Neurobiology

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Neurulation

A.1.1 State the neural tube of embryonic chordates...

Neurulation: Development of a dorsal nerve cord in chordates, which occurs during the third week of embryonic development

  • Neural plate (thickened area of ectoderm cells on the dorsal surface) grows in a cranial-caudal direction
  • The lateral edges of the neural plate elevate, forms neural groove, then separates from the rest of the ectoderm and fuses to form the neural tube
  • Neural crest cells, formed during the closure of the neural tube contribute to the formation of the peripheral nervous system
  • The channel inside the neural tube persists as a narrow canal in the centre of the spinal cord

https://www.youtube.com/watch?v=lGLexQR9xGs

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Spina Bifida

Application: Incomplete closure of the embryonic n...

  • Each vertebrae has a strong centrum that provides support and a thinner vertebral arch, which encloses and protects the spinal cord.
  • Centrum develops on the ventral side of the neural tube at an early stage in embryonic development.
  • Tissue migrates from both sides of the centrum around the neural tube and normally meets up to form the vertebral arch.
  • Spina bifida occurs if the two parts of the arch don't become properly fused together and leave a gap.
  • It is most common in the lower back and varies in severity from very mild with no symptoms to severe and debilitating.

Possibly caused by the embryonic neural tube closing up completely when it is formed from the neural groove ?

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A.1.2 State that neurons are initially produced by

Neuro-ectodermal cells develop in the neural plate, which develops into the neural tube with continued proliferation of cells by mitosis and differentiation along the pathways leading to the cells becoming functioning neurons.

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Skill: Annotation of a diagram of embryonic tissue

Xenopus - (clawed) frog

See notes

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Neurons and axons

A.1.3 Immature neurons migrate to a final location.

Immature neurons = cell body w cytoplasm, nucleus

Mature, functional neurons do not normally move, but their axons/dendrites can regrow if damaged.

A.1.4 An axon grows from each immature neuron in response to chemical stimuli.

Chemical stimuli determine neuron differentiation and the direction of the growing axon in the developing embryo. Many smaller dendrites that bring impulses from other neurons to the cell body may also develop.

A.1.5 Some axons extend beyond the neural tube to reach other parts of the body.

Axons carry impulses to other neurons/cells that act as effectors (muscle/gland cells).

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Synapses

A.1.6 A developing neuron forms multiple synapses.

Synapses develop between neurons and other cells.

A.1.7 Synapses that are not used do not persist.

When transmission occurs at a synapse, chemical markers are left that cause it to be strengthened. Synapses that are inactive do not have these markers - neural pruning

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Neuroplasticity

A.1.8 Describe neural pruning as the loss of unused neurons.

Neural pruning: the elimination of part of a neuron or the whole cell

Newborn babies have more neural connections than adults in some parts of their brains, which suggests some neurons are lost in childhood

Evidence for the removal of dendrite and axon branches = neurons that are not used destroy themselves by apoptosis

A.1.9 Explain how the plasticity of the nervous system allows it to change with experience.

Neuroplasticity describes the lasting ability of the brain to form and lose connections between neurons throughout life. Although, there is a higher degree of plasticity up to age 6

https://www.youtube.com/watch?v=5KLPxDtMqe8

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Strokes

Application: Events such as strokes may promote re...

(Ischemic) stroke: disruption of blood supply to a part of the brain

  • Blockage of blood vessels - lack of blood flow to the affected area
  • Rupture of blood vessels - leakage of blood in affected area

During a stroke part of the brain is deprived of sufficient oxygen and glucose. If cell respiration ceases in neurons ...

https://www.youtube.com/watch?v=gdOHAcaqZjE

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The Brain

A.2.1 The interior part of the neural tube expands to form the brain.
Cephalization: development of a head

Draw and label a diagram of the brain.

(http://www.ib.bioninja.com.au/_Media/brain.jpeg)

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Parts of the brain

A.2.2 Name the different parts of the brain that have specific roles.

Application: Swallowing, breathing and HR as examples of activities coordinated by the medulla (oblongata)

See diagram:
Medulla oblongata
Cerebellum
Hypothalamus
Pituitary gland
Cerebral hemispheres

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Investigating brain function: Animal experiments

Application: Use of animal experiments, autopsy, lesions and fMRI to identify the role of different brain parts.

Animal experiments:

  • Have yielded valuable information but ethical issues concerned due to possible distress
  • Procedures involve removing parts of the skull or carrying out experiments on the brain that result in different behaviours
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Investigating brain function: Lesions (and autopsy

Application: Use of animal experiments, autopsy, lesions and fMRI to identify the role of different brain parts.

Lesions (and autopsy)

  • Injuries to a specific part of the brain, which have provided more direct insights into functioning of the human brain
  • Strokes and accidents that damage just one area of the brain give info about what the area controls
  • People who have injuries to a particular area of the brain can survive, can have their personality altered (e.g. Phineas Gage, frontal lobes)
  • Many lesions due to tumours, strokes or accidents have been investigated by carrying out an autopsy and relating the position of the lesion to observed changes in behaviour and capacities
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Investigating brain function: fMRI

Application: Use of animal experiments, autopsy, lesions and fMRI to identify the role of different brain parts.

fMRI - functional MRI:

  • fMRI scans monitor blood flow to different areas of the brain as a subject carries out different tasks
  • As a region of the brain becomes more active more blood flows to it

1. MRI image of the brain is taken with the subject still

2. Series of lower res scans taken as the subject is exposed to certain stimuli

3. Subjects in fMRI exps asked to remain still in the scanner as they respond to stimuli

4. Scans reveal which areas of the brain are active and help to show how it is working

Can reveal tumours, Parkinson's, brain injury

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Areas of the brain: specific functions

Application: Visual cortex, Broca's area, nucleus accumbens as areas of the brain with specific functions.

