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Biomedicine II > 13: Nervous System I > Flashcards

13: Nervous System I Flashcards

1
Q

The Nervous System

A
  • A network of fibres which span the body, co-ordinating a diverse range of voluntary and involuntary actions.
  • Transmits signals between parts of the body.
  • Rapidly responds to changes within the internal and external environment.
  • Works alongside the endocrine system to maintain homeostasis.
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2
Q

Central Nervous System (CNS)

A

Consists of brain and spinal cord

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3
Q

Peripheral nervous system (PNS)

A

Peripheral nerves (nerves not in the CNS)

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4
Q

Nervous System Functions

A

Sensory
Integration
Motor

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5
Q

Sensory

A
  • Detects internal and external environmental changes (e.g. proprioception, sensation/touch).
  • Impulse carried by sensory neurons.
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6
Q

Integration

A
  • Processes sensory information by analysing, storing & making decisions.
  • Abundant in the brain ( ‘perception’).
  • Carried by interneurons.
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7
Q

Motor

A
  • Produces a responseto sensory information (perception) to effect change.
  • Impulse carried by motor neurons.
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8
Q

Peripheral Nervous System

A

Somatic nervous system

Autonomic nervous system

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9
Q

Somatic nervous system

A

Controls voluntary muscles and transmits sensory information to the CNS

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10
Q

Autonomic nervous system

A

Controls involuntary body functions.
• The ANS works automatically and involuntarily to maintain homeostasis.
•
The hypothalamusis the highest control centre over autonomic motor neurons.
•
Affects organs, glands, cardiac and smooth muscles.

Includes:
Sympathetic nervous system
Parasympathetic nervous system

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11
Q

Sympathetic nervous system

A

Arouses body to expend energy
• ‘Fight or flight’ response.
• Thoraco-lumbar innervation.

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12
Q

Parasympathetic nervous system

A

Calms body to conserve and maintain energy
• ‘Rest and digest’.
• Cranio-sacral innervation.

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13
Q

Parasympathetic nervous system

A

Calms body to conserve and maintain energy
• ‘Rest and digest’.
• Cranio-sacral innervation.

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14
Q

Eye (pupil)

A

S: Dilation
PS: Constriction

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15
Q

Lungs

A

S: Bronchodilation
PS: Bronchoconstriction

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16
Q

Heart

A

S: Heart rate and blood pressure increased
PS: Heart rate and blood pressure decreased

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17
Q

Gastrointestinal Tract

A

S: Decreased motility and secretions
PS: Increased motility and secretions

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18
Q

Liver

A

S: Conversion of glycogen to glucose
PS: Glycogen sythesis

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19
Q

Adrenal Glands

A

S: Releases adenaline
PS: No involvement

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20
Q

Enteric Nervous System

A
  • The ‘Brain’ of the GIT, containing around 100 million neurons.
  • Functions independently but regulated by the autonomic nervous system.
  • Links with the CNS via the sympathetic and parasympathetic nerve fibres (Vagusnerve) –involuntary.
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21
Q

Enteric Nervous System: Nerve types

A

Sensory neurons
Motor neurons
Interneurons

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22
Q

Sensory neurons

A

Sensory neurons monitor chemical changes (via chemo-receptors) in the GI tract and stretching (stretch receptors) of its walls.

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23
Q

Motor neurons

A

Motor neurons govern motility and secretions of the GIT and associated glands

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24
Q

Interneurons

A

Connect the 2 plexus

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25
Q

Nervous tissue cells

A

Neurons

Neuroglia

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26
Q

Neurons

A
  • Neurons process and transmit information.
  • Structural and functional units of the nervous system. They are electrically excitable.
  • Lots of different types of neurons (most diverse cell type in the body).
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27
Q

Neuroglia (glial cells)

A
  • Glial cells are supporting cells that nourish, support and protect neurons.
  • There are 6 types of glial cell.
  • More numerous than neurons, making up 90% of brain volume.
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28
Q

Neuroglia (glial cells)

A
  • Glial cells are supporting cells that nourish, support and protect neurons.
  • There are 6 types of glial cell.
  • More numerous than neurons, making up 90% of brain volume.
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29
Q

