Module 10: Central and Somatic Nervous Systems Flashcards
Describe the protective structures of the spinal cord.
The spinal cord is protected by:
- Meninges
- Cushion of cerebrospinal fluid
The meninges are three coverings that run continuously around the spinal cord and brain:
- The outermost layer is the dura mater.
- The middle layer is the arachnoid.
- The innermost meninge is the pia mater, a thin, transparent connective tissue layer that adheres to the surface of the spinal cord and brain.
The vertebral column provides a bony covering of the spinal cord.
Describe the general features of the spinal cord.
In an adult, the spinal cord begins as a continuation of the medulla oblongata and terminates at about the second lumbar vertebra.
The end portion of the spinal cord is called the cauda equina.
The 31 pairs of spinal nerves are named and numbered according to the region and level of the spinal cord from which they emerge.
The spinal cord includes 8 pairs of cervical nerves, 12 pairs of thoracic nerves, 5 pairs of lumbar nerves, 5 pairs of sacral nerves, and 1 pair of coccygeal nerves.
Describe the structure of the spinal cord visible in a transverse section.
The anterior median fissure and the posterior median sulcus penetrate the white matter of the spinal cord and divide it into the right and left sides.
The gray matter of the spinal cord is shaped like the letter H or a butterfly and is surrounded by white matter.
- consists primarily of cell bodies of neurons and neuroglia and unmyelinated axons and dendrites of association and motor neurons.
- divided into the lateral, posterior and anterior horns
- contain cell bodies of neurons.
The central canal runs the length of the spinal cord and contains cerebrospinal fluid.
Anterior to the gray commissure is the anterior white commissure,
- connects the white matter of the right and left sides of the spinal cord.
The white matter consists of bundles of myelinated axons of motor and sensory neurons.
- divided into columns.
- Each column contains distinct bundles of nerve axons that have a common origin or destination and carry similar information.
- These bundles are called tracts. Sensory (ascending) tracts conduct nerve impulses toward the brain.
- Motor (descending) tracts conduct impulses down the cord
- They are named by the region they occupy
- lateral column
- posterior column
- anterior column
Branching out of the spinal cord is the Posterior (dorsal) root of the spinal nerve and the anterior (ventral) root or spinal nerve.
- the posterior root contains the posterior root ganglion
- responsible for sensory input to the CNS
- The Anterior root contains motor nerve nerves which have 2 motor axons somatic or autonomic depending on the stimulus
Define plexus, and name the four principal plexuses.
Shortly after passing through its intervertebral foramen, a spinal nerve divides into several branches.
- A network of branching nerves is called a plexus
The cervical plexus
- supplies the skin and muscles of the head, neck, and upper part of the shoulders
- connects with some cranial nerves
- supplies the diaphragm.
The brachial plexus
- constitutes the nerve supply for the upper extremities
- several neck and shoulder muscles.
The lumbar plexus
- supplies the anterolateral abdominal wall, external genitals, and part of the lower extremities.
The sacral plexus
- supplies the buttocks, perineum, and part of the lower extremities.
Describe the structure of a spinal nerve.
Spinal nerves, part of the peripheral nervous system, connect the CNS to sensory receptors, muscles, and glands.
The 31 pairs of spinal nerves
- named and numbered according to the region and level of the spinal cord from which they emerge.
Spinal nerves connect to the spinal cord through an anterior and a posterior root.
- the posterior root contains sensory axons
- the anterior root contains motor axons
- a spinal nerve is a mixed nerve, at least at its origin.
There are 3 coverings on a spinal nerve which are composed of connective tissue
- Epineurium
- surrounds entire nerve
- Perineurium
- surrounds each fascicle
- fascicles are bundles of axons
- surrounds each fascicle
- Endoneurium
- surround each axon within a nerve
- The Dura Mater fuses with the epineurium
Many Blood vessels are found within the perineurium and the endoneurium layers
Discuss the two principal functions of the spinal cord.
The spinal cord has two principal functions.
The white matter tracts are highways for nerve impulse conduction to and from the brain.
Sensory information travels up the spinal cord to the brain along two main routes on each side of the cord. These routes are called pathways or tracts.
Motor information travels from the brain down the spinal cord to effectors (muscles and glands) along two main descending tracts.
The gray matter receives and integrates incoming and outgoing information. The signals processed in the gray matter are called spinal reflexes.
Discuss in detail a reflex arc.
The gray matter of the spinal cord serves as an integrating centre for spinal reflexes.
A reflex is a fast, predictable, automatic response to changes in the environment that helps to maintain homeostasis.
A reflex arc is the simplest type of pathway; pathways are specific neuronal circuits and thus include at least one synapse.
The five functional components of a reflex arc are the following:
- Receptor
- Sensory neuron
- Integrating centre neuron
- Motor neuron
- Effector
Reflexes help to maintain homeostasis by permitting the body to make rapid adjustments to homeostatic imbalances.
Identify the principal parts of the brain.
The principal parts of the brain are the:
- Brain stem
- pons, midbrain, medulla oblongata
- Diencephalon
- thalamus, hypothalamus, pineal gland
- Cerebellum
- Cerebrum
Describe how the brain is protected.
The brain is protected by the:
- Cranial bones
- Cranial meninges
The cranial meninges, continuous with the spinal meninges, are called:
- Dura mater
- Arachnoid
- Pia mater
Blood flows to the brain mainly through blood vessels that branch from the cerebral arterial circle (circle of Willis) at the base of the brain. Veins return blood from the head to the heart.
Although the brain comprises only about 2% of the total body weight, it utilizes about 20% of the oxygen used by the entire body. The brain is one of the most metabolically active organs of the body, and the amount of oxygen it uses varies with the degree of mental activity.
