Intro to neuroanatomy and the brain Flashcards
What are the brain and spinal cord surrounded by
Three layers of connective tissue meninges, from the outer layer in is goes dura mater, arachnoid mater and pia mater.
Dura mater
Outermost meningeal layer which forms a sheaf around the spinal cord that extends from the foramen magnum to the lower border of the second sacral (S2) border
Arachnoid matter
Intermediate meningeal layer that is also extended from foramen magnum to the lower border of second sacral (S2) vertebra.
Pia mater
Deepest meningeal layer that is inseparable from the surface of the spinal cord. Below the spinal cord, it continues as a thread-like structure called filum terminale, which is attached to the dorsal aspect of the 1st coccygeal vertebra. Lateral extensions of pia mater are called the denticulate ligaments.
Afferent (sensory) fibres
Convey nerve impulses to the CNS from the sense organs and receptors
Efferent (motor) fibres
Convey nerve impulses from the CNS to the effector organs
Function of nervous system
Sensory function- gathers information from inside and outside the body
Transmission- sends sensory information to the processing area of the brain and spinal cord
Integrative function- processes information to determine the best response
Motor function- sends information to effectors for muscular contraction and glandular secretion
Three main parts of brain
- Forebrain = Telencephalon (cerebrum) and diencephalon (thalamus, hypothalamus)
- Midbrain
- Hindbrain = Pons, Medulla oblongata & Cerebellum
The brainstem
A collective term for the midbrain, pons and medulla oblongata
What surrounds the brain
The three meninges, which are continuous with the spinal cord. The cerebrospinal fluid surrounds the brain and is found in the subarachnoid space.
Cerebrum
Largest part of the brain. Consists of a left and right hemisphere connected by a mass of white matter called the corpus callosum. Each hemisphere has a frontal, temporal, parietal and occipital lobe.
Myelin
Spiral extension of the glial plasma membrane which wraps around the axon.
Process of myelination
- Oligodendrocytes extend processes to interact with the axon of the neuron.
- Once in contact, reorganization of the cytoskeleton occurs to allow lateral and radial expansion so that they wrap around the axon.
- The extension of the processes occurs in the innermost region, closer to the axon.
- In addition to extension, compaction of these multi-layer membrane starts in the outermost layers.
- Ion channels cluster in the region where there is no axon-glial interaction, forming the node of Ranvier.
Myelination on PNS and CNS
In PNS there is one myelin per schwann cell, the schwann cell and axon are very close together. In CNS there are multiple myelin per oligodendrocytes, the oligodendrocyte and axon are far apart.
Nerves
Made of neurons. Have both cranial and spinal nerves that are made of sensor and motor neurons
Structure of spinal nerve
The dorsal root (sensory neurone) and ventral root (motor neurone) separate from the interneuron in the grey matter, they exit via the dorsal and ventral horn respectively. Further along the motor and sensory meet to form the spinal nerve which then divides in the dorsal and ventral ramus. Both ramus contain a mix of sensory and motor nerves.
Division of the nervous system
Contains the peripheral nervous system (PNS) which has 12 cranial nerves and 31 spinal nerves. It contains the central nervous system (CNS) which has the brain and spinal cord
Sympathetic nervous system summary
Prepares the body for emergencies so ‘fight or flight’. Part of the autonomic nervous system
Parasympathetic nervous system summary
Conserves and restores energy ‘rest or digest’. Part of the autonomic nervous system
Somatic nervous system
Controls voluntary movement
Structure of a neuron
Dendrites- receives signals from other cells
Cell body- organises and keeps the cell functional
Cell membrane-protects cells
Nucleus- controls cell
Axon- transfers signal to other cells
Axon hillock- generates impulse in the neuron
Node of ranvier- allows diffusion of ions
Schwann cell- produces the myelin sheath
Myelin sheath-increases signal speed
Axon terminal- forms junction with other cells
Why is myelination useful
Increases nerve impulse speed. The myelinated regions have no ion channels so no action potentials are generated. The action potential now have to jump between the nodes of ranvier which is quicker
Ependymal cell
Simple cuboidal epithelial cells that line fluid filled passageways within the brain and spinal cord. Produce the cerebrial spinal fluid, this helps to maintain pH and acts as a cushion to protect the nervous system.