Nerve

A

A bundle of one or more neurons

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30
Q

Neuron parts

A
  • Cell body & Dendrites.
  • Axon.
  • Myelin sheath & nodes of Ranvier.
  • Terminal endings.
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31
Q

Action potential

A
  • Neurons possess electrical excitability: the ability to create a nerve impulse or “action potential”.
  • A stimulus is anything able to generate an action potential. The stimulus can be internal or external.
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32
Q

Neurons: Cell Body & Dendrites

A
  • Cell bodies consists of a nucleus and cell organelles.
  • Cell bodies are known collectively as grey matter.
  • Collections of cell bodies clustered together are referred to as:
  • Nucleiin the CNS –form structural and functional groups in the brain.
  • Gangliain the PNS.
  • Dendrites are the receiving portion of the cell. They communicate with other neurons/dendrites
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33
Q

Neurons: Axons

A
  • Axons are long, thin cylindrical projections that carry nerve impulses towards another neuron, away from the cell body.
  • Length varies from <1mm (in CNS) to approx. 1m (sciatic nerve).
  • Axons are covered by a membrane called the axolemma.
  • The ends are called axon terminals.
  • Axon bundles are called tractsin the CNS and nervesin the PNS.
  • If injured, axons can regenerate at a rate of1-2mm per day.
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34
Q

Neurons: Myelin Sheath

A
  • A multi-layered lipid & protein covering around the axons.
  • The myelin sheath electrically insulates the axon & increases the speed of nerve conduction.
  • Formed by glial cells (schwann cells (PNS) & oligodendrocytes(CNS)) in the embryo, continuing through childhood and peaking in adolescence.
  • Each cell wraps about 1mm of length repeatedly (up to 100 layers) in a myelinated axon.
  • Gaps in myelin sheath are called nodes of Ranvier.
  • Vitamin B12 is a co-factor needed for the production of myelin.
35
Q

Grey & White Matter

A

When observing a region of the brain or spinal cord, some regions appear white whilst others appear grey.

36
Q

Grey Matter

A

Grey matter is mostly composed of cell bodies. It also contains dendrites and unmyelinated axons.

37
Q

White Matter

A

White matter is composed primarily of myelinated axons. The whitish colour of the myelin is what gives the region its name

38
Q

Neuroglia

A
  • Neuroglia (or ‘glial cells’) are non-excitatory. They surround and bind neurons. Neurons would not function without glial cells.
  • Glial cells are far smaller than neurons, but are 50x more prevalent. They can multiply and divide (unlike neurons).
  • After trauma, glia fill spaces left by damaged neurons (an important concept with growth of tumours -> ‘gliomas’).
39
Q

Neuroglia: Functions

A
  • Surround neurons & hold them in place.
  • Supply nutrients & oxygen to neurons.
  • Destroy pathogens & remove dead neurons.
40
Q

Neuroglia: Types

A

There are six types of neuroglia:
• 4 in the Central Nervous System: Astrocytes, oligodendrocytes, microglia & ependymal cells.
• 2 in the Peripheral Nervous System: Schwann cells and Satellite cells.

41
Q

Astrocytes

A
  • Star-shaped with branching processes.
  • Most numerous and largest neuroglia (in CNS).
  • Hold neurons to their blood supply (physical support).
  • Contribute to the blood brain barrier.
42
Q

Oligodendrocytes

A

• Glial cells that myelinate axons in the CNS.

43
Q

Microglia

A
  • Derived from monocytesthat migrate to the CNS before birth.
  • Resident immune cells in brain: ‘phagocytic’.
  • Mobile in the brain and multiply with damage.
44
Q

Ependymal Cells

A
  • Epithelial cells which line the walls of the:
  • 4 Ventricles of the cerebrum.
  • Central canal of spinal cord.
  • Produce cerebrospinal fluid (csf) and beat their cilia to circulate csf.
45
Q

Schwann Cells

A
  • Schwann cells produce myelin around the axons of neurons in the peripheral NS.
  • This insulates the axon, increases the speed of nerve impulse conduction & participates in axon regeneration.
  • Most dendrite connections & myelination completed by age of 3. So, malnutrition in infancy = irreversible damage.
  • The unmyelinated gaps along a neuron are called Nodes of Ranvier.
46
Q

Satellite Cells

A

• Surround cell bodies in PNS ganglia, providing structural support & exchange substances.

