Neurophysiology and Behaviour - OPTION E Flashcards
Neural development
= processes that generate, shape, and reshape the nervous system, from the
earliest stages of embryogenesis to the final years of life.
Simplified breakdown of human fertilization process
zygote –> morula –> (blastocoele) –> blastula,
Non-simplified breakdown of human fertilization process
After fertilization the zygote divides, soon forming an embryo with a cluster of 16-32 cells called a morula.
After the 64-cell stage, this ball develops an inner cavity, called the blastocoele, thus becoming a blastula, and about 7 to 8 days after fertilization, the embryo becomes implanted in the uterine wall.
During the formation of the gastrula some cells of the blastula soon being moving toward the interior of the blastocoele to form distinct layers – mesoderm, ectoderm and the endoderm. The nervous system is derived from the ectoderm—the outermost tissue layer—of the embryo. The notochord (dorsal cord) – appears – eventually becomes vertebrate.
Neurulation =
folding process in vertebrate embryos, which includes the transformation of the neural
plate into the neural tube.
The embryo at this stage is termed the neurula.
Around the third week of gestation, the notochord sends a molecular signal that causes the cells of the ectoderm just above it to thicken into a column called the neural plate. The neural plate begins to invaginates (folds inward) to form the neural groove along the back of the embryo which then closes to form the neural tube. This develops into the brain and the spinal nerve cord.
Spina bifida =
closing of the neural tube = event in the development of the nervous
system –> When the neural tube fails to close correctly, serious birth defects can result.
Spina bifida, which occurs in about 1 of every 1000 births. It is caused by a malformation of the caudal portion of the neural tube. This malformation in turn results in a malformation of the lower vertebrae that
often leaves the spinal cord exposed, makes it vulnerable to injury, and limits use of the legs and feet.
Spina bifida causing a gap higher up the back is more likely to cause paralysis of the lower limbs and mobility difficulties compared with gaps in the middle or at the base of the back. A baby is more likely to have cognitive symptoms if he or she develops hydrocephalus, due to excess spinal fluid in the brain.
Spina bifida appears to be associated with …
Spina bifida appears to be associated with a deficiency of folic acid. This vitamin should be available in sufficient quantities in the pregnant mother’s food, but if her diet is poor or imbalanced, the resulting shortage of folic acid can be serious enough to interfere with the formation of the neural tube.
Development of Neurons and the Central Nervous System
- Cell division in the neural tube produces large numbers of cells that differentiate into neurons.
- Axons grow out of immature neurons in response to chemical stimuli.
- Some axons extend beyond the neural tube to reach other parts of the body.
- Some immature neurons migrate to their final location and become sensory or motor neurons.
- Developing neurons form multiple synapses with other neurons.
- It is a use it or lose it approach – if the neurons and synapses are not being used the neural pruning occurs – this is an example of the plasticity of the nervous system throughout life.
Both the brain and spinal cord develop from the neural tube. As the embryo grows the neural tube lengthens. The anterior develops into the brain, the rest forms the spinal cord. This happens before birth.
Using Animal Models for research - benefits
Some animals commonly used include :
- Caenorhabditis elegans (flatworm),
- Drosophila melanogaster (fruit fly)
- Danio rerio (zebra fish)
- Xenopus laevis (African claw frog)
- Mus musculus (mouse)
Research and experimentation into embryonic development and many aspects of physiology is difficult to do with humans. Model organisms are in vivo models and are widely used to research human disease when human experimentation would be unfeasible or unethical. eg. it may involve damage to the developing embryo or to the adult.
–> made possible by the common descent of all living organisms, and the conservation of
metabolic and developmental pathways and genetics over the course of evolution.
Studying model organisms can be informative, but care must be taken when extrapolating from one organism to another. Animals are
used to build understanding and to research treatments for developmental diseases.
Using Animal Models for research - concerns
Humans have different biochemical pathways and what may work for a mouse or a frog may not work for humans “if you are a mouse with cancer, we can cure you”
ethics of animal experimentation have resulted in scientists finding alternative methods to test drugs, cosmetics and diseases.
Basic overview of the brain
The brain is one of the largest organs in the body.
It is protected by the skull, the meninges (membranous coverings), and the cerebrospinal fluid (CSF).
The brain is the control centre for the body.
Cerebellum:
controls automatic (unconscious) functions such as movement and balance.
It is an ancient part of the brain that we have in common with mammalian brain.
Brain Stem:
the central trunk of the mammalian brain, consisting of the medulla oblongata, pons, and midbrain, and continuing downwards to form the spinal cord. The autonomic nervous system controls involuntary processes in the body using centres located mainly in the brain stem.
