The Nervouse System Flashcards
The two parts of the nervous system
• The nervous system consists of the brain, the spinal cord and the peripheral nerves that run throughout the body. It is usually viewed as comprising two parts.
• The first part, called the central nervous system (or CNS), consists of the brain and spinal cord.
• The second part, called the peripheral nervous system (or PNS), consists of the peripheral nerves that exit the spinal cord to the rest of the body
The spinal cord
• The spinal cord is a long bundle of nerves that extends from the base of the brain down the middle of the
back
• Information from the peripheral nerves enters the spinal cord and then travels up to the brain. Similarly, information from the brain may travel down the spinal cord to the peripheral nerves
Grey matter and white matter
• Grey matter, is home to neural cell bodies, axon terminals, and dendrites, as well as all nerve synapses.
• The white matter of your brain and spinal cord is composed of bundles of axons. These axons are coated with myelin
• In diseases such as multiple sclerosis the immune system attacks the Schwann cells which destroys the myelin sheath and the cell bodies
Peripheral nerves
• Peripheral nerves have two important functions.
• First, peripheral nerves respond to sensory stimuli, including those that are painful and carry sensory information from all over the body to the central nervous system.
• Second, some peripheral nerves carry information from the central nervous system back to the rest of the body
The difference
• Your sympathetic and parasympathetic nervous systems have opposite roles.
• While your sympathetic nervous system carries signals that put your body’s systems on alert, your parasympathetic carries signals that relax those systems.
• The two systems work together to keep your body in balance
Neurons
• Neurones are specialised cells of the nervous system which carry electrical impulses around the body. There are different types of neurones but have the same basic features
• Neurones have a long fibre known as an axon
• They have a cell body that contains the nucleus and other cellular structures
• The end of the axon, known as the axon terminal, contains many nerve endings
• The nerve endings at the axon terminal allow neurones to connect to many other neurones which receive impulses from the axon terminals; this forms a network for easy communication
• Some neurones are myelinated, their axon is insulated by a myelin sheath (Schwann cells) with small uninsulated sections along its length (called nodes of Ranvier)
Types of neuron
• Sensory neurons – transmit nerve impulses from a receptor to an intermediate or motor neuron . They have one dendron that is often very long that carries the impulse towards the cell body and one axon that carries it away from the cell body
• Motor neurons – transmit nerve impulses away from an intermediate or relay neuron to an effector such as a gland or muscle. They have a long axon and many short dendrites
• Intermediate or relay neurons – transmit impulses between neurons – sensory to motor is an example. They have many one axon but many dendrites
The structure and function of the neuron
• Cell body – contains all the organelles including the nucleus and a large amount of rough ER to produce proteins and neurotransmitters
• Dendrons – extension of the cell body which subdivide into smaller branched fibres call dendrites that carry nerve impulses towards the cell body.
• Axon- the long single fibre that carries nerve impulses away from the cell body towards the axion terminal
• Schwann cells – surround the axon providing protection and electrical insulation. They are wrapped in layers around the axon
• Myelin sheath – forms a covering to the axon and is made up on the membranes of the Schwann cells . Membranes are made of myelin which is rich in lipids – neurons with this sheath are known and myelinated neurones – white matter
• Nodes of Ranvier – constrictions between adjacent Schwann cells where there is no myelin sheath. They are 2-3um long and occur every 1-3mm in humans
Summary of the structure
• A nerve is a bundle of axons carrying information. For example, the optic nerve is a bundle of axons all carrying visual sensory information from the eye in the form of electrical energy.
• You may also have heard the term ‘nerve impulse’. This refers to the electrical signal that is transmitted by the neuron along its axon and allows rapid transmission of information all around the nervous system. This is more properly known as an ‘action potential’
Receptor cells
Receptor cells - these act as transducers – converting sensory information into electrical energy for the
specific stimulus they detect
Which is which?
Light energy - electrical energy
kinetic energy - electrical energy
chemical changes -electrical energy
The nerve impulse
• A nerve impulse may be defined as a ‘self propagating wave of electrical activity that travels along an axon membrane’.
