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Class Notes Nervous System
Lecture 1 Nerves, Neurons and Neuroglia CNS and PNS Function of the Nervous System Supporting Cells (Neuroglial Cells) Neuron Structure Classification of Neurons Membrane Potential Nerve Impulse Synapse All or None and processing of Info Types of Nerves Nerve Pathways Meninges Spinal Cord Brain Brain Stem
¯Introduction: A. The nervous system is composed of neurons and neuroglia.
- Neurons transmit nerve impulses along nerve fibers to otheneurons
- Nerves are made up of bundles of nerve fibers.
- Neuroglia carry out a variety of functions to aid and protect components of the nervous system.
B. Organs of the nervous system can be divided into the central nervous system (CNS), made up of the brain and spinal cord, and the peripheral nervous system (PNS), made up of peripheral nerves that connect the CNS to the rest of the body.
C. The nervous system provides sensory, integrative, and motor functions to the body.
- Motor functions can be divided into the consciously controlled somatic nervous system and the unconscious autonomic system.
¯ General Functions of the Nervous System
A. Sensory receptors at the ends of peripheral nerves gather information and convert it into nerve impulses.
B. When sensory impulses are integrated in the brain as perceptions, this is the integrative function of the nervous system.
C. Conscious or subconscious decisions follow, leading to motor functions via effectors.
¯Supporting cells
A. Classification of Neuroglial Cells
- Neuroglial cells fill spaces, support neurons, provide structural frameworks, produce myelin, and carry on phagocytosis. Four are in the CNS and the last in the PNS.
- Microglial cells are small cells that phagocytize bacterial cells and cellular debris.
- Oligodendrocytes form myelin in the brain and spinal cord.
- Astrocytes are near blood vessels and support structures, aid in metabolism, and respond to brain injury by filling in spaces.
- Ependyma cover the inside of ventricles and form choroid plexuses within the ventricles.
- Schwann cells are the myelin-producing neuroglia of the peripheral nervous system.
¯Neuron Structure
A. A neuron has a cell body with mitochondria, lysosomes, a Golgi apparatus, chromatophilic substance (Nissl bodies) containing rough endoplasmic reticulum, and neurofibrils.
B. Nerve fibers include a solitary axon and numerous dendrites.
- Branching dendrites carry impulses from other neurons (or from receptors) toward the cell body.
- The axon transmits the impulse away from the axonal hillock of the cell body and may give off side branches.
- Larger axons are enclosed by sheaths of myelin provided by Schwann cells and are myelinated fibers. a. The outer layer of myelin is surrounded by a neurilemma (neurilemmal sheath) made up of the ytoplasm and nuclei of the Schwann cell. b. Narrow gaps in the myelin sheath between Schwann cells are called nodes of Ranvier.
- The smallest axons lack a myelin sheath and aunmyelinated fibers.
- White matter in the CNS is due to myelin sheaths in this area.
- Unmyelinated nerve tissue in the CNS appears gray.
- Peripheral neurons are able to regenerate because of the neurilemma but the CNS axons are myelinated by oligodendrocytes thus lacking neurilemma and usually do not regenerate.
¯ Classification of Neurons
- At threshold, action potential is reached.
- Action potential may be reached when a series of subthreshold stimuli summate and reach hreshold. D. Action Potential
- At threshold potential, membrane permeability to sodium suddenly changes in the region of stimulation.
- As sodium channels open, sodium ions rush in, and the membrane potential changes and becomes depolarized.
- At the same time, potassium channels open to allow potassium ions to leave the cell, the membrane becomes repolarized, and resting potential is reestablished.
- This rapid sequence of events is the action potential.
- The active transport mechanism then works to maintain the original concentrations of sodium and potassium ions.
- At the same time, potassium channels open to allow potassium ions to leave the cell, the membrane becomes repolarized, and resting potential is reestablished.
- This rapid sequence of events is the action potential.
- The active transport mechanism then works to maintain the original concentrations of sodium and potassium ions.
¯ Nerve Impulse
A. A nerve impulse is conducted as action potential is reached at the trigger zone and spreads by a local current flowing down the fiber, and adjacent areas of the membrane reach action potential.
B. Impulse Conduction
- Unmyelinated fibers conduct impulses over their entire membrane surface.
- Myelinated fibers conduct impulses from node of Ranvier to node of Ranvier, a phenomenon called saltatory conduction.
- Saltatory conduction is many times faster than conduction on unmyelinated neurons.
C. All-or-None Response
- If a nerve fiber responds at all to a stimulus, it responds completely by conducting an impulse (all-or-none response).
- Greater intensity of stimulation triggers more impulses per second, not stronger impulses.
