Nervous System Flashcards
What percentage do Glial cells approximately, constitute to the cellular elements of the nervous system?
90%
What are the several types of Glial cells found in the nervous system?
- Astrocytes
- Microglia
- Oligodendrocytes
- Ependyma
What are Astrocytes?
Astrocytes are star shaped cells that are found in the CNS.
These cells are important in establishing the Blood Brain Barrier this isolates the brain from the blood, which may contain substances that may be damaging to neurons.
What is the blood brain barrier?
The brain receives blood from arteries it sort of acts as a filter keeping harmful substances out.
What are Microglia?
Microglia are phagocytic cells which remove for example damaged neurons and microorganisms.
They form part of an active immune system which is important since antibodies cannot cross the BBB
What are Oligodendrocytes?
Oligodendrocytes form the myelin sheath of CNS neurons.
In the peripheral nervous system this role is performed by Schwann cells.
What are Ependyma?
Ependyma are glial cells which line the ventricles of the brain and form cerebrospinal fluid. (CSF)
What are Neurons
Neurons are the functional cellular element of the nervous system in that they are capable of generating and transmitting action potentials.
What are the 3 functional regions of neurons?
Neurons consist of 3 functional regions:
- Dendritic (input from other neurones or sensory receptors)
- Somatic (Cell body typical of all cells)
- Axonic (Output to other neurons or effectors e.g. muscles and glands.)
Structural classification is based on the projections originating from the cell body
What is the structural classification of neurones?
Unipolar - one projection (e.g. peripheral sensory neurones)
Bipolar - Two projections (e.g. retinal neurons)
Multipolar - many projections (e.g. motor neurones)
Functional classification is based on the direction of action potential movement
What is Functional classification groups?
Afferent (sensory) neurons - Action potential travelling towards the CNS
Efferent (Motor) neurons - Action potentials travelling away from the CNS
Interneurons - Action potentials travelling within the CNS
What does the highly extensive dendritic branching allow?
It allows inputs from other neurons
How many inputs may a typical neuron recieve?
Up to 10 000 inputs
What does a myelin sheath, formed from Schwann cells, increase?
The conduction velocity of an action potential.
What is an axon terminal?
Axons terminate at an axon terminal, which form the pre synaptic regions of a synapse and allows communication with other neurons or structures.
The Structure Of A Neuron
Fig 2.3 PG 14
How can you measure the potential difference (Voltage) across the membrane of an individual neuron?
By placing once microelectrode in the cytoplasm of the cell whilst leaving a second one on the outside of the cell.
What is the potential difference?
At resting state there is a difference in charge across the neurone membrane: the inside of the neurone is more negatively charged than outside.
The difference in charge is called a potential difference.
What is the resting membrane potential?
The resting potential is the potential difference across a neuron’s membrane when it’s at rest. In a resting neuron, the inside is more negatively charged than the outside, with a resting potential of about -70 mV to -80mV
How is the resting membrane potential estabilished?
The RMP is established by 3 factors:
- A difference between the composition of fluids within the neuron (intracellular fluid) and that outside of the neuron (extracellular fluid)
- Differential permeability of the neuronal membrane to different ions found in the ICF and ECF.
- The presence of transport proteins in the membrane which are capable of moving ions across the membrane.
What ions are the ICF and ECF rich in?
The ICF is rich in K+ ions and poor in Na+ ions - concentrations are approximately 140mM and 5mM respectively
In the ECF these concentrations are essentially reversed.
However note that through the presence of other ions in both the ICF and ECF , the number of positive and negative ions in the 2 compartments are equal.
At rest, what is the neuronal membrane permeable to?
K+ ions
What does the presence of K+ leak channels ensure?
It ensures that K+ ions are free to move down their outwardly acting concentration gradient from the ICF to the ECF
As K+ ions move what happens to the charge?
As K+ ions move they take a positive charge with them and leave behind their corresponding associated negative charge. As this happens a potential difference begins to be established.
Why is it that once K+ ions move out it becomes increasingly difficult for subsequent ions to move out?
This is because of the development of an inwardly acting electrical gradient.
At some point, the outwardly acting concentration gradient is equal and opposite to the inwardly acting electrical gradient
What is this called and what happens?
