Neurology and Neuroscience Flashcards
What are the components of the CNS and the PNS?
The central nervous system (CNS) consists of the two cerebral hemispheres, the brainstem, the cerebellum and the spinal cord. The peripheral nervous system (PNS) consists of the nerve fibres originating from the CNS.
Outline the structure of the cerebral hemispheres
The cerebral hemispheres (also known as the telencephalon) have a distinctive convoluted surface appearance where the ridges are called gyri (singular gyrus) and the valleys are called sulci (singular sulcus).
What are the 4 functionally distinct regions/lobes comprising the cerebral hemispheres?
1) Frontal: Responsible for executive functions such as personality
2) Parietal: Contains the somatic sensory cortex responsible for processing tactile information
3) Temporal: Contains important structures such as the hippocampus (short term memory), the amygdala (behaviour) and Wernicke’s area (auditory perception & speech)
4) Occipital: Processing of visual information
What are the components of the brainstem?
The brainstem consists of the midbrain, pons and the medulla in descending order. These structures have a multitude of important functions (e.g. control of respiration & heart rate) and are the target or the source of all the cranial nerves.
Outline the location and function of the cerebellum
The cerebellum is located towards the dorsal region of the CNS and is attached to the brainstem. It has an important role in motor coordination, balance and posture.
Outline the location and function of the spinal cord
The spinal cord extends down from the medulla and acts as a conduit for neural transmission but can coordinate some reflex actions.
What is a mature neuron?
A mature neuron is a non-dividing excitable cell, with a heterogeneous morphology, whose main function is to receive and transmit information in the form of electrical signals. Although neurons come in a variety of different shapes and size, they do share a number of common features.
Outline the morphology of the 4 types of neurons
1) Unipolar: 1 axonal projection
2) Psuedo-unipolar: Single axonal projection that divides into two
3) Bipolar: 2 axonal projections from the cell body
4) Multipolar: Numerous axonal projections from cell body
> Pyramidal cells: ‘pyramid’ shaped cell body
> Purkinje cells: GABA neurons found in the cerebellum
> Golgi cells: GABA neurons found in the cerebellum
What are the functions of the soma, axon and dendrites?
1) Soma (cell body, perikaryon)
> Contains nucleus, & ribosomes
> Contains neurofilaments which are important in maintains the structure structure, as well as useful for transportation
2) Axon
> Long process (aka nerve fibre) - originates from soma at axon hillock
> Can branch off into ‘collaterals’
> Usually covered in myelin
3) Dendrites
> Highly branched cell body - NOT covered in myelin
> Receive signals from other neurons
Other than neurons, what other cells are found in the CNS?
1) Astrocytes: the most abundant cell type in the mammalian brain. They function as structural cells and are known to play an important role in cell repair, synapse formation, neuronal maturation and plasticity.
2) Oligodendrocytes & Schwann cells: oligodendrocytes are the myelin producing cells of the CNS, whilst Schwann cells perform the same function in the PNS. Each oligodendrocyte cell body sends out numerous projections that form internodes of myelin covering the axons of neurons. Whilst each oligodendrocyte is capable of myelinating a number of axons a Schwann cell only myelinates a single axonal segment.
3) Microglia & Ependyma: microglial cells are specialised cells that are similar to macrophages and they perform immune functions in the CNS. Ependymal cells are epithelial cells that line the fluid filled ventricles regulating the production and movement of cerebrospinal fluid.
What is the resting membrane potential (RMP)?
This is an ionic imbalance between the extracellular fluid and the intracellular fluid of a neuron with an unequal distribution of the major physiological ions. These concentrations are determined by the activities of a variety of membrane bound channels and transporters. The resting membrane potential due almost entirely to the movement of K+ ions out of the cell.
How are the major physiological ions distributed in RMP?
1) Sodium (Na+):
> Intracellular conc. 5-15 mM
> Extracellular conc. 140-155 mM
2) Potassium (K+):
> Intracellular conc. 140-160 mM
> Extracellular conc. 2-5 mM
3) (Ca2+):
> Intracellular conc. ~0.0001 mM
> Extracellular conc. 1-2 mM
4) Chloride (Cl-):
> Intracellular conc. 5-10 mM
> Extracellular conc. 70-140 mM
5) Organic Phosphates (-):
> Intracellular conc. 130 mM
> Extracellular conc. 3 mMWhat is the electromotive force (emf)?
The relative concentrations of the major physiological ions is one of the factors that gives the cell membrane an electromotive force (emf), a potential difference between the inside and the outside of the cell. As the cell membranes are impermeable to the major physiological ions, transportation is regulated by channels & pumps.
How is the RMP of cells calculated?
Conventionally the outside of the cell is referred to as the zero reference point and has a voltage of 0mV, the inside of the cell (in particular the area immediately adjacent to the cell membrane) has a negative membrane potential of around -50 to -90 mV in neurones. Thus neurones are said to have a resting membrane potential (RMP) of around -70 mV.
What is an action potential?
If, the membrane potential becomes more negative, the cell is hyperpolarised. If the membrane potential becomes more positive the cell is depolarised. When a cell is sufficiently depolarised an action potential is generated, where there is a brief depolarisation spike in the membrane potential (to around +10 mV) before returning back to the RMP. This action potential is transmitted along the membrane and axon by means of cable transmission and it is the ability to propagate action potentials, which makes neurons ‘excitable’.
Outline the role of voltage-gated ion channels in action potentials
At resting membrane potential (RMP), voltage-gated Na+ channels (VGSCs) and voltage-gated K+ channels (VGKCs) are closed. Membrane depolarisation leads to a change in the ionic channel configuration, allowing VGSCs to open, causing an Na+ influx and further depolarisation. VGKCs open at a slower rate than VGSCs and lead to repolarisation, due to the efflux of K+ from the cell membrane.
Outline the function of Na+-K+-ATPase (pump)
Action potentials leave a Na+ & K+ imbalance that needs to be restored. This is accomplished using Na+-K+-ATPase (pump), which restores the ion gradients. It does this in 3 steps.
1) Resting configuration: Na+ enters vestibule and upon phosphorylation, Na+ ions are transported through the protein pump.
2) Active configuration: An exchange of energy, using ATP, allows Na+ to be removed from cell and K+ enters the vestibule.
3) Pump returns to resting configuration: This allows K+ to be transported back into the cell.
What is saltatory conduction?
The action potential spreads along the axon by ‘cable transmission’. The myelin sheath on axons prevents the action potential from spreading, due to its high resistance & low capacitance. The Nodes of Ranvier are small gaps of myelin spread intermittently along axons. The action potential can ‘jump’ between nodes by saltatory conduction. However, action potentials are unable to ‘jump’ across the gap at the axon terminal.
What are synapses?