¯The Synapse
A. Nerve impulses travel from neuron to neuron along complex nerve pathways.
B. The junction between two communicating neurons is called a synapse; there exists a synaptic cleft between them across which the impulse must be conveyed.
C. Synaptic Transmission
- The process by which the impulse in the presynaptic neuron is transmitted across the synaptic cleft to the postsynaptic neuron is called synaptic transmission.
- When an impulse reaches the synaptic knobs of an axon, synaptic vesicles release neurotransmitter into the synaptic cleft.
- The neurotransmitter reacts with specific receptors on the postsynaptic membrane.
D. Excitatory and Inhibitory Actions
- Neurotransmitters that increase postsynaptic membrane permeability to sodium ions may trigger impulses and are thus excitatory.
- Other neurotransmitters may decrease membrane permeability to sodium ions, reducing the chance that it will reach threshold, and are thus inhibitory.
E. Neurotransmitters
- At least 50 kinds of neurotransmitters are produced by the nervous system, most of which are synthesized in the cytoplasm of the synaptic knobs and stored in synaptic vesicles.
- When an action potential reaches the synaptic knob,calcium ions rush inward and, in response, some synaptic vesicles fuse with the membrane and release their contents to the synaptic cleft.
- Enzymes in synaptic clefts and on postsynaptic membranes rapidly decompose the neurotransmitters after their release.
- Destruction or removal of neurotransmitter prevents continuous stimulation of the postsynaptic neuron.
¯ Impulse Processing
A. How impulses are processed is dependent upon how neurons are organized in the brain and spinal cord.
B. Neuronal Pools
- Neurons within the CNS are organized into neuronal pools with varying numbers of cells.
- Each pool receives input from afferent nerves and 1processes the information according to the special characteristics of the pool.
¯ Impulse Processing
A. How impulses are processed is dependent upon how neurons are organized in the brain and spinal cord.
C. The middle meninx, the arachnoid mater , is thin and lacks blood vessels.
- It does not follow the convolutions of the brain.
- Between the arachnoid and pia maters is a subarachnoid space containing cerebrospinal fluid.
D. The innermost pia mater is thin and contains many blood vessels and nerves.
- It is attached to the surface of the brain and spinal cord and follows their contours.
¯Brain
A. The brain is the largest, most complex portion of the nervous system, containing 100 billion multipolar neurons.
B. The brain can be divided into the cerebrum (largest portion and associated with higher mental functions), the diencephalon (processes sensory input), the cerebellum (coordinates muscular activity), and the brain stem (coordinates and regulates visceral activities).
C. Structure of the Cerebrum
- The cerebrum is the largest portion of the mature brain, consisting of two cerebral hemispheres.
- A deep ridge of nerve fibers called the corpus callosum connects the hemispheres.
- The surface of the brain is marked by convolutions, sulci, and fissures.
- The lobes of the brain are named according to the bones they underlie and include the frontal lobe, parietal lobe, temporal lobe, occipital lobe.
- A thin layer of gray matter, the cerebral cortex, lies on the^ outside of the cerebrum and contains 75% of the cell bodies in the nervous system.
- Beneath the cortex lies a mass of white matter made up of myelinated nerve fibers connecting the cell bodies of the cortex with the rest of the nervous system.
D. Functions of the Cerebrum
- The cerebrum provides higher brain functions, such as interpretation of sensory input, initiating voluntary muscular movements, memory, and Integrating information for reasoning.
- Functional Regions of the Cerebral Cortex a. The functional areas of the brain overlap, but the cortex can generally be divided into motor, sensory, and association areas. b. The primary motor areas lie in the frontal lobes, anterior to the central sulcus and in its anterior wall.
c. Broca's area, anterior to the primary motor cortex, coordinates muscular activity to make speech possible. d. Above Broca's area is the frontal eye field that controls the voluntary movements of the eyes and eyelids. e. The sensory areas are located in several areas of the cerebrum and interpret sensory input, producing feelings or sensations. f. Sensory areas for sight lie within the occipital lobe. g. Sensory and motor fibers alike cross over in the spinal cord or brain stem so centers in the right hemisphere are interpreting or controlling the left side of the body, and vice versa. h. The various association areas of the brain analyze and interpret sensory impulses and function in reasoning, judgment, emotions, verbalizing ideas, and storing memory. i. Association areas of the frontal lobe control a number of higher intellectual processes. j. A general interpretive area is found at the junction of the parietal, temporal, and occipital lobes, and plays the primary role in complex thought processing.
E. Hemisphere Dominance
- Both cerebral hemispheres function in receiving and analyzing sensory input and sending motor impulses to the opposite side of the body.