This is called the K+ electrochemical equilibrium.
At this point a potential difference has been established.
The value approximates to but is not identical to the measured resting membrane potential.
What is the principal extracellular ion?
The principal extracellular ion is Na+
There is an inwardly acting Na+ concentration gradient.
However membrane permeability to Na+ is only a hundredth of that to K+
Therefore there is a small movement of Na+ ions which brings a positive charge back into the neuron.
This acts to reduce the value of the potential difference generated by the movement of K+ alone.
Over time the neuron loses K+ and gains Na+
Within the membrane there is a Na+/K+-ATPase Pump
What is its function?
Within the membrane there is a Na+/K+-ATPase Pump which transports 3 Na+ ions out in exchange for 2 K+ ions in.
This restores the concentration gradient of these ions, upon which the resting membrane potential depends.
Since there is an unequal exchange of ions, the pump makes a small contribution to the RMP itself.
What are Action Potentials?
Action potentials are the ‘currency’ of the nervous system.
An action potential is a rapid sequence of changes in the voltage across a membrane.
Action potentials is basically where the inside of the neuron changes from being negatively charged to positively charged and back to negatively charged all within 2ms
What does a typical intracellular recording of an AP look like?
Fig 2.4 PG 16
How can an AP be initiated?
Action potentials may be initiated by activity in other neurons, by stimulation of sensory receptors or experimentally by the application of an electrical or drug induced stimulus.
What are the 3 Phases of an action potential?
An action potential can be divided into 3 phases:
- Depolarization
- Repolarization
- After - Hyperpolarization
What is depolarization?
The depolarization phase takes the interior of the neuron from an RMP of approximately -80mV to a value of +30mV. The mechanism responsible for this is the rapid opening of the membrane Na+ channels and the influx of Na+ ions.
When does depolarization take place?
Depolarization: This is the initial phase of the action potential and starts when the previously resting membrane potential reaches the threshold level at-55mV.
What happens at the peak of the action potential?
(What is the repolarizing phase?)
At the peak of the action potential, the Na+ channels close and a series of K+ channels open. As a result, the membrane potential begins to decrease as K+ ions and thus positive charge leave the neuron.
What is the after - hyperpolarization phase
Compared with the Na+ channels, the K+ channels which open during repolarization are slow to close.
As a result, the loss of K+ ions exceeds that necessary to return the neuron to the RMP.
Therefore there is a temporary drop in membrane potential below the RMP.
What happens at the end of the action potential?
At the end of the AP a neuron has gained Na+ ions and lost K+ ions.
The activity of the Na+/K+-ATPase Pump restores these ions to their original pre - AP location and the overall membrane potential is returned to the RMP.
In order to generate an AP what must the membrane potential reach?
The Threshold
What is the threshold?
The threshold is generally about 15mV above the RMP.
If the RMP reaches this value the Na+ channels open instantaneously and depolarization occurs.
What is the refractory period?
The refractory period is a period of time during which the neuron is either unresponsive to a second stimulus or an increased stimulus intensity is required to generate a second AP.
What is the absolute refractory period?
The absolute refractory period is the time during which a neuron is unresponsive to a second stimulus, regardless of intensity. This period occurs at the same time with the depolarization phase moving into the repolarization phase of the AP
It corresponds to the period when the Na+ channels are inactivated.
What is the inactivation of the Na+ channels?
Inactivation of Na+ channels follows their rapid opening and closure during depolarization.
From being inactivated they become closed and are then able to open again.
What is the relative refractory period?
The relative refractory period corresponds to the depolarizing and after - hyperpolarizing phase of the AP. During this phase a second stimulus, of greater magnitude than that which generated the first AP, may generate a second AP.
How are regions of depolarized membrane affected?
Regions of depolarized membrane depolarize the adjacent region, which depolarizes the next, and so on.
What does the presence of the relative refractory period ensure?
The presence of the relative refractory period ensures that the movement of the AP is unidirectional.
What are the 2 important factors which influence the speed of conduction of the AP?
The 2 important factors that influence the speed of conduction of the AP are the diameter of the axon (the larger the faster) and the presence of a myelin sheath
Where does depolarization occur in myelinated neurones?