These are the small gaps that exist between two neurones are known as synapses. The synapse itself is a junction consisting of a pre-synaptic nerve terminal (e.g. the axon terminal), which is separated from the postsynaptic cell (e.g. the dendrite of another neurone) by an extracellular space known as the synaptic cleft. Since the electrical signal cannot jump over the synaptic cleft it is converted into a chemical signal to cross the synapse and then back into an electrical signal on the post-synaptic cell.
Outline the process of neurotransmission at the synapse
1) Propagation of the action potential (AP):
> AP is propagated by VGSCs opening
> Na+ influx leads membrane depolarisation and AP ‘moves along’ the neuron
> VGKC opening allows K+ efflux and repolarisation
2) Neurotransmitter (NT) release from vesicles:
> AP opens voltage-gated Ca2+ channels at presynaptic terminal, leading to Ca21 efflux
> Ca2+ binds to vesicles containing the neurotransmitter
> The vesicles bind to the presynaptic membrane and release the neurotransmitter into the synaptic cleft by exocytosis
3) Activation of postsynaptic receptors:
> NT binds to receptors on post-synaptic membrane
> Receptors modulate post-synaptic activity
4) Neurotransmitter reuptake:
> NT dissociates from receptor and can be
1) Metabolised by enzymes (e.g. cholinesterase for acetylcholine) in synaptic cleft
2) Recycled by transporter proteins
Outline communication between neurons
The communication between neurons can be either autocrine or paracrine. Both types of communication involve the use of neurotransmitters.
Outline the 3 types of synapses
1) Axodendritic synapse: connection between presynaptic terminal and the neuronal dendrite
2) Axosomatic synapse: connection between presynaptic terminal and the neuronal soma
3) Axoaxonic synapse: connection between presynaptic terminal and the neuronal axon
What is the neuromuscular junction?
This is a specialised structure incorporating axon terminal & muscle membrane allowing unidirectional paracrine chemical communication between peripheral nerve & muscle.
Outline the process of paracrine communication in the neuromuscular junction
1) Action potential propagated along axon (Na+ & K+), leads to Ca2+ entry at presynaptic terminal.
2) Ca2+ entry causes acetylcholine (ACh) release into synaptic cleft.
2) ACh binds to nicotinic ACh receptors (nAChR) on skeletal muscle and leads to a change in end-plate potential (EPP).
4) A miniature EPP indicates quantal ACh release.
What is the sarcolemma?
This is the skeletal muscle membrane, where nicotinic ACh receptors (nAChR) activation leads to depolarisation and an action potential (AP). T-tubules are continuous with the sarcolemma and are closely connected to sarcoplasmic reticulum, the action potential travels through T-tubules.
What is the sarcoplasmic reticulum?
This surrounds myofibrils, which are contractile units of muscle. The sarcoplasmic reticulum stores Ca2+, only releasing it following sarcolemma depolarisation. The Ca2+ then causes myofibril contraction and muscle contraction.
Outline some disorders of the neuromuscular junction
1) Botulism: Botulinum toxin (BTx) irreversibly disrupts stimulation-induced ACh release from presynaptic nerve terminal.
2) Myasthenia Gravis (MG): an autoimmune disorder where antibodies are directed against ACh receptor. This causes fatigable weakness, meaning that it becomes more pronounced with repetitive use.
3) Lambert-Eaton myastenic syndrome (LEMS): an autoimmune disorder where antibodies are directed against voltage-gated calcium channels (VGCC).
What is the Nernst Equation?
This equation allows the equilibrium potential (E) to be calculated:
E (mV) = (RT/zF)*ln(X2/X1), where:
R = gas constant
T = Temperature in Kelvin (assume 310K)
z = charge on ion (-1 for Cl-, +2 for Ca2+)
F = Faraday’s number - charge per mol of ion
ln or log = natural logarithm (log to base e) or convert to common log
X2 = intracellular ion concentration
X1 = extracellular ion concentration
Why is the Nernst Equation not totally reliable?
The equilibrium potential of potassium (EK) and sodium (ENa) are theoretical values. In reality biological membranes are not uniquely selective for an ion. Membranes have mixed and variable permeability to all ions (but, for neurones at rest K+»_space; Na+). A typical resting membrane potential (Em) is -70 mV and not -90 mV which is EK. Each ion’s contribution to membrane potential is proportional to how permeable the membrane is to the ion at any time.
What is the Goldman-Hodgkin-Katz (GHK) equation?
This equation describes the membrane potential (Em) more accurately; P is permeability or channel open probability (0 = 100% closed, 1 = 100% open, 0.5 = open 50% of time), Subscript on P indicates the ion, [K+], [Na+] and [Cl-] represent concentration and the subscript i or o indicates inside or outside the cell.
Define the different terms describing membrane potential
1) Depolarisation: membrane potential becomes more positive, towards 0 mV.
2) Overshoot: membrane potential becomes positive.
3) Repolarisation: membrane potential becomes more negative, towards the resting membrane.
4) Hyperpolarisation: membrane potential decreases beyond the resting membrane potential.
What are graded potentials?
External stimulus or neurotransmitters causes a change in membrane potential. The change in membrane potential is graded in response to the type (depolarisation or hyperpolarisation) or strength of stimulation. Graded potentials produce the initial change in membrane potential that determines what happens next – they initiate or prevent action potentials. Graded potentials tend to decay from the stimulus site, along the length of the axon, as the charge ‘leaks’ from the axon and the size of the potential change decreases along the axon.
How do action potentials occur?
Action potentials (AP) occur when a graded potential reaches a threshold for the activation (opening) of Na+ channels resulting in an “all-or-nothing” event. These occur in excitable cells (mainly neurons and muscle cells but also in some endocrine tissues). Action potentials play a central role in cell-to-cell communication and can be used to activate intracellular processes.
Outline the permeability of axon membranes
Their permeability depends on conformational state of ion channels. The ion channels are opened by membrane depolarisation, inactivated by sustained depolarisation and closed by membrane hyperpolarisation/repolarisation. When membrane permeability of an ion increases it crosses the membrane down its electrochemical gradient. Movement changes the membrane potential toward the equilibrium potential for that ion.
Outline the 5 phases of the action potential
Phase 1 - Resting membrane potential:
> Permeability for PK > PNa
> Membrane potential nearer equilibrium potential for K+ (-90mV) than that for Na+ (+72mV)
Phase 2 - Depolarising stimulus:
> The stimulus depolarises the membrane potential
> Moves it in the positive direction towards threshold
Phase 3 - Upstroke:
> Starts at threshold potential
> Increase in permeability of Na+ (PNa) because voltage-gated Na+ channels open quickly
> Increase in permeability of K+ (PK) because voltage-gated Na+ channels start to open slowly
> Membrane potential increases toward the Na+ equilibrium potential
Phase 4 - Repolarisation:
> PNa decreases because the voltage-gated Na+ channels close
> PK increases as more voltage-gated K+ channels open and remain open
> Membrane potential decreases toward the K+ equilibrium constant
- At the start of repolarisation is the absolute refractory period, where the Na+ channel activation gate is open and the Na+ inactivation gate is closed. In this period, no new action potential can be triggered.