• It is a temporary reversal of the electrical potential difference across the axon membrane.
• The reversal is between two states
• The resting potential
• The action potential
The resting potential
• This is the resting state of the neuron when it is not being stimulated
• Due to the uneven distribution of sodium ions (Na+) and potassium ions (K+) there is a difference in voltage between the inside and outside of the axon. It is polarised.
• The inside is slightly more negative than the outside membrane potential of a resting neuron is by convention given a negative value, typically around 65-70 millivolts (mV). -70mV
An action potential (AP)
• An AP is only triggered if the membrane potential at the axon hillock reaches a certain threshold, typically around –55 mV.
• The membrane potential has to become ‘less negative’, moving from a resting membrane potential of –70 mV to the less negative value of –55 mV before the action potential is triggered by the energy of the stimulus.
• If the membrane potential does not reach -55mV then the nerve doesn’t transmit the signal so is called a failed initiation
The mechanism of the action potential is divided into 2 phases:
• Depolarization
• Repolarisation
Summary of stages
When the neuron is at resting potential there is more Na+ outside the neuron and more K+ inside. The membrane potential is -70 mV. For an action potential to be triggered the membrane potential must reach a threshold value of -55 mV
• The first phase of the action potential is called depolarisation. During this phase voltage-sensitive Na+ channels open and Na+ floods into the neuron, down its concentration gradient. This causes a change in the membrane potential to +30 mV i.e. the inside now becomes more positive than the outside.
• The second phase of the action potential is called repolarisation. During this phase voltage-sensitive K+ channels open and K+ moves out of the neuron, down its concentration gradient but causes an overshoot (hyperpolarisation) where the neuron cannot fire again (refectory period)
• A sodium-potassium pump restores the original distribution of ions so that the neuron is ready to produce another action potential.
Passage of an action potential
• The action potential moves rapidly along the axon as a travelling wave of depolarisation followed by a wave of repolarisation before another action potential can be created.
• The process is slightly different in myelinated and non myelinated neurons due to the action of the presence of the myelin sheath so we just need to know how these both work.
• Myelinated axons use a method called saltatory conduction
Saltatory conduction
• The myelin sheath as an insulator which prevents action formation as ions cannot be pumped in from the extracellular fluid through the sheath.
• At the nodes of Ranvier action potentials can occur so the action potentials occur here and jump to the next node – as a result the action potential travels faster in myelinated neurons.
• In a unmyelinated neuron the depolarisation takes place all along the axon so travel at speeds of 0.5 to 10 m/s, myelinated axons can conduct at velocities up to 150 m/s
Synapses
• The junction where one neuron can chemically signal to another neuron is known as a synapse.
• The neuron sending the chemical signal is referred to as the presynaptic neuron and the neuron receiving the chemical signal is referred to as the postsynaptic neuron.
• Normally, it is the axon terminal of the presynaptic neuron that ‘synapses with’ the dendrites of the postsynaptic neuron
Synapses 02
• As each action potential arrives at the axon terminal of the presynaptic neuron it triggers the release of a small amount of specific signalling chemicals from the terminals – neurotransmitters - because they transmit signals across the synapse between neurons.
• The neurotransmitter diffuses across the synapse where it may attach to proteins called receptors in the cell membrane of the postsynaptic neuron. If enough neurotransmitters of the right type attach to
receptors on the postsynaptic neuron, a new action potential may be triggered in this neuron
Neurotransmitters – excite or inhibit
• When a neurotransmitter binds to receptors on the postsynaptic neuron it can either excite or inhibit the postsynaptic neuron.
• If the neuron is excited it is more likely to produce an action potential.
• If it is inhibited it is less likely to produce an action potential, if an action potential is not produced, the signal essentially stops at this point.
Examples: Glutamate and GABA.
• Glutamate is an excitatory neurotransmitter, which means that when it binds to a neuron it will make that neuron more likely to produce an action potential.
