A.1 Neural development

Question 1

Neural tube

Answer 1

a hollow structure from which the brain and spinal cord form. It runs the length of the dorsal side of the body.

Question 2

Key to neural development are the processes of

Answer 2

cell division and differentiation, which respectively allow the growth of the organism and an increase in complexity as relatively unspecialised cells become increasingly specialised, e.g. ectoderm becomes the neural tube which contains cell that eventually differentiate to become neurons, once formed the neural tube elongates to match the growth of the embryonic organism.

Question 3

Developmental neuroscience

Answer 3

aims to discover how nervous systems are formed during the development of adults from embryos.

Experimentation is essential and, due to the often invasive or damaging nature of the investigations, it is unethical to perform many experiments in humans.

Animal models are useful in research for several key reasons:
Less ethical concerns
Nervous systems less complex – easier to observe
Development is quicker
External development of the embryo – easier to observe

Question 4

process of neurulation in Xenopus

Answer 4

1. Ectoderm tissue differentiates to form the neural plate and neural plate border. The Notocord is dervied from the mesoderm tissue.
2. The neural plate folds inwards and downwards. The notocord is ‘pushed’ downwards by the folding. Eventually the neural plate borders meet to form the neural crest.
3. The closure of the neural tube separates the neural crest from the ectoderm. The neural crest cells will develop (differentiate) to form the majority of the peripheral nervous system, e.g. the ganglions seen in the second image.
4. Mesoderm differentiate to become Somites, which will eventually give rise to parts of the skeleton and muscle systems, including the vertebrae.
   The neural tube will eventually form the brain and spinal cord.
   The Notocord degenerates to eventually become the intervertebral discs.

Question 5

Spina Bifida – a neural tube defect

Answer 5

Spina bifida is caused by a gap in the neural tube: the gap arises if infolding of part of the neural plate, to form the neural tube during neurulation, is incomplete.

Spina bifida can occur at any location along the spine and symptoms can vary from none to severe and debilitating.

Question 6

Migration of immature neurons

Answer 6

This is a distinctive feature of the development of the nervous system.

Neural migration can occur by use of contractile actin filaments moving the cell and it’s organelles in a given direction.

Migration is particularly important in brain development. Mature neurons do not move, but regrowth can occur if dendrites or axons become damaged.

Question 7

Axon growth in immature neurons

Answer 7

Axons grow out from each immature neuron. In some cases the axon grows out of the neural tube to other parts of the embryo, e.g. a motor neuron needs to connect to muscle fibres. An axon in humans maybe up to 1m long and longer in larger animals.

Question 8

Axons

Answer 8

long, narrow outgrowths from the cell body. They carry the impulse from neuron to neuron.

Question 9

Chemical stimuli

Answer 9

determine the direction and length of the axon, i.e. differentiation neurons, during the growth of the embryo.

Question 10

Connections

Answer 10

between neurons are highly branched: neurons commonly possess multiple dendrites for receiving impulses from different neurons and multiple terminal ends for passing signals to different neurons.

Question 11

Formation of synapses

Answer 11

When a growing axon reaches the cells it is intended to connect with a synapses are developed; the synapses consists of specialised membranes from the two cells separated by a narrow gap (cleft).

Most neurons develop multiple synapses at both the initiating dendrites and terminal ends. This allows complex patterns of communication.

Question 12

Synapse

Answer 12

structure that permits a neuron to pass an electrical or chemical signal to another neuron or effector cell (e.g. muscle or gland).

Question 13

Neural pruning

Answer 13

the process of synapse elimination, by retraction of the axons from unwanted synaptic connections.

Question 14

Apoptosis

Answer 14

of neurons involves the killing of the cell and elimination of all connections associated with the neurons.

Question 15

Many synapses are formed during

Answer 15

fetal and early childhood development, but new synapses can be formed at any stage of life.

Question 16

“use it or lose it”:

Answer 16

When a synapse is used it is reinforced and unused synapses weaken. Synapses that remain unused are eventually pruned by the retraction of the axon.

Question 17

Neuroplasticity

Answer 17

a term that describes change to the brain throughout an individual’s life course.

Neuroplastic change can occur at small scales, e.g. changes to individual neurons, large whole-brain scale, e.g. in response to injury brain function may move to a new area of the brain.

Question 18

Growth of axons and dendrites is as much a part of neuroplasticity as

Answer 18

pruning and apoptosis. Re-growth of axons can be up to 5 mm per day.

Question 19

Neuroplasticity is seen throughout the life of an organism, but

Answer 19

brain show a much higher degree of plasticity during early childhood.

Question 20

Brain injuries and neuroplasticity

Answer 20

Conditions in the environment, such as social interactions, can play a role in brain cell survival and the formation of synapses.

In response to injury*, or disease, rearrangements in brain function involve changes in the connection between linked neurons.

Undamaged axons can ‘sprout’ nerve endings and connect with damaged cells to repair damage or with undamaged nerve cells to re-route neural pathways and brain function.

For example:
Each brain hemisphere has it’s own tasks, if one brain hemisphere is damaged, the other hemisphere sometimes can take over the functions of the damaged one (e.g. hand control).
The brain can also respond to a lack of sensory input by enhancing the processing of other sensory inputs, e.g. blind individuals use areas of the visual cortex to process hearing or touch.

Question 21

A.2 The human brain