Chapter 10: Nervous system 1 A&P

Small cells that engulf and destroy microbes and cellular debris to protect the central nervous system are called

microglia

Ganglia

nerve cell bodies grouped together outside the central nervous system

Neurons

nerve cell that transmits impulses

Neuroglia

nerve cells that perform support and protection

The body's control center and communication network, which directs the functions of the body's organs and systems, is the

nervous system

A nerve is a bundle of _____.

neurons

Nucleus and Horns

Two other terms are of note: a nucleus is a mass of nerve cell bodies and dendrites inside the CNS, consisting of gray matter; horns are the areas of gray matter in the spinal cord

Unmyelinated vs. myleinated

An unmyelinated nerve fiber conducts an impulse over its entire length, but the conduction is slower than that along a myelinated fiber. A myelinated fiber is insulated by the myelin sheath, so transmission occurs only at the nodes of Ranvier between adjacent Schwann cells. Action potentials and inflow of ions occur only at these nodes, allowing the nerve impulse to jump from node to node, and the impulse travels much faster. An impulse on a myelinated motor fiber going to a skeletal muscle could travel about 120 meters per second, while an impulse on an unmyelinated fiber would travel only 0.5 meter per second.

Adrenalin/epinephrine

a neurotransmitter also called epinephrine used by the autonomic nervous system; a hormone

Spinal meninges

a series of connective tissue membranes specifically associated with the spinal cord

Meninges

a series of connective tissue membranes that surround the brain and spinal cord

Acetylcholinesterase

an enzyme in the postsynaptic neuron that breaks down acetylcholine.

Somatic nervous system

conducts impulses from the brain and spinal cord to skeletal muscle, causing us to respond or react to changes in our external environment

Autonomic nervous system (ANS)

conducts impulses from the brain and spinal cord to smooth muscle tissue, cardiac muscle tissue and glands

Dendrites

consist of one dendrite and one axon. Dendrites are short and branched, like the branches of trees. These are the receptive areas of the neuron and a multipolar neuron will have many dendrites. extension of the nerve cell body that receive nerve impulse.

Afferent peripheral system

consists of afferent or sensory neurons that convey information from receptors in the periphery of the body to the brain and spinal cord

Efferent peripheral system

consists of efferent or motor neurons that convey information from the brain and spinal cord to muscles and glands

Being a _____ center and a _____ network is a function of the nervous system

control, communication

Motor neurons are part of the _____ _____ _____.

efferent nervous system

Ependymal cells

line the fluid-filled ventricles of the brain; produce and move cerebrospinal fluid through the CNS

Action potential

when a muscle generates its own impulse to contract

The nerve glue is the _____ cell

neuroglial

Dura mater

(which means tough mother) the outermost spinal or cranial meninx, . It forms a tough outer tube of white fibrous connective tissue

Functions of the Spinal Cord

A major function of the spinal cord is to convey sensory impulses from the periphery to the brain and to conduct motor impulses from the brain to the periphery. Ascending nerve tracts of the spinal cord carry sensory information from body parts to the brain, and descending tracts conduct motor impulses from the brain to muscles and glands.

Integrating

A second principal function is to provide a means of integrating reflexes. A pair of spinal nerves is connected to each segment of the spinal cord. Each pair of spinal nerves is connected to that segment of the cord by two pairs of attachments called roots. The posterior or dorsal root is the sensory root and contains only sensory nerve fibers. It conducts impulses from the periphery (like the skin) to the spinal cord. These fibers extend into the posterior or dorsal gray horn of the spinal cord

Subdural space

Between the dura mater and the arachnoid is a space called the subdural space, which contains serous fluid. Between the arachnoid and the pia mater is the subarachnoid space. it is here that the clear, watery cerebrospinal fluid circulates. The meninges do not attach directly to the vertebrae. they are separated by a space called the epidural space. this space contains loose connective tissue and some adipose tissue that acts as a protective cushion around the spinal cord.

There are _____ pairs of cervical nerves

Eight

The three types of ions involved in nerve impulses are _____ (K), _____(Na) and _____ (Cl).

Potassium, sodium, chloride

Other types of Reflexes

Reflexes also occur within our bodies to help maintain homeostasis. Heartbeat rate, digestion and breathing rates are controlled and maintained by reflexes concerned with involuntary processes. Coughing (the choke reflex), sneezing, swallowing and vomiting are other examples of automatic subconscious reactions to changes within or outside our body

When the outside of a nerve is positively charged and the inside is negatively charged, the condition is known as the _____.

Resting potential

The _____ nervous system conducts impulses from the brain and spinal cord to skeletal muscle, thereby causing us to respond or react to changes in our external environment.

Somatic

The speeding up of activity is a function of the _____ division of the ANS

Sympathetic

Spinal Nerves

The 31 pairs of spinal nerves arise from the union of the dorsal and ventral roots of the spinal nerves. All the spinal nerves are mixed nerves because they consist of both motor and sensory fibers. Most of the spinal nerves exit the vertebral column between adjacent vertebrae. They are named and numbered according to the region and level of the spinal cord from which they emerge

Nucleus

a mass of nerve cell bodies and dendrites inside the central nervous system

Pia Mater

The innermost spinal meninx is known as the pia mater or delicate mother. It is a transparent fibrous membrane that forms a tube around and adheres to the surface of the spinal cord (and brain). it contains numerous blood vessels and nerves that nourish the underlying cells.

Arachnoid Mater

The middle spinal meninx is called the arachnoid mater or spider layer. It forms a delicate connective membranous tube inside the dura mater.