Thalamus:
main relay centre. All sensory messages enter here before they are sent to the cerebrum.
Hypothalamus:
maintains homeostasis, such as temperature and blood sugar, and coordinates the nervous
and endocrine systems, controls pituitary gland.
Medulla oblongata:
controls automatic and homeostatic activities, Such as:
- swallowing involving involuntary muscle contraction, and vomiting,
- breathing rate in response to chemoreceptors in blood vessels responding to carbon dioxide levels,
- heart rate altered by stimulating the heart pacemaker nerve (sino atrial node) to increase or decrease the heart contractions.
Pituitary gland:
secretes hormones produced by the hypothalamus, regulating many body functions. Eg. FSH, LH, Growth hormone, Prolactin
Cerebral cortex (cerebrum)
The cerebrum forms a larger proportion of the brain and is more highly developed in humans than other animals. The human cerebral cortex has become enlarged principally by an increase in total area with
extensive folding to accommodate it within the cranium.
Cerebrum is divided into two cerebral hemispheres (left/right).
Left and Right Hemispheres
The two hemispheres communicate information to each other through the corpus callosum (white matter)
right hemisphere controls the muscles on
the left side of the body, while the left hemisphere controls the muscles on the right side of the human body. Because of cross over wiring, damage to one side of the brain affects the opposite side of the body.
In general, the left hemisphere is dominant in language: logic and exact mathematical computations.
The right hemisphere is mainly in charge of spatial abilities, face recognition and processing music and visual imagery.
Hemisphere –> lobes
Each is then divided into four lobes: frontal, occipital, temporal and parietal lobes.
Is involved in complex higher order functions such as memory, emotion, language, reasoning and sensory processing.
Sensory cortex (Somatosensory)
receives sensory inputs especially touch. Left somatosensory cortex receives sensory information from right side of body etc.
Motor cortex
controls voluntary muscle contractions. (Left–right cross over)
Visual cortex
processes visual stimuli by rod and cone receptors in retina of eye. (Left–right cross over). Also includes pattern recognition, speed, direction judgement.
Brocas Area
controls production of speech.
If a person has damage to this they can not put sounds together to create meaning.
Wernickes area
controls understanding of written and spoken language
Nucleus accumbens
deep in the frontal cortex.
It acts as pleasure or reward centres of the brain. Stimuli such as food, sex, good times cause the release of dopamine that give a feeling of wellbeing.
But, drugs such as nicotine, alcohol, cocaine and heroin also cause the release of dopamine in the nucleus accumbens, and in some cases, these drugs cause much more dopamine release than ‘natural,’ non-drug
rewards.
Ventricles:
Cavities containing cerebrospinal fluid, which absorbs shocks and delivers nutrients.
Meninges:
membrane covering which protects the brain hemispheres.
Body size and Brain size = correlation
There is a strong positive correlation between brain and body mass. The higher above the correlation line the larger brain in relation to body mass.
Methods Used to Study the Brain - Animal experiments:
Many experiments are done on animals to determine brain operation. Often primates are used to gain insight into human brain.
The animal is kept alive, its skull is open and various section of the brain are stimulated,
removed, altered etc and the effect on the animal is recorded during and after.
Improved imaging technology is reducing the use of this method in some areas of research.
Methods Used to Study the Brain - Brain damage - Lesions = Autopsies
Damaged areas of the brain called lesions are caused accident, stroke and tumours.
The loss of brain function gives insight into the areas that are controlled by the brain.
Autopsies after death can locate the areas of
damage and lesions.
Brain damage - Lesions
Damaged areas of the brain called lesions are caused accident, stroke and tumours.
Methods Used to Study the Brain - FMRI (functional magnetic resonance imaging)
Active parts of the brain have increased blood flow so a person may be given a stimulus eg. picture of food, and a scan is taken to determine which parts of the brain become active.
Although specific functions can be
attributed to certain areas, brain imagery
shows that some activities are spread in
many areas and that the brain can even
reorganize itself following a disturbance
such as a stroke.
stimulus =
is a change in the environment (internal or external) that is detected by a receptor and elicits a response.
Human sensory receptors are classified as - Mechanoreceptors:
sensory receptors that respond to mechanical pressure.
Eg. Pacinian corpuscles in the skin and hair cells in the cochlea are the most sensitive mechanoreceptors in transducing air pressure waves into sound. eg, muscles, skin
Human sensory receptors are classified as -Chemoreceptors:
sensory receptors that respond to chemicals.
Eg. olfactory receptor neurons in the nose
and taste buds on the tongue. Chemicals in the air are detected by the many different olfactory receptors.