In myelinated neurons, depolarization only occurs at the nodes of Ranvier therefore conduction velocity is increased.
What is a synapse?
A synapse is the gap between an axon terminal and another neuron or structure (e.g. muscle cells).
What is the organisation of a synapse?
Figure 2.6 Page 18
How do synapses operate?
Synapses may operate either electrically or chemically.
Chemical neurotransmission is most common.
How do Calcium ions (Ca2+) enter the axon terminal?
Action potentials reach the axon terminal and result in its depolarization.
This facilitates the opening of voltage - gated Ca2+ channels, which allows entry of calcium ions into the axon terminal
What happens if there is a rise in intracellular Ca2+ levels?
The rise in intracellular Ca2+ levels activates a variety of intracellular protein kinases
What does the activation of kinases result in?
The activation of kinases results in the fusion of the vesicle containing the neurotransmitter with the membrane of the axon terminal and release of the neurotransmitter into the synapse.
What is a neurotransmitter?
A neurotransmitter is a chemical messenger that carries signals between neurons, or nerve cells, and other target cells in the body
What does the neurotransmitter bind to which leads to a change in the membrane potential?
The neurotransmitter binds to selective receptors on the post - synaptic membrane and results in a change in membrane potential. These changes may be either excitatory - a depolarizing response caused by the influx of Na+ ions
or
hyperpolarizing - caused by the loss of further K+ ions or the influx of Cl- ions
What happens if changes to the membrane potential are short lived and decrease in size as they move away from the point of initiation?
They are transmitted towards the axon hillock. If by the time they reach the axon hillock, they are sufficient to raise the membrane potential to threshold, an AP will be initiated in the second neurone
What happens after the detachment of the neurotransmitter from its receptor?
The final step in the process is the detachment of the neurotransmitter from its receptor followed by metabolism and inactivation.
Metabolism may occur in the synapse (extracellular) or it may be intracellular following active uptake of the neurotransmitter into neurons or glial cells.
What is the brain enclosed with?
The brain is enclosed within the bones of the skull and is covered by the three further membranes - the pia mater, the arachnoid mater and the dura mater.
There is a space between the pia mater and the arachnoid matter - the subarachnoid space.
What is the cerebrospinal fluid? (CSF)
And where is it located?
The subarachnoid space contains cerebrospinal fluid (CSF).
CSF is produced by the ependymal cells, which line the lateral ventricles of the brain.
It has a number of roles including that of acting as a ‘shock absorber’.
What is the general organization of the nervous system?
Figure 2.7 Page 20
Label a sagittal section of the brain showing its principal components?
Figure 2.8 Page 20
What are the regions of the brainstem?
The brainstem is the most fundamental region of the brain. It is composed of 3 regions:
- The medulla
- The pons
- The midbrain
All neural activity between the brain and the rest of the body passes through the brainstem.
What are ‘centres’ in the medulla?
The medulla contains a number of ‘centres’ (groups of neurons with similar roles) which coordinate cardiovascular and ventilatory activity.
For example, it contains the medullary chemoreceptors, which monitor levels of CO2 and adjust ventilation accordingly.
It contains other centres which regulate swallowing and sneezing amongst other activities.
The medulla is also site of origin of the 5 of the 12 cranial nerves.
What are the cranial nerves?
The cranial nerves are with one exception (the 10th cranial nerve - the vagus ), responsible for the innervations (sensory and motor) of structures in the head and neck.
These nerves are responsible for a number of cranial reflexes (e.g. the papillary reflex - when light is shone on the eye, the pupil reflexly constricts to regulate the amount of light entering the eye).
Innervations - the process of supplying nerves to an organ or part of the body
What are the pyramids on the medulla?
The medulla also contains structures on its surface called the pyramids.
These are triangular - shaped collections of neurons which originate in cortical regions of the brain and terminate in the spinal cord.
These neurons are involved in regulating motor activity - they terminate on and influence motor neurons, which control skeletal muscle.
This arrangement is termed the corticospinal pathway. Fibres within this pathway decussate near the boundary between the medulla and spinal cord.