- Later in repolarisation, the absolute refractory period continues, but both the activation and inactivation gates of Na+ are closed.
Phase 5 - After hyperpolarisation
> At rest voltage-gated K+ channels are still open
> K+ continues to leave the cell down the electrochemical gradient
> Membrane potential moves closer to the K+ equilibrium - some voltage-gated K+ channels then close
> Membrane potential returns to the resting potential
- There is a relative refractory period, where some Na+ channels have recovered from inactivation – allowing the voltage-gated Na+ channels to open. However, a stronger than normal stimulus required to trigger an action potential.
Outline the regenerative relationship between the permeability of sodium (PNa) and the equilibrium potential of the membrane (Em)
Once threshold potential is reached an action potential is triggered. APs are “all-or-nothing” events, so once triggered a full-sized action potential occurs due to positive feedback. Following an action potential, there is a refractory state where the membrane is unresponsive to threshold depolarisation until the voltage-gated Na+ channels recover from inactivation.
What is passive propagation?
Adjacent to the formation of an action potential, is an area of local depolarisation caused by local current flow. The adjacent region then moves from its resting membrane potential and gradually depolarises towards a threshold value. Once the threshold value is reached, voltage-gated sodium channels will open. This leads to the gradual movement of the action potential down the length of the axon. Voltage-gated ion channels are mostly located at the Nodes of Ranvier.
Which factors affect conduction velocity?
Membrane/internal resistance of the axon, the insulation of the axon and the diameter of axon alters propagation distance and velocity. The greater the diameter and insulation of an axon, the slower the potential decays. This is because larger diameter axons have lower resistance, so ions move faster – conduction velocity is proportional to the square root of the axon diameter. For instance, in small diameter, non-myelinated axons, the conduction velocity is 1 m/s, whereas it is 120 m/s in large diameter, myelinated axons.
Which disease states are associated with reduced axon diameter and myelination?
1) Reduced axon diameter: re-growth after injury)
2) Reduced myelination: multiple sclerosis and diphtheria, cold, anoxia, compression and drugs (some anaesthetics block sodium channels).
What is pharmacology?
This is the study of how chemical agents (drugs) can influence the function of living systems. It is better defined as a chemical substance that interacts with a specific target within a biological system to produce a physiologic effect.
What questions must be asked when considering how individual drugs produce their effects?
1) What is the target for the drug?
2) Where is the effect produced?
3) What is the response produced after interaction with this target?
What is the most common use of opioids in medicine?
Opioids (e.g. morphine) are commonly used as analgesics. They target a part of the brain called the peri-aqueductal grey region.
What do opioids interact with?
Opioids bind opioid receptors, which are exogenous compounds. Endorphins are endogenous compounds released by the body that can activate opioid receptors. These opioid receptors exist in many different locations in the brain and throughout the body.
What causes side effects?
They can be produced by drug action:
1) On other targets in the same tissue or other tissues
2) On the same target in other tissues
3) Dependent on the dose of the drug administered
How is the safety of drugs determined?
The “safest” drugs are those where there is a large difference between the dose required to induce the desired effect and the dose requires to induce side effects/adverse effects. For instance, pramipexole, a dopamine receptor agonist, is used to treat Parkinson’s disease. It has a similar structure to dopamine, acting as an agonist, it tries to mimic the effects of dopamine. At low doses, it has this therapeutic effect, but as the dose increases serotonegic side effects are seen, as it is structurally similar enough to serotonin to bind to its receptors. So as dose increases, selectivity tends to decrease.
What is the key component of most drugs?
The majority of drug targets are proteins. There are 4 main classes of drug targets:
1) Receptors
2) Enzymes
3) Transport proteins
4) Ion channels
What are the 4 most commonly prescribed drugs?
1) Atorvastatin (type of statin) - acts on an enzyme (i.e. HMG-CoA reductase)
2) Amlodipine - acts on an ion channel, blocking the calcium ion channel, allowing some vasodilation, reducing blood pressure in hypertension
3) Salbutamol (found in blue asthma inhalers) - binds to a Beta-2 adrenergic receptor in the lung, which bronchodilates
4) Citalopram (selective serotonin reuptake inhibitors; antidepressant ) - acts on a transport protein, by blocking the serotonin reuptake protein
What is drug selectivity?
To be an effective therapeutic agent, a drug must show a high degree of selectivity for a particular drug target (ie the lock and key hypothesis).
Why might selectivity be more important to drugs than endogenous compounds (e.g. dopamine)?
Neurotransmitters are very specifically delivered to their drug target. Endogenous compounds like dopamine, noradrenaline and serotonin are released from the end of nerves that release the neurotransmitter specifically into the synapse that can affect the relevant drug target. Typically drugs are administered orally and gets into the bloodstream,nowhere it can be distributed to the relevant tissue to produce an effect. This means that is can be distributed to any tissue in the body, so needs high specificity so as to not bind to the wrong target.
What are side effects?
A side effect is an effect produced by the drug that is secondary to the intended effect. If that side effect has negative health consequences, then it is also termed an adverse effect. The two terms are often used interchangeably, since most side effects have some sort of negative health consequence from minor (e.g. runny nose) to major (e.g. heart attack).
Outline the diversity of neurotransmitters
Enormous diversity in variety of transmitters and their receptors including Amino acids (e.g. glutamate, γ-aminobutyric acid [GABA], glycine [Gly]), Amines (e.g. noradrenaline [NA] and dopamine [DA]) and Neuropeptides (e.g. opioid peptides). Vary in abundance from nM to mM CNS tissue concentrations. May mediate rapid (µs - ms) or slower effects (secs). Neurons receive multiple transmitter influences which are integrated to produce diverse functional responses.
Outline the steps of neurotransmitter release
Activation of transmitter release is calcium dependent and requires RAPID transduction/electromechanical transduction (200ms), describing the process linking the Ca2+ ion channels opening to the exocytosis of neurotransmitters:
1) Membrane depolarisation
2) Ca2+ channels open
3) Ca2+ influx
4) Vesicle fusion
5) Vesicle exocytosis
6) Transmitter release
How do rapid release rates of neurotransmitters occur?
Synaptic vesicles are filled with neurotransmitter (T) and the influx of calcium causes the vesicles to becomes docked in the synaptic zone on the membrane. The vesicles are then primed to release the neurotransmitter and fuse fuse with the presynaptic membrane. Special proteins (vesicular proteins such as SNARE proteins) on the vesicle and presynaptic membrane enable fusion & exocytosis. Protein complex formation between vesicle, membrane and cytoplasmic proteins to enable both vesicle docking and a rapid response to Ca2+ entry leading to membrane fusion and exocytosis.
