Biological - The Nervous System Flashcards
Describe the evolution of the nervous system.
*Nervous tissue is found in most
species of multicellular animals.
*Some single-celled organisms have
electrical information transmission.
*600 m years ago - sponges: no
neurons but synaptic junctions.
*Jellyfish - diffuse nerve nets, no central nervous system.
What is a bilaterian?
Animals with bilateral symmetry, eg worm. We still follow this pattern/structure.
What is Cephalisation?
Nervous tissue concentrated toward one end of an organism.
Central Nervous System vs Peripheral Nervous System.
– Central nervous system (CNS) – the brain and the spinal cord.
– Peripheral nervous system (PNS) – NS other than brain and spinal cord (e.g., nerves from sense organs to the CNS).
What did Cajal argue and find about nerve cells?
Early work on nerve cells, end of 19th century discussion was is it one tissue or separated into single cells. Cajal argued separate cells and won, we now know they are separate cells.
▪ neurons are separable, i.e., that
there is a small gap between the tips
of one neuron’s fibers and the next
neuron
▪ => the nervous system consists of
individual neurons
What are the two main types of cell in the nervous system?
▪ Neurons / nerve cells
– Cerebellum: 70 billion
– Cerebral cortex: at least 12-15 billion
– Spinal cord: 1 billion
▪ Glia /glial cells/ neuroglia (from Greek ‘glue’)
– smaller than neurons; exceed neurons in numbers (old
estimate: x 10; recent estimate: x 1.2)
– most common glia: oligodendrocytes (76%), astrocytes
(17%) and microglia (6%)
What do Glia cells do?
▪ Glia are essentially support cells
in the NS and have many vital
roles:
1. Provide structure, i.e., surround neurons and hold them in place (astrocytes)
2. Insulate nerve cells with
myelin sheaths (oligodendrocytes in the CNS, Schwann cells in the PNS)
3. Supply nutrients and oxygen to neuron (astrocytes)
4. Removal of dead neuronal tissue & immune defence of the CNS (microglia: phagocytes)
5. During development, glial cells provide scaffolds for neurons to migrate to their final destinations
(radial glia)
6. Modulate neurotransmission in the synapses
Describe neuron structure.
Cells in the nervous system that specialise in performing information-processing tasks
▪ Each neuron consists of a body cell (soma), and fibres
(dendrites and an axon).
▪ Soma contains the cell nucleus and “machinery”
▪ Cell nucleus - contains the cell’s genetic material organized as DNA
molecules
▪ Cell “machinery”
– mitochondria – performs metabolic activities, extracts energy from
nutrients
– ribosomes - protein production
– endoplasmic reticulum – transports proteins to other locations
– Golgi Apparatus, Nissl Bodies, etc
Dendrites (Greek “tree”) branching fibres receiving information from other neurons.
Axon– a thin fibre transmits information to other neurons. The axon can be few μm or 1m long. In its distal part an axon has many branches (similar to dendrites). Each of them swells at its tip, forming a presynaptic terminal (end bulb or bouton).
Myelin sheath – insulating layer
Speeds up electrical transmition.
What are the different types of neurons?
Afferent (sensory) neurons (Arrive) –carry information from receptors.
Efferent neurons (Exit) – carry signals away to the effectors muscles or glands.
- Descending (efferent nerves): from the CNS (brain, spinal cord) to effectors (muscles or glands);
- Ascending (afferent nerves): from
sensory organs (receptors) to the CNS.
Interneurons –connect other neurons.
How does neural activity work?
The membrane of a neuron maintains an electrical gradient (the difference in electrical charge) between the inside and outside of the cell.
Resting potential: The electrical potential inside the neuron is slightly lower than outside (average –70 mV).
The transmission of information through the neuron is realized by the change of its potential.
It has two stages:
1. transmission of information from
the exterior through dendrites to the cell body;
2. transmission from the cell body
through the axon out of the neuron.
Stage 1 - Dendrites
*Many „postsynaptic potentials” –changes in electrical potential (strong or weak) moving toward the centre. Potentials from all dendrites sum up.
Stage 2 – cell’s body -> YES/NO
*If the sum is strong enough – the neuron „fires” (action potential, always the same strength).
There are two types of synapses:
excitatory synapses, inhibitory synapses. According to the type of chemical released in the synaptic gap, the information tends to excite or inhibit the postsynaptic neuron.
Excitatory synapse -> excitatory postsynaptic potential.
EPSP is a change in polarization moving along dendrites toward the cell’s body.
EPSP is positive-> provokes depolarization (decreases polarization).
Inhibitory synapse -> inhibitory postsynaptic potential.
IPSP is a change in polarization moving along dendrites toward the cell’s body.
