Unit 3 Compendium Review

Outline:

1. Nervous System

central nervous system

peripheral nervous system

2. Nervous Tissue

neurons

neuroglia

3. Nerve Impulse

resting potential

action potential

4. Limbic System and higher mental functions

memory and learning

language and speech

Nervous System

There are two major divisions of the nervous system. The central nervous system and the peripheral nervous system. The two systems work together and are connected to each other. The nervous system has three specific functions: 1. The nervous system receives sensory input- sensory receptors in skin and other organs respond to external and internal stimuli by generating nerve impulses that travel by way of the PNS to the CNS. 2. The CNS performs integration-the CNS sums up the input it receives from all over the body. 3. The CNS generates motor output-nerve impulses from the CNS go by way of the PNS to the muscles and glands. The spinal cord and brain make up the CNS, where sensory information is received and motor control is initiated. Both the spinal cord and the brain are protected by bone. The spinal cord is surrounded by vertebrae and the brain is enclosed by the skull. The spinal cord extends from the base of the brain through a large opening in the skull called the foramen magnum and into the vertebral canal formed by openings in the vertebrae. The spinal nerves project from the cord between the vertebrae that make up the vertebral column. Intervertebral disks seperate the vertebrae and if a disk slips a bit and presses on the spinal cord pain results. The central nervous system contains cerebrospinal fluid as do the meninges that protect the spinal cord. The spinal cord provides a means of communication between the brain and the peripheral nerves that leave the cord. When someone touches your hand sensory receptors generate nerve impulses that pass through sensory fibers to the spinal cord and up ascending tracts to the brain. The gate control theory of pain proposes that the tracts in the spinal cord have "gates" and that these "gates" control the flow of pain messages from the peripheral nerves to the brain.

The peripheral nervous system contains the nerves. Nerves are designated as cranial nerves when they arise from the brain and spinal nerves when they arise from the spinal cord. All nerves take impulses to and from the CNS. The cell body and the dendrites of neurons are in the CNS or ganglia. The axons of neurons project from the CNS and form the spinal cord. Humans have 12 pairs of cranial nerves attached to the brain. Some cranial nerves are sensory nerves-that is they contain only sensory fibers, some are motor nerves that contain only motor fibers, and others are mixed nerves that contain both sensory and motor fibers. Cranial nerves are largely concerned with the head, neck, and facial regions of the body. The spinal nerves of humans emerge in 31 pairs from either side of the spinal cord. The roots of a spinal nerve physically seperate the axons of sensory neurons from the axons of motor neurons. The dorsal root of a spinal nerve contains sensory fibers that conduct impulses inward from sensory receptors. All spinal nerves are called mixed nerves because they contain both sensory and motor fibers.

Nervous Tissue

Nervous tissue contains two types of cells: neurons and neuroglia cells. Neurons are the cells that transmit nerve impulses between parts of the nervous system: neuroglia support and nourish neurons. The three type of neurons are sensoryneurons, interneurons, and motor neurons. A sensory neuron takes nerve impulses from a sensory recepter to the CNS. Sensory receptors are special structures that detect changes in the environment. An interneuron lies entirely within the CNS. Interneurons can receive input from sensory neurons and also from other interneurons in the CNS. They sum up all the nerve impulses received from these neurons before they communicate with motor neurons. A motor neuron takes nerve impulses away from the CNS to an effector.Neurons vary in appearance but all of them have three parts: a cell body, dendrites, and an axon.

Nerve Impulse

Nerve impulses convey information within the nervous system. The nerve impulse is studied by using excised axons and a voltmeter to measure voltage. The voltmeter allows us to measure the potential difference between two sides of the axonal membrane. When the axon is not conducting an impulse the voltmeter records a membrane potential of about -65mV indicating that the inside of the neuron is more negative than the outside. This is called resting potential because the axon is not conducting an impulse. The resting potential correlates with a difference in ion distribution on either side of the axonal membrane. Action potential is a rapid change in polarity across an axonal membrane as the nerve impulse occurs. If a stimulus causes the axonal membrane to depolarize to a certain level called threshold an action potential occurs in an all-or-none manner. The action potential requires two types of gated channel proteins in the membrane. One gated channel protein opens to allow Na+ to pass through the membrane and another opens to allow K+ to pass through the membrane.

Limbic System and Higher Mental Functions

The limbic system is an evoluntionary ancient group of linked structures deep within the cerebrum that is a functional grouping rather than an anatomical one. The limbic system blends primitive emotions and higher mental functions into a united whole. It accounts for why activities such as sexual behavior and eating seem pleasurable and also why mental stress can cause high blood pressure. Two significant structures within the limbic system are the amygdala and the hippocampus. The amygdala in particular can cause experiences to have emotional overtones and it creates the sensation of fear. This center can use past knowledge fed to it by association areas to assess a current situation and if necessary trigger the fight-or-glight reaction. The hippocampus plays a crucial role in learning and memory. The hippocampus region acts as an information gateway during the learning process determining what information about the world is to be sent to memory and how this information is to be encoded and stored by other regions in the brain.

Memory is the ability to hold a thought in mind or to recall events from the past ranging from a word we learned only yesterday to an early emotional experience that has shaped our lives. Learning takes place when we retain and utilize past memories. There are different types of memory: short-term memory, long-term memory, semantic memory, episodic memory and skill memory. Semantic memory is things like numbers, and words. Episodic memory is things like persons and events. Skill memory is involved in performing motor activities such as riding a bike or playing ice hockey. When a person first learns a skill more areas of the cerebral cortex are involved than after the skill is perfected.

Language depends on semantic memory. It appears that the left hemisphere plays a role of great importance in language functions. So when we see a word this is what happens: the word is seen in the visual cortex, information concerning the word is interpreted in Wernicke's area, information from Wernicke's area is transferred to Broca's area and then information is transferred from Broca's area to the primary motor area.

Autonomic System is also in the PNS. The autonomic system regulates the activity of cardiac and smooth muscles and glands. The system is divided into the sympathetic and parasympathetic divisions. Activation of these two systems generally causes opposite responses. Although their functions are different the two divisions share some features: 1. They function automatically and usually in an involuntary manner. 2. They innervate all internal organs. 3. They utilize two neurons and one ganglion for each impulse.

The nerves in the somatic system serve the skin, skeletal muscles, and tendons. The somatic system includes nerves that take sensory information from external sensory receptors to the CNS and motor commands away from the CNS to the skeletal muscles.