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All of portage modules BIOD 152
Intro to the Nervous System
Notice: To optimize your learning in this course, we advise that you complete the labs and modules as indicated in the BIOD 152 Lab Schedule. The nervous system receives and processes information and sends out signals to the muscles and glands to elicit an appropriate response. In this way, the nervous system integrates and controls the other systems of the body. In the human nervous system, the central nervous system (Figure below) includes the brain and the spinal cord (dorsal nerve cord), which lie in the midline of the body.
The skull protects the brain and the vertebrae protect the spinal cord. The central nervous system can send signals or impulses to and receive impulses from the peripheral nervous system. The peripheral nervous system includes all nerves not in the brain or spinal cord which are the cranial nerves that connect directly to the brain and the spinal nerves which project from either side of the spinal cord. The peripheral nervous system connects all parts of the body to the central nervous system and can be divided into a sensory or afferent division and a motor or efferent division. The peripheral nervous system receives impulses from the sensory organs via the afferent division and then relays signals or impulses from the central nervous system to muscles and glands via the motor or efferent division. The efferent division can be further divided into the somatic system and the autonomic system. The somatic system nerves control skeletal muscles, skin, and joints. The autonomic system nerves control the glands and smooth muscles of the internal organs and are not generally under conscious control and can be divided into two systems: the sympathetic system which activates and prepares the body for vigorous muscular
- What are the 2 divisions of the autonomic nervous system? sympathetic and parasympathetic
- What is the function of the sympathetic nervous system? to arouse and activate muscles for activity, stress and emergency
- What is the function of the parasympathetic nervous system? To calm and lower activity. Permits digestion and conserves energy
Neurons
Neurons (Figure below) are nerve cells that vary in size and shape. They do not undergo mitosis (cell division), require enormous amounts of fuel, are able to survive just minutes without oxygen, and can last an entire human lifetime. Neurons all have three parts: the dendrites, the cell body, and the axon. The neuron cell body, which synthesizes all nerve cell products, consists of a large nucleus with surrounding cytoplasm containing the normal organelles. The dendrites are numerous short extensions that emanate from the cell body which receive information from other neurons conducting those nerve impulses toward the cell body. The single axon, on the other hand, conducts nerve impulses away from the cell body to its axon terminals where it is emitted across a synapse to the dendrite of another neuron. Axons can vary in length being very short or as long as three feet, the length of the axon which extends from the bottom of the spine to the big toe. Axons are composed of cells like the cell body but lack rough endoplasmic reticulum, depending on the cell body for necessary proteins. The peripheral nerve axon is coated in short sections called Schwann cells which are mainly composed of a white fatty layer called the myelin sheath rolled around the axon which insulates the nerve fiber from others and increases the speed of nerve impulses. There are also unmyelinated fibers, which are common in the gray matter of the brain and spinal cord, in which the Schwann cells do not wrap around the axon but are just loosely associated with the axon. The Schwann cell insulating sections are not continuous, having gaps between them called Nodes of Ranvier. At these exposed nodes, the nerve impulse is
forced to jump to the next node in a manner called salutatory conduction, greatly increasing the nerve impulse transmission along the axon. The cell body contains the nucleus and other organelles typically found in cells with the exception of centrioles (since it is not capable of mitosis). One of the main functions of the cell body is to manufacture neurotransmitters, which are chemicals stored in secretory vesicles at the end of axon terminals. When neurotransmitters are released by the axon terminal vesicles, they participate in the transmission of the nerve impulse from one neuron to another. Problem Set 2:
- Identify the parts of the neuron shown in the diagram below. a. Dendrite b. Cell body c. Nucleus d. Axon e. Myelin sheath f. Schwann cell g. Node of Ranvier h. Axon terminal
- List 3 unusual characteristics of neurons. Do not undergo mitosis. Need oxygen to survive. Require a lot of energy and fuel to survive.