Visual cortex (part of the cerebral cortex):
located at the back of the brain, responsible for processing information, including speed and direction of moving objects

Broca's area (part of the left cerebral hemisphere):
responsible for speech production

Nucleus accumbens (one in each hemisphere):
reward centre of the brain, releasing dopamine when stimulated (e.g. through pleasurable food, sex, cocaine)

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Autonomic nervous system

A.2.3 The autonomic nervous system controls involuntary processes in the body using centres located (mainly) in the medulla oblongata/brain stem.

Autonomic nervous system:
sympathetic and parasympathetic, these often have contrary effects on involuntary process

Example:

  • parasympathetic nerves causes an increase in blood flow to the gut wall during digestion/absorption
  • sympathetic nerves cause a decrease in blood flow during fasting or when blood is needed elsewhere
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Cerebral cortex: outer layer of cerebral hemispher

A.2.4 The cerebral cortex forms larger proportion of the brain and is more highly developed in humans than other animals.

  • Only found in mammals

A.2.5 Describe how the human cerebral cortex has become enlarged principally by an increase in total area with extensive folding to accommodate it within the cranium.

  • Enlarged in humans by extensive folding

A.2.6 State that the cerebral hemispheres are responsible for higher order functions

  • Responsible for higher order functions (learning, speech, memory and emotions)
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Left vs right ?

A.2.7 Describe how the left cerebral hemisphere receives sensory input from sensory receptors in the right side of the body and the right side of the visual field in both eyes and vice versa for the right hemisphere.

(in terms of motor neurons)

A.2.8 State that the left cerebral hemisphere controls muscle contraction in the right side of the body and vice versa for the right hemisphere.

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Brain metabolism

A.2.8 State that brain metabolism requires large energy inputs

The brain uses more energy than any other human organ (accounting up to 20% of the body's total input)

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Models

Use models as representations of the real world: the sensory homunculus and motor homunculus are models of the relative space human body parts occupy on the somatosensory cortex and the motor cortex

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Identifying parts of the brain

Skill: Identification of parts of the brain in a photograph, diagram or scan of the brain.

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Pupil reflex

Application: Use of pupil reflex to evaluate brain damage.

Pupil reflex: constriction of the pupils caused by the contraction of circular muscles in the iris, occurs when bright light shines into the eye, rapid reflex action protects the retina from excess light, which could damage it

Brain dead: patient with severe damage to the medulla oblongata is unlikely to recover as this region controls the vital life functions (e.g. breathing, HR)

The pupil reflex is controlled by the brain and not the spinal cord, and is therefore used to determine whether a person is brain dead or not. When light stimulates photoreceptors in the retina, impulses pass along the optic nerve to the medulla oblongata. Circular muscles stimulated to contract, radial muscles relax.

Doctors can test for brain death by shining a light in the patient's eye, testing for pupil reflex. Other reflexes (e.g. blinking, eye movement) can also be tested.

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Correlation between body and brain size

Skill: Analysis of correlations between body size and brain size in different animals

  • Will probably be data based
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Receptors

A.3.1 State that receptors detect changes in the environment

See notes

Sensory cells translating chemical, electromagnetic, and chemical stimuli into action potentials that our nervous system can make sense of

https://www.youtube.com/watch?v=mFm3yA1nslE

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Rods and Cones

A.3.2 State rods and cones are photoreceptors located in the retina

A.3.3 State that rods and cones differ in their sensitivities to light intensities and wavelengths

Compare rod and cone cells

One cone cell per bipolar cell
Several rod cells per bipolar cell
(see ppt)

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Diagram of the human eye

Skill: Labelling a diagram of the structure of the human eye.

Need to do this

http://www.highlands.edu/academics/divisions/scipe/biology/faculty/hargett/2121rev5/eydiqz.htm

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Organise

https://www.youtube.com/watch?v=o0DYP-u1rNM

Frequency determines hue
Amplitude refers to brightness

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Bipolar and ganglion cells

A.3.4 State that bipolar cells send the impulses from rods and cones to ganglion cells.

A.3.5 State that ganglion cells send messages to the brain via the optic nerve

Photoreceptors (rod and cone cells) in the retina convert light into nerve impulses. The impulses pass to bipolar cells, which relay the signal to the optic nerve via ganglion cells.

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Right to left visual

A.3.6 State that the information from the right field of vision from both eyes is sent to the left part of the visual cortex and vice versa.

There are no rod and cone cells where the optic nerve meets the retina, so any light which falls on this 'blind spot' is not perceived - brain fills in the rest

(see ppt)

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The Ear

A.3.7 Explain how structures in the middle ear transmit and amplify sound

A.3.8 State that sensory hairs of the cochlea detect sounds of specific wavelengths

A.3.9 Describe

A.3.10 State

Application: Red-green...

Application: Detection...

Application: Use of...

Skill: (Drawing and) labelling a diagram of the structure of the human eye

Skill: Annotation

Skill: Labelling

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