IPSP is negative-> provokes hyperpolarization (increases polarization).
Postsynaptic potentials (excitatory and inhibitory) move along the dendrite toward the body cell. They become smaller and may vanish before reaching the body cell.
Summation:
Postsynaptic potentials sum up when meet other postsynaptic potentials or if followed by others.
If the ‘free potential’ is low (does not reach the threshold level) it dies.
If the ‘free potential’ is high (reaches the threshold level) it provokes a sudden and massive electric excitation at the proximal part of the axon: the ‘action potential’ (spike).
What are the two types of summation during neural transmission?
Over space – from different dendrites.
Over time – from the same dendrites.
How is the size of an action potential determined?
Trick question!
The amplitude of an action potential is independent of the amount of current which produced it, i.e. larger currents do not create larger action potentials
▪ The amplitude of an action potential is constant for a given
axon (e.g., +30 mV or +40mV).
➔ action potentials are all-or-none: they either occur fully or do not occur at all.
How do action potentials move down the neuron?
▪ First action potential – on the axon hillock.
▪ Action potentials move down an axon towards another cell using saltatory conduction.
– APs hop along the axon recurring at successive nodes of Ranvier ➔ fast propagation.
– myelin prevents any charge leakage through the axon.
Excitatory & inhibitory postsynaptic potentials.
Neuron’s have a spontaneous firing rate.
▪ IPSPs decrease the rate of APs in the postsynaptic neuron.
▪ (if above threshold) EPSPs increase
the rate of APs in the postsynaptic
neuron.
What are neurotransmitters?
Chemical particles used to transmit the information through the synapse.
The amount of neurotransmitter is not stable.
What happens when an action potential reaches the end of the axon?
When an action potential reaches the end of an axon, the depolarization opens calcium gates in the presynaptic membrane. It provokes a sudden release of neurotransmitters into a synaptic cleft.
Are synapses fast or slow?
Slow transmission.
Describe a neurotransmitters journey.
The neurotransmitter diffuses across the synaptic gap to reach the postsynaptic membrane, where it
is attached to the receptors.
When a neurotransmitter binds to the receptor, gates open in the postsynaptic membrane.
The quantity of neurotransmitters released impacts the robustness of changes in the postsynaptic membrane.
What is the neurotransmitter lock and key concept?
NTs are chemicals, they have physical shapes. Need the proper shape to open the gate. Sometimes similar shapes can do the job. Antagonists work against and block transmission, even with proper ‘key’ for gate.
What is the function of a synapse?
The role of a synapse is to send chemical information through the gap and open ions’ gates in the postsynaptic neuron.
How many different neurotransmitters do humans have?
Humans have around 100 different
neurotransmitters.
Which ions are related to EPSP and IPSP?
ESPS = Na+, open the Na+ gate and enter.
IPSP = Cl- (Open Cl- gate, and enter), K+ (Open K+ gate, K+ leave).
Examples of NTs and whether they are excitatory or inhibitory.
Excitatory neurotransmitters:
* Glutamate, epinephrine, norepinephryne
Inhibitory neurotransmitters:
* GABA, glycine
Excitatory / Inhibitory:
* acetylcholine, dopamine, serotonin
What are the two types of receptor?
Ionotropic – open „fast” gates.
(glutamate – excitatory, GABA – inhibitory)
As soon as the neurotransmitter binds to a receptor on the membrane, the ion gates open.
The process starts in 1ms and lasts for 20ms.
Eg vision and hearing.
Metabotropic – open „slow” gates.
A sequence of metabolic changes in a larger area of the cell. The process starts in 30ms and lasts for seconds, minutes or longer.
Eg taste, hunger, and fear.
What are neuromodulators?
*The chemicals that affect metabotropic receptors are often called neuromodulators, to distinguish them from the fast effects of the neurotransmitters at ionotropic synapses.
*Chemicals which may activate metabotropic receptors:
dopamine, norepinephrine, serotonin, neuropeptides and sometimes glutamate and GABA
too.
Neuromodulatory causes NTs to intervene and diffuse around, may impact chemical activity.
How many types of NTs do neurone release and respond to?
The majority of neurons
release two (or more) types of
neurotransmitters. Each neuron may respond to all types of neurotransmitters received at the
postsynaptic membrane of different synapses.
What determines the strength of a synapse, and how may this change?
The size of postsynaptic potential, which is related to number of NTs and receptors.
Synaptic strength may vary over time:
*Short-term changes (seconds, minutes)
*Long-term changes (memory and learning effects, brain plasticity)
Intensive use of a synapse may lead to its potentiation -> stronger effects on postsynaptic membrane.