What are neurotoxins?
Vesicular proteins are targets for neurotoxins:
1) Alpha latrotoxin (from black widow spider) stimulates transmitter release to depletion
2) Zn2+-dependent endopeptidases inhibit transmitter release
3) Tetanus toxin (C tetani) causes spasms & paralysis
4) BOTULINUM TOXIN (C botulinum) causes flaccid paralysis.
What are the 2 main groups of neurotransmitter receptors?
1) Ion channel-linked receptors: these are fast response (msecs) and they mediate all fast excitatory and inhibitory transmission. Examples include:
> CNS: Glutamate, γ-aminobutyric acid (GABA)
> NMJ: Acetylcholine (ACh) at nicotinic receptors
2) G-protein-coupled receptors: these are slow response (secs/mins). Their effectors may be enzymes (adenyl cyclase, phospholipase C, cGMP-PDE) or channels (e.g. Ca2+ or K+).
> CNS and PNS: ACh at muscarinic receptors, dopamine (DA), noradrenaline (NA), serotonin (5HT) and neuropeptides (e.g. enkephalin).
Outline ion channel-linked receptors
These are rapid activation and rapid information flow (inhibitory or excitatory). Multiple subunit combinations (usually 5 subunits) leads to distinct functional properties. Examples include Nicotinic cholinergic receptors (nAChR), glutamate (GluR), GABA (GABAR), glycine (GlyR) receptors. The glutamate receptor is responsible for mediating depolarisation, excitation of the central nervous system and GABA-A receptor allows Cl- to flow into the cell, leading the the hyperpolarisation of the post synaptic membrane. This is responsible for inhibition in the central nervous system.
Outline different postsynaptic potentials of glutamate receptors
1) Excitatory neurotransmitter receptor: allows an influx of Na+ into the postsynaptic membrane, generating an excitatory postsynaptic potential (EPSP), that causes the membrane to increase then return to normal after 5 milliseconds.
2) Inhibitory neurotransmitter receptor: GABA binds to the receptor, opening a Cl- channel and allowing the Cl- into the postsynaptic membrane. This causes a fall in the membrane potential called inhibitory postsynaptic potential (IPSP).
What are the 2 main types of glutamate receptors?
1) AMPA receptors (α-Amino-3-hydroxy-5-methyl-4-isoxazole propionic acid): these are ion channel-linked receptors that mediate the majority of majority excitatory synapses. They have a rapid onset, offset and desensitisation.
2) NMDA receptors (N-methyl-D aspartate): these are permeable to NA+ and Ca2+ ions. They provide slow component of excitatory transmission and serve as coincidence detectors which underlie learning mechanisms.
Outline the steps that occur at an excitatory glutamate synapse
1) Glutamate synthesised from glucose via TCA cycle & transamination.
2) Glutamate reversibly binds postsynaptic receptors (linked to ion channels).
3) Rapid uptake of glutamate by excitatory amino acid transporters (EAATs).
4) Glutamate enzymatically modified by glutamine synthetase to glutamine in glial cells.
What are seizures?
Seizures are caused by abnormal cell firing, due to excess glutamate in the synapse. Electroencephalography (EEG) measures electrical activity in the brain.
What is epilepsy?
One of the commonest neurological conditions affecting 50 million people worldwide. Characterised by recurrent seizures due to abnormal neuronal excitability. Despite advances in modulating seizure generation and propagation the disease can be disabling. 25-30% refractory to treatment. A new generation of drugs targeting the GABA synapse have proved beneficial.
Outline the steps that occur at an inhibitory GABA synapse
1) GABA synthesised by decarboxylation of glutamate by glutamic acid decarboxylase (GAD).
2) GABA reversibly binds post-synaptic receptors (linked to ion channels).
3) Rapid uptake of GABA by GABA transporters (GATs).
4) GABA enzymatically modified by GABA-transaminase (GABA-T) to succinic semialdehyde (glial cells & GABA nerve terminals).
Outline the importance of the GABA receptor
The pentameric organisation of the GABA receptor provides pharmacologically important binding domains. Different drugs bind to different GABA receptor subunits. Drugs that facilitate GABA transmission are:
2) Antiepileptic
2) Anxiolytic
3) Sedative
4) Muscle relaxant
What is the autonomic nervous system?
It is a sub-division of the peripheral nervous system, not under conscious control. It controls non-skeletal peripheral function, such as: the cardiac muscle (heart), smooth muscle, internal organs and skin.
Outline the arc of the nervous system
1) The 2 main types of afferent neurones, somatic sensory (external environment) and visceral sensory (internal environment), relay information towards the peripheral nervous system (e.g. cranial/spinal nerves).
2) Information enters the spinal chord via the dorsal horn. The information can then be relayed up to the central nervous system (e.g. brain and spinal chord).
3) Information is then relayed from the CNS and PNS to the efferent neurones, somatic motor and visceral motor. The visceral motor neurones form the autonomic nervous system (ANS).
What are the 2 arms of the autonomic nervous system/visceral motor neurones?
1) Parasympathetic nervous system: deals with routine maintenance, tends to control “rest and digest” functions. This system’s pre-ganglionic neurons emerge from the brain stem and the sacral portion of the spinal chord, known as the craniosacral outflow.
2) Sympathetic nervous system: deals with mobilisation and increased metabolism, tends to control “flight or fight” functions. This system has its outflow in thoracolumbar region of the spinal chord, cells emerge only from T1 down to L2.
Both arms often innervate the same tissues and have
opposing/antagonistic effects.
Outline the different effects of the autonomic nervous system in the body
1) Pupil:
> Parasympathetic - constriction
> Sympathetic - dilation
2) Heart:
> Parasympathetic - decreases rate and contractility
> Sympathetic - increases rate and contractility
3) Stomach:
> Parasympathetic - increases motility and secretions
> Sympathetic - decreases motility and secretions
4) Lungs:
> Parasympathetic- bronchoconstrict
> Sympathetic - bronchodilate
5) Liver:
> Parasympathetic - increase bile release
> Sympathetic - increase glucose release
6) Bladder:
> Parasympathetic - contraction
> Sympathetic - relaxation
Which arm of the ANS controls blood vessels tone?
The sympathetic nervous system controls blood vessel tone – both vasoconstriction and vasodilation.
What are the visceral motor and sensory neurones
1) Visceral motor neurones originate in the hypothalamus. These visceral motor neurons project to the brainstem or the spinal cord where they synapse with autonomic neurons (parasympathetic or sympathetic). Outflow to core and body wall and control pupils, sweat glands, salivary glands, heart muscle, airways. Thoracolumbar (T1-L2) and craniosacral outflow (cranial nerves III, VII, IX, X).