47
Q

Nerve Impulses

A

2 types:
Graded potential
Action potential

48
Q

Graded potential

A

For short distance communication
Occur in dendrites and cell body
Amplitude proportional to stregnth and stimulus (no threshold)
Longer duration

49
Q

Action potential

A

For long distance communication
Propagated down axon
All-or-nothing (has threshold)
Shorter duration

50
Q

Action potential

A

For long distance communication
Propagated down axon
All-or-nothing (has threshold)
Shorter duration

51
Q

Potential facilitation

A
  1. Specific ion channels can open and close when stimulated.

2. Electrical difference across the cell membrane (‘resting potential’).

52
Q

Nerve Impulses: Ion Channels

A
  • These are transport channels for ions created by transmembrane proteins within the neuron membranes.
  • When ion channels open they allow specific ions to move through the membrane across a concentration gradient e.g. Na+ channels allow Na+ through.
  • Channels open in response to a stimulus which changes the permeability of the membrane to Na+& K+
  • Stimuli include changes in voltage, chemicals (hormones), mechanical pressure.
53
Q

Nerve Impulses: Resting Potential

A
  • Neurons at rest possess an electrochemical gradient across the cell membrane.
  • The resting potential is created by a build up of negative ions on the inside of the cell membrane, relative to the extracellular fluid which contains more positive ions.
  • The separation of charges across a cell membrane creates potential energy.
  • This resting potential is approximately -70mV.
  • Cells exhibiting a membrane potential are said to be polarised or “charged”.
  • The extracellular fluid is rich in Na+and Cl–ions and carries a positive charge.
  • The intracellular fluid is rich in K+and large negatively charged proteins and phosphates which cannot leave the cell. Thus carries a negative charge inside the cell.
  • As the Na+ and Cl– try to move back to equalise the charge, the separation of charges is maintained by the sodium-potassium pump which pumps 3Na+ out for every 2K+it pumps back in (using ATP).
54
Q

Nerve Impulses: Action Potential

A
  • An action potential is the formation of a nerve impulse.
  • It is a series of events which reverses the membrane potential and then restores it to its resting state.
  • It is then propagated down the axon in an “all-or-nothing” fashion meaning there is no reduction of the signal as it travels.
55
Q

Action Potential: Phases

A
  1. Depolarisation: The negative membrane potential (-70mV) becomes positive and reaches +30mV.
  2. Repolarisation: The membrane is then restored to –70mV.
56
Q

Action Potential: Depolarisation

A
  • Depolarisation is triggered by stimulation of a nerve ending.
  • Depolarisation must reach a threshold value of -55mv in order to generate an action potential.
  • Na+channels open allowing Na+to flood intothe cell up to about +30mV (so at the peak of the action potential, the inside of the membrane is 30mV more positive than the outside).
  • A positive charge builds up inside the cell.
57
Q

Action Potential: Repolarisation

A
  • K+ channels open much more slowly than Na+ channels so just as the Na+ channels are closing the K+ channels open.
  • This allows K+ to flood out of the cell, restoring the membrane potential to –70mV.
58
Q

Action Potential: Refractory period

A
  • Period of time after repolarisation in which a nerve cannot generate another action potential because Na+ & K+ are on the wrong sides of the membrane.
  • During this period, the Na-K pump pumps 3 Na+out and 2 K+ back into the cell to restore the resting potential.
  • Absolute refractory period: Even a strong impulse cannot generate an action potential.
  • Relative refractory period: Larger than normal stimulus needed to generate an action potential.
59
Q

Unmyelinated Axons

A

• ‘Conduction’ describes the movement of a nerve impulse along the axon of a neuron.
Unmyelinated axons:
• No myelin sheath around the nerve.
• The membrane becomes depolarised in a continuous conduction away from the cell body down the axon.
• Step-by-step depolarisation & repolarisation occurs of each adjacent segment of cell membrane. This occurs in one direction only.
eg Olfactory Nerve

