Class Test Revision BHE PDF

**Class test revision** Cornea- transparent layer at the front of the eye which refracts light as it enters the eye Conjunctiva- thin film at the front of the eye which provides defence from infection and foreign bodies Pupil- aperture at in the centre of the eye which allows light to enter. The...

**Class test revision** Cornea- transparent layer at the front of the eye which refracts light as it enters the eye Conjunctiva- thin film at the front of the eye which provides defence from infection and foreign bodies Pupil- aperture at in the centre of the eye which allows light to enter. The pupil constricts in bright light and dilates in dark settings to regulate the amount of light entering Iris- band of pigmented muscle surrounding the pupil. Contains the muscle which dilates and constricts the pupil Choroid- layer of blood vessels beneath the sclera which provide nourishment to the retina Ciliary muscles- muscles attached to the lens which control its shape during accommodation Ciliary body produces aqueous humour Lens- biconvex tissue which refracts light onto the retina. Can change shape to view objects in distance or up close Sclera- tough and opaque layer of tissue on the exterior of the eye which provides shape Aqueous Humour- watery liquid found in the anterior chamber between the cornea and the iris. Supplies nutrients to the lens and cornea Vitreous humour- gelatinous liquid found in the vitreous chamber, maintains shape and keeps the retina attached to the choroid Retina- layer at the back of the eye, contains photoreceptors rods and cones. Receives light and converts it to a signal which is sent to the brain along the optic nerve Fovea- area of high cone density in the retina responsible for high-definition colour vision Optic disc- stalk of the optic nerve, blind spot, does not contain any rods or cones Fibrous tunic- cornea sclera Vascular tunic- choroid iris ciliary body make up the uvea Inner layer- retina and optic nerve Tarsal plate- fold of connective tissue in eyelids containing meibomian glands which produce oily layer of tears Nucleus- control centre of the cell containing all genetic information. Genetic information is stored in chromosomes. DNA has a double helix shape so lots of info can be stored in a small space Nucleolus- structure within the nucleus which synthesises ribosomal RNA. It is not surrounded by a membrane. Nuclear envelope- surrounds the nucleus, made up of phospholipid bilayer and nuclear pores to allow substances to enter and leave. Helps the nucleus maintain it shape Ribosome- made up of a large and small subunit. Ribosomes contain rRNA and proteins. They are the site of protein synthesis. They can be free in the cytoplasm or attached to the Rough Endoplasmic reticulum. RER- the surface of the membrane is rough as there are many ribosomes attached. Produces secretory proteins and phospholipids SER- smooth as there are less ribosomes attached to the surface. Detoxifies harmful compounds and produces phospholipids Golgi apparatus- site of protein folding after they have been produced in the ribosome. Proteins are packaged and sent to different locations within and out with the cell. Folded surface so large surface area. Vesicles transport molecules Vesicles transport materials within the cytoplasm and form during phagocytosis, exocytosis and endocytosis. Lysosomes- contain digestive enzymes which are used during phagocytosis to engulf pathogens. Arise from Golgi apparatus Mitochondria- site of ATP production in the cell. High energy cells will have many mitochondria present. Site of Krebs cycle and electron transport chain The cytoskeleton provides support and structure to the cell. It is made up of microtubules, micro filaments and intermediate filaments. Centrosomes are made up of 2 centrioles which organise the microtubules and chromosome distribution Cell membrane- surrounds the cytoplasm of the cell and controls what enters and leaves, made up of a phospholipid bilayer, membrane proteins and cholesterol. Bilayer has hydrophilic heads and hydrophobic tails and is held together by hydrogen bonds. Membrane pores allow substances to enter and leave the cell. Proteins are held to the cell membrane by transmembrane domains. Prokaryotic- no nucleus usually bacterial cells Passive transport- diffusion of molecules down the concentration gradient, from a high to low concentration, does not require ATP. Nonpolar molecules move in this way Facilitated diffusion- uses transmembrane proteins to move molecules, down the gradient, no ATP required. Polar or ionic molecules Osmosis- movement of water down the gradient, high to low conc. Water moves to the side of the membrane with the higher solute concentration. Hypertonic solution- high solute concentration. If cell is placed in