Outcome 3 Nervous System Flashcards
Nervous system
Consists of two principle divisions
-central nervous system (CNS) (brain and spinal cord)
-peripheral nervous system (PNS) (the nerves of the body)
🔹subdivision of the PNS is the autonomic nervous system (ANS), which consists of structures that regulate the body’s automatic or involuntary functions for ex heart rate, secretions of chemicals by glands
Cells of the nervous system
- neurons, or nerve cells (conduct impulses)
- glia, which are support cells, (glia support neurons)
Neurons
Each neuron consists of three parts: a main part called the neuron CELL BODY, one or more branching projections called DENDRITES, and one elongated projection known as an AXON
- the three types of neurons are classified according to the direction in which they transmit impulses
1. Sensory neurons
2. Motor neurons
3. Interneurons
Dendrites
-the processes or projections that transmit impulses TO the neuron cell bodies or axons
Axons
- are the processes that transmit impulses AWAY from the neuron cell bodies or dendrites
- surrounded by a segmented wrapping of a material called myelin
- myelin is a white, fatty substance formed by schwann cells that wrap around some axons outside the CNS
- such fibers are called myelinated fibers
Sensory neurons
- transmit impulses to the spinal cord and brain from all parts of the body
- also called afferent neurons
Motor neurons
- transmit impulses in the opposite direction of sensory neurons
- transmit impulses away from the brain and spinal cord
- they do not conduct impulses to all parts of the body but only to two kinds of tissue: muscle and glandular epithelial tissue
- also called efferent neurons
Interneurons
- conduct impulses from sensory neurons to motor neurons
- often also connect with each other to form complex, central networks of nerve fibers
- also called central or connecting neurons
Nodes of ranvier
Indentations between adjacent Schwann cells
The outer cell membrane of a Schwann cell is called _____
- neurilemma
- axons in the brain and spinal cord have no neurilemma and this is clinically significant because it plays an essential part in the regeneration of cut and injured axons
- therefore the potential for regeneration in the brain and spinal cord is far less than it is in the peripheral nervous system
Glia aka neuroglia
-do not specialize in transmitting impulses
-special type of supporting cells
-one function is to hold the functioning neurons together and protect them
-also the regulation of neuron function, therefore they act not just as physical “glue” but also help bring the various functions of the nervous tissue together into a coordinated whole
-one of the most common types of brain tumour called a glioma develops from them
Vary in size and shape:
-astrocytes
-microglia
-oligodendrocytes
Astrocytes (type of glia)
- look somewhat like stars because of the threadlike extensions that jut out from their surfaces
- their threadlike branches attach to neurons and to small blood vessels, holding their structures close to each other
- the “feet” of the astrocytes form a wall around the outside of the blood vessels in the nervous system. This astrocyte wall, along with the vessel wall, forms a structure known as the blood-brain barrier (BBB).
Microglia (a type of glia)
- smaller than astrocytes
- usually remain stationary, but in inflamed or degenerating brain tissue, they enlarge, move about, and act as microbe eating scavengers
- they surround the microbes, draw them into their cytoplasm, and digest them
- also help cleanup cell damage resulting from injury or disease
Oligodendrocytes (a type of glia)
- help hold nerve fibers together and also serve another and probably more important function- they produce the fatty myelin sheath that envelops nerve fibers located in the brain and spinal cord
- each oligodendrocyte can form part of the myelin sheath around several axons whereas Schwann cells wrap entirely around only one axon
Disorders of nervous tissue
-multiple sclerosis: characterized by myelin loss in central nerve fibers and resulting conduction impairments
-tumours
🔹general name for nervous system tumours is neuroma
🔹most neuromas are gliomas, glial tumours
🔹multiple neurofibromatosis: characterized by numerous benign tumours
Nerve
- nerve: Bundle of peripheral axons
- tract: bundle of central axons
- white matter: brain or cord tissue composed primarily of myelinated axons (tracts)
- gray matter: brain or cord tissue composed primarily of cell bodies and unmyelinated fibers
Nerve coverings
- nerve coverings: fibrous connective tissue
- endoneurium: surrounds individual fibers within a nerve
- perineurium: surrounds a group (fascicle) of nerve fibers
- epineurium: surrounds the entire nerve
CNS
- its two major structures, the brain and spinal cord are found along the midline of the body
- brain is protected in the cranial cavity of the skull
- spinal cord is surrounded by the spinal cavity by the vertebral column
- brain and spinal cord are also covered by protective membranes called meninges
Divisions of the brain
The brain consists of the following major divisions, named in ascending order beginning with the most inferior part 1. Brainstem 🔹medulla oblongata 🔹pons 🔹midbrain 2. cerebellum 3. diencephalon 🔹hypothalamus 🔹thalamus 🔹pineal gland 4. cerebrum
Brainstem
-consists of, named in ascending order, the medulla oblongata, pons, and midbrain
-structure: white matter with bits of grey matter scattered through it
-functions: all three parts of brainstem are two-way conduction pathway
-MEDULLA OBLONGATA: two-way conduction pathway between the spinal cord and higher brain centres; cardiac, respiratory, and vasomotor control center
🔹upward extension of the spinal cord, lies just inside the cranial cavity above the large hole in the occpital bone called the foramen magnum