60
Q

Myelinated Axons

A
  • Myelin is an insulator, preventing ionic currents from crossing the cell membrane.
  • Instead, at the ‘nodes of Ranvier’, there are high concentrations of Na+gates. This causes the current to appear to jump from node to node (‘saltatory conduction’).
  • Action potentials ‘leap’ across long segments of the myelinated axon, leading to much faster conduction.
  • Saltatory conduction is far more energy efficient, as less ATP is needed for the sodium-potassium pumps.
  • Action potentials are also conducted slower at lower temperatures.
61
Q

Continuous conduction

A

Unmyelinated
Step-by-step depolarisation
Slower
Less energy efficient

62
Q

Saltatory Conduction

A

Myelinated
‘Leaps’ of depolarisation
Faster
More energy efficient

63
Q

Local anasthetics

A

Local Anaesthetics block Na+gates, preventing them from opening and therefore stopping an action potential from being formed, inhibiting the nerve from transmitting the ‘pain’ message.

64
Q

Local anasthetics

A

Local Anaesthetics block Na+gates, preventing them from opening and therefore stopping an action potential from being formed, inhibiting the nerve from transmitting the ‘pain’ message.

65
Q

Synapses

A
  • Neurons are not continuous and have gaps between them called ‘synapses’.
  • The ends of axon terminals are called synaptic end bulbs.
  • The space between the synaptic end bulbs & post-synaptic neuron is the synaptic cleft, which is filled with interstitial fluid.
  • The nerve impulse is carried across the synaptic cleft by chemical messengers called ‘neurotransmitters’. These are stored in synaptic vesicles.
  • Neurotransmitters can have either excitatory or inhibitory effects on the post-synaptic neuron.
66
Q

Synapses: Signal Transmission

A
  1. An action potential arrives at the synaptic end bulb. The depolarisation phase causes calcium (Ca2+) channels to open, sending Ca2+ into the synaptic bulb.
  2. Increase in Ca2+concentration causes exocytosis of synaptic vesicles: neurotransmitters are released into the synaptic cleft.
  3. The neurotransmitters diffuse across the synapse and bind to receptorson the post-synaptic neuron.
  4. This opens the ion channels,generating an action potential in the post-synaptic nerve.
67
Q

Neurotransmitters

A
  • A neurotransmitter (NT) is a chemical messenger that gets released from a pre-synaptic terminal, causing an effect on the post-synaptic cell.
  • More than 100 NT’s been be identified so far.
  • One way to classify neurotransmitters is whether they have an excitatory or inhibitory action on the post-synaptic neuron
  • Following a nerve impulse, neurotransmitters need to be inactivated and removed for the process to be able to start again. This can occur bydiffusion, enzymes breakdown (e.g. MAO) or through re-absorption.
68
Q

Neurotransmitter Types

A
  1. Amino Acids e.g. glutamate, GABA.
  2. Monoamines e.g. dopamine, serotonin.
  3. Peptides (neuropeptides) e.g. endorphins.
  4. Unique molecules e.g. acetylcholine.
69
Q

Neurotransmitters: Excitatory

A

Causes depolarisationof the post-synaptic neuron.
Opens the Na+ ion channels
Inner membrane becomes more positive

70
Q

Neurotransmitters: Inhibitory

A

Causes hyperpolarisation of the post-synaptic neuron
Opens the K+ ion channels
Inner membrane becomes more negative

71
Q

Glutamate

A
  • Glutamate (glutamic acid) is a major excitatory NT in the CNS. It plays a major role in memory and learning.
  • Glutamate is initially produced from the amino acid Glutamine.
72
Q

Gamma Aminobutyric Acid (GABA)

A
  • GABA is the major inhibitory NT in the brain, produced from glutamate.
  • The conversion of glutamate to GABA is vitamin B6 dependent.
  • 1/3 of brain synapses use GABA. It is essential in preventing neural over-activity.
  • The drug ‘diazepam’ (Valium) enhances GABA.
73
Q

Serotonin

A
  • Serotonin (5-Hydroxytryptamine) is produced from the amino acid ‘Tryptophan’.
  • 95% of serotonin is produced in the enteric nervous system (digestive tract), whilst the remainder is located in the Central Nervous System.
  • Serotonin has a vital role in the GIT. It is involved in intestinal motility (peristalsis) and epithelial cell secretion.
  • Serotonin also plays a role in attention, sleep and pain regulation.
  • Serotonin is removed from a synapse by the enzyme ‘MAO’.
  • A change in serotonin levels is the result of something, NOT the cause.
74
Q