hypertonic solution it will shrivel Hypotonic- low solute concentration. If a cell is placed in a hypotonic solution it will swell and burst. Isotonic- same concentration on both sides of the membrane. Extrusion- maintains osmotic balance in a cell, water is expelled by vacuoles contracting Active transport- movement of charged molecules from low to high concentration, against the gradient. Requires ATP. Glucose travels in this way Uniporters- transport one molecule at a time Symporters- transport more than one molecule in the same direction Antiporters- transport more than one molecule in opposite directions Sodium potassium pump- transports 3 sodium out of the cell and takes 2 potassium molecules in. this is an antiporter. Requires ATP for the protein to change shape. 3 sodium bind and ATP causes the protein to change shape. Protein now has low affinity for sodium, so they are released into the outside of the cell. The protein now has a high affinity for the potassium ions so 2 of these bind. ATP changes the shape of the protein again and it now has a low affinity for potassium, so they are released into the cell. Coupled transport- uses ATP indirectly. Uses the energy released when a molecule moves by diffusion to provide energy for the active transport of another. A symporter is used Endocytosis- movement of substances into a cell. Example- cholesterol Requires energy Phagocytosis- cell engulf pathogens to destroy them (particulate matter) Pinocytosis- cell takes in only fluid Receptor mediated- molecules are taken in after they bind to a receptor Exocytosis- movement of substances out of the cell, requires energy Can be used to secrete hormones, neurotransmitters and digestive enzymes 8 cranial bones- frontal, parietal, occipital, sphenoid, ethmoid, temporal Suture- joints between bones, immoveable, fuse the skull bones together Foramen- opening in the bone allowing nerves and veins to pass through to the brain Sinus- hollow air-filled space, lined with mucosa, protects from infection, inflammation is sinusitis Fossa- hollow scoop in skull bones which can house tissues and glands Fissure- gaps or grooves between bones, blood vessels and nerves can pass Fontanel- infant skull softer spots before sutures become immoveable Frontal bone- bone of the forehead- makes up the upper wall of the orbit, articulates with the parietal and temporal bones, contains an air-filled frontal sinus Parietal bones- one on either side of skull, articulate with frontal bone, occipital and temporal bones Coronal suture- between parietal and frontal Sagittal suture-between left and right parietal bones Lambdoid suture- between parietal and occipital Squamous suture- between parietal and temporal Occipital bone- back of the skull, contains foramen magnum for spinal cord to pass through, protects the brain stem and attachment site for neck and back muscles Temporal bones- below parietal bones, floor of the cranium. Squamous and petrous portion. Petrous houses middle and inner ear structures. Sphenoid bone- inner keystone bone of the skull, butterfly shaped. Articulates with all other cranial bones. Sella turcica in centre which has a fossa to house the pituitary gland Ethmoid bone- fragile inner bone, separates the nasal cavity form the brain cavity and contains ethmoid sinus Suture- immoveable joints that fuse cranial bones together. Fossa- hollow scoop in the bone to house tissues and glands Foramen- opening in the skull bone to allow nerves and veins to access the brain Fontanel- soft spots in infant skull Sinus- hollow air-filled pocket lined with mucosa, protects from infection 8 cranial bones- frontal, parietal, temporal, occipital, sphenoid, ethmoid Frontal- anterior portion of skull, bone of the forehead, contains frontal sinus, forms the upper wall of the orbit Parietal- one on either side of skull, articulates with frontal temporal and occipital Coronal suture- between frontal and parietal Sagittal suture- between left and right parietal Lambdoid suture- between occipital and parietal Squamous suture- between parietal and temporal Occipital- base portion of the skull, contains foramen magnum to allow spinal cord to pass. Protects the brain stem and provides sites of attachment for neck and back muscles Temporal- one on either side below parietal bones. Contains squamous and petrous portion. Petrous houses the inner and middle ear. Sphenoid- inner keystone bone, articulates with all other cranial bones. Contains Sella turcica, fossa which houses pituitary gland Ethmoid- inner fragile bone, separates nasal cavity from brain cavity, contains ethmoid sinuses. The DNA of a cell is found in the nucleus. DNA is arranged in a double helix shape. Eash nucleotide is made up of a sugar phosphate backbone and a base. The bases of DNA are held together by weak hydrogen