A.1: Neural Development

Animal Models in Neuroscience Neuroscience is all about understanding how the nervous system works and is formed from embryo to adults. Understanding can lead to the treatment of various diseases of the nervous system. However, because our nervous systems are quite complex and because of ethical reasons, neuroscientists often study the nervous systems of animals

Animal Models Flatworm – they mature quickly and have a low, fixed number of cells as an adult Fruit Fly – breed easily, mature quickly, and only 4 chromosomes Zebrafish – tissues practically transparent

Animal Models American clawed frog – large eggs, easily manipulated House mouse – shares many human diseases

Development of the Neural Tube Neurulation: the process in which chordates* develop a dorsal nerve cord in the early stages of development. For humans, it occurs during the 1st month of pregnancy. *Chordates: a phylum that humans belong to

Neurulation (p 514) An area of the ectoderm develops into the neural plate The neural plate folds inwards forming a groove along the back of the embryo and separates from the rest of the ectoderm This is the neural tube that elongates as the embryo grows The space inside will be the centre of the spinal cord

Neurulation in Xenopus Annotate a diagram of embryonic tissues in Xenopus laevis (African clawed frog) See page 515

Spina bifida A condition caused by the incomplete closure of the embryonic neural tube. It usually occurs in the lower back. Can be mild (with no apparent symptoms – severe (with debilitating symptoms that can lower ability to walk, result in problems with bladder and bowel control)

Spina Bifida

Development of Neurons There are billions of neurons in your body. As the neural tube is produced, cells undergo mitosis and differentiation occurs leading to cells turning into neurons. Proliferation continues as the spinal cord and brain develop.

Development of Neurons MYTH: You are born with all the neurons you will ever have. You do not produce more neurons after birth Cell division stops in most parts of the nervous system before birth, however, there are may parts of the brain where new neurons are produced during adulthood!

Migration of Neurons Migration of neurons in important in brain development. Some neurons are produced in one parts of the developing brain need to migrate to another part. The cytoplasm and organelles in it are moved from one end of the neuron to anther (similar to how an Amoeba moves with contractile actin filaments) https://www.youtube.com/watch?v=ZRF-gKZHINk

Migration of Neurons Mature, functional neurons do not normally move, though their axons and dendrites may regrow if damaged

Development of Axons An immature neurons contains a cell body with a cytoplasm and nucleus. An axon is a long extension of the cytoplasm from the cell body which carries nerve impulses to another neuron. Only one axon develops on each neuron (though it can be highly branched at the end plate)

Development of Axons Chemical stimuli determine neuron differentiation. (Dendrites are shorter and numerous and bring impulses from other neurons to the cell body)

Growth of Axons Axons grow at their tips Within the CNS they are short and make connections between other neurons in the CNS Some neurons within the PNS develop very long axons (1m+ in humans) and take impulses to other neurons or effector cells (muscles or glands)

Axons can me many meters long in some animals

Axons As long as the cell body of the neuron remains intact, the axon may be able to regrow if damaged or severed outside the CNS. Can be as rapid as 5 mm /day Control of muscles and return of sensation may occur with time after the initial injury.

Development of Synapses A developing neuron forms multiple synapses. The minimum is 2 (one to bring impulses in at the dendrites, the other to pass them on at the axon end plate) Most neurons develop multiple synapses (some in the brain develop hundreds) allowing for complex patterns of neural communication.

Development of Synapses

Development of Synapses Synapse development involves special structures being assembled in the membranes on either side of the synapse and in the synaptic cleft between them

Elimination of Synapses Synapses often disappear if they are not used. When synaptic transmission occurs, chemical markers are left that cause the synapse to be strengthened. (ie: increasing the number of receptors on post synaptic membrane) Inactive synapses will not have these markers and will become weaker and are eventually eliminated.

Neural Pruning Neural Pruning: the elimination of a neuron (or part of it) There are more neurons in some parts of newborn babies brains than in adults. This indicates that some neurons are lost during childhood. Neurons that are not used destroy themselves by apoptosis.

Did you know? New born babies have ~11.2 million neurons in the thalamus Adult humans have ~6.43 in the thalamus

Plasticity of the Nervous System The nervous system changes with experiences Connections between neurons can be changed by growth of axons and dendrites Formation of new synapses Pruning of dendrites, axons, or whole neurons

Plasticity of the Nervous System Plasticity continues throughout life, but much higher from birth to age 6. Plasticity occurs because of experiences and the way the nervous is utilized by the individual It’s the basis for forming new memories and reasoning.

Strokes During a stroke, part of the brain is deprived of sufficient oxygen and glucose. Neurons may become irreparably damaged and die. Minor strokes may be hardly noticed. Of major strokes, 1/3 will full recover, 1/3 survive with a disability

Strokes Recovery from strokes will happen within the 6 months following a stroke It involves parts of the brain taking on new functions to make up for the damaged areas. New synapses form In this way, a stroke patient may relearn aspects of speech, writing, motor skills, etc.