• GABA (Gamma Amino Butyric Acid) is an inhibitory neurotransmitter, which means that when it binds to a neuron it will make that neuron less likely to produce an action potential
Neurotransmitters and behaviours
• In depression, medications called selective serotonin reuptake inhibitors (SSRIs) can be used to boost serotonin levels by stopping the body from reabsorbing serotonin, leaving more serotonin to pass messages between nerve cells. This can help some people manage symptoms alongside other treatments such as CBT
• Neurotransmitters contribute to nearly every function in the human body. An appropriate balance of neurotransmitters can help prevent certain health conditions, such as depression, anxiety, and Parkinson’s disease.
What is Parkinson’s ?
• Parkinson’s disease is a progressive disorder that is caused by degeneration of nerve cells in the part of the brain called the substantia nigra, which controls movement.
• These nerve cells die or become impaired, losing the ability to produce an important chemical called dopamine.
• Dopamine is neurotransmitter.
• Neurotransmitters move across the synapse to transmit an action potential to the next neuron
Dopamine
• Dopamine operates in a delicate balance with other neurotransmitters to help coordinate the millions of nerve and muscle cells involved in movement.
• Without enough dopamine, this balance is disrupted, resulting in tremor (trembling in the hands, arms, legs and jaw); rigidity (stiffness of the limbs); slowness of movement; and impaired balance and coordination – the hallmark symptoms of Parkinson’s
Cell death in the SN
• The cause of cell death of the dopamine producing neurons in the SN is caused by the accumulation of a protein - Alpha-synuclein
• This protein clumps together inside the cell into Lewy Bodies within cells that take up space and disrupt cellular function and causes cell death.
• As a result not enough dopamine is produced and this leads to the development of physical symptoms as well as depression and anxiety to hallucinations, memory problems and dementia can also develop
Treatments – drugs
• Parkinson’s drugs can have a range of effects to relieve symptoms but also cause side effects:
• increase the amount of dopamine in the brain - Levodopa
• act as a dopamine substitute, stimulating the parts of the brain where dopamine works. Dopamine Agonists
• block the action of other factors (enzymes) that break down dopamine in the blood to reduce the level lost in the blood and tissues – (COMT and MOA-Bs)
• Anticholinergic medications that reduce the difference in levels between neurotransmitters and relieve symptoms
Levodopa - The gold standard of Parkinson’s therapy
• Levodopa works by crossing the blood-brain barrier, where it is converted into dopamine to increase levels in the brain.
• It is now combined with an enzyme inhibitor called carbidopa. The addition of carbidopa prevents levodopa from being metabolized in the anterointernal tract, liver and other tissues, allowing more of it to reach the brain. Smaller doses – less side effects
• For most patients, levodopa reduces the symptoms of slowness, stiffness and tremor. It is especially effective for patients that have a loss of spontaneous movement and muscle rigidity.
• This medication, however, does not stop or slow the progression of the disease
Surgery - Deep Brain Stimulation (DBS)
• Small electrodes which are implanted to provide an electrical impulse to the deep parts of the brain involved in motor function.
• The electrodes are usually placed on one side of the brain. An electrode implanted in the left side of the brain will control the symptoms on the right side of the body and vice versa. Some patients may need to have stimulators implanted on both sides of the brain.
• This form of stimulation helps rebalance the control messages in the brain, thereby suppressing tremor.
• DBS of the subthalamic nucleus or globus pallidus may be effective in treating all of the primary motor features of Parkinson’s and may allow for significant decreases in medication doses
Supportive therapies
• Physiotherapist – help with posture and movement problems.
• Speech and language therapist - help with swallowing problems, speech and writing.
• Occupational therapist - everyday tasks if they become difficult, such as moving around your home.
• Complimentary therapies – little evidence but can be helpful for some
Impacts on body systems and mental health
• Difficulties with the muscular-skeletal system
• Difficulties with chewing, swallowing and speech
• Difficulties with digestion
• Difficulties with mental health and memory
• Difficulties with the cardiovascular system