bonds. Adenine pairs with thymine and cytosine pairs with guanine. DNA is made up of one coding strand and one template strand. The order of bases determines the structure and function of proteins produced. Strands of DNA run antiparallel Proteins- binding, catalysis, structure Genes can be switched on and off in a cell to perform different functions. Transcription- RNA polymerase moves along the DNA strand. It creates a complimentary copy of the template stand. This is the mRNA strand. The base thymine is replaced with uracil. Transcription occurs in the nucleus. The mRNA strand is then sent to the ribosome for translation. Translation- occurs in the ribosome. tRNA carries the correct amino acids to the ribosome. The mRNA code is read in triplets and a chain of amino acids is produced which folds into a protein. Initiation- major groups come together Elongation- amino acids are added in order Termination- the end of the process is signalled RNA splicing- introns are removed and exons are retained. The exons are the coding regions Housekeeping genes- switched on all the time Proteins are formed in the ribosome and then are sent to the Golgi apparatus for packaging. The proteins are then sent to various destinations inside and outside the cell. Mutation- permanent change in the DNA of an organism which can result in dysfunctional protein, or no protein being formed. Examples of mutations- retinitis pigmentosa 1. During transcription, a complimentary copy of the non-coding DNA strand is created. RNA polymerase moves along the strand and replicates the DNA. The base thymine is replaced by uracil. The mRNA carries the complimentary strand to the ribosome for translation. Before it leaves the nucleus, RNA splicing occurs. This is when introns are removed and the coding regions, exons, are retained. 2. Translation occurs in the ribosome. tRNA carries the correct amino acids to the ribosome. The mRNA strand is read in triplets. A chain of amino acids is produced which folds into a protein. 3. The Golgi apparatus folds and packages the proteins so they can be sent to other destinations in the cell 4. Retinoblastoma, retinitis pigmentosa, colour blindness 1. Performing normal function and not dividing G0 or G1 2. Entering the cell cycle to divide 3. Normal programmed cell death Mitosis- cell division for growth and repair Cell division Interphase- g1, S, g2 G1- organelles duplicate and centrosomes replicate S- DNA replicated, cell committed to division G2- protein synthesis and centrosome replication complete Mitotic phase- mitosis and cytokinesis Prophase- chromosomes become visible, mitotic spindles form, centrosomes move to opposite poles, nucleolus and nuclear envelope disappear Metaphase- centromeres line up at the metaphase plate Anaphase- the paired chromatids divide at the centre and the mitotic spindles pull them to opposite poles Telophase- nuclear envelope and nucleolus reform, mitotic spindles disappear, chromosomes no longer visible Cytokinesis- cleavage furrow forms and the cytoplasm splits into 2 portions forming the new cell Promoter genes stimulate cell division Repressor genes inhibit cell division Too much division- growths or tumour Not enough division- loss of tissue mass 1. Normal function without dividing is G0, interphase when the organelles and centrosomes replicate is G1, DNA replication is S, protein synthesis is G2, followed by cell death 2. Prophase, the chromosomes become visible and mitotic spindles form. The centrosomes move to opposite poles of the cell. The nucleolus and the nuclear envelope disappear 3. Promotor genes are switched on to stimulate cell division. Repressor genes are activated to inhibit division. Apoptosis- programmed cell division, allows unwanted cells to be eliminated from an organism without causing harm to the cells around it. Apoptosis can occur to control dividing cell populations or to get rid of diseased or damaged cells. Blocking signals- promote cell survival Promoting signals- promote cell death, override survival signals Signals activate caspase enzymes which destroy the cell, cell then engulfed by macrophages Intrinsic pathway- damage within the cell Extrinsic pathway- receptors bind to the cell which trigger its death Necrosis- cell death due to injury or disease, cell contents swell and burst, debris accumulates, triggers inflammatory response, causes damage to other cells, uncontrolled Cancer cells evade apoptosis by downregulating death signals or amplifying anti apoptotic machinery 1. Cell death by apoptosis is controlled. The debris of the dead cell is engulfed by macrophages and some of the contents are recycled. Apoptosis does not cause any damage to surrounding cells as it is contained and controlled 2. During necrosis, the cell contents swell and rupture. The debris accumulates and forms pus. This triggers the inflammatory response, and the cell death becomes uncontrolled. 