Nervous System Lecture 4
Neurons Functional unit of the nervous system Also called the nerve cell Soma or body Axon Dendrites Neuroglial cells support cells Schwann cells produce myelin in PNS Oligodendrocytes produce myelin in CNS Astrocytes nourishment of nerve cells Microglial cells regenerative processes
Neurons Classification (SAME) Sensory or afferent Motor or efferent Interneurons neither sensory nor motor Nerve signals Through nerve action potential An electrochemical message of neurons All or None principle
Short review of the nervous system anatomy CNS Brain Spinal cord Ventricles CSF White matter and grey matter PNS Dorsal root ganglion
Establishment of the Nervous System Neural plate neural tube formation of the brain and spinal cord Cephalization = the neural tube becomes markedly enlarged cephalically, while the caudal portion remains roughly the same size Brain area exhibits 3 regional divisions Prosencephalon (Forebrain) Mesencephalon (Midbrain) Rhombencephalon (Hindbrain)
Neuroepithelium Primordial cells of the nervous system Cells on the neural tube, are called neuroepithelial cells These cells have a high degree of mitotic activity. Neuroepithelial cells migrate to the external limiting membrane (outer basement membrane of the neural tube) and begin to divide. These cells travel from the outer wall to the inner wall as they undergo mitosis.
Neuroepithelium At the end of mitosis, they return back to the external limiting membrane, where they shall become mature cells, called neuroblasts, or resume mitotic division as neuroepithelial cells. Neuroblasts lose their ability to undergo mitosis and migrate toward the outer wall of the neural tube. These neuroblasts begin to produce processes which are the forerunners of the axons and dendrites. Other neuroepithelial cells differentiate into neuroglial cells or glioblasts or ependymal cells.
Neuroepithelium As the neuroepithelium matures, it can be divided into 3 layers Ventricular or ependymal layer Innermost layer Contains cells still in the mitotic cycle Eventually becomes the ependyma (epithelial lining of the central canal of the spinal cord and the ventricular system of the CNS) Mantle layer Marginal layer
Neuroepithelium Mantle layer Middle layer composed of neuroblasts Becomes the gray mater Marginal layer Outermost layer Contains mostly cell processes and few cell bodies Becomes the white matter
Neuroblasts Do not undergo mitosis anymore Forerunners of the nerve cells Initially bipolar (unipolar in humans) Connected to the inner and outer borders The attachment to the central luminal border retracts making them unipolar (apolar in humans) The unipolar cells then accumulate large amounts of rough endoplasmic reticulum to create several cytoplasmic processes, becoming multipolar neuroblasts. (axon and dendrite)
Gliablasts or Glioblasts Become the supporting cells of the CNS and PNS called neuroglial cells Neuroepithelial cells differentiate into gliablasts, which eventually separate into: Astrocytes Oligodendrocytes Retain the ability to divide, even in the adult.
Ependymal cells Innermost layer of cells (Ventricular layer) Becomes the lining of the central canal and ventricles Area where CSF circulates
Development of the Central Nervous System SPINAL CORD BRAIN
Development of the spinal cord Neuroepithelial cells continue to grow and differentiate, eventually producing the characteristic butterfly shape Mantle layer = inner gray matter Marginal layer = outer white matter (color is due to myelin) Longitudinal (up-down) connections from brain to spinal cord Lumen of the neural tube decreases in size and becomes the central canal, with the ependymal layer inside.
Development of the spinal cord Gray matter divides the white matter of the spinal cord into 4 areas, through its dorsal and ventral horns Dorsal = afferent paths (sensory) Ventral = efferent paths (motor) 2 lateral areas = afferent paths (pain and temperature) Differentiation into 4 areas is due to presence of the notochord Intermediate neurons are also formed their axons grow from the dorsal area to the ventral area (forming the basic reflex arc)
Development of the spinal cord During early development, the spinal cord and the vertebral bodies grow together, and the spinal nerves are directly aligned with the openings of the corresponding vertebral body. After some time, there is differential growth of the vertebral column and spinal cord. The spinal cord lags behind the vertebral column, making the distal spinal cords stretched out. This produces the cauda equina of the spinal cord.