Dopamine

A
  • Dopamine is located in several areas of the brain, including the substantia nigra.
  • Synthesised from the amino acid ‘tyrosine’.
  • Plays a key role in movement, reward mechanisms, regulating muscle tone, cognition and emotion.
  • Dopamine also acts as an inhibitor for prolactin release from the anterior pituitary gland.
  • The removal of dopamine from a synapse is by re-uptake or using the enzymes MAO & COMT.
  • Dopamine depletion is associated with Parkinson’s disease.
75
Q

Monoamines

A

Adrenaline and Noradrenaline
Typeof neurotransmitter: Monoamine
Produced from: Tyrosine
Primary action: Excitatory, opens Na+channels
Location: Sympathetic NS, motor neurons, brain & adrenal medulla.
Role: Major excitatory neurotransmitter (also hormones)
Removal: Re-uptake or degradation by enzymes monoamine oxidase (MAO) & catechol-oxygen-methyl transferase (COMT).

76
Q

Neuropeptides

A
  • Neuropeptides are small proteins acting as neurotransmitters and hormones.
  • Common neuropeptides include endorphins, enkephalins, dynorphins & substance P.
  • They may act as neuromodulators-substances that do not propagate nerve impulses directly, but instead exert regulatory effects on synaptic receptors.
  • Enkephalins, endorphins and dynorphins are opioids (body’s natural analgesics). They are released after exercise.
  • Substance P enhances the feeling of pain.
77
Q

Acetylcholine (ACH)

A

Primary action: Excitatory (inhibitory in the vagus nerve).
Location: Major NT in the parasympathetic nervous system, CNS and neuromuscular junction.
Role: Muscle contractions, cognition.
Removal: Degraded by the enzyme acetylcholinesterase.
Associations: Alzheimer’s, botulinum toxin (‘botox’) blocks Ach.

78
Q

NitricOxide (NO)

A

Primary action: Excitatory.
Formed from: Arginine.
Role: Vasodilation. Exists for less than 10 seconds.
Pharmacology: Usedfor angina (GTN). Viagra enhances NO.

79
Q

Neurotransmitter Breakdown Enzymes

A 
Monoamine oxidase (MAO)
Catechol-O-methyl transferase(COMT)
80
Q

Monoamine oxidase (MAO)

A
  • Found in neurons and astrocytes.
  • Involved in the breakdown of monoamines:
  • Serotonin
  • Adrenaline
  • Noradrenaline
  • Dopamine
81
Q

Catechol-O-methyl transferase(COMT)

A

Catalyses the breakdown of:
• Adrenaline
• Noradrenaline
• Dopamine

82
Q

Nerve Sensation: Touch

A
  • Touch excites agraded potential in sensory nerve endings (Meissner’s corpuscles).
  • The graded potential triggers the axon of a sensory neuron to form an action potentialwhich travels into the CNS.
  • Neurotransmitters are released at synapses where there are interneurons.
  • Perception occurs in the brain (primary somatosensory area) the interpretation of touch occurs.
83
Q

Nerve Sensation

A
  • ‘Spinal nerves’ carry impulses to and from the spinal cord. There are 31 pairs of spinal nerves.
  • The spinal nerve is named based on the spinal level it originates from e.g. L5.
  • Spinal nerves combine to form named ‘peripheral nerves’, such as the sciatic nerve.
  • If a spinal nerve is injured (e.g. by a disc prolapse, bone spur, tumour), it can cause pain and altered sensation (tingling/numbness) in the associated distribution. This distribution is called a ‘dermatome’.
84
Q

Neuroregeneration

A

• Neurons have limited powers of regeneration. These are mostly dependent upon the location.
Peripheral Nervous System (PNS):
• Peripheral nerve fibres do regenerate if Schwann cells and the cell body are intact. This is also dependent on scar tissue present.
Central Nervous System (CNS):
• CNS nerve fibres cannot regenerate. Oligodendrocytes and astrocytes inhibit re-growth so scar tissue is formed instead.
• After the foetal period there is an absence of growth stimulating factors.
• Clean up of debris is slow (no macrophages).

Biomedicine II (16 decks)

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