🔹consists of grey and white matter. in the medulla bits of grey matter mix intricately with white matter. in the spinal cord grey and white matter do not intermingle
-PONS: two way conduction pathway between areas of the brain and other regions of the body; influences respiration
-MIDBRAIN: two way conduction pathway; relay for visual and auditory impulses
Cerebellum
- second largest part of the brain
- muscle coordination; maintenance of equilibrium and posture; assists cerebrum
- folded grey matter composes the thin outer layer, forming a large surface area of nervous connections that allow for a huge amount of information processing
- white matter tracts form most of the interior
- plays an essential part in the production of normal movements, perhaps has an overall coordinating function for the whole brain
Diencephalon
HYPOTHALAMUS: regulation of body temperature, water balance, sleep-cycle control, appetite, and sexual arousal, heartbeat, constriction and dilation of blood vessels
-the posterior pituitary gland, the stalk that attaches it to the undersurface of the brain, and areas of grey matter located in the side walls of a fluid-filled space called the third ventricle are extensions of the hypothalamus
-exerts major control over virtually all internal organs
-some neurons in the hypothalamus make the hormones that the posterior pituitary gland secretes into the blood
THALAMUS: sensory relay station from various body areas to cerebral cortex; emotions and alerting or arousal mechanisms
-each enlarged end lies in a lateral wall of the third ventricle
PINEAL GLAND: adjusts output of melatonin in response to changes in external light, to keep the body’s internal clock on time
Cerebrum
- largest and uppermost part of the brain
- has ridges and grooves
- ridges: convolutions, or GYRI
- grooves: SULCI
- deepest sulci are called fissures, longitudinal tissue divides the cerebrum into right and left halves or hemispheres. These halves are almost separate structures except for their lower mid portions, which are connected by a structure called the corpus callosum
- two deep sulci subdivide each cerebral hemisphere into four major lobes and each lobe into numerous convolutions
- a thin layer of grey matter called the cerebral cortex is made up of neuron dendrites and cell bodies; this makes up the surface of the cerebrum
- white matter makes up most of the interior, within this white matter are a few islands of grey matter known as basal nuclei or basal ganglia, whose functioning is essential for producing automatic movements and postures (Parkinson’s disease is a disease of the basal nuclei)
- sensory perception, emotions, willed movements, consciousness, memory
Spinal cord: structure
- lies inside spinal column in the spinal cavity
- extends from the occipital bone down to the bottom of the first lumbar vertebra or T12
- place your hands on your hips, and they will line up with your fourth lumbar vertebra, your spinal cord ends just above this level
- spinal cord provides two way conduction paths to and from the brain
- ascending tracts: conduct impulses up the cord to the brain
- descending tracts: conduct impulses down the cord from the brain
Spinal cord: functions
- it switches or transfers incoming sensory impulses to outgoing motor impulses thereby making it possible for a reflex to occur
- carry impulses to and from the brain
Coverings and fluid spaces of the brain and spinal cord
- nervous tissue is not sturdy, even moderate pressure can kill nerve cells, so nature safeguards the chief organs made of this tissue (spinal cord and brain) by surrounding them with a tough, fluid cushioned set of membranes called meninges. The meninges are then surrounded by bone
- three layers of the spinal meninges are:
1. Dura mater
2. Pia mater
3. Arachnoid mater
Dura mater (meninges)
-the tough outer layer, that lines the vertebral canal
Pia mater (meninges)
-the innermost membrane covering the spinal cord itself
Arachnoid mater
-membrane between the dura and pia mater
Meningitis
- inflammation or infection of the meninges
- severe headaches and neck pain
Cerebrospinal fluid (CSF)
- fluid fills the subarachnoid spaces between the pia mater and arachnoid mater in the brain and spinal cord, this fluid is called CSF
- also fills spaces in the brain called cerebral ventricles
- one of the body’s circulating fluids
- forms continually from fluid filtering out of the blood in a network of brain capillaries known as the choroid plexus and into the ventricles
- this fluid forms continually from blood, circulates, and is resorbed into blood
Peripheral nervous system
-nerves connecting the brain and spinal cord to other parts of the body constitute the peripheral nervous system (PNS)
Cranial nerves
- 12 pairs of cranial nerves emerge from the undersurface of the brain, extending mostly from the brainstem
- their fibers conduct impulses between the brain and structures in the head and neck and in the thoracic and abdominal cavities
Lumbar puncture
- the meninges, the fluid containing membranes surrounding the brain and spinal cord, extend beyond the spinal cord, an anatomical fact that is most convenient in regard to being able to perform lumbar punctures without putting the spinal cord at risk of injury
- withdrawal of some CSF from the subarachnoid space in the lumbar region of the spinal cord
- needle gets inserted just above or below the fourth lumbar vertebra
- sample should be yellow ish and clear
Hydrocephalus
“Water on the brain”
-abnormal accumulation of CSF in the ventricles of the brain
-communicating hydrocephalus is caused by an inability to absorb CSF into the venous sinuses, or an overproduction of CSF
-non communicating hydrocephalus is caused by an obstruction in the flow of CSF
-may be congenital or acquired
Symptoms and complications:
-dilation of the ventricular system leading to compression atrophy of the brain
-appears as dilated ventricles