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CHAPTER 8

The nervous system ~

Section 8.1:

• Nervous system: one of the most amazing designs in God’s living creation

  • Neurobiologist: a biologist who studies the nervous system

  • Neurologist: a physician who specializes in disorders of the nervous system

• Central nervous system: includes the brain and the spinal cord

  • Peripheral nervous system: consists of the nerves that branch from the brain and spinal cord

• Central nervous system:

  • Brain: the principal organ of the nervous system

  • Protected by the cranium

• Spinal cord: thick bundle of nerve fibers located within the spinal cavity

  • Spinal nerves: transmit nerve signals to and from the rest of the body

  • Cranial nerves: branch directly from the brain stem and transmit nerve signals to and from the eyes, ears, mouth, face, and scalp

  • Meninges: composed of an outer layer called the dura mater, a middle layer called the arachnoid mater, and an inner layer called the pia mater

    • Dura mater: one of the strongest tissues of the body; serves primarily as a flexible protective layer

    • Arachnoid mater: composed of thin fibers, like the fibers of a spider’s web, that form an intricate three­-dimensional network around the brain

    • Pia mater: forms a delicate covering that rests directly against the brain and spinal cord

  • Cerebrospinal fluid: clear fluid that circulates through the fibers of the arachnoid mater, cushioning the brain when you bump your head

  • Glial cells: support and insulate nerve tissue

  • Neurons: the actual nerve cells

    • Cell body: contains a nucleus (which controls its metabolic activities) and most of the nerve cell’s cytoplasm

    • Dendrite: a short, branched extension of the cell that receives nerve impulses from other neurons and conducts them toward the cell body

    • Axon: a long extension that relays nerve impulses from the cell body to other neurons

    • Nerve impulses travel in one direction in a nerve fiber–from the dendrites toward the cell body and from the cell body to­ward the axon

  • Gray matter: found within the brain and spinal cord; consists largely of the cell bodies of neurons and is gray because the cell bodies lack the white, specialized covering known as myelin

    • Myelin: white, specialized covering

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  • White matter: composed of axons and glial cells that are white because of their myelin content

  • Ganglia: masses of cell bodies

  • Plexus: a network of interconnected nerve fibers going to or from a region

    • Brachial plexus: located at the back of the neck and shoulder; branches to form the median nerve and other arm nerves

  • Nerve center: group of cell bodies in the brain or spinal cord

    • For example, the nerve centers in the medulla oblongata of the brain stem control your breathing

  • Sensory neurons: neurons that transmit information to the central nervous system from the senses of sight, hearing, taste, touch, and smell, as well as those that transmit pain signals

  • Motor neurons: neurons that relay signals from the central nervous system to the other parts of the body

  • Interneurons: found only in the central nervous system; relay signals between neurons or groups of neurons and are responsible for the processing of information by the brain, like the logic circuits of a computer

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  • Blood-brain barrier: protect the central nervous system from being permanently damaged every time you get sick

    • Microglia: help protect the brain from infection

• Meningitis: disease caused when invading microorganisms enter the nerve tissue and infect the meninges

  • Coma: a state of prolonged unconsciousness

  • Poliomyelitis (polio): a serious disease that attacks the spinal cord

    • It is caused by a virus that enters the motor neurons of the spinal cord (the anterior horn cells) and destroys some of them

• Peripheral nervous system:

  • Peripheral nervous system: consists of nerves

  • Nerves: bundles of nerve fibers (axons) branching from the brain and spinal cord and connecting the central nervous system to the extremities of the body

    • Sciatic nerve: one of the longest nerves in the body; connects the leg extremities with the spinal cord in the lower back

  • There are twelve pairs of cranial nerves (branching directly from the brain) and thirty-­one pairs of spinal nerves (connected to the brain through the spinal cord)

  • Mixed nerves: nerves contain both sensory and motor fibers

  • Sensory nerve fibers: carry impulses from light, taste, sound, touch, and pain from other parts of the body to the spinal cord and brain for analysis

  • Motor nerve fibers: carry impulses from the brain and spinal cord to produce action in muscles and organs

  • Autonomic nervous system: the part of the peripheral nervous system that controls the heart and other internal organs

    • Hypothalamus: controls the sympathetic and parasympathetic divisions of the autonomic nervous system

  • Sympathetic division: generally responds to your body’s needs during increased activity and in emergency situations by causing heartbeat and breathing rates to increase, thus supplying more blood to your body so that you can respond quickly

  • Parasympathetic division: acts as a balance to counteract the actions of the sympathetic division

Section 8.2:

• Structure of a Human Nerve

  • Median nerve: controls the muscles of the forearm and the muscles and skin of the hand

• Like the other nerves of the peripheral nervous system, the median nerve consists of bundles of nerve fibers surrounded by connective tissues.

• Nerve cells

• Neurons: responsible for the process of nerve impulse conduction

• Neurons, unlike most other body cells, rarely reproduce.

• Schwann cells: produce layers of myelin sheathing that acts much like the insulation on an electrical wire

• In the brain and spinal cord, myelin sheaths for axons are provided by special glial cells called oligodendrocytes instead of by Schwann cells

• Defective impulse transmission:

• Multiple Sclerosis: a disease of the brain and spinal cord; occurs when the body’s immune system attacks the glial cells that provide myelin sheaths for nerve cell axons

• Paralysis: the inability of the muscles to move

• How neurons work

  • Action potential:  a neuron is triggered to fire, a brief pulse of positive charge sweeps through the neuron and races down the axon like the fuse of a firecracker

• Synaptic Transmission: 

  • Synapse: an enclosed junction between two neurons or a neuron and another cell

• Neuromuscular junction:  a synapse between a neuron and a muscle fiber

• Neurotransmitter: tis a chemical that tis released when the action reaches the synapse

• Inhibitors of nerve impulses:

• Botulinum toxin: powerful poison that is responsible for the most deadly type of food poisoning

• Clostridium botulinum: manufactures the Botulinum toxin

• Parkinson’s disease: affects the patient’s control of posture and movement

• Dopamine: a lack of this can cause Parkinson’s disease 

• Reflex Action

• Reflex: a quick, automatic response; simplest act of the nervous system

• Reflex arc:  simplest nerve pathway