Human sensory receptors are classified as - Thermoreceptors:
sensory receptors that respond to relative changes in temperature.
Warm and cold receptors in the skin play a part in sensing environmental temperature.
Human sensory receptors are classified as - Photoreceptors:
sensory receptors that respond to light,
most commonly referring to a specialized type of neuron found in the retina of vertebrate eyes that is capable of photo transduction
nociceptor =
Damaged tissues release prostaglandins, which stimulate a nociceptor pain
neurons.
The passage of impulses from the pain receptors travel to the sensory areas of the cerebral cortex. The feeling of pain is due to these areas of the cerebral cortex
stimulated by: chemical, thermal, or mechanical event that has the potential to damage body tissue,
Photoreception - The Structure and Function of the Human Eye
The human eye is essentially a three layered structure comprising an outer fibrous layer (the schlera and cornea), a middle vascular layer (the choroids, ciliary body and iris), and inner retina (neurons and photoreceptor cells).
The shape of the eye is maintained by the
fluid filled cavities (aqueous and
vitreous humors), which also assist in
light refraction.
Eye colour is provided by the pigmented
iris. The iris also regulates the entry of
light into the eye through the contraction
of circular and radial muscles.
Vision - two stages (simplified):
formation of the image on the retina and generation and conduction of
nerve impulses.
Vision - expanded explination
When light reaches the retina, it is absorbed by the photosensitive pigments associated with the membranes of the photoreceptor cells (the rods and cones). The pigment molecules are altered by the absorption of light in such a way as to lead to the generation of nerve impulses. It is these impulses that are conducted via nerve fibers in the optic nerve to the visual processing centre of the cerebral cortex.
Vision - Cells of the Retina
Light falls on the pigment epithelium of the retina at the back of the eye. This stimulates the rods and cones. An impulse (action
potential) is generated which moves through the cells to the optic nerve.
Vision - Rods
specialised for vision in dim light.
They sensitive to all visible wavelengths versus three types sensitive to red, blue and green light. The passage of impulses is from a group of rod cells to a single nerve fibre in the optic nerve.
Vision - Cones
specialized for colour vision and high visual acuity.
Cone density and visual acuity are greatest in
the central fovea (rods are absent here). Best in bright light.
Vision - cones - 3 types = trichromatic colour vision.
There are three classes of cones, each with a maximal response in either short (blue), intermediate (green) or long (yellow-green) wavelength light. The yellow-green cone is also sensitive to the red part of the spectrum and is often called the red cone.
The differential responses of the cones to light of three different wavelengths is called trichromatic colour vision.
vision - Colourblindness
inability or decreased ability to see colour or precieve colour differences.
It occurs when one or more of the cone cells are in fewer numbers
The most usual cause is a fault in the development of one or more sets of retinal cones. This type of color blindness is usually a sex-linked condition.
vision - Bipolar cells
in the retina combine the impulses from rod or cone cells and send them on to ganglion cells of the optic nerve.
vision - Ganglion cells
send messages to the brain via the optic nerve.
optic chiasma.
The left and right optic nerves meet at a
structure called the optic chiasma. Here, all
the neurons that are carrying impulses from
the half of the retina nearest to the nose
cross over to the opposite optic nerve.
Beyond the optic chiasma, the neurons continue to the thalamus, where the information is processed. It is then carried to the visual cortex at the back of the brain, where further processing leads to formation of images.
contralateral processing
the left optic nerve carries information from the right half of the field of vision and vice versa. This is called contralateral processing is due to the optic chiasma, where the right brain processes information from the left visual field and vice versa.
The Ear and Hearing (Mechanoreception) - Eardrum:
Eardrum vibrates as sound waves from the air hit and transmits the vibrations to the middle ear.
The Ear and Hearing (Mechanoreception) - Bones of the middle ear:
These small bones called ossicles (anvil, hammer and stirrip) act as levers,
amplifying sounds by 20 times and then transmit vibrations to the oval window.
The Ear and Hearing (Mechanoreception) - Oval Window:
Transmits sound waves to the fluid filling the cochlea which moves the Round window.
Hence waves can move across the fluid in the cochlea.
The Ear and Hearing (Mechanoreception) - Hair cells in the cochlea:
The cochlea is a spiral tube with receptors called hair cells. Sound waves in the
fluid vibrate the hair cells at particular frequencies and wavelengths. This causes the hair cells to send messages via the auditory nerve to the brain.
The Ear and Hearing (Mechanoreception) - Auditory nerve:
Impulses caused by sound perception are transmitted to the brain via the auditory nerve.
