organization of nervous system Flashcards
What are interneurons?
have short axons and produce direct effects only in their immediate neighbourhood
What apparatus does the cell body of a neuron contain?
The nucleus, Golgi apparatus, ribosomes and mitochondria, nissl bodies
what proportion of the neurons of the cerebral cortex are pyramidal cells?
60%
what are Nissl Bodies?
Rough Endoplasmic reticulum densely packed with ribosomes
What is the purpose of Nissl Bodies?
They allow high rates of protein synthesis.
What is the range of cell body size?
5- 120 micrometers
What are dendrites
Highly branched extensions of the cell body
What is the length of axons?
up to 1 mm
What surface area of the neuron do the dendrites usually constitute?
Up to 90% of the surface area of the neuron
How are the dendrites of a neuron arranged?
They are arranged in the cell-typical pattern called a dendritic tree
What do the dendrites of spiny neurons have?
Dendrites on spiny neurons are covered with hundreds of tiny projections called dendritic spines
What are nerves lacking dendritic spines called?
Aspiny Neurons
what is the site of origin of an axon called?
Axon Hillock
What is the range of diameters of the axons?
0.2 - 20 micrometers in humans
What is the range of lengths of an axon?
from a few micrometers to over a meter.
what are the branches of axons usually called
Axon collaterals
What are the swollen ends of the axon called?
terminals/ boutons
What do the ends of the axons contain?
Mitochondria and vesicles
Some axons have swellings long their length, what are they called?
Varicosities
two differences in the anatomy of an axon vs dendrite
Axons are less highly branched an do not have protein synthetic machinery
How are the proteins made in the cell body transported into and along the axon?
Via axoplasmic transport
Axon terminals are rich in which organelle?
Mitochondria
How can nerve cells be classified?
They are classified by their structure, connections and neurotransmitters
What are neurons with one; two; or more than two neurites called?
Unipolar, bipolar and multipolar neurons respectively.
Where are bipolar neurons found?
In the retina where they synapse with photoreceptors
What are sensory neurons in the dorsal root ganglion called?
Pseudounipolar (start as bipolar cells but two neurites fuse.
Why are some cortical cells called stellate cells?
star-like appearance of dendritic trees
What is the structure of a pyramidal cell
Pyramidal shaped cell body and a dendritic tree
What is unique about Purkinje cells?
Their dendrites form a two-dimensional array
What are examples of PROJECTION neurons
Principal, relay or Gogi type 1, pyramidal, purkinje neurons
What are projection neurons?
Neurons which have axons extending into other regions of the nervous system
Examples of interneurons
local circuit, Golgi type II, stellate cells
What type of neuron does the nervous system receive inputs from?
Afferent neurons
How does the nervous system project to other regions/ effector organs
Via efferent neurons
What neurotransmitter do the pyramidal cells secrete
Glutamate
What are sensory neurons?
afferent neurons capable of responding directly to physical stimuli
What neurotransmitter do stellate and purkinje cells secrete?
y-aminobutyrate
What is the estimated number of neurons in the human brain
86 billion
How many neurons in the cerebral cortex vs cerebellum
16 billion in cerebral cortex, 69 billion in the cerebellum
Overall what is the glia/ neuron ratio in the human brain
close to 1. Glial cells outnumber the neurons almost four-fold in the cerebral cortex and in other regions, glia are heavily outnumbered
How can astrocytes by distinguished from neurons
Astrocytes lack Nissl bodies, and contain specific astrocyte marker molecules.
Astrocytes invest neurons and synapses leaving a gap of just…. across?
20nm
what are end feet?
astrocyte processes which butt onto capillaries or the Pia mater
what is a glial membrane?
layer produced by the endfeet covering the surface of peripheral nerves and CNS
How are astrocytes coupled together
via electrical synapses
What are the 10 functions of astrocytes?
- Controlling potassium concentration
- Uptake or synthesis of precursors of NT
- Terminating actions of small transmitter molecules
- provide neurons with metabolic energy
- detoxification of ammonia and free radicals
- regulates BBB
- regulates synapse formation in the developing brain + production of new neurons in adult brain
- Radial glial cells: guide neurons to proper destinations
- Gliotransmission
What is gliotransmission?
Astrocytes release transmitters under some circumstances (eg glutamate and adenosine) and have a role in information processing. But they are not excitable
How do astrocytes detoxify ammonia?
via the ornithine-arginine cycle
Which cells produce the myelin sheath?
Oligodendrocytes in the CNS and Schwann cells in the PNS
which are the immune system cells?
Microglia.
where are microglia derived from?
Bone marrow monocytes
what is the role of microglia during development?
secrete growth factors, guide axons, stimulate differentiation of other glial cells and the formation of blood vessels. They can transform into macrophages which phagocytose debris generated by programmed cell death
what is the role of microglia in adults?