3. Cancer cells contain genetic alterations meaning they are able to amplify anti apoptotic machinery or downgrade the death signals 1. Apoptosis is necessary to get rid of diseased or damaged cells and also to control dividing cell populations. It is a natural part of the cell cycle, and maintains a balance between cell proliferation and cell death 2. Apoptosis is controlled by blocking and promoting signals. Blocking signals promote cell survival. Promoter signals promote cell death and can override the survival signals 3. During apoptosis, the promoter signals activate the enzyme caspase. This then destroys the cell contents. It is engulfed by a macrophage and some contents are recycled 4. Apoptosis is controlled, recycles contents and does not cause damage to other cells. Necrosis is uncontrolled, causes damage to other cells and no contents are recycled. 5. If too much apoptosis occurs- loss of tissue mass. If not enough occurs- growth of tumours 1. Cells can perform their normal function without dividing, can enter the cell cycle to divide or can be involved in programmed cell death 2. Somatic- one cell produces 2 identical daughter cells, reproductive- one cell produces unique haploid daughter cells 3. Cell division is necessary to replace dying cells and maintain the balance between cell proliferation and cell death. Cells divide for growth, renewal and repair 4. Cell cycle- interphase and mitotic phase. In interphase G0, the cell organelles and centrosomes replicate. In S the DNA replicates. In G2 the centrosome replication is complete. In mitosis: prophase- chromosomes become visible and mitotic spindles form. Centromeres move to opposite poles and nucleolus and envelope disappear. Metaphase- the chromosomes line up at the metaphase plate. Anaphase- the paired chromatids separate and are pulled to opposite poles of the cell by the mitotic spindles. Telophase, the chromosomes are no longer visible, nucleolus reforms and spindles disappear. Cytokinesis- cytoplasm splits into 2 equal portions. 5. Done above 6. Controlled by genes. Promoter genes are switched on to stimulate cell division. Repressor genes are switched on to inhibit division 7. Uncontrolled cell division- tumour growth or retinoblastoma **Fertilisation**- day 1, the sperm fuses with a mature ovum to create a diploid zygote. Gametes are haploid and contain one set of chromosomes. The zygote contains 2 sets of chromosomes- 46. The head of the sperm contains acrosomal enzymes which allow the sperm to dissolve and penetrate the outer layer of the ovum. The head and tail of the sperm disconnect as the head fuses with the oocyte's plasma membrane. The nuclei of the ovum and the sperm head are called pronuclei. The pronuclei fuse to form a diploid zygote. **Cleavage and Morula**- the first cleavage occurs at day 2. The zygote undergoes rapid mitotic division. It divides into 2 cells. Cleavage continues until the zygote is 16 cells. This forms a solid ball of cells called the morula at day 4. **Blastocyst**- at end of day 4 morula enters unterine cavity. Blastocyst formed when it is 32 cells. **Embryoblast**- known as inner cell mass, will become the embryo **Trophoblast**- will become outer chorionic sac for waste and nutrient exchange **Blastocyst cavity**- filled with fluid **Implantation**- day 5, the blastocyst hatches from the zona pellucida, enzymatically forms a hole and squeezes through. Zona pellucida must be shed in order for implantation to happen Day 6 the blastocyst lightly attaches to the wall of the endometrium. Day 7 attaches more firmly and womb lining becomes more vascularised (more blood vessels and thickens) **Fertilisation** On day 1, the sperm and the mature ovum fuse to form a zygote. The gametes are haploid and contain one set of chromosomes. The zygote is diploid and contains 46 chromosomes. The head of the sperm contains acrosomal enzymes which allow it to dissolve and penetrate the oocyte's cell membrane. When the head of the sperm fuses with the membrane, the tail of the sperm breaks off. The nuclei of the sperm head and the ovum are called pronuclei. The pronuclei fuse to form a diploid zygote. **Cleavage and morula** At day 2, the zygote undergoes rapid mitotic division. The first cleavage occurs, and the zygote divides into 2 cells. By the end of day 3, the zygote is made up of 16 cells. This forms the morula which is a solid ball of 16 cells. **Blastocyst** At day 4 the morula enters the uterine cavity. When it is 32 cells, it becomes a blastocyst made up of 2 main cell populations. The embryoblast is also known as the inner cell mass. It will form the future embryo. The trophoblast will form the outer chorionic sac for nutrient and waste