Neurodevelopment and neuronal plasticity

Cell birth/ proliferation

this is a massive process- 250000 neurones born a minuet at its peak. neural tube initially one cell thick touching on both ends. as the neural tube widens, the extensions of the cells elongate still holding on to the outer wall.

process of cell birth/ proliferation continued

neurogenesis does not take place with neuronal division- neurones do not divide. immature cells called stem cells divide to form progenitor (precursor) cells. each progenitor cell can be a neuroblast or a glioblast

The brains nursery (Santiago Ramon y Cajal)

early observations noticed that cells undergoing mitosis were always c,,ooser to the inner surface of the neural tube, known as the ventricular zone.

Ventricular system

The nerual tube gives rise to the ventricular system in a mature brain. the lining of our ventricles contain stem cells. an abundance of neurones will be created during early development, more than we will ever have as adults.

Cell Migration

the movement of the newly formed cells towards the outer layers. the cortex develops in an inside-out manner, seen across species. Occurs with the help of chemical signals (immunoglobulins and cytokines) and physical support provided by the raidal glia- cells climb along radial glia with the help of extensions

Rakic et al 2009 cell migration

A primative map predisposes cells born in a certain region to migrate to a certain locatrion of the cortex

How do neuroees migrate

most of themshimmy up glial poles. some migrate tangentially and migrate from areas such as the basal ganglia and amygdala.

extensive migration of young neurones into the infant frontal lobe

a large wave of neurones are still migrating in the frontal cortex after birth. most prominent in the first few month of life (tyoically 3 up to 7). most of these will become inhibitory GABAergic interneurones.

Differentiation and maturation

once they arrive at their destination immature neruones begin to express genes that will allow them to become a particual type of cell. they start to form an axon and dendrites that give them a distinct shape.

dendritic development

dendritic arborization (branching) frowth of dendritic spines

Induction in stage 3

omgoing cell-cell interations via the secreation of chemicals where cells influence the fate of their neighbouring cells- this is induction. if immature cells are removed from a given region, they will be replaced by subsequent neurones that will arrive in this area and aqcuire the same characteristics. because of this ability of immune cells to differentiate into any type of cell based on the area of characteristics (pluripotency) they can be used theraputically to help tackle diseases such as parkinsins. once cells differentiate and mature, they lose this property.

Synaptogenesis and synaptic pruning

synaptogensis guided by variety of cues. the growing end of the axon is called a growth cone (Santiago Ramon y Cajal 1890) which waas characterised as a battering ram endowwed with exquisite chemical sensitivity with rapid ameboid movements. axons extended by adding microtubules to the tip of the axon.

growth cones develop thin extensions known as filopodia.

growth cones are attracted to chemicals released from target sites (Roger Sperry 1944) cell adhesion molecules (CAMs). Tropic molecules.

Synaptogenesis

once successful contact has been made, axon and target induce eachother to construct machinery to help them attatch to one another and to form a synapse

when synapses are formed, sluggish and slow in firing compared t mature brains but get faster with time

majority of synapses take place after birth and continue to rearrange themselves throughout life.

Filopodia advance by adhering to other cells or by sensing their way around

Filopodia can make physical contact with other cells or they can be chemically guided. proteins in membrane serve as receptors that recognise various molecules to which they will adhere to or not.

growth cones detect and select among a wide range of guidance cues

both contact guidance and chemotropism can be either attractive or repulsive to the growth cone.

Synaptic Pruning

Successful synapses are those which are active and thus maintained and strengthened. those not successful are eminimated- synaptic pruning. the determining factor is experience - use it or lose it principle. due to the ability of the brain to constantly form new synapses and prune others there is plasticity.

Synaptic rearrangement- Purves and Hadley 1985

Occurs throughout life and is related to learning or experience

Cell death

first noticed by Viktor Hamburg 1900-2001 but initially dissmissed. saw chick embryos had about 2000 motor neurones shortly after egg was laid but after 1 week intubation number dropped to 12000. now verified within humans

Apoptosis

Programmed cell death- PCD. Conserved process in animals and plants. aptosis causes seperation of fingers in humans and webbing in ducks. when axons reach their targets, they form synapses with several cells there is overabundance. some neurones will have to go as there is more than we will eventually need.