2) Relay sensory information from the core (viscera). These nerves detect pain, fullness, blood pressure. These nerves enter the spinal chord via the regions T1-L2, S2-S4 and back via the cranial nerves IX and X.
What are pre- and post-ganglionic neurons?
In general, autonomic neurons consist of two neurons – a pre-ganglionic and a post-ganglionic neuron. A ganglion is a nerve cell cluster or group of nerve cell bodies.
1) PNS: tends to have long pre-ganglionic fibres, but short post-ganglionic fibres. Ganglions close to (or embedded within) effector tissues.
2) SNS: tends to have short pre-ganglionic fibres, but long post-ganglionic fibres. Ganglions close to spinal cord.
Which arm of the ANS innervates the adrenal gland?
This is the exception to the 2 neuron structure of autonomic neurons. It is innervated by the SNS, there is no pre-ganglionic neuron as there is no ganglia.
Which neurotransmitters are released by the ANS?
1) PNS: acetylcholine (ACh) is released by both the pre-ganglion neurons and the post-ganglion neurons.
2) SNS: acetylcholine (ACh) is released by the pre-ganglion neurons and generally noradrenaline (NA) is released by the post ganglion neurons, but there are exceptions (e.g. sweat glands and blood vessels, where acetylcholine is the neurotransmitter released).
What does the adrenal gland release?
The adrenal gland secretes a hormone, not a neurotransmitter. This hormones is adrenaline (and some noradrenaline). It is secreted straight into the bloodstream and not into a synapse.
What is the enteric nervous system?
This is a complex neural network found in the gut, often thought of as a “mini brain”. This is because there is a lot of nervous control of the gut that can occur independent of the brain.
Which arm of the ANS innervates the lungs?
There are parasympathetic nerves innervating the lung tissue, but there are no sympathetic neurons innervating the lung tissue, at all. Although the SNS cannot control the lung via neurotransmitter, it can do it by the release of adrenaline, which causes bronchodilation.
What is the micturition reflex?
The sensory information received in the bladder is the pressure within the bladder as it slowly fills with urine, which is then relayed up to the brain. The information is then relayed back down to the efferent nervous system to the autonomic nervous system, to influence parasympathetic and sympathetic function. The PNS controls the detrusor muscle, which controls the bladder and the SNS controls the internal sphincter. As pressure slowly builds up in the bladder and reaches a certain point, that sensory information is relayed up to the brain. The PNS then contracts the detrusor, to squeeze the bladder and the PNS relaxes the sphincter, allowing urine to leave the bladder. The somatic nervous system also gives a voluntary level of control around urination.
Which receptors are found at all autonomic ganglia?
Ion channel receptor are found on here as they have a fast response (msecs) and mediate all fast excitatory and inhibitory transmission. These are always nicotinic acetylcholine receptors (nACh). Nicotinic acetylcholine receptors are also found within the adrenal gland.
What are Nicotinic acetylcholine receptors?
Nicotinic acetylcholine (nACh) receptors mediate the responses to acetylcholine released from preganglionic fibres at all autonomic ganglia. In addition, they also mediate the response to acetylcholine released by sympathetic nerves innervating the adrenal medulla. Different receptors mediate the effects of neurotransmitters released from postganglionic fibres.
What are muscarinic and adrenal receptors?
1) Muscarinic Ach receptors – respond to Ach release from post-ganglionic PNS fibres.
2) Adrenergic receptors – respond to noradrenaline (NA) release from post-ganglionic SNS fibres or adrenaline via blood.
Both types of receptors are found in the heart, muscarinic receptors slowing down the heart rate and adrenergic receptors speeding up the heart rate.
Outline the biosynthesis and metabolism of acetylcholine
1) Choline + acetyl CoA are enzymatically converted by choline acetyl transferase.
2) Acetylcholine is then packaged into vesicles.
3) An action potential causes Ca2+ influx and exocytosis of the acetylcholine.
4) Receptor activation (Muscarinic or nicotinic)
5) Acetylcholine is rapidly degraded by acetylcholinesterase in the synapse.
Choline is taken up into presynaptic terminal by choline uptake protein.
Outline the biosynthesis and metabolism of noradrenaline
1) Tyrosine is converted to DOPA by tyrosine hydroxylase. DOPA is converted to dopamine by DOPA decarboxlase.
2) Dopamine is then packaged into vesicles with dopamine β hydroxylase, forming noradrenaline as the product.
3) An action potential causes Ca2+ influx and exocytosis of the noradrenaline.
4) Receptor activation (Adrenergic)
5) Removal of neurotransmitter from synapse via uptake into pre-synaptic terminal or glial cell; can be metabolised in the synapse prior to uptake.
Outline the biosynthesis and metabolism of adrenaline in the adrenal gland
1) Tyrosine is converted to DOPA by tyrosine hydroxylase. DOPA is converted to dopamine by DOPA decarboxlase.
2) Dopamine is packaged into vesicles with dopamine β hydroxylase. Noradrenaline is the product.
3) Noradrenaline converted to adrenaline in the cytoplasm by phenylethanol methyl transferase.
4) Action potential causes Ca2+ influx and exocytosis of the adrenaline hormone.
5) Adrenaline diffuses into capillary and is transported to tissues in the blood.
What three structures is the brain composed of?
1) Forebrain, composed of:
> Cerebral hemisphere
> Diencephalon - composed of a large nucleus, the thalamus, inferior to which is the hypothalamus
2) Midbrain
3) Hindbrain, composed of:
> Pons
> Medulla
> CerebellumWhat are the 4 lobes of the brain?
1) Frontal: Regulating and initiating motor function, language, cognitive functions (executive function [e.g. planning], attention, memory).
2) Parietal: Sensation (touch, pain), sensory aspects of language, spatial orientation and self-perception.
3) Temporal: Processing auditory information.
4) Occipital: Processing visual information.
What is the limbic lobe?
This lobe includes the amygdala, hippocampus, mamillary body, and cingulate gyrus. It is concerned with learning, memory, emotion, motivation and reward.
What is the insular cortex (lobe)?
This lies deep within lateral fissure. It is concerned with visceral sensations, autonomic control, and interoception, auditory processing, visual-vestibular integration.
Outline the meninges
The meninges is composed of three layers:
1) Dura mater – thick, composed of 2 layers:
> Periosteal - layer of periosteum
> Meningeal - durable, dense fibrous membrane , peels away from Periosteal layer at various points to form subdural venous sinuses.
2) Arachnoid mater - thin, transparent, fibrous membrane.
3) Pia mater - thin, translucent and mesh-like
What is cerebrospinal fluid (CSF)?
It is produced in the choroid plexuses (modified epithelial cells) of the lateral, 3rd and 4th ventricles. It occupies ventricular system and sub-arachnoid space. There is around 125 ml of CSF and 500 ml, in total, is produced each day. It is reabsorbed via arachnoid villi (granulations) into superior sagittal sinus.