exchange. The blastocyst cavity is filled with fluid **Implantation** At day 5 the blastocyst hatches from the zona pellucida by enzymatically creating a hole and squeezing through it. The zona pellucida must be shed form implantation to occur. At day 6, the inner cell mass lines up with the wall of the endometrium and the blastocyst attaches lightly. At day 7 it attaches more firmly, and the endometrium becomes more vascularised and thickens. Week 2- the embryoblast and the trophoblast differentiate Embryoblast becomes epiblast and hypoblast Trophoblast becomes cytotrophoblast and syncytiotrophoblast Epiblast- primitive ectoderm Hypoblast- primitive endoderm. Cytotrophoblast- inner layer Syncytiotrophoblast- outer layer in direct contact with maternal blood, facilitates implantation Epiblast and hypoblast form bilaminar embryonic disc At end of week 2 bilaminar disc is connected to trophoblast by a connecting stalk which becomes the future umbilical cord. **Week 3- Gastrulation** Formation of a primitive streak generates body axes. Bilaminar embryo becomes trilaminar. ICM becomes 3 germ layers. The hypoblast is replaced by the endoderm Ectoderm- outer layer- skin nervous system, nose, ears Mesoderm- middle layer- bones connective tissue Endoderm- inner layer- lungs liver and pancreas Embryonic disc folds ventrally to create a trilaminar embryonic disc- hollow tube of 3 germ layers Primitive streak is formed by migrating epiblast cells **Neurulation** Neurulation is the start of the nervous system formation. At day 16 mesodermal cells migrate towards the head of the embryo forming a hollow tube called the notochordal process. At day 22-24 the notochordal process is a solid cylinder of cells called the notochord. The notochord is necessary to induce the formation of the neural plate. Precursor to vertebral backbone. 1. At day 19 the neural plate is formed in a cranial to caudal direction. Wider at the top and will become the brain. Narrower towards the bottom will become the spinal cord. 2. At end of week 3, edges of neural plate rise up and move together to form neural folds enclosing a space called the neural groove 3. Neural folds fuse to form neural tube which is the precursor to the CNS. 4. Neural crest cells migrate away from the tube. Crest cells can differentiate into many cell types and form structures of the PNS Neural ectoderm produces neurons, glial cells Surface ectoderm forms exterior layers of skin Ends of neural tube completely close at the end of week 4 Somites- as neural tube closes notochord degenerates and mesodermal cells differentiate into somites Rostral to caudal direction, cuboidal mesodermal tissue, form axial skeleton **Brain Vesicle Formation** Week 4- head end of neural tube develops into 3 primary brain vesicles Prosencephalon- forebrain Mesencephalon- midbrain Rhombencephalon- hindbrain At week 5 secondary vesicles are formed Prosencephalon becomes telencephalon and diencephalon Rhombencephalon becomes metencephalon and myelencephalon (spinal cord) Week 8- cerebrospinal fluid and 5 main regions of CNS 1. Prosencephalon 2. Diencephalon 3. Mesencephalon 4. Rhombencephalon 5. Spinal cord **Simple diffusion** Movement of molecules down the concentration gradient from high to low concentration. Does not require ATP. Small molecules or non-polar molecules can move in this way. **Facilitated Diffusion** Movement of molecules down the gradient through a transmembrane protein. Does not require ATP. Polar or ionic molecules can move in this way **Osmosis** Diffusion of water from high to low concentration. Water moves to the side of the membrane with the higher solute concentration. If solution has a high solute concentration it is hypertonic. If it has a low solute concentration it is hypotonic. Solutions are isotonic if they have the same concentration of solute. If a cell is placed in a hypertonic solution it will shrivel. If it is placed in hypotonic solution it will swell and burst. Osmosis doesn't require ATP. **Active Transport** Movement of molecules against the concentration gradient. Requires ATP. Charged ions and molecules like glucose can move in this way **Coupled Transport** ATP is used indirectly. Energy released from the diffusion of one molecule is used for the active transport of another. Uniporters- move one molecule in one direction Symporters- move 2 molecules in the same direction Antiporters- move 2 molecules in opposite directions **Sodium Potassium Pump** 3 sodium ions leave the cell, and 2 potassium ions enter. Energy provided by ATP causes the protein to change shape giving high affinity for sodium. The sodium molecules leave the ell and protein changes shape to have higher affinity for potassium. Changes shape again and potassium enters the cell. **Bulk Transport** Exocytosis- movement of molecules