Microgilial cells
Phagocytes that move through the nervous tissue removing unwanted substances
Main cell types of the brain
Neurones, astrocytes, oligodendrocytes, microglia and ependymal cells
Oligodendrocytes
Cells with sheet like processes that wrap around axons. Acts as the mylin sheath in the CNS, which acts as an insulator, speeding up the nervous transmission.
Astrocyte
Star shaped cells with projections that anchor them to capillaries. They form the blood brain barrier, which isolates the CNS from general circulation. They recycle the neurotransmitter and supply nutrients.
What affects movement of ions
Concentration gradient (chemical potential), electrical potential (net charge) and the presence of specific ion channels
How is resting potential maintained
The sodium-potassium pump, moves 3 sodium ions out of the cell whilst moving two potassium ions into the cell, against their concentration gradient. A leaky K+ channel then lets K+ out of the cell. Overall, this causes intracellular compartment to be more negative then the extracellular space. The resting membrane potential is -70mV.
What is equilibrium potential
The membrane potential that exactly balances the concentration gradient of the ions across the membrane. When the equilibrium potential and membrane potential are close together ions will stop moving even if there is a concentration gradient. Meaning positive ions will not move into a more positive space or down a positive equilibrium potential
Brief summary of generating an action potential
Stimulus -> Depolarisation (cell becomes positive) -> Repolarisation (cell becomes negative) -> Hyperpolarisation (cell becomes more negative) -> resting membrane potential
Depolarisation
A stimulus opens some of the voltage-gated sodium channels. Na+ flows down its concentration gradient into the axon, increasing the positivity inside the cell. The membrane potential reaches the threshold potential (-55mV) and more voltage gated Na+ channels open. This allows more Na+ to move into the axon, depolarising the axon. The inside of the cell is now more positive then the outside.
Repolarisation
The voltage gated Na+ channels close when the membrane potential is positive, Na+ stops moving into the axon. 6. At +40mV the voltage gated K+ channels open, allowing K+ ions to flow down its concentration gradient out of the axon. The axon become more negative as it loses positive charge. This is repolarization of the membrane.
Hyperpolarisation
Voltage gated K+ channels close when the membrane potential reaches -70mV. However, the voltage gated K+ channels close slowly resulting in more K+ lost, the membrane is hyperpolarised. Meaning its slightly more negative then its resting potential. As the sodium/potassium ATPase and the leaky channel continue to work the membrane potential eventually goes back to its resting potential at -70mV
Propagation of an action potential
In response to a signal the soma end of the axon depolarises. The depolarisation spreads down the axon as it jumps between the nodes of ranvier. The first part of the membrane repolarises, because Na+ channels are inactivated and K+ channels have opened, the membrane cant depolarise again. The action potential moves down the axon repeating the process, the depolarisation can move in both directions but not back on itself
How does the arrival of an action potential at a terminal bouton trigger neurotransmitter release
- When the action potential arrives at the terminal axon it causes a change in the membrane potential, causing the voltage gated calcium channels to open.
- This allows calcium ions to enter the axon down a concentration gradient.
- The calcium causes the fusion of synaptic vesicles with the presynaptic membrane.
- The neurotransmitter will be released by exocytosis into the synaptic cleft.
- The neurotransmitter will diffuse across the synaptic cleft to bind to the ligand-gated sodium channels on the post-synaptic membrane.
- This will cause the opening of the channels allowing sodium ions to flow down the concentration gradient into the post synaptic neuron.
Membrane potential
Uneven distribution of ions, resulting in a potential difference across the membrane. Its the electrical potential of the intracellular space takeaway the electrochemical potential of the extracellular space
Why is concentration gradient important in nerve cells
A concentration gradient has to be established for the ions to move across the membrane. The ion channels also have to be open for the ions to move through them. A negative electrical potential will attract positive ions.
Synapses
A region where communication happens between two neurons (a presynaptic neuron and a postsynaptic neuron), they are separated by a synaptic cleft.
Electrical synapse
Two neurons tied together by a protein called a connexon. The connexon is two gap junctions tied together, they form a pore allowing ions to move from pore to another. Signal cant be modulated. Whatever change that happens in the presynaptic neuron i.e. depolarisation also occurs in the post synaptic neuron. Less delay and faster transmission.