A blood-brain barrier (BBB) protects brain cells from harmful substances and pathogens by serving as a selective barrier to prevent the passage of many substances from the blood to the brain.
Explain the formation, circulation, and reabsorption of cerebral spinal fluid.
Cerebrospinal fluid (CSF)
- clear, colourless liquid that protects the brain and spinal cord against chemical and physical injuries
- carry oxygen, glucose, and other needed chemicals from the blood to neurons and neuroglia. Removes waste and toxic substances
Formation:
- Four CSF-filled cavities called ventricles exist within the brain.
- filtration by networks of capillaries called choroid plexuses (found in the ventricles)
Circulation:
- circulates through the subarachnoid space, ventricles, and central canal.
- There are 4 ventricles
- 2 lateral
- thrid
- forth
- each ventricle is connected with one another as well as with the central canal and subarachnoid space.
- There are 4 ventricles
- Materials that enter the CSF from the choroid capillaries cannot leak between the surrounding ependymal cells, which constitute the blood-cerebrospinal fluid barrier that permits certain substances to enter the fluid but excludes others and protects the brain and spinal cord from harmful elements.
Reabsorption:
- Most of the CSF is absorbed by the arachnoid villi of the superior sagittal blood sinus.
- Reabsorption normally occurs at the same rate at which CSF is produced in the choroid plexuses, thereby maintaining a relatively constant CSF volume and pressure.
Describe the structure and functions of the brain stem.
The brain stem is located between the spinal cord and the diencephalon. The brain stem consists of three regions:
Medulla oblongata
Pons
Midbrain
The medulla oblongata, or just medulla, is continuous with the upper part of the spinal cord and contains portions of both motor and sensory tracts.
Functional regions include nuclei that are reflex centres for the regulation of heart rate, respiratory rate, vasoconstriction, swallowing, coughing, vomiting, sneezing, and hiccupping; the first three are considered vital reflexes.
The pons is located superior to the medulla. It connects the spinal cord with the brain and links parts of the brain with one another by way of tracts.
The pons relays nerve impulses related to voluntary skeletal movements from the cerebral cortex to the cerebellum.
The pons also contains the pneumotaxic and apneustic areas, which help control respiration and the respiratory centre in the medulla.
The midbrain connects the pons and diencephalon. The midbrain also conveys motor impulses from the cerebrum to the cerebellum and spinal cord, sends sensory impulses from the spinal cord to the thalamus, and regulates auditory and visual reflexes.
The midbrain helps to regulate muscle tone, alerts the cortex to incoming sensory signals, and is responsible for maintaining consciousness and awakening from sleep (reticular activating system [RAS]).
Describe the components and function of the diencephalon.
The diencephalon, extending from the brain stem to the cerebrum, consists of the:
- Thalamus
- Hypothalamus
- Pineal gland
The thalamus is located superior to the midbrain and contains nuclei that serve as relay stations for all sensory impulses, except smell, to the cerebral cortex.
The thalamus also registers the conscious recognition of pain and temperature and some awareness of light touch and pressure. It plays an essential role in awareness and the acquisition of knowledge, which is called cognition.
The hypothalamus is found inferior to the thalamus.
It controls many body activities, and is one of the major regulators of homeostasis.
The six major functions of the hypothalamus include:
- Control of the autonomic nervous system (ANS)
- Control of the pituitary gland
- Regulation of emotion
- Regulation of hunger and thirst
- Regulation of temperature
- Regulation of circadian rhythm
In conjunction with the hypothalamus, the pineal gland secretes melatonin to influence diurnal cycles.
Describe the structure and function of the cerebellum.
The cerebellum occupies the inferior and posterior aspects of the cranial cavity; it has two hemispheres and a central, constricted vermis.
The cerebellum is attached to the brain stem by three pairs of cerebellar peduncles.
The cerebellum functions to coordinate skeletal muscle contractions and to maintain normal muscle tone, posture, and balance.
Describe the components and function of the cerebrum.
The cerebrum is the largest part of the brain. It provides us with the ability to read, write, speak, plan, and create.
The cerebral cortex, the surface layer of the cerebrum, is 2–4 millimetres (mm) thick, and is composed of gray matter. The cortex contains billions of neurons. It also has gyri (convolutions), deep grooves called fissures, and shallower sulci.
The cerebral white matter—tracts that connect parts of the brain with itself and other parts of the nervous system—lies beneath the cortex.
The longitudinal fissure separates the cerebrum into right and left halves called hemispheres. Internally, the cerebrum is connected by the corpus callosum, a bundle of transverse white fibres.
Each cerebral hemisphere is further subdivided into four lobes by sulci or fissures. These cerebral lobes—frontal, parietal, temporal, and occipital―are named after the bones that cover them.
The insula, a fifth part of the cerebrum, lies deep within the cerebrum under the parietal, frontal, and temporal lobes; it cannot be seen in an external view of the brain.
The basal ganglia are paired masses of gray matter in each cerebral hemisphere. They are responsible for helping to control muscular movements.
The limbic system is found in the cerebral hemispheres and diencephalon. It functions in emotional aspects of behaviour and memory, and is associated with pleasure and pain.
The functional areas of the cerebral cortex are the following:
- Sensory areas that enable perception
- Motor areas that enable movement
- Association areas that enable memory, emotion, and so on
Somatic sensory pathways relay information from sensory receptors to the cerebral cortex.
Somatic motor pathways relay signals from the brain stem to the skeletal muscles.
State the number of cranial nerves, and describe the classification system used to identify them.
Twelve pairs of cranial nerves originate from the brain.
The pairs are named primarily on the basis of their distribution, and they are numbered by order of their attachment to the brain.