How is cerebrospinal fluid (CSF) different from plasma?
1) Lower pH of 7.33, compared to plasma’s 7.41
2) Less glucose, around 60 mg/dL compared to 90 mg/dL in plasma
3) Fewer proteins, around 0.035 gm/dL compared to 7.0 gm/dL in plasma
4) Same Na+ concentration of 135 mEq/L
5) Lower K+ concentration of 2.8 mEq/L compared to 5 mEq/L in plasma
Outline the anatomy of the spinal chord
1) The Dorsal horn and Ventral horn form part of the grey matter of the spinal chord.
2) Many afferent axons relaying sensory information come into the dorsal horn, via the dorsal roots.
3) The white matter surrounds the grey matter and is largely filled with axons travelling superiority and inferiorly in the spinal chord.
4) Dorsal rootlets and Ventral rootlets are bundles of very small fibres.
5) The main Ventral root takes fibres away from the spinal chord and is found on the anterior aspect of the spinal chord.
6) The mixed spinal nerve is the point at which the dorsal and ventral roots combine, meaning that there are both sensory and motor axons.
7) The dorsal root ganglion contains cell bodies of primary sensory neurones, these are the neurones which have, for example, their endings in skin and joints and convey information about joint position, pain, temperature and general sensation.
Outline the structure of the spinal chord
The spinal chord is curved within the vertebral column and follows the shape of the vertebral column. The spinal chord is composed of different segments/regions – each gives rise to a pair of mixed spinal nerves, meaning one comes out from the left side of the chord and one comes out from the right side of the cord. There are 31 pairs of spinal nerves in total:
> 8 cervical segments
> 12 thoracic segments
> 5 lumbar segments
> 5 sacral segments
> 1 coccygeal segment
The nerves of the segments emerge between adjacent vertebrae, known as intervertebral foramina. The Relationship between nerves emergence and the foramina through which they emerge changes between cervical and thoracic regions:
> Nerves C1-C7 emerge above vertebrae
> Nerves C8-Co1 emerge below vertebrae
What are spinal chord enlargements?
Throughout the spinal chord, the shape changes and the proportion of grey and white matter changes also. Enlargements in the upper part of the chord are:
> Cervical enlargement – increased innervation of upper limbs
> Lumbar enlargement – increased innervation of lower limbs
What is the major descending pathway?
The major descending pathway for voluntary movement is the corticospinal pathway, with both lateral and ventral tracts. It is composed of upper motor neurons in the primary motor cortex and lower motor neurons in the brainstem (innervates head and neck) or the spinal cord (innervates the limbs and the body). About 85% of fibres decussate (cross) to the opposite side of the chord, in the medulla. Only 15% stay in the same side. Tracts are found in both hemispheres and both sets of tracts that form the lateral corticospinal tract have come from the opposite (contralateral) hemisphere. The tracts descending on the anterior side, one on each hand of the spinal chord, stay on the same side as the side that the upper motor neurones originate from.
> Lateral corticospinal tract - generally supplies limb muscles
> Anterior corticospinal tract - generally supplies trunk muscles
What are the major ascending pathways?
The main pathways for sensation are the dorsal column pathway (posterior aspect of the spinal chord) and the spinothalamic tract.
> The dorsal column pathway is for fine touch, vibration and proprioception (position) from the skin and joints.
> The lateral spinothalamic pathway is for pain and temperature.
> The ventral spinothalmic pathway is for crude touch on the skin.
Where is the primary motor cortex found?
The central sulcus runs from about 2/3 of the way back off from the top of the brain to the back, forwards and inferiorly, almost down to the lateral fissure. The gyrus sitting in front of the central sulcus is the pre-central gyrus and the gyrus that sits behind it is the post-central gyrus. The primary motor cortex is found in the pre-central gyrus.
What is somatotopy?
This is when the body is mapped out onto different surfaces of the brain (e.g. on the primary motor cortex), indicating where an electrical stimulus would cause different parts of the body to move. Descending tracts tend to be bundled so that there is still some somatotopic representation. As it descends, the tract forms a structure called the internal capsule, with other nuclei surrounding it.
What is the corticobulbar tract?
This is the pathway that goes from the primary cortex to the muscles of the face. They have upper motor neurones in the primary cortex and lower motor neurones in the cranial cavity, in the brain stem in nuclei of the cranial nerves:
> Oculomotor - extra-ocular muscles
> Trochlear - extra-ocular muscles
> Trigeminal - muscles of mastication
> Abducens - extra-ocular muscles
> Facial - muscles of facial expression
> Hypoglossal - muscles of the tongue
What are the 4 brainstem motor tracts?
These are extra-pyramidal tracts that supply muscles more automated in their responses. They all have their origins in the brainstem.
1) Vestibulospinal - provides information about head movement and position and mediates postural adjustments.
2) Tectospinal - orientation of the head and neck during eye movements.
3) Reticulospinal - control of breathing and emotional motor function.
4) Rubrospinal - innervate lower motor neurons of the upper limb.
Where is the primary somatosensory cortex found?
This is found in the post-central gyrus, which is posterior to the central sulcus of the brain.
What are the 2 principal somatosensory ascending pathways?
1) Dorsal (posterior) column pathway:
> Mechanical - fine discriminative touch, pressure, vibration and proprioception (sense of own positioning)
2) Spinothalamic pathway:
> Mechanical, chemical and thermal - crude touch, pain and temperature
How is the dorsal somatosensory column pathway structured?
Fibres enter the spinal chord via the dorsal horn and enter the ascending dorsal column pathways.
> Information is conveyed from lower limbs and body (below T6) travel ipsilaterally along the gracile tract.
> Information conveyed from upper limbs and body (above T6) travel ipsilaterally along the cuneate tract.
How do fibres travel in the somatosensory pathways?
1) Fibres come into the primary neuron which travels on the same side of the spinal chord and synapses in the medulla:
> First synapse of the gracile tract is in the gracile nucleus.
> First synapse of the cuneate tract is in the cuneate nucleus.
2) Second order axons decussate in the caudal medulla and form the contralateral medial lemniscus tract, and synapse in the thalamus.
3) Third order neurons from the thalamus project to the somatosensory cortex. The size of the somatotopic areas is proportional to density of sensory receptors in that body region (somatosensory homunculus).
Outline the somatosensory spinothalamic (anterolateral) pathway
1) The primary neuron comes into the spinal chord via the dorsal horn and rather than going up the same side, it synapses onto the 2nd order neurons in the spinal chord at about that level that it comes into the spinal chord.
2) The 2nd order neuron then crosses to the other side (contralateral) of the spinal chord, where it synapses onto the 3rd order neuron in the thalamus.