A nerve consists of hundreds of thousands of axons (#3) wrapped together in a connective tissue. In the peripheral nervous system the cell bodies of neurons (#2) are grouped together in masses called ganglia which are part of a single nerve. The neurons are also accompanied by non-nerve "supporting" cells known collectively as neuroglial cells which include (as shown in the diagram below) ependymal cells (#1), oligodendrocytes (#4), astrocytes (#5) and microglial cells (#7). The functions of these supporting cells are as follows: ependymal cells (circulate cerebrospinal fluid and allow fluid exchange between brain, spinal cord and CSF), oligodendrocytes (insulation of central nervous system axons), astrocytes (control chemical environment of neurons) and microglial cells (protect CNS by scavenging dead cells and infectious microoganisms). Neurons can be classified as to their structure and function. Structurally, neurons are classified according to the number of extension from their cell body, as multipolar, bipolar and unipolar neurons. Multipolar neurons, the most common type in humans found as motor neurons or interneurons within the CNS, have three or more extensions, one axon and many dendrites. Bipolar neurons, found as receptors cells in the visual and olfactory systems, have two extensions, one axon and one dendrite. Unipolar neurons, found as sensory neurons in the peripheral nervous system, have one extension which branches into two, one central process running to the CNS and another peripheral process running to the sensory receptor. Functionally, neurons are classified as sensory or afferent neurons, motor or efferent neurons and association
or interneurons. Most sensory neurons are unipolar and carry impulses from receptors in the skin or internal organs toward the CNS. Most motor neurons are multipolar and carry impulses from the central nervous system to muscle fibers or glands. Interneurons are usually multipolar and found within the central nervous system only. They transmit impulses between sensory and motors neurons conveying messages between various parts of the central nervous system, such as from one side of the brain or spinal cord to the other, or from the brain to the spinal cord, and vice versa. Problem Set 3:
- List the four types of support neuroglial cells and a function of each. Ependymal cells, circulate cerebrospinal fluid and allow fluid exchange between the brain, spinal cord and CSF. Oligodendrocytes, insulate CNS axons Astrocytes, control chemical environment of neurons Microglial cells, protect CNS by scavenging dead cells and infectious microorganisms
- List the 3 structural classes of neurons, and describe the structure and an example of each. Multipolar, which is most common in humans have three or more extensions, one axon and many dendrites. They are found as motor neurons or interneurons. Bipolar, have two extensions, one axon and one dendrite. They are found as receptor cells in the visual and olfactory systems. Unipolar, have one extension which branches into two. One central process running to the CNS and another to PNS running to the sensory receptors. Are found as sensory neurons in the PNS.
- List the 3 functional classes of neurons, and describe the structure and an example of each. Sensory neurons, are unipolar and carry impulses from receptors in the skin or internal organs towards CNS. Motor neurons, multipolar and carry impulses from CNS toward muscle fibers and glands. Interneurons, are multipolar and found within CNS only and transmit impulses between sensory and motor neurons conveying messages between various parts of the CNS, such as from one side of the brain or spinal cord to the other side.
Action Potentials
Neurons are specialized to conduct electric impulses called action potentials. The nerve impulse is an electrochemical charge moving along an axon created by the movement of unequally distributed ions on either side of an axon’s plasma membrane. At rest the plasma membrane is said to be polarized, meaning that one side has a different charge than the other side. When the axon is not conducting an impulse, this difference in electrical charge is called resting potential and is equal to about -70mV (millivolts). This means that the charge on the inside of the axon's cell membrane is 70 millivolts less than the outside of the membrane. This is maintained by a sodium-potassium pump which uses active transport to carry ions across the plasma membrane.
The action potential travels along the length of an axon like a wave. It is self-propagating because the ion channels are prompted to open whenever the membrane potential decreases (depolarizes) in an adjacent area. An action potential is an all-or-nothing response either occurring or not. Since no variation exists in the strength of a single impulse, we distinguish the difference in intensity of a sensation (minor pain/major pain) by the number of neurons stimulated and the frequency with which they are activated. An impulse passing from one vertebrate nerve cell to another always moves in only one direction and there is a very short delay in transmission of the nerve impulse from one neuron to another. Neurons do not touch. There is a minute fluid-filled space, called a synapse, between the axon terminal of the sending (presynaptic) neuron and the dendrite of the receiving (postsynaptic) neuron.
The transmission of nerve impulses is electrochemical in nature as chemicals called neurotransmitters allow the signal to jump the synaptic gap. When a nerve impulse reaches the end of an axon, voltage-gated calcium channels open. As Ca+2 rushes in, it causes vesicles containing the neurotransmitter to fuse with the plasma membrane and release the neurotransmitter into the synapse. When the neurotransmitter released binds with a receptor on the next neuron, Na+ channels in the receiving dendrites open. Depolarization occurs and the impulse is carried. Acetylcholine and norepinephrine are well-known neurotransmitters, active in both the peripheral nervous system and the central nervous system. Once a neurotransmitter has been released into a synapse, it has only a short time to act. Some synapses contain enzymes that rapidly inactivate the neurotransmitter. For example, the enzyme acetylcholinesterase, or simply cholinesterase, breaks down acetylcholine. In other synapses, the synaptic ending rapidly absorbs the neurotransmitter, possibly for repacking in synaptic vesicles or for chemical breakdown. The short existence of neurotransmitters in the synapse prevents continuous stimulation (or inhibition) of postsynaptic membranes. Problem Set 4:
- What is the technical term used to describe a nerve impulse and what causes the impulse? is an electrochemical charge moving along the axon created by the movement of unequally distributed ions on the either side of the axon's plasma membrane.
- An axon's membrane is polarized with a resting potential of -70 mV. Explain what this means and what maintains this resting potential. The axon plasma membrane is polarized, meaning that one side has a different charge than the other side. This difference called a resting potential means that the charge on the inside of the axon's cell membrane is 70 mV less than the outside of the membrane. Resting potential means no signals are being sent. The axon is maintained at -70mV by a sodium-potassium pump using active transport carries ions across plasma membrane and because 3Na+ are pumped out and 2k+ are pumped in a relative positive charge develops and is maintained on the outside of the membrane.
