemical gradient and this stops because of the electrochemical gradient

Equilibrium Potential

- the magnitude of cell's membrane voltage at equilibrium and is calculated using the Nernst equation
- the membrane potential when the electrical gradient is strong enough to attract the ions back across the membrane, thereby stopping the net flow of ions driven by the chemical gradient

Modeling the Resting Potential

- the concentration of KCl is higher in the inner chamber and lower in the outer chamber
- because of potassium channels, potassium diffuses down its concentration gradient to the outer chamber
- negative charge builds up inside because Cl stays behind, thereby slowing the outflow of potassium
- at equilibrium, the electrical and chemical gradients for potassium balance each other

Membrane potentials change when a neuron responds to a stimulus

- gated ion channels open
- opening gate potassium channels increases the magnitude of the membrane potential: more negative so toward -90mV (hyperpolarization)
- opening gated sodium channels decreases the magnitude of the membrane potential: more positive, so toward +62mV (depolarization)

Action potential

- sequential opening and closing of voltage-gated ion channels
- a rapid depolarization followed by repolarization that is propagated with fidelity along the axon of a nerve cell
- these result in long-distance signaling
- in mammals, the threshold for an action potential is -55mV

Graded potentials

- small changes in membrane potential that either depolarize or hyperpolarize
- by themselves, the propagate a few millimeters before dying out

Action Potential mechanism

1) Resting state: the gated Na+ and K+ channels are closed. Ungated channels maintain the resting potental
2) Depolarization: the cell is depolarized to the threshold voltage in response to an excitatory signal causing voltage gated Na+ channels to open, Na+ flows into the neuron due to its strong electrochemical gradient allowing for further depolarization
3) Rising phase of the action potential: once +35 mV is reached, the action potential occurs and Na+ channels are inactivated and K+ channels open
4) Falling phase: K+ flows out of the neuron due to its strong electrochemical gradient, repolarizing the cell to a hyperpolarized state; the K+ channels then close
5) Undershoot: the Na+/K+ ATPase brings the neuron back to the resting potential and restores the Na+ and K+ gradients

Conduction of an action potential

- Action potentials are initiated at the axon hillock and travel through the axon
- Na+ influx during the rising phase depolarizes the neighboring region of the axon membrane, reaching threshold: action potential
- This process is repeated along the length of the axon, which results in the movement of the impulse from the cell body to the synaptic terminus
- Brief inactivation of Na+ channels, called the refractory period, prevents the backflow of information
- Frequency of firing conveys the strength of the signal

Saltatory Conduction

- vertebrate animals
Schwann cells in the PNS and oligodendrocytes in the CNS insulate the axons using myelin
- voltage-gated ion channels are only in the gaps between the myelin sheaths called nodes of Ranvier
- Transfer speed: 100 m/sec or faster
- Faster conduction -\> few ion channels need to be activated and deactivated

Continuous Conduction

- invertebrate animals
- axons are not insulated
- voltage-gated channels throughout
- Transfer speed: 5cm to 30m per sec
- to increase speed of conduction, axon are made thicker

Chemical synapses

- chemical neurotransmitters released by the presynaptic neuron are received by the postsynaptic cell
- most common

Electrical synapses

- electric currents flow from one neuron to another via special gap junctions
- these mediate rapid, unvarying behaviors like heart contractions
- least common

Post-synaptic potentials

1. An action potential arrives, depolarizing the presynaptic membrane
2. The depolarization opens voltage-gated channels, triggering an influx of Calcium
3. The elevated Calcium concentration causes synaptic vesicles to fuse with the presynaptic membrane, releasing neurotransmitter into the synaptic clef
4. The neurotransmitter bind to ligand-gated ion channels in the postsynaptic membrane. Binding triggers opening, and allows Na+ and K+ to diffuse through

Ionotropic receptors

- at many chemical synapses, the receptor is a ligand-gated ion channel
- binding results in a grade potential called a postsynaptic potential

Excitatory ionotropic receptors

- when the channel is permeable to both K+ and Na+, it depolarizes
- causes an Excitatory postsynaptic potential (EPSP)