Neural Darwinism

many neurones will not form active synapses and will be eliminated

Apoptosis active

cells that undergo this are expressing genes that enable them to die; death genes (caspases)

Rita Levi-Montalcini

Proteins secreted by target cells promote the survival and growth of neurones- survival signals. Nerve Growth Factor (NGF)

what neurones live and die

there are several such NDF proteins. the family of these factors is named neurotrophic factors. in order to survive and avoid apoptosis, neurones need neurotrophins (growth factors) from its target cells and active communication with other neurones which leads to strengtheming of the synapses

Myelination

process where glia form a fatty sheath that covers the axons of neurones. Myelin speeds up the transmission of neural impulses and allows for its saltatory conduction. first occurs the the spinal cord then the hindbrain midbrain and forebrain. myelination is a slow process that gradually occurs for decades depending on the region. in the cortex it continues until adulthood.

Powell 2006

adolescence is a period of increased synaptic pruning. the prefrontal cortex is still immature while others are better developed such as the limbic system.

Gogtay et al 2004

did brain scans of 4-25 yr olds every 2 years and found that grey matter thickens in childhood but then it gradually begins to thin out. synaptic pruning starts from the back to front by early adulthood. increase in white matter (myelination) which peaks in adulthood. perhaps another phase of use it or lose it. completes earlier in girls than boys. environmental influence is oerly important.

Specter 2001

in songbirds, there is a steady replacement of neurones in the singing area. found a huge pool of labelled cells and many cells were new neurones. every bird no matter the age was creating thousands of them each day.

Bedard and parent 2004

Olfactory epithelium contains cells that continuously divide to provide new olfactory sensory neurones and replace damaged ones

olfactory bulb

cells produced in the subventricular zone SVZ of the lateral ventricles migrate to replace interneurones in the adult olfactory bulb

RMS

the long path of migration towards the olfactory bulb is called the rostral migratory stream (Altman, 1969; curtis et al 2007)

more on RMS

Newborn cells from the SVZ migrate to the olfactory bulb and become interneurones. astrocytes wrap around the mitigating neurones to create a pipeline that keeps them on the right path. this occurs throughout life.

Altman & Das 1965

The granular layer of the denate gyrus of the hippocampus was the first neurogenic area to be discovered. new neurones are created and added to the denate gyrus throughout life

Prefrontal cortex

very few adult born neurones in the cortex which are created in the svz but much is unknown. Neurogenesis can be induced by injury depending on its extent

recovery from injuries

Recovery is better in younger brains than older brains and it is better in the periphery than ot is the brain. mechanisms of recovery involve the branching of dendrites, a process known as collateral sprouting- new branches formed by non damaged axons attatch to vacant spots of dendrites and cell bodies. cells secrete neurotrophons that allow sprouting to occur. synapses form fast in the forst two weeks of recoery

brain adaptions

people who have ben blind since infancy have enhanced tactile and auditory ability. in cases of amblyobia we can intervene eg eyepatch and reinstate good vision. this is neuroplasticity

Sadato et all 1996;1998

researchers asked blind and sighted people to feel braille letters or other items and asked whether they feel the same or different. Pet scans indicated substantial activity in occipital cortex of blind people when doing this task.

Burton et al 2002

fMRI with blind participants showed robust activity i n the visual cortex when reading braille

Weeks et al 2000

Blind people recruit areas usually used for vision to perform auditoy localization tasks

Rosenzweig & Bennet 1980

rats raised in an enriched environment develop a thicker cortex and have increased dendritic branching. Much is the enhancement was due to physical activity increased dendritic branching was correlated with improved ability to learn

Konrad Lorenz 1930s- imprinting

a period where the brain us most sensitive to a specific experience

critical periods

absence of visual stimuli can lead to blindness or lack of exposure to language at an early age may lead to the inability to use language - genie. same for musical development motor skills ect

Bateson 1979

sensitive periods could be conceived of as a brief opening of a window of vulnerability of need and also of opportunity