Decussation means crossing over - hence neurons which have originated in the right motor cortex control the left side of the body and vice versa.
What is the job of the second group of cortical neurons?
There is a second group of cortical neurons which terminate in the medulla - these are termed the corticobulbar pathway.
This pathway is responsible for controlling the motor neurones that innervate structures in the head and neck (e.g. the tongue)
What is the pons?
Situated above the medulla is the pons.
- The pons has centres which are concerned with the regulation of ventilation
- It is the site of origin of a further 4 cranial nerves.
In many respects the role of the pons is broadly similar to that of the medulla.
What is the midbrain?
The uppermost portion of the brainstem is the midbrain
-The midbrain is the site of origin of 3 cranial nerves
- It contains a number of nuclei concerned with motor activity (e.g. the subtantia nigra).
What are the 2 paired regions of the midbrain?
The midbrain contains 2 paired regions known as the inferior and superior colliculi.
The inferior colliculi receive an input from auditory neurons and are responsible for the startle reflexes, which respond to sound.
The superior colliculi receive an input from the visual system and are responsible for eye movements. (e.g. fixation of gaze.)
What is the reticular formation?
Scattered throughout the brainstem is the reticular formation - it represents a functional region rather then anatomical one
It plays a role in a number of aspects of brain function, including those determining the level of arousal and the sleep - wake cycle and also a number of reflexes. (e.g. baroreceptor reflexes)
What is the diencephalon?
The diencephalon is formed from the thalamus and the hypothalamus
What is the role of the thalamus?
The thalamus plays an important role in the processing of all sensory information except olfaction.
It also participates in neural activity related to movement and emotional behaviours.
Olfaction - Olfaction is the sense of smell, or the ability to perceive chemical stimuli in the air
What is the role of the hypothalamus and where is it located?
The hypothalamus lies beneath the thalamus and is involved in a number of important physiological responses including:
- Thermoregulation
- The regulation of eating and drinking
- An interface between the nervous and endocrine systems (A number of hormones are produced here which influence endocrine activity in the pituitary gland)
What is the cerebellum?
The cerebellum, which is Latin for’ little brain’ is located posterior to the brainstem and inferior to the cerebrum.
Despite its relatively small size, this region of the brain is densely packed with neurons - 50% of the neurons within the brain are found here.
What is the function of the cerebellum?
- It acts as a comparator, ensuring that movements directed by the motor cortex actually occur. It does this by comparing feedback from proprioceptors with the plan of movement initiated by the motor cortex
- It is involved in the learning of new motor skills.
What is the cerebrum?
The cerebrum is the largest part of the brain. It is divided into 2 hemispheres (connected at the corpus callosum), which are each divided into 4 lobes:
Parietal
Occipital
Frontal
Temporal
What is the outermost layer of the cerebrum called?
The outermost layer of the cerebrum is termed the cerebral cortex.
This is the grey matter. The cortex is characterized by a highly folded appearance
The peaks of the folds are known as gyri whilst the troughs are known as sulci.
What is Grey matter?
Grey matter is a type of tissue in your brain and spinal cord (central nervous system) that plays a crucial role in allowing you to function normally from day to day.
What is the motor cortex?
The motor cortex is a region of the brain that controls voluntary movement.
What are cortical regions?
Cortical regions are involved in the perception of sensory information, in planning voluntary movement, in language and in sophisticated cognitive functions.
What lies beneath the cortex in the white matter of the cerebrum?
Subcortical nuclei which are extensively interconnected and also interconnect with regions in the thalamus and the brainstem.
Together they form the basal ganglia whose role is in the execution of voluntary movement.
This arrangement is known as extrapyramidal motor system.
What is the spinal cord?
The spinal cord originates from the medulla of the brainstem.
It is about 45cm long and 1.5cm diameter
Originating from the spinal cord, how many spinal nerves are there?
There are 31 pairs of spinal nerves - these branch extensively and ultimately form the peripheral nervous system.
Spinal nerves:
- cervical thoracic
- lumbar
- sacral
- coccygeal
What does cross section of the spinal cord look like
FIG 2.9 PG 23
What is the separation at the level of the spinal cord?
There is separation of the sensory neurone and the motor neurones.