out of the cell. Requires energy Endocytosis- movement of molecules into the cell. Phagocytosis takes in a pathogen to destroy it. Pinocytosis takes in fluid. Receptor mediated takes in substances after they bind to a receptor on the cell surface. CNS- brain and spinal cord PNS- nerves branching off and going to limbs and extremities Somatic- contains sensory and motor neurons. Sensory goes from sense organs to the CNS. Motor goes from CNS to muscles and glands Autonomic- contains sympathetic and parasympathetic Sympathetic- increases breathing and heart rate Parasympathetic- decreases breathing and heart rate Enteric- controls digestion and gut processes Neurons transmit and receive electrochemical messages. Electrochemical messages are passed to the CNS to generate an impulse. Neurons are made up of a cell body, dendrite input and axon output. Multipolar- contains many dendrites and one axon Bipolar- contains one dendrite and one axon Unipolar- contains one process with a dendrite at one end and an axon at the other Neuroglia support neurons and produce the myelin sheath Astrocytes- maintain correct chemical environment for neurons Microglia- provide defence against infection Ependymal cells- produce the cerebrospinal fluid Satellite cells- support neurons in the PNS Schwann cells- produce myelin sheath in the PNS Oligodendrocytes- produce myelin sheath in the CNS No regeneration of cells is possible in the CNS Myelin sheath insulates the axon and increases the speed of nervous impulses. Grey matter in the brain is unmyelinated. White matter is myelinated. G0- cell is performing its normal function without dividing Cell Cycle **Interphase** G1- organelles and centrosomes replicate S- DNA replicates G2- Proteins are synthesised and centrosome replication is complete **Mitotic Phase** Prophase- the chromosomes become visible. Mitotic spindles appear. Centromeres move to opposite poles of the cell. The nuclear envelope and nucleolus disappear. Metaphase- the chromosomes line up at the centre along the metaphase plate Anaphase- the mitotic spindles pull the paired chromatids apart and they move to opposite poles of the cell Telophase- the chromosomes are no longer visible; the mitotic spindles disappear and the nucleolus and nuclear envelope reform Cytokinesis- the cytoplasm of the cells separate. A cleavage furrow forms and the cytoplasm splits into 2 equal portions Mitosis happens for growth, renewal and repair. It produces 2 identical daughter cells. It is controlled by genes. When promoter genes are switched on, this stimulates the cell to divide. When repressor genes are switched on, this inhibits cell division. Too much division heads to tumours or excessive growths. Not enough division leads to loss of tissue mass. **Fertilisation** The sperm fuses with the mature ovum to form a zygote. The gametes are haploid and contain one set of chromosomes. The zygote is diploid and contains 2 sets of chromosomes. The head of the sperm contains acrosomal enzymes which allow it to dissolve and penetrate the oocyte's cell membrane. When the sperm has fused with the ovum's cell membrane, the tail detaches. The nuclei of the sperm and the ovum are known as pronuclei. The pronuclei fuse to form the diploid zygote. **Cleavage and Morula** The zygote undergoes a period of rapid mitotic division. At the end of day 1, the first cleavage occurs, and the zygote divides into 2 cells. At the end of day 3, a solid ball of 16 cells is formed called the morula. **Blastocyst** At day 4-5, the ball of cells enters the uterine cavity. When the morula is 32 cells thick it forms the blastocyst. The blastocyst is made up of 2 cell populations. The embryoblast will become the future embryo. The trophoblast will form the outer chorionic sac for waste and nutrient exchange. The blastocyst cavity is filled with fluid. **Implantation** At day 6, the blastocyst lines up with the wall of the endometrium and attaches lightly. At day 7, the blastocyst attaches more firmly, and the endometrium becomes vascularised. **Week 2** The cell layers of the blastocyst differentiate. The embryoblast becomes the epiblast and the hypoblast. The trophoblast becomes the cytotrophoblast and the synctiotrophoblast. The epiblast and hypoblast form the bilaminar embryonic disc. **Week 3** Gastrulation occurs. A primitive streak forms at the head end of the embryo. This generates body axes and forms the 3 germ layers. The hypoblast is replaced by the endoderm. Ectoderm and mesoderm are formed. This forms trilaminar disc when it folds ventrally at the edges **Week 4** Neurulation occurs. Neural plate is formed, and this rises at the edges to form the neural groove. As edges join together this forms neural tube. Neural crest cells break off from the neural tube and differentiate into PNS structures.