13: Nervous System I Flashcards
The Nervous System
- 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.
Central Nervous System (CNS)
Consists of brain and spinal cord
Peripheral nervous system (PNS)
Peripheral nerves (nerves not in the CNS)
Nervous System Functions
Sensory
Integration
Motor
Sensory
- Detects internal and external environmental changes (e.g. proprioception, sensation/touch).
- Impulse carried by sensory neurons.
Integration
- Processes sensory information by analysing, storing & making decisions.
- Abundant in the brain ( ‘perception’).
- Carried by interneurons.
Motor
- Produces a responseto sensory information (perception) to effect change.
- Impulse carried by motor neurons.
Peripheral Nervous System
Somatic nervous system
Autonomic nervous system
Somatic nervous system
Controls voluntary muscles and transmits sensory information to the CNS
Autonomic nervous system
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
Sympathetic nervous system
Arouses body to expend energy
• ‘Fight or flight’ response.
• Thoraco-lumbar innervation.
Parasympathetic nervous system
Calms body to conserve and maintain energy
• ‘Rest and digest’.
• Cranio-sacral innervation.
Parasympathetic nervous system
Calms body to conserve and maintain energy
• ‘Rest and digest’.
• Cranio-sacral innervation.
Eye (pupil)
S: Dilation
PS: Constriction
Lungs
S: Bronchodilation
PS: Bronchoconstriction
Heart
S: Heart rate and blood pressure increased
PS: Heart rate and blood pressure decreased
Gastrointestinal Tract
S: Decreased motility and secretions
PS: Increased motility and secretions
Liver
S: Conversion of glycogen to glucose
PS: Glycogen sythesis
Adrenal Glands
S: Releases adenaline
PS: No involvement
Enteric Nervous System
- 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.
Enteric Nervous System: Nerve types
Sensory neurons
Motor neurons
Interneurons
Sensory neurons
Sensory neurons monitor chemical changes (via chemo-receptors) in the GI tract and stretching (stretch receptors) of its walls.
Motor neurons
Motor neurons govern motility and secretions of the GIT and associated glands
Interneurons
Connect the 2 plexus
Nervous tissue cells
Neurons
Neuroglia
Neurons
- 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).
Neuroglia (glial cells)
- 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.
Neuroglia (glial cells)
- 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.
Nerve
A bundle of one or more neurons
Neuron parts
- Cell body & Dendrites.
- Axon.
- Myelin sheath & nodes of Ranvier.
- Terminal endings.
Action potential
- 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.
Neurons: Cell Body & Dendrites
- 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
Neurons: Axons
- 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.
Neurons: Myelin Sheath
- 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.
Grey & White Matter
When observing a region of the brain or spinal cord, some regions appear white whilst others appear grey.
Grey Matter
Grey matter is mostly composed of cell bodies. It also contains dendrites and unmyelinated axons.
White Matter
White matter is composed primarily of myelinated axons. The whitish colour of the myelin is what gives the region its name
Neuroglia
- 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’).
Neuroglia: Functions
- Surround neurons & hold them in place.
- Supply nutrients & oxygen to neurons.
- Destroy pathogens & remove dead neurons.
Neuroglia: Types
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.
Astrocytes
- 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.
Oligodendrocytes
• Glial cells that myelinate axons in the CNS.
Microglia
- Derived from monocytesthat migrate to the CNS before birth.
- Resident immune cells in brain: ‘phagocytic’.
- Mobile in the brain and multiply with damage.
Ependymal Cells
- 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.
Schwann Cells
- 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.
Satellite Cells
• Surround cell bodies in PNS ganglia, providing structural support & exchange substances.
Nerve Impulses
2 types:
Graded potential
Action potential
Graded potential
For short distance communication
Occur in dendrites and cell body
Amplitude proportional to stregnth and stimulus (no threshold)
Longer duration
Action potential
For long distance communication
Propagated down axon
All-or-nothing (has threshold)
Shorter duration
Action potential
For long distance communication
Propagated down axon
All-or-nothing (has threshold)
Shorter duration
Potential facilitation
- Specific ion channels can open and close when stimulated.
2. Electrical difference across the cell membrane (‘resting potential’).
Nerve Impulses: Ion Channels
- 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.
Nerve Impulses: Resting Potential
- 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).
Nerve Impulses: Action Potential
- 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.
Action Potential: Phases
- Depolarisation: The negative membrane potential (-70mV) becomes positive and reaches +30mV.
- Repolarisation: The membrane is then restored to –70mV.
Action Potential: Depolarisation
- 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.
Action Potential: Repolarisation
- 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.
Action Potential: Refractory period
- 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.
Unmyelinated Axons
• ‘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
Myelinated Axons
- 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.
Continuous conduction
Unmyelinated
Step-by-step depolarisation
Slower
Less energy efficient
Saltatory Conduction
Myelinated
‘Leaps’ of depolarisation
Faster
More energy efficient
Local anasthetics
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.
Local anasthetics
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.
Synapses
- 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.
Synapses: Signal Transmission
- An action potential arrives at the synaptic end bulb. The depolarisation phase causes calcium (Ca2+) channels to open, sending Ca2+ into the synaptic bulb.
- Increase in Ca2+concentration causes exocytosis of synaptic vesicles: neurotransmitters are released into the synaptic cleft.
- The neurotransmitters diffuse across the synapse and bind to receptorson the post-synaptic neuron.
- This opens the ion channels,generating an action potential in the post-synaptic nerve.
Neurotransmitters
- 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.
Neurotransmitter Types
- Amino Acids e.g. glutamate, GABA.
- Monoamines e.g. dopamine, serotonin.
- Peptides (neuropeptides) e.g. endorphins.
- Unique molecules e.g. acetylcholine.
Neurotransmitters: Excitatory
Causes depolarisationof the post-synaptic neuron.
Opens the Na+ ion channels
Inner membrane becomes more positive
Neurotransmitters: Inhibitory
Causes hyperpolarisation of the post-synaptic neuron
Opens the K+ ion channels
Inner membrane becomes more negative
Glutamate
- 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.
Gamma Aminobutyric Acid (GABA)
- 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.
Serotonin
- 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.
Dopamine
- 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.
Monoamines
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).
Neuropeptides
- 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.
Acetylcholine (ACH)
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.
NitricOxide (NO)
Primary action: Excitatory.
Formed from: Arginine.
Role: Vasodilation. Exists for less than 10 seconds.
Pharmacology: Usedfor angina (GTN). Viagra enhances NO.
Neurotransmitter Breakdown Enzymes
Monoamine oxidase (MAO) Catechol-O-methyl transferase(COMT)
Monoamine oxidase (MAO)
- Found in neurons and astrocytes.
- Involved in the breakdown of monoamines:
- Serotonin
- Adrenaline
- Noradrenaline
- Dopamine
Catechol-O-methyl transferase(COMT)
Catalyses the breakdown of:
• Adrenaline
• Noradrenaline
• Dopamine
Nerve Sensation: Touch
- 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.
Nerve Sensation
- ‘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’.
Neuroregeneration
• 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).