Sensory neurones with an input to the spinal cord arise from a range of sensory receptors e.g. pain, temperature, touch etc
Their dendrites are in the peripheral of the body
Their cell bodies are collected together in the dorsal root ganglia whilst their axons terminate in the grey matter of the spinal cord.
How can the grey matter be divided?
The grey matter can be divided into a number of layers each with a specific function.
For example, layers 1 - 6 in the dorsal horn are where the axons of the sensory neurones terminate
How does motor output leave the spinal cord?
Motor output from the spinal cord leaves via the ventral roots.
The dorsal and ventral roots merge to form spinal nerves, which since they contain both sensory and motor neurones are said to be mixed nerves.
What does the grey matter of the spinal cord act as?
The grey matter of the spinal cord acts as an ‘integrating centre’ for spinal reflexes.
An integrating centre is a region of the brain or spinal cord that receives and interprets sensory information, and then coordinates a response to a stimulus
What is a reflex?
Reflexes are a relatively simple automatic predefined responses to stimuli.
The basic organization of a reflex arc
Receptor - afferent neurone - spinal cord - efferent neuron - effector
What is white matter?
The white matter contains bundles of axons which convey information from the periphery to the brain or from the brain to the periphery. - The ascending and descending tracts respectively
What do the tracts convey?
The tracts convey specific types of information.
The spinocerebellar tract conveys information to the cerebellum about information on the position of joints and tendons etc
What are typical spinal reflexes?
Tendon or myotatic reflexes such as the knee jerk reflex.
What are two types of sensory input?
Somatosensory and special senses
What is somatosensory input?
Somatosensory input arises from within the body and signals such information as temperature, touch, pressure, pain and proprioception (i.e. information about body and position.)
What are special senses?
Special senses are focused in the head and include vision, taste, smell, hearing and vestibular sensation.
What are sensory receptors?
The role of sensory receptors is to transduce sensory information - i.e. to convert one source of energy to another.
Therefore photoreceptors in the eye ‘convert’ light to action potentials.
Sensory receptors can be classified in several ways
What are:
Interoreceptors
Exteroreceptors
Interoreceptors gather information about changes in the internal environment
(e.g. pressure of blood in the arterial system).
Exteroreceptors gather information about the external environment
(e.g. photoreceptors in the retina)
What are mechanoreceptors?
Mechanoreceptors detect changes in mechanical deformation. This may include touch and pressure in the skin, which are detected by structures called Meissner’s corpuscle and Merkel’s disc.
Mechanoreceptors also include the baroreceptors found in the walls of some arteries and which monitor blood pressure.
What are chemoreceptors?
Chemoreceptors detect changes in the composition of the body.
They include chemoreceptors which detect changes in O2 and CO2 and signal appropriate changes to ventilation.
What are thermoreceptors?
Thermoreceptors detect changes in temperature. These receptors are capable of detecting both cold and hot.
What does the term polymodal mean?
Some receptors can be considered as polymodal which means they respond to more then one type of stimulus. A good example of this is Nociceptors which are responsible for detecting pain. They respond to both chemical and mechanical stimuli.
What is the process of transduction?
The process of transduction involves the production of a receptor potential.
transduction is the process of converting a stimulus into an action potential by a sensory receptor. The process involves a number of steps, including:
Stimulus: A chemical or physical stimulus changes the membrane potential of a sensory receptor.
Conversion: The receptor converts the stimulus energy into an electrical signal.
Transmission: The signal is carried from the site of the stimulus to the brain.
Perception: The brain integrates the sensory signals into a meaningful whole.
What is a receptor potential?
A receptor potential is an electrical signal that occurs when a sensory receptor is activated, and is the beginning of all sensory signals
A receptor potential is created when the membrane potential surpasses a threshold level. It’s a depolarizing event that’s caused by an inward flow of current
What is a rapidly adapting receptor?
They show a rapid reduction in their output - thermoreceptors display this type of activity
What is a slowly adapting receptors?
They show a much slower reduction in their activity in response to prolonged stimulation.
In the face of prolonged stimulation, what do sensory receptors display?
They display a reduction in their response. I.e. They are said to adapt.