Describe neuron plasticity and synaptic pruning.
Connections change, adapt, and grow, eg in visual cortex from 0-24 months.
Pruning:
Very intensive in 2-16 years old humans.
*Elimination of unnecessary synapses is beneficial.
*Learning - > pruning
If too many synapses, cannot manage. Pruning very important for healthy development.
What happens to used NTs?
Once the neurotransmitter has activated the receptors, its action must be terminated (to stop
effect on the receptor & to allow frequent responding).
1/ Reuptake is a process, which brings neurotransmitters back to the presynaptic neuron (by chemicals called transporters). This recycling process concerns serotonin and
the catecholamines (dopamine,
nerepinephryne, epinephrine).
2/ Other neurotransmitters are broken down (for example acetylcholine).
3/ Big neurotransmitters (peptide) simply diffuse.
Glial cells can reabsorb neurotransmitters at some synapses
(& influence synaptic activity by granting or withholding such absorption).
Mood Disorder Treatment.
- Selective serotonin reuptake inhibitors (SSRIs).
- Reuptake of serotonin form the synaptic cleft is reduced.
- Serotonin may bind to receptors several times.
Drug addiction.
Drug addiction is a brain disease that produces profound modifications
in human behaviour.
Dopamine = reward
*In healthy people alcohol increases dopamine release.
*In alcoholics
-> reduced number of dopamine receptors
-> reduced dopamine release
-> craving
Risk of addiction
*Not everyone runs the same risk.
*Dopamine uses several different receptor types.
*People at risk of addiction are characterised by a specific balance of different types of receptors.
Research in progress.
What is the function of the nervous system?
1) Control of vital functions (breathing, heart rate, digestion etc.)
2) Control of movement
3) Reaction to external events for survival
4) Cognitive/ emotional processes
What are the two types of nervous system in the Peripheral Nervous System?
1) Somatic nervous system
nerves which convey information from sense organs to CNS, and from CNS to muscles.
2) Autonomic nervous system
controls internal organs like heart,
intestines, etc.
Describe the somatic nervous system.
*Ascending pathways (afferent):
Bottom -> up; sensory information;
*Descending pathways (efferent):
Top –> down; control of movements;
From brain/spinal cord to muscles.
*Top – down connections (to muscles)
Motor cortex located anterior to the central sulcus elicits all voluntary movements.
Two paths:
1. Dorsolateral tract (pyramidal).
Controls movements in peripheral parts such as hands, fingers, and
toes. Controls the contralateral part of the body.
2. Ventromedial tract.
Controls more proximal parts of the body: muscles of neck, shoulders, and trunk. Is responsible for bilateral movements like walking. Controls both (left and right) sides of the body.
- Bottom – up tracts, (from the body):
Two paths:
1. Dorsal column medial lemniscal pathway; Carries information
About touch, vibration,
Proprioception.
2. Spinal – thalamic tract; Carries information about pain, and
temperature.
Describe the autonomic nervous system.
Out of our conscious control.
*Sympathetic nervous system
*Parasympathetic nervous system
▪ The sympathetic division of the
autonomic nervous system
prepares the body for action
during times of threat and
prepares the body for muscular
exertion or stressful activities
(“fight or flight”)
▪ The parasympathetic division is
active during times of relaxation
and rest and dominates in
controlling the body for metabolic
“business as usual”
What is homeostasis?
*Stability of biological processes inside the body: body temperature,
chemical characteristic (ph) of body fluids, osmotic solution of body fluids, blood volume, glucose concentration in blood.
How does homeostatic regulation work?
*The centre of homeostasis regulation is hypothalamus.
*Hypothalamus organizes autonomic nervous system work, and projects to higher brain areas.
*Negative feedback: if discrepancy from the set point is detected, the remedial action is maintained until the homeostasis is regained.
*2 mechanisms:
*Neurohormonal
*Behavioural
What does the central nervous system consist of?
1) Brain (within the skull),
2) Spinal cord (within the spinal column).
Brain consists of:
White matter -
*Interhemispheric connections (corpus callosum, anterior commissure).
*Cortico-subcortical pathways connect cortex to subcortical areas, motor areas in brainstem and
spinal cord (up-down).
*Subcortical-cortical pathways: from brainstem, sensory areas in thalamus to the cortex (down-up).
*Association pathways between different areas in the same hemispheres.
Corpus Callosum - Wide bundle of neural fibres, connecting right and left hemispheres.
How does the nervous system develop?
*During embryonic development of humans: a neural tube undergoes cephalisation to form
initially three, then five vesicles (bubbles).
*Phylogenesis- the sequence of events involved in the evolutionary development of a species.
*Ontogenesis - the process of an individual organism grow.