Chemical synapse
There is a synaptic cleft between the two neurons. Communication happens through neurotransmitters. They cause more delay and slower transmission of signal, but they can be modulated. As the charge can be different in the presynaptic neuron when compared to the post synaptic neuron.
Neuromuscular junction
Specialised synapse that consists of a presynaptic neuron (motor neuron) and muscle fibre, use acetylcholine as the nuerotransmitter.
Metabotropic receptors
Coupled to a G protein, leads to a signalling cascade so will be slower
Ionotropic receptors
Coupled to an ion channel, opens or closes this ion channel, is faster then metabotropic
Excitatory neurotransmitter
They bind to ligand gated sodium channels, in the post-synaptic neuron causing an increase in membrane potential and leading to depolarisation, as it becomes more positive.
Inhibitory neurotransmitters
eg GABA, they cause hyperpolarisation in the postsynaptic neuron. The bind to receptors opening the chloride channels, causing Cl- to enter the axon. This tends to keep the membrane polarised meaning it is less likely that you will get an action potential. It will become more negative.
Adrenaline (CNS neurotransmitter)
Fight or flight
Norepinephrine/ Noradrenaline (CNS neurotransmitter)
Synthesised from dopamine, has postsynaptic effects depending on the receptor it binds to. All are metabotropic receptors. Involved in fight or flight.
Dopamine (CNS neurotransmitter)
Important in the reward centre and motor control, all of them are metabotropic. D1 receptor family is excitory, the D2 receptor family is inhibitory
Serotonin (CNS neurotransmitter)
Causes happiness, only one ionotropic serotonin receptor, that is 5-HT^3, the rest are metabotropic.
GABA (CNS neurotransmitter)
Calms you down, helps you get to sleep. Most common inhibitory transmitter in the brain, have both receptor subtypes. The inotropic receptors are faster at mediating the effects of glutamate
Glycine (CNS neurotransmitter)
Most common inhibitory transmitter in the spinal cord, have both receptor subtypes but the ionotropic receptors are faster at mediating the effects of glutamate
Acetylcholine (CNS neurotransmitter)
Found at neuromuscular junction, involved in contraction of skeletal muscle as well as learning and memory. The only neurotransmitter released by the preganglionic neuron at the ganglia. Nicotine receptors are ionotropic whilst muscarinic receptors are metabotropic.
Glutamate (CNS neurotransmitter)
Most common excitatory neurotransmitter in the brain and has both receptor subtypes, the ionotropic receptor mediates the effect of glutamate faster
Endorphins (CNS neurotransmitter)
Improves your mood after exercise
Receptors in the automatic nervous system
The cholinergic and adrenergic receptors. The adrenergic separates in alpha and beta, the cholinergic separates into muscarinic and nicotine subtypes. The alpha, beta and muscarinic receptors are metabotropic but the nicotinic receptors are ionotropic. These types of receptors are always after the postsynaptic neuron. The presynaptic neuron always releases ACh which binds to N2 receptors on the postsynaptic neuron.
Adrenergic receptor
Binds to epinephrin and the Norepinephrine. It has two subtypes the alpha and the beta. The alpha separates into alpha one and alpha two, and the beta separates into beta one and beta two. Both beta receptors are attached to a G alpha s protein
Adrenergic beta receptor
When norepinephrine binds to the beta receptors the GalphaS protein is activated which goes on to activate adenyl cyclase (AC), which turns ATP into cAMP. The cAMP will act as the second messenger, amplifying the message downstream
Adrenergic alpha 1 receptor
The alpha 1 receptor is coupled with the G alpha Q protein. When the norepinephrine binds to G alpha 1 receptors, G alpha Q is activated which activates Phospholipase C, which will generate DAG and IP3 from the phospholipid. This will cause stimulation of cellular processes.
Adrenergic alpha 2 receptors
Alpha 2 is coupled with the G alpha I protein. When the norepinephrine binds to G alpha 2 receptors, the G alpha I protein is activated which is inhibitory. This inhibits AC meaning no CAMP is produced, with no activation of the message downstream.