Inhibitory ionotropic receptors

- when the channel is permeable to only K+ or only Cl-, it hyperpolarizes
- causes an Inhibitory postsynaptic potential (IPSP)

Metabotropic receptor

- in some chemical synapses, the receptor is a GPCR
- receptor activated a signal transduction cascade that involves a second messenger
- Metabotropic receptors are slower, but have a longer response

Summation of post-synaptic potentials

(a) Subthreshold, no summation -\> not enough to cross threshold

(b) Temporal summation -\> two rapid EPSPs at the same synapse add up to trigger an action potential

(c) Spatial summation -\> two simultaneous EPSPs at different synapses add up to trigger an action potential

(d) Spatial summation of EPSP and IPSP -\> IPSPs negate the reception of EPSP

Neurotransmitters can...

bind multiple receptors, exerting very different effects

Stopping neurotransmitter signal

- neurotransmitters can often bind multiple receptors, exerting very different effects
- enzymatic hydrolysis of the neurotransmitter
- recapture by the presynaptic neuron
- simple diffusion

Acetylcholine (ACh)

- muscle stimulation, memory formation, and learning
- at the neuromuscular junction, it binds an ionotropic receptor and induces skeletal muscle contraction; it is degraded by acetylcholinesterase
- in cardiac muscle, it binds to a metabotropic receptor and reduces the heart rate

Glutamate

formation of long-term memory

Dopamine and Serotonin

affect sleep, mood, attention, and learning

Nitric Oxide

relaxes smooth muscle

Nervous system complexity

- nerve net
- cephalization

Nervous system in vertebrate animals

CNS: brain and spinal cord (integration)
PNS: nerves and ganglia (sensing)

Brain

- grey matter is on the outside
- white matter is on the inside
- cerebrospinal fluid in ventricles and periphery

Spinal cord

- grey matter is on the inside
- white matter is on the outside
- cerebrospinal fluid in ventricles and periphery

Grey matter

neuron cell bodies

White matter

bundled axons

Cerebrospinal fluid

protection and homeostasis

Motor system (efferent)

signals skeletal muscles for both voluntary and involuntary movements

Autonomic nervous system

- manages involuntary process
- three components:
- Sympathetic division
- Parasympathetic division
- Enteric division

Sympathetic division

- corresponds to arousal and energy generation including fight or flight
- most nerves exit the CNS midway along the spinal cord and from synapses just outside the spinal cord
- neurotransmitter: norepinephrine

Parasympathetic nervous system

- Promotes calming and a return to self-maintenance
- most nerves exit at the base of the brain and form synapses near or within an internal organ
- neurotransmitter: acetylcholine

Enteric division

- active in the digestive tract, pancreas, and gallbladder
- neurotransmitter: mostly acetylcholine

Cerebrum

- largest part of brain, and is divided into left and right hemispheres
- controls skeletal muscle contraction and is the center for learning, emotion, memory, and perception
- each side has frontal, temporal, occipital, and parietal lobes

Cerebellum

coordinates movement and balance, learns motor skills

Diencephalon

- Thalamus: received sensory input going to the cerebrum
- Hypothalamus: neuroendocrine gland; superchiasmatic nuclei control the circadian clock
- Pineal gland: source of melatonin

Brainstem

Midbrain: receives and integrates sensory information and sends it to the forebrain, coordinates visual reflexes

Medulla and pons: transfer information from PNS to the midbrain and forebrain; control homeostatic functions (e.g. breathing and circulation) and large-scale body movements (e.g. running)

The limbic system

- controls emotion
- interacts with other regions of the brain to generate and experience emotions
- includes the amygdala, hippocampus, and parts of the thalamus

Amygdala

the most important structure involved in emotional memory

Cortex

- the outer layer of the cerebrum
- where cognitive functions reside
- within the cortex are sensory, association, and motor areas

Sensory information

- arrives at primary sensory areas of the cortex via the thalamus
- once processed, information moves to the prefrontal cortex for decision making
- for example, motor commands are generated via the motor cortex

Cerebral cortex

- voluntary movements and cognitive functions (where memories are stored)
- in the motor cortex (frontal lobe) and somatosensory cortex (parietal lobe), neurons are arranged by the region of the body that they serve
- the size of the region is proportional to the amount of activity needed