- cease being mobile unless nervous system damage occurs—> they proliferate, revert to macrophage lifestyle and are responsible for the inflammatory process
What are the subdivisions of the peripheral nervous system?
somatic, autonomic and enteric nervous system
What constitutes the somatic nervous system in humans?
31 Pairs of spinal nerves, each pair arising from a single segment of the spinal cord, and 12 pairs of cranial nerves which come from the brain
What information do the afferent axons of the somatic nervous system carry?
Carry sensory information from skin, muscles, joints and viscera, they are usually wired to mechanoreceptors/ nociceptors/ thermoreceptors
How many cranial nerves are mixed (containing sensory and motor fibres)
4 (5,7,9,10)
Which spinal nerve is not mixed?
C1- motor; Cx 1- sensory
what forms a spinal nerve
Each spinal nerve is formed from a ventral and dorsal root.
where do the cell bodies of the primary afferent neurons lie?
they lie within the dorsal root ganglia just outside the spinal cord.
What constitutes a nerve fibre of a peripheral nerve?
Nerve fiber consists of an axon together with accompanying Schwann cells. several unmyelinated axons are invested by a single glial cell
What is the connective tissue surrounding individual nerve fibres called?
endometrium
What are the bundles of individual nerve fibres In the PNS called?
Fasciculi
What are the nerve fasciculi surrounded by?
Perineurium
What is a nerve?
A nerve is one or several fasciculi all encapsulated bu a connective tissue epineurium
What two classification systems are used to classify PNS axons
The Erlanger and Gasser system is used to classify both afferents and efferents. the Lloyd and Hunt scheme is used exclusively to define afferent axons.
What is the autonomic nervous system?
Visceral motor system. Target tissues are smooth muscle, cardiac muscle, endocrine and exocrine glands, liver, kidney and adipose tissue
What are the synapses of autonomic neurons with their target cells Called?
Neuro-effector junctions
Where are the cell bodies of the preganglionic neurons of ANS situated?
In the motor nuclei of cranial nerves or the intermediolateral horn of the thoracic and lumbar spinal cord
What type of axons are the preganglionic neurons of the ANS?
Myelinated Type B fibres which secrete acetylcholine
where do preganglionic axons of the ANS synapse with with postganglionic neurons?
Autonomic ganglia
What type of axons do the postganglionic neurons have ?
Unmyelinated type C fibers
Where do the sympathetic ganglia lie?
they lie close to the spinal cord in one of two locations:
- Paired paravertebral chains that run parallel to the vertebral column in the neck and down the posterior wall and abdomen
- in subsidiary ganglia of autonomic plexuses adjacent to major blood vessels.
What neurotransmitter do most postganglionic sympathetic neurons secrete?
noradrenaline
what do the cells of the adrenal medulla secrete
adrenaline (in response to the preganglionic sympathetic fibres that supply it.
Where are parasympathetic autonomic ganglia situated?
they are all subsidiary ganglia located close to the target organ
What neurotransmitter do all postganglionic parasympathetic axons secrete?
acetylcholine
Which is the only major organ that does not have parasympathetic supply?
liver
why is the parasympathetic system less extensive than the sympathetic division?
only a few specialised blood vessels have parasympathetic innervation, all blood vessel smooth muscle receives a sympathetic supply.
Where does the Myenteric (Auerbach’s) plexus lie?
Between the longitudinal and circular smooth muscle layers and extends the whole length of the gut
Where does submucosal/ Meissners plexus lie?
in the submucosa and extends from the pylorus of the stomach to the anus
What is the function of the enteric nervous system?
The enteric nervous system acts autonomously to coordinate gut motility and secretion (modified by both divisions of the ANS)
How is the spinal gray matter organised?
10 columns running through the cord can be distinguished on the basis of cell size. On transverse section these appear as REXED LAMINAE.
Which lamina of the spinl cord do nociceptor afferents synapse on?
Lamina II
What neurons do lamina VII and IX house?
preganglionic autonomic neurons and motor neurons, respectively
What are the three main structural components of the brain?
- Tracts or pathways i.e white matter
- Nuclei within the white matter (clusters of neuron cell bodies)
- cortex: thin rind with high density of cell Bodies
When can the division of the brain into hindbrain, midbrain and forebrain be discerned in the human embryo?
by the end of the 4th week
What does the hindbrain consist of?
medulla, pons, cerebellum
what is the dominant part of the telencephalon?
The cerebrum- two cerebral hemispheres linked across the midline by the corpus callosum
How many axons constitute the corpus callosum?
10 to the power of 6
what are gyri?