- Most people exhibit hemisphere dominance for the language- related activities of speech, writing, and reading.
- The left hemisphere is dominant in 90% of the population, although some individuals have the right hemisphere as dominant, and others show equal dominance in both hemispheres.
- The non-dominant hemisphere specializes in nonverbal functions and controls emotions and intuitive thinking.
- The basal ganglia are masses of gray matter located deep within the cerebral hemispheres that relay motor impulses from the cerebrum and help to control motor activities by producing inhibitory dopamine.
- Basal ganglia include the caudate nucleus, the putamen, and the globus pallidus.
F. Ventricles and Cerebrospinal Fluid
- The ventricles are a series of connected cavities within the cerebral hemispheres and brain stem.
- The ventricles are continuous with the central canal of the spinal cord, and are filled with cerebrospinal fluid.
- Choroid plexuses, specialized capillaries from the pia mater, secrete cerebrospinal fluid. a. Most cerebrospinal fluid arises in the lateral ventricles.
b. The medulla oblongata also houses nuclei that control visceral functions, including the cardiac center that controls heart rate, the vasomotor center for blood pressure control, and the respiratory center that works, along with the pons, to control the rate and depth of breathing. c. Other nuclei in the medulla oblongata are associated with coughling, sneezing, swallowing, and vomiting.
- Reticular Formation a. Throughout the brain stem, hypothalamus, cerebrum, cerebellum, and basal ganglia, is a complex network of nerve fibers connecting tiny islands of gray matter; this network is the reticular formation b. Decreased activity in the reticular formation results in sleep; increased activity results in wakefulness. c. The reticular formation filters incoming sensory impulses.
I. Cerebellum
- The cerebellum is made up of two hemispheres connected by a vermis.
- A thin layer of gray matter called the cerebellar cortex lies outside a core of white matter.
- The cerebellum communicates with other parts of the central nervous system through cerebellar peduncles.
- The cerebellum functions to integrate sensory information about the position of body parts and coordinates skeletal muscle activity and maintains posture.
¯Spinal Cord
A. The spinal cord begins at the base of the brain and extends as a slender cord to the level of the intervertebral disk between the first and second lumbar vertebrae.
B. Structure of the Spinal Cord
- The spinal cord consists of 31segments, each of which gives rise to a pair of spinal nerves.
- A cervical enlargement gives rise to nerves leading to the upper limbs, and a lumbar enlargement gives rise to those innervating the lower limbs.
- Two deep longitudinal grooves (anterior median fissure and posterior median sulcus) divide the cord into right and left halves.
- White matter, made up of bundles of myelinated nerve fibers (nerve tracts), surrounds a butterfly-shaped core of gray matter housing interneurons.
- A central canal contains cerebrospinal fluid.
C. Functions of the Spinal Cord
- The spinal cord has two major functions: to transmit impulses to and from the brain, and to house spinal reflexes.
- Tracts carrying sensory information to the brain are called ascending tracts; descending tracts carry motor information from the brain.
- The names that identify nerve tracts identify the origin and termination of the fibers in the tract.
- Many spinal reflexes also pass through the spinal cord.
¯Peripheral Nervous System
A. The peripheral nervous system (PNS) consists of the cranial and spinal nerves that
arise from the central nervous system and travel to the remainder of the body.
B. The PNS is made up of the somatic nervous system that oversees voluntary activities, and the autonomic nervous system that controls involuntary activities.
C. Cranial Nerves
- Twelve pairs of cranial nerves arise from the underside of the brain, most of which are mixed nerves.
- The 12 pairs are designated by number and name and include the olfactory, optic, oculomotor, trochlear, trigenimal, abducens, facial, vestibulocochlear, glossopharyngeal, vagus, accessory, and hypoglossal nerves. D. Spinal Nerves
- Thirty-one pairs of mixed nerves make up the spinal nerves.
- Spinal nerves are grouped according to the level from which they arise and are numbered in sequence, beginning with those in the cervical region.
- Each spinal nerve arises from two roots: a dorsal, or sensory, root, and a ventral, or motor, root.
- The main branches of some spinal nerves form plexuses.
- Cervical Plexuses a. The cervical plexuses lie on either side of the neck and supply muscles and skin of the neck.
- Brachial Plexuses a. The brachial plexuses arise from lower cervical and upper thoracic nerves and lead to the upper limbs.
- Lumbrosacral Plexuses a. The lumbrosacral plexuses arise from the lower spinal cord and lead to the lower abdomen, external genitalia, buttocks, and legs.
¯ Nerve Pathways
A. The routes nerve impulses travel are called pathways, the simplest of which
is a reflex arc.