Cholinergic receptors
Separates into the muscarinic and the nicotine subtypes. The muscarinic separates into M1-M5 and the nicotinic separates in to N1 and N2. The N1 receptor is the muscle receptor as it’s found on the membrane of skeletal muscles. The M2 is the neurone receptor as its found on the membranes of the neurones. Binds to acetycholine.
Cholinergic receptors M1, M3, M5
When Ach binds to M1, M3, M5 receptors the G alpha Q protein is activated which activate PLC, converting the membranes phospholipids into IP3 and DAG (diglyceride). Causing stimulation of the cellular processes. Receptor is coupled with G alpha Q
Cholinergic receptors M2, M4
When ACh binds to M2,M4 the G alpha I protein is activated. This inhibitory protein inhibits adenyl cyclase (AC). Meaning no AMP is produced and the message is not amplified meaning no activation of the cellular processes
Cholinergic receptors N1 and N2
Nicotinic receptors are coupled with ion channels, when ACh binds to N1 and N2 the ion channel opens. Meaning the ions flow into the cell down the concentration gradient, leading to depolarisation and skeletol muscle contraction, as it’s the muscle receptor.
What is the adrenergic alpha 1 receptor used for?
Higher affinity for norepinephrine then epinephrine. Found on vascular smooth muscle, contraction causes vasoconstriction
What is the adrenergic alpha 2 receptor used for?
Higher affinity for norepinephrine then epinephrine. Found on the pre-synaptic membrane as feedback control for norepinephrine
What is the adrenergic beta 1 receptor used for?
Equal affinity for norepinephrine and epinephrine. Found on the cardiac myocyte, contraction causes increase in cardiac output. Causes depolarisation of SA node
What is the adrenergic beta 2 receptor used for?
Higher affinity for epinephrine then norepinephrine. Found on the vascular and bronchial smooth muscle, relaxation causes vasodilation and bronchodilation
The auotonomic nervous system
Split into sympathetic and parasympathetic division. Organised in a two neuron model. The preganglionic neuron forms a synapse with the postganglionic neuron which forms a synapse with the effector organ. The synapse between the two neurons is the autonomic ganglion, this is a collection of cell bodies).
Autonomic nervous system neurons
Organised in a two neuron model. The preganglionic neuron forms a synapse with the postganglionic neuron which forms a synapse with the effector organ. The synapse between the two neurons is the autonomic ganglion, this is a collection of cell bodies). Preganglionic neuron only release Ach which binds to the N2 receptor
Sympathetic nervous system lots of info
Fight or flight. It has a thoraco-lumbar origin, from T1-T12 and L1-L2. Normally uses an adrenergic receptor after postsynaptic neuron. The ganglia is located in the sympathetic chain, far away from the effector organ. The postganglionic neuron will release Norepinephrine most of the time. In the sweat glands it releases ACh instead, the receptor on the effector cell will be M3 on the sweat glands. In the adrenal gland you have no postganglionic neuron.
Parasympathetic nervous system (lots)
Rest and digestion. Has a cranio-sacral origin, it is made of 4 cranial nerves which originate from the brainstem and S2-S4. The ganglia is located near or in the target organ. The postganglionic neuron will release Ach to an M2 receptor
How is contraction and relaxation of smooth muscle regulated
It is regulated by the autonomic nervous system and hormones. Regulation depends on location
Oxytocin
Stimulates contraction of uterus
Progesterone
Inhibits contraction of uterus, released in pregnancy
Angiotensin 2
Stimulates vasoconstriction
Epinephrine
Stimulates vasoconstriction but inhibits bronchoconstriction
How the autonomic muscle regulates the heart muscle
Parasympathetic response- slows rate
Sympathetic response- increases rate
How the autonomic muscle regulates vascular smooth muscle
Parasympathetic response- N/A
Sympathetic response- constriction or dilation
How the autonomic muscle regulates the salivary gland
Parasympathetic response- watery secretion
Sympathetic response- mucus and enzymes
How the autonomic muscle regulates the adrenal medulla
Parasympathetic response- N/A
Sympathetic response- release of epinephrine and norepinephrine
How the autonomic muscle regulates the lungs
Parasympathetic response- bronchoconstriction
Sympathetic response- bronchodilation
How the autonomic muscle regulates pupils
Parasympathetic response- constricts
Sympathetic response- dilates