The folds of the cerebrum
what are the creases between gyri called
sulci
how many cell layers make up the neocortex?
6
What are Brodmann areas?
These are the cortical regions mapped out on the basis of differences in cellular makeup and relative thickness of the layers. Brodman map corresponds quite well to how functions are localised in the cortex
why does the sensory cortex have a thick layer IV?
Because of the large number of thalamic inputs
what are the functions of the cerebral cortex?
most often associated with the planning and execution of intentional movement, sensory perception, and cognitive functions
what are the regions of cerebral cortex not specifically devoted to sensory or motor activities called?
Association cortex
within the core of each hemisphere lie clusters of nuclei forming components of which two neural systems?
The extrapyramidal system and limbic system
what makes up the extrapyramidal system (responsible for organising stereotyped patterns of movement)
basal ganglia: striatum, sub thalamus and substantia nigra
what makes up the limbic system?
Nuclei: amygdala, septal nucleus and mammillary bodies
cingulate cortex
hippocampus
What separates the Pia mater and arachnoid mater
subarachnoid space
What is the space between vessel walls of superficial cerebral vessels and the Pia mater called? and what is continuous with?
The perivascular (Virchow-robin) space; continuous with the subarachnoid space.
Where is the pia mater surrounding the superficial cerebral vessels lost?
at the cerebral capillaries
what are expanded regions of subarachnoid space called?
Cisterns
what are arachnoid villi
Small herniations of arachnoid villi that protrude through the dura into the venous sinuses
How does bulk flow of csf into blood occur at the arachnoid villi?
Via mesothelial tubes in the arachnoid villi that act as one-way valves, closing when the pressure of the venous sinus exceeds that of subarachnoid space.
what is the subdural space?
Potential space between the dura mater and arachnoid mater. It is traversed by cerebral veins entering the venous sinuses in the dura.
What is the flow of CSF from lateral ventricles?
Lateral ventricles –> foramen of munro –> third ventricle –> aqueduct of sylvius –>fourth ventricle –> SAS –> arachnoid villi –> venous sinuses
what makes up the choroid plexus?
Cuboidal epithelium covering core of highly vascular pia mater.
what is the rate of csf secretion in adults?
500 cm(3)per day
steady state volume of csf?
100-150 cm(3), with 30cm (3) contained in ventricles
compared with blood plasma, what is the make-up of csf?
Csf has higher NA, CL and HCO3 concentrations, lower K, urea, glucose and amino acid concentrations. Protein concentration 1000fold lower, higher ionic concentration, same osmolality.
Which pump on choroid epithelium pumps sodium into the csf
NA+, K+ ATP-ase on apical border.
what is the significance of the NA/K/ATPase pump on choroid epithelium?
It generates a sodium gradient that drives two secondary active transport mechanisms bringing NA across basolateral border- NA/ H exchange and a NA/CL symport. CL- influx in turn drives CL/HCO3 anti port.
How is bicarbonate formed intercellularly in choroid plexus?
Formed by hydration of CO2, reaction greatly accelerated by high levels of carbonic anhydrase present in the choroid plexus.
What is the metabolic function of csf?
By equilibrating with brain extracellular fluid, unwanted metabolites (choline, dopamine and serotonin metabolites, urea, creatinine and K) are transported into the blood via villi or choroid plexus.
What are the threee mechanical effects pf csf/ meninges that protect the brain?
- Subarachnoid space is fluid filled compartment in which the brain floats: effective weight of the brain reduced from 1350 - 50 g
- Adjustments to csf and meninges prevent changes in intracranial pressure due to alterations in cerebral blood flow.
- Meninges support brain and the csf reduces the force with which the brain impacts side of cranium when the head moves
What provides the blood-brain barrier?
brain capillary endothelial cells, coupled together by tight junctions
what surrounds brain capillaries and what do they secrete?
astrocyte end feet, secrete growth factors (eg Angiopoietin 1) that promote efficacy of tight junctions.
Where do circumventricular organs lie?
lie on the blood side of the blood brain barrier
how are circumventricular organs separated from the rest of the brain ?
ependymal cells that are coupled together by tight junctions
what is the purpose of circumventricular organs and the lack of BBB.
Allows posterior pituitary to secrete oxytocin and vasopressin directly into systemic circulation, allows brain to measure osmolality, or concentrations of specific ions or molecules for homeostatic functions.
Which substances can passively diffuse across the plasma membranes of endothelial cells?
water, gases which are water or lipid soluble (O2 or volatile general anaesthetics), and lipophilic molecules (steroids)
How is the BBB able to actively exclude a wide range of potentially neurotoxic lipophilic compounds?
through a transport protein in the plasma membrane of endothelial cells, P-glycoprotein.