Language information movement

when repeating a spoken word
auditory cortex (temporal lobe) --\> Wernicke's area (temporal lobe) --\> Broca's area (frontal lobe) --\> Motor cortex (frontal lobe)

Lateralization

the specialization of the left and right sides of the cerebrum

Cognitive lateralization

- the left side is more adept to language, math and logical operations
- the right side is more adept to recognize faces and patterns, spatial relations, and non-verbal thinking

Motor and sensory lateralization

- the left side of the brain controls the ride side of the body
- the right side controls the left side of the body

Corpus callosum

enables the right and left sides to communicate

Basal nuclei

centers for planning and learning movement sequences

Cell death

- during embryogenesis, neurons that do not reach an appropriate location undergo apoptosis
- half the neurons formed in the embryo are eliminated

Synapse elimination

- a developing neuron forms numerous synapses, and the activity of the neuron stabilizes some synapses and destabilizes others
- by the end of embryonic development, more than half the synapses have been eliminated

Neuronal plasticity

modifying connections between neurons at the synapse

Connection strength

connections can be strengthened or weakened in response to activity

Response strength

The strength of the post-synaptic response can be strengthened or weakened in response to activity

Hippocampus

where we hold short term memory; this information is released if it becomes irrelevant

Long-term memory

held within the cerebral cortex, where new data is associated with already stored memory

Long-term potentiation (LTP)

- a process involved in memory formation
- a lasting increase in the strength of synaptic transmission

LTP establishment

- activity at nearby synapses depolarizes the membrane of the post-synaptic cell; the NMDA receptor is unblocked and responds to glutamate
- calcium and sodium flow into the cell
- the influx of calcium induces AMPA glutamate receptors to insert themselves into the membrane

Prior to LTP

the NMDA glutamate receptor opens in response to glutamate but is blocked by Magnesium, so nothing happens

Synapse exhibiting LTP

glutamate activates AMPA receptors (1), triggering depolarization (2). This unblocks the NMDA receptors (3). Together, the AMPA and NMDA receptors trigger postsynaptic potentials strong enough to initiate action potentials without input from other synapses (4).

Neurological disorders due to dopamine signaling

Schizophrenia: caused by an overactive dopamine system in the brain
- symptoms are alleviated by drugs that block dopamine receptors

Parkinson's disease: involves the death of neurons that normally release dopamine at synapses
- symptoms are alleviated by L-dopa, a drug that crosses the blood-brain barrier and is converted into dopamine by the enzyme, dopa decarboxylase

Drugs that stimulate the reward system

- neurons that function in the reward system release dopamine
- as addiction to drugs that affect the reward system develops, long-lasting changes to the reward circuitry results in craving that is independent of pleasure
- addictive drugs include alcohol, cocaine, nicotine, and heroin

Sensory processes

- Sensory reception
- Transduction
- Transmission
- Perception

Sensory reception

detection of a stimulus by sensory cells
- sensing a stimulus opens or closes ion channels
- Non-neuronal sensory receptor cells form chemical synapses with afferent neurons

Sensory receptor cells

- sensory neurons
- non-neuronal cell that regulates a neuron

Sensory cell arrangement

- singly
- sensory organs

Sensed stimuli

Outside of the body - heat, light, pressure, chemicals

Inside of the body - blood pressure, body position

Sensory transduction

- conversion of a stimulus to a change in the membrane potential (receptor potential)
- receptor potentials are graded potentials that by summation, trigger action potentials

Sensory transmission

- receptor potential initiates an action potential that is transmitted from the PNS to the CNS (nerve impulse)
- the size of the receptor potential increases with the intensity of the simulus
- frequency of action potentials encodes the strength of the stimulus
- integration of sensory information begins in the sensory receptor cell, by summation

Sensory perception

- a stimulus is processed when action potentials reach the brain, generating the perception of the stimulus
- these action potentials travel along neurons that are dedicated to a particular stimulus
- the brain distinguishes between stimuli by the path action potentials take

Amplification

the strengthening of a sensory signal during transduction; may involve 2nd messengers

Sensory adaptation

a decrease in responsiveness to continued stimulation