3) The 3rd order neuron travels from the thalamus to the part of the sensory cortex corresponding to that body region.
> Pain and temperature sensations ascend within the lateral spinothalamic tract.
> Crude touch ascends within the anterior spinothalamic tract.
Outline neuron synapse in the dorsal horn
1) Primary afferent axons terminate upon entering the spinal cord.
2) Second order neurons decussate immediately in the spinal cord and form the spinothalamic tract.
Outline the function of somatic nerves
1) Somatic afferent nerves convey information from skin, skeletal muscle and joints.
2) Somatic efferent nerves convey information to skeletal muscles.
Define dermatome
This is an area of skin that is supplied by a single spinal nerve.
Define myotome
This is a group of muscles innervated by a single spinal nerve.
What are visceral (autonomic nerves)?
1) Visceral afferent nerves carry information from the viscera (thoracic, abdominal and pelvic organs).
2) Visceral efferent nerves can be divided into sympathetic and parasympathetic
3) Sympathetic efferent nerves innervate the viscera (organs) and periphery (vasculature and sweat glands).
4) Parasympathetic efferent nerves innervate the viscera (organs) only.
What is the difference between a ganglion, a nucleus and a plexus?
1) A ganglion is a collection of cell bodies outside the CNS.
> All afferent (somatic and visceral) fibres have their cell bodies in spinal ganglia.
> Visceral efferent nerves synapse in a peripheral ganglion.
2) A nucleus is a collection of cell bodies inside the CNS.
3) A plexus is a network of interconnecting nerves.
Outline the structure of visceral nerves
1) Peripheral nerves arranged in fasciculi
> Individual fascicles are covered in perineurium
> Individual axons are covered in endoneurium
2) Three layers of connective tissue
3) External vascular layer - epineurium
Outline the classification of peripheral nerves
There are 2 different classification systems:
1) One based on conduction velocity
> Uses letters A, B and C - with A being the fastest
2) One based on axonal diameter (sensory nerves only)
> Uses Roman numerals I-IV – with I the largest diameter
- Myelinated axons are much larger in diameter and have the fastest conduction velocities (e.g, myelinated motor neurones to skeletal muscle).
- Unmyelinated, thin axons have the slowest conduction velocity (e.g. Postganglionic autonomic fibres; sensory from free nerve endings for pain and temperature; smell).
Outline sensory receptors
These can detect external or internal information.
- They can be classified by source of stimulus:
1) External
> Exteroceptors - pain, temperature, touch and pressure
2) Internal
> Proprioceptors - movement and joint position
> Enteroceptors - movement through the gut and blood pH
They can also be classified by mode of detection:
1) Chemoreceptors - detector molecules which bind to receptor (e.g. in olfactory bulb)
2) Photoreceptors - detect light in the retina
3) Thermoreceptors - detect temperature in the skin
4) Mechanoreceptors - mechanical opening of ion channels (e.g. touch receptors in skin)
5) Nocireceptors - detect tissue damage, interpreted as pain
What are proprioceptors?
These are receptors found in:
1) Muscle spindles - detect changes in muscle length. Muscle spindles are small sensory organs contained within skeletal muscles which detect whether the muscle has been stretched. They are the basis for simple reflex actions.
2) Golgi tendon organs - detect changes in the tension of tendons. The more the muscle contracts against a nine, the more tension that is created in the tendon, and the more impulses are provided back to the spinal chord by the Golgi tendon organs.
3) Joint receptors - found in joint capsules, detect the start and end of movement
These overall give a sense of where the body is in space and the angles of the joints, providing information about whether joints are flexed without needing to look.
What is the neuromuscular junction?
This is a specialised synapse, found between a motor neuron and a muscle fibre.
What is a motor unit?
This is a single motor neuron together with all the muscle fibres that it innervates. It is the smallest functional unit with which to produce force. Humans have approximately 420,000 motor neurons and 250 million skeletal muscle fibres. On average each motor neuron supplies about 600 muscle fibres. Stimulation of one motor unit causes contraction of all the muscle fibres in that unit.
What is a reflex action?
This is an involuntary coordinated pattern of muscle contraction and relaxation elicited by peripheral stimuli.
Outline the reflex arc of the tendon jerk reflex
1) Stretching stimulates the sensory receptor muscle spindle)
2) The sensory neuron is activated, travelling to the spinal chord in the ventral or anterior horn
3) Within the integrating centre (spinal chord), the sensory neuron activates the lower motor neuron
4) The lower motor neuron is then activated, which innervates the same muscle. At the same time, multiple branches of the sensory neurone cause the activation of additional interneurons, in between the afferent and efferent neurons, which cause inhibition of the antagonist muscle, to help with the reflex action.
5) The effector (same muscle) contracts and relieves the stretching
Outline the anatomy of sympathetic outflow to the periphery nervous system
There are no sympathetic nerves that emerge up in the cervical spinal chord or the low lumbar cord. In the thoracolumbar chord, an extra horn of grey matter is found in the middle, called the lateral/anterolateral horn, contains cell bodies of the sympathetic flow. Because they are efferent fibres they emerge from the anterior aspect of the chord and go off into structures called paravertebral trunks or sympathetic ganglia/chains. The sympathetic chains runs either side, outside the protection of the vertebral column and lie at the back of the chest. If the innervation is going out to supply the periphery, the neurons can either:
1) Synapse onto a secondary/post-ganglionic neuron, which can travel out with the mixed spinal nerve to be distributed to the skin that it needs to innervate.
2) Travel down without synapsing to another sympathetic ganglia, whether it will synapse on that post-ganglonic neuron, which will be distributed to skin skin.
What are ramus communicans?
These are small connecting nerves found between the sympathetic ganglia and the main nerves. There are two types: the white (pre-ganglionic neuron has a myelin sheath) and gray ramus (unmyelinated neuron at the mixed spinal nerve).
Outline the anatomy of sympathetic outflow to the heart
Between T1-T4, sympathetic nerves can be seen. The pre-ganglionic neurons go out towards the sympathetic chain, via the white ramus, synapses and sends an unmyelinated post-ganglionic nerve into the heart. The collection of nerves surrounding the heart is the cardiac plexus. Similarly to the sympathetic outflow to the periphery, these neurons also have their synapses in sympathetic chains sitting at either side, contains cell bodies of post-ganglionic neurones. They don’t necessarily synapse at the level that the nerves emerge from the spinal chord.
Outline the anatomy of sympathetic outflow to the viscera
Thoracic spinal nerves have pre-ganglionic neurones which into the sympathetic chains, however, the chains have no synapses. The pre-ganglionic nerves simply travel straight down to the organ via splanchnic nerves. These nerves collect together to form nerves called the greater, lesser and least splanchnic nerves, when they reach the organs. When they get close to the organs, they synapse in ganglia sitting around the aorta, called pre-aortic ganglia, where the cell bodies of the post-ganglion in neurons are found. These then send their fibres out to the organs themselves.