- What causes the difference in intensity of a sensation? The difference in intensity of a sensation is due to the number of neurons stimulated and the frequency with which they are activated.
- How is an impulse passed from one nerve cell to another? There is a minute fluid-filled space, called a synapse, between the axon terminal of the sending neuron and the dendrite of the receiving neuron. When a nerve impulse reaches the end of an axon, neurotransmitters are released into the synapse. These bind with a receptor on the next neuron, opening Na+ gates in the receiving dendrite which causes depolarization and the impulse is carried.
- What prevents continuous stimulation of a nerve synapse and how is this accomplished? Self propagation, channels opening in a wave like action down the axon Once sodium gates starts to open, the rest do and continues going
Peripheral Nervous System
The peripheral nervous system lies outside the central nervous system. The peripheral nervous system is made up of nerves, which are part of either the somatic system or the autonomic system. The somatic system contains nerves that control skeletal muscles, skin, and joints. The autonomic system contains nerves that control the smooth muscles of the internal organs and the glands. Humans have twelve pairs of cranial nerves attached to the brain. Cranial nerves are either sensory nerves (having long dendrites of sensory neurons only), motor nerves (having long axons of motor neurons only), or mixed nerves (having both long dendrites and long axons). With the exception of the vagus nerve, all cranial nerves control the head, neck, and face. The vagus nerve controls the internal organs.
The first cranial nerve is the olfactory. It is a sensory nerve responsible for the sense of smell. The second cranial nerve is the optic. It is a sensory nerve responsible for the sense of sight. The third cranial nerve is the oculomotor. It is a motor nerve responsible for eye movement. The fourth cranial nerve is the trochlear. It is a motor nerve also responsible for eye movement. The fifth cranial nerve is the trigeminal. It is a motor and sensory nerve. It is responsible for chewing or mastication and sensation of the face, nose, and mouth. The sixth cranial nerve is the abducens. It is a motor nerve responsible for eye movement. The seventh nerve is the facial. It is a motor and sensory nerve. It is responsible for facial expressions and sensation of the tongue. The eighth cranial nerve is the vestibulocochlear. It is a sensory nerve responsible for hearing and balance. The ninth cranial nerve is the glossopharyngeal. It is a motor and sensory nerve. It is responsible for swallowing and taste. The tenth cranial nerve is the vagus. It is a motor and sensory nerve. It is responsible for digestion, regulation of heart rate, and sensation of the digestive tract. The eleventh nerve is the accessory. It is a motor nerve and is responsible for the rotation of the head and shrugging of the shoulders. The twelfth cranial nerve is the hypoglossal. It is a motor nerve that is responsible for tongue movements. Humans have thirty-one pairs of spinal nerves. There are eight pairs of cervical (cranial) nerves, twelve pairs of thoracic nerves, five pairs of lumbar nerves, five pairs of sacral nerves, and one
The phrenic nerve is the most important nerve of the cervical plexus and supplies both motor and sensory fibers to the diaphragm. Irritation of this nerve causes hiccups and severing this nerve would cause paralysis of the diaphragm and require use of a ventilator (mechanical respiratory). The saying “three, four, five keeps the diaphragm alive” is an easy way to remember that the phrenic nerve arrives from the ventral rami of C 3 -C 5.
Five nerves that originate from the ventral rami of C 5 -T 1 issue from the Brachial plexus. The axillary nerve supplies three muscles: the deltoid (a muscle of the shoulder), the teres minor (one of the rotator cuff muscles) and the long head of the triceps brachii (an elbow extensor). The axillary nerve also carries sensory information from the shoulder joint. The radial nerve supplies the triceps brachii muscle of the arm, as well as 12 muscles in the forearm and the associated joints and overlying skin. The median nerve supplies flexor muscles of the forearm and the skin of the first three and a half fingers. Compression of the median nerve in the carpal tunnel causes carpal tunnel syndrome or decreased sensation in the first three and a half fingers. The musculocutaneous nerve supplies the flexor muscles of the arm. The ulnar nerve supplies part of the flexor muscles of the forearm, wrist, and hand as well as the skin of half the ring finger and pinky finger. If the ulnar nerve is damaged it results in a condition known as claw hand, the inability to open the fourth and fifth fingers.
The somatic nervous system includes all nerves that serve the musculoskeletal system and the exterior sense organs, including the skin. Exterior sense organs (and skin) are receptors, which receive environmental stimuli and then initiate nerve impulses. Muscle fibers are effectors, which bring about a reaction to the stimulus.
Problem Set 5
- How many pairs of cranial nerves do humans have and what do most of them (except for one) control? All the cranial nerves control the head, face and neck except for the vagus nerve.
- For each of the following cranial nerves, list its name, type and what it controls. Name Type Controls What? 1st Olfactory Sensory Sense of smell 2nd Optic Sensory Sense of sight Eye movement 3rd Oculomotor Motor 4th Trochlear Motor Eye movement