Development of the spinal cord At birth, the spinal cord ends at the level of L3-L4 In adulthood, the spinal cord, ends at the level L2-L3
Development of the brain Similar to spinal cord development, but the nerve cell bodies migrate to the outer layer of cells to form an outer gray matter and an inner white matter Migration is through radial glial cells a form of astrocyte that have long processes, extending from the outer wall to the inner wall Neuroblasts travel along these cells to reach their final destination in the outer rim of the brain.
3 Primary Vesicles 5 Secondary Vesicles Final areas Prosencephalon Telencephalon Cerebral hemispheres (neopallium) Corpus striatum Limbic system (archipallium) Diencephalon Optic vesicles Thalamus Hypothalamus Epiphysis (pineal gland) Infundibulum and Rathke s pouch (posterior pituitary gland) Mesencephalon Mesencephalon Corpora quadrigemina Tegmentum Cerebral peduncle Rhombencephalon Metencephalon Cerebellum Pons Myelencephalon Medulla
Functions - Prosencephalon Cerebrum Voluntary motor and sensory functions Association areas = areas of higher brain function Analysis, abstract thinking, etc. Limbic system = emotion, spatial and short term memory Cingulate gyrus = order of complex behaviors Hippocampus = recent memory Amygdala = aggressive behavior and fear Hypothalamus = autonomic nervous system Thalamus
Functions - Prosencephalon Thalamus Relay station (sensory) Hypothalamus Regulates body temperature, water balance, appetite, thirst, production of hormones, etc Pineal gland Affects melanocytes, circadian rhythm (sleep and long-term reproductive cycles for other animals)
Functions - Mesencephalon Corpora quadrigemina Superior colliculi = some visual function Inferior colliculi = some auditory function
Functions - Rhombencephalon Medulla oblongata Route for ascending and descending pathways Center for visceral, auditory, proprioceptive reflexes. Reflex centers for respiration, heartbeat and intestinal motility Pons Carry impulses from cerebellum to cerebrum and back Cerebellum Maintains equilibrium and balance Refinement of motor action = coordination
Formation of the Brain Ventricles and CSF Lumen of neural tube becomes the central canal in the spinal cord In the brain, the central canal expands into 4 areas to make the ventricles 2 lateral ventricles (telencephalon) Third ventricle (diencephalon) Fourth ventricle (rhombencephalon) Blood vessels on the lateral walls of the lateral ventricles and roofs of the 3 rd and 4 th ventricles produce the choroid plexus, which forms the CSF
Flow of CSF in the CNS 1. Choroid plexus in lateral ventricles 2. Foramen of Monro 3. Third ventricle (and choroid plexus) 4. Sylvian aqueduct 5. Fourth ventricle (and choroid plexus) 6. Foramen of Luschka and Magendie 7. Subarachnoid space 8. Arachnoid villi superior sagital sinus venous drainage
Meninges Outer covering of the brain and spinal cord Dura mater Outermost Mesodermal in origin Arachnoid layer Neural crest in origin (ectoderm) Middle layer Absorbs the CSF via arachnoid villi Pia mater Innermost layer Neural crest in origin (ectoderm)
Development of the Peripheral Nervous System Spinal nerves Cranial nerves
Spinal Nerves Cell bodies in the ventral horn of the spinal cord sends out axons (motor) to the periphery Sensory neurons travel from the periphery to enter the spinal cord via the dorsal root. The sensory nerve cell body is located outside the spinal cord, in an area called the dorsal root ganglion, formed from neural crest cells. An interneuron shall then transmit the signal to the brain or to an efferent nerve cell for a quick response.
Formation of myelin
Cranial Nerves Number Name Function Type I Olfactory Smell Sensory II Optic Vision Sensory III Occulomotor Eye movement Motor IV Trochlear Eye movement Motor V Trigeminal Facial sensation Chewing Mixed VI Abducens Eye movement Motor VII Facial Facial movement and taste Mixed VIII Acoustic Hearing Sensory IX Glossopharyngeal Swallowing and gag reflex Mixed X Vagus Swallowing and upper palate and gag reflex Mixed XI Spinal Accessory Turns head Motor XII Hypoglossal Tongue movement Motor
Sex Hormones and Brain Development Mammalian brains are inherently female (similarly with development of the sex organs) Exposure of the CNS to testosterone during a critical period of development, results in the embryo s being irreversibly imprinted with male characteristics Male rats that have been castrated at birth show female mating characteristics Female rats that have been given testosterone at birth show male mating behavior