Outline the anatomy of parasympathetic outflow to the viscera
Also known as the craniosacral outflow, this is via some cranial nerves (ie cranial nerves III, VII, IX and X). There are specialised ganglia where the pre-ganglionic neurones synapse with the post ganglionic neurons to go off and do different things such as cause lacrimation (tear production), constrict pupils, saliva secretion via parotid glands or saliva production via the submandibular and sublingual salivary glands. The vagus nerve travels out of the cranial cavity and supplies the heart with parasympathetic outflow and some gut structure. Some of the sacral pre-ganglionic nerves go to some of the pelvic viscera, to carry out functions such as election or activation of pelvic viscera, such as the bladder and rectum.
Which structures form the brainstem?
1) Midbrain
2) Pons
3) Medulla
Outline the structure and function of the brainstem
1) All ascending and descending pathways of the cerebral cortex pass though the brainstem.
2) Many pathways synapse here in relay nuclei or arise directly from or synapse in other brainstem nuclei.
3) The cerebellum is connected to brainstem by 3 cerebellar peduncles (white fibre tracts).
4) The brainstem contains nuclei of 9 of the 12 pairs of cranial nerves – III to XII, except XI, the spinal accessory nerve.
Outline the superior internal aspect of the brainstem
1) The tectum (roof): sits behind the central canal or the ventricular system.
2) The tegmentum: anterior the the tectum.
3) The base: anterior the the tegmentum.
Outline the posterior aspect of the brainstem
1) The pineal gland: a midline situated gland above two pairs of bumps on the back of the midbrain.
2) The superior colliculi: the pair of bumps directly beneath the pineal gland.
3) The inferior colliculi: the pair of bumps directly beneath the superior colliculus.
4) The dorsal column: in the medulla, it farting the pathways for touch and proprioception from the body.
Outline the anterior aspect of the brain stem
1) The optic chiasm: the crossing of some parts of the optic nerves to the contralateral side.
2) Pituitary stalk: posterior to the optic chiasm.
3) Paired mammillary bodies: inferior to the pituitary stalk.
4) Cerebral peduncle: large fibre tracts, inferior to the paired mammillary bodies , that bring pathways down from the cerebral cortex. NOT TO BE CONFUSED WITH CEREBELLAR PEDUNCLES.
5) Medullary pyramids: found on the medulla oblongata
6) Pyramidal decussation: the point at which medullary pyramid fibres cross from one side to the other.
Outline the structure of the midbrain
1) Aqueduct
2) Superior and inferior colliculi
3) Cerebral peduncles: large white fibre tracts that contain corticospinal and corticobulbar tracts, taking voluntary information from the primary motor cortex to the lower motor neurones.
4) Cranial nerves III and IV: the oculomotor and trochlear nerves respectively emerge at this level
Outline the structure of Pons
1) The larger 4th ventricle
2) The three pairs of cerebellar peduncles, called the middle cerebellar peduncle. The superior and inferior cerebellar peduncle are above and below this peduncle, respectively
3) Cranial nerves V, VI, VII and VIII: the trigeminal, abducens, facial and vestibulocochlear nerves, respectively, emerge at this level
Outline the structure of the open (upper) medulla
1) It is known as the open medulla as there is no roof to the 4th ventricle
2) The inferior olivary nucleus
3) Cranial nerves IX, X and XII: the glossopharyngeal, vagus and hypoglossal nerves, respectively, emerge at this level
Outline the structure of the closed (lower) medulla
1) The 4th ventricle tapers down to form the central canal
2) Gracile and cuneate nuclei
3) Dorsal columns: inferior to the gracile and cuneate nuclei, they carry fine touch and proprioception up towards their respective nuclei in the primary somatosensory cortex
4) Decussation of the pyramids: some fibres cross from one side of the medulla to the contralateral side
Outline the structure of the medulla as a whole
1) Considered a rostral continuation of the spinal cord
2) Anterior surface contains pyramids, containing the descending tracts from the cerebral cortex to the spinal chord
3) Posterior surface has dorsal columns
4) The posterior surface can be divided into open and closed regions, depending on whether there is a cerebellum present or not, as well as the nature of the 4th ventricle
5) The closed region of the medulla has an extension of the central canal of the spinal cord - the 4th ventricle tapers down to become the central canal
6) The central canal widens to form the 4th ventricle in the open medulla, with the cerebellum immediately posterior to that
7) The rostral sections is found closest to the top of the medulla and the caudal section closest to the bottom
8) The motor pathways crossing at the caudal section are the corticospinal fibres, which have the upper motor neurone in the motor cortex and the lower motor neurones in the anterior horn of the spinal chord
Where does blood supply to the CNS and brainstem come from?
The blood supply to the brain stem comes from 2 principal anterial supplies: the anterior and posterior. The circle of Willis is supplied via 3 main cerebral arteries, but the supply for those arteries comes from anterior and posterior parts:
1) The anterior part - derived from the vessels of the internal carotid system. This supplies most of the cerebral hemispheres.
2) Posterior part - derived from the vertebrobasilar system, composed of the vertebral arteries which join to form the basilar artery. This supplies most of the brain stem.
Which arteries form the vertebrobasilar system?
From the feet towards the front of the head:
1) Vertebral artery
2) Posterior spinal artery
3) Anterior spinal artery
4) Posterior inferior cerebellar artery
5) Anterior inferior cerebellar artery
6) Basilar artery
7) Superior cerebellar artery
Which arteries for the Circle of Willis?
From the feet towards the front of the head:
1) Posterior cerebral artery
2) Posterior communicating artery
3) Middle cerebral artery
4) Carotid artery
5) Anterior cerebral artery
Which nuclei are found in the brain stem?
Both sensory and motor nuclei are found on both sides of the brain stem:
1) Cranial nerve V: the trigeminal nucleus, has a number of different nuclei that receive sensory information. It therefore has different points within the brain stem where nerves synapse into second order neurones.
How can cranial nerves be grouped?
All cranial nerves can be grouped according to their functional components:
1) General somatic afferent (GSA) - Fibres carry general sensation from skin, muscles, joints of head and neck
2) General somatic efferent (GSE) - Fibres innervate skeletal muscles
3) General visceral afferent (GVA) - Fibres carry sensation from viscera of head, neck, thorax and abdomen
4) General visceral efferent (GVE) - Fibres are the preganglionic parasympathetic neurons to cranial, thoracic and abdominal viscera
What are the additional cranial nerve components in the brain stem?
1) Special somatic afferent (SSA) - Fibres carry special senses of hearing and balance
2) Special visceral afferent (SVA) - Fibres carry taste sensation
3) Special visceral efferent (SVE) - Innervate skeletal muscles of the jaw, face, larynx and pharynx