What is adequate stimulus?
Each receptor responds more favourably to a given stimulus which is sometimes called adequate stimulus.
How does somatosensory information enter the spinal cord?
It enters the spinal cord via the dorsal roots of spinal nerves and is transmitted to the brain via the spinothalamic tracts.
What does the autonomic nervous system do?
Initiates change internally e.g. contraction of gut smooth muscle to aid digestion.
What does motor output from the CNS activate?
It brings about the activation of effectors, producing an overt response.
What does somatic motor output refer to?
The activation of skeletal (voluntary) muscle.
How is the production of motor activities a complex process?
It involves several regions of the brain.
Much human movement is initiated voluntarily, but some aspects occur relatively automatically once initiated (e.g. walking)
Where does the planning of motor activity begin?
The planning of motor activity begins in the premotor cortex. This is the cortex associated with the motor cortex.
This plan, or intention to move is relayed to the motor cortex itself. By itself, the plan to initiate movement is not enough. 2 other brain regions are important the basal ganglia and the cerebellum
What is the role of the cerebellum?
Integrate the timing and coordination of the desired movements.
Why is the basal ganglia required?
The basal ganglia is required to activate the appropriate motor cortical neurons necessary for movement, whilst at the same time inhibiting unwanted motor cortical activity.
How do the cerebellum and basal ganglia feed back to the motor cortex?
Via the thalamus
Finally, what is the necessary output of the motor cortex?
Its relayed to the nuclei in the brainstem which give rise to descending motor pathways - the medial pathways and the lateral pathways.
What is the medial pathway?
The medial pathways are concerned with the maintenance of an appropriate posture, amongst other things
What is the lateral pathways?
The lateral pathways are pathways from the motor cortex to the motor neurons in the spinal cord, which, when activated produce movement in the head and limbs.
What does activation of the autonomic nervous system (ANS) produce?
It produces movement which is not recognised at the level of consciousness and which for the most part is not seen.
What is the ANS?
The ANS is effectively the motor system which produces responses in the viscera - essentially, smooth muscle responses - in order to ensure visceral homeostasis.
What are the 2 divisions of the ANS?
The sympathetic ANS (SANS)
The parasympathetic ANS (PANS).
Generally these 2 have opposing actions
SANS is considered to be the ‘fear, flight, fight’ system whilst PANS is considered to be the ‘rest and digest’ system
What does the pathway between CNS and effectors in the ANS consist of?
2 neuron chain.
What are pre ganglionic neurons?
Pre- ganglionic neurons have their cell bodies in the CNS. Their axons leave via the ventral roots and travel in the spinal nerves.
They leave the spinal cord and synapse with post - ganglionic neurones.
Some pre - ganglionic neurons travel in cranial nerves rather than spinal nerves
What are post - ganglionic neurones?
Post ganglionic neurones have their cell bodies located in peripheral autonomic ganglia, and their axons terminate on effector structures - the viscera and blood vessels
What is the arrangement of neurons in the autonomic nervous system?
Fig 2.1 Pg 28
What do the pre ganglionic neurons of the SANS do?
Pre - ganglionic neurons of the SANS leave the spinal cord via thoracic and lumbar spinal nerves.
They release the neurotransmitter acetylcholine, which stimulates the post - ganglionic neurons.
The post ganglionic neurones release the neurotransmitter noradrenalin onto effector structures.
What do the pre - ganglionic neurons of the PANS do?
The pre ganglionic neurones of the PANS leave the spinal cord via the sacral spinal nerves and also some of the cranial nerves.
They release the neurotransmitter acetylcholine onto the post ganglionic neurons.
The post ganglionic neurons release the neurotransmitter acetylcholine onto effector structures.
Whats the effect of the activation of the SANS on heart rate?
Produces an increase in HR and force of contraction while activation of PANS produces opposite response
This is a dual antagonistic innervation
What is a single innervation?
E.g. vascular smooth muscle in arteries and arterioles.
There is a single innervation from the SANS which when activated produces vasoconstriction
What is dual non - antagonistic innervation?
E.g. the male reproductive system
Activation of SANS is necessary for ejaculation
Activation of PANS is involved in erectile responses of the penis.