Other Point of Attachment

The other point of attachment of the spinal nerve to the cord is the anterior or ventral root and this is the motor root. It contains motor nerve fibers only and conducts impulses from the spinal cord to the periphery (like muscles). It connects with the anterior or ventral gray horn of the spinal cord

Sensory or afferent neuron

a neuron in contact with receptors, it detects changes in the external environment; also called afferent neuron

The Spinal Cord and Nerves

The spinal cord begins as a continuation of the medulla oblongata of the brainstem. Its length is approximately 16 to 18 inches. Its diameter varies at different levels because it is surrounded and protected by bone (the vertebrae) and by disks of fibrocartilage (the intervertebral disks). It is made up of a series of 31 segments, each giving rise to a pair of spinal nerves. In addition to the above protection, the spinal cord (as well as the brain) is further protected by the meninges, a series of connective tissue membranes. Those associated specifically with the spinal cord are called the spinal meninges.

By Name and Number

There are 8 pairs of cervical nerves, 12 pairs of thoracic nerves, 5 pairs of lumbar nerves, 5 pairs of sacral nerves and a single pair of coccygeal nerves The spinal nerves are also numbered according to the order (starting superiorly) within the region. Thus, the 31 pairs are: 1.C1 through C8 (cervical) 2.T1 through T12 (thoracic) 3.L1 through L5 (lumbar) 4.S1 through S5 (sacral) 5.Cx (coccygeal)

Reflex

an involuntary reaction to an external stimulus. This is experienced when we prick our finger on a rose thorn and immediately pull away from the source of pain. The reflex allowed us to respond much more quickly than if we had to consciously think about what to do and interpret the information in the CNS. A reflex then is an involuntary reaction or response to a stimulus applied to our periphery and transmitted to the CNS.

The middle layer, or "spider layer," of the meninges is called the ______.

arachnoid mater

Synapses

areas where the terminal branches of an axon are anchored close to, but not touching, the ends of the dendrites of another neuron. These synapses are one-way junctions that ensure that the nerve impulse travels in only one direction. This area is called a synaptic cleft. Other such areas of synapses are between axon endings and muscles or between axon endings and glands. An impulse continuing along a nerve pathway must cross this gap

Nerve tracts of the spinal cord that carry sensory information from body parts to the brain are known as which type of nerve tract?

ascending

Nerve

bundle of nerve cells or fibers located outside the CNS. Most nerves are white matter. Nerve cell bodies that are found outside the CNS are generally grouped together to form ganglia. Because ganglia are made up primarily of unmyelinated nerve cell bodies, they are masses of gray matter

Glial cells

cells that perform support and protection

The control center for the entire system is the _____ _____ _____.

central nervous system

The nerve fibers on the nerve cell that are the receptive areas of the neuron are known as:

dendrites

Nerve tracts of the spinal cord that conduct motor impulses from the brain to the muscles and glands are known as which type of nerve tract?

descending

The sensory root of the spinal cord is the _____ _____.

dorsal or posterior root

The _____ _____, which means "tough mother," is the outermost layer of the meninges.

dura mater

Schwann cells/ Synapses

form myelin sheaths around nerve fibers in the peripheral nervous system

Schwann cells/neurolemmocytes

form myelin sheaths around nerve fibers in the peripheral nervous system. Single cells called Schwann cells, also called neurolemmocytes surround the axon at specific sites and form the fatty myelin sheath around the axons in the peripheral nervous system.

Nodes of Ranvier/neurofibral nodes

gaps in the myelin sheath; also called neurofibral nodes

White matter

groups of myelinated axons from many neurons supported by neuroglia. White matter forms nerve tracts in the CNS

Posterior or dorsal gray horn

part of the spinal cord; also known as dorsal gray horn

The transparent fibrous membrane that forms a tube around and adheres to the surface of the spinal cord and brain is called the _______.

pia mater

Anterior or ventral root

point of attachment of the spinal nerve to the cord, also known as the motor root

Microglial cell

small cells that protect the central nervous system by (whose role is to engulf) engulfing and destroying microbes like bacteria and cellular debris

Central Nervous System

the control center for the whole system. It consists of the brain and spinal cord. All body sensations and changes in our external environment must be relayed from receptors and sense organs to the CNS to be interpreted (what do they mean) and then, if necessary, acted on (such as move away from a source of pain or danger)

Axon terminals

the endings of axons

Pia mater

the innermost spinal or cranial meninx

Membrane or resting potential

the ionic and electrical charge around a nerve fiber that is not transmitting an impulse; also called resting potential

Axon

the long extension of a nerve cell body; a neuron has only one axon. A nerve cell, however, will have only one axon, which begins as a slight enlargement of the cell body called the axonal hillock. The axon is a long process or fiber that begins singly but may branch and at its end has many fine extensions called axon terminals that contact with dendrites of other neurons. Numerous mitochondria and neurofibrils are in the axon

Arachnoid mater

the middle spinal or cranial meninx

Posterior or dorsal root

the sensory root containing only sensory nerve fibers; also known as dorsal root

A bundle of nerve fibers located inside the central nervous system is known as a ___

tract

Two Kinds of Nerve Fiber

•There are two kinds of nerve fibers on the nerve cell: •dendrites (DEN-drightz) •axons

An involuntary reaction to an external stimulus is known as a (n)

reflex

A _____ _____ is the simplest pathway able to receive a stimulus, enter the central nervous system for immediate interpretation, and produce a response

reflex arc

Repolarization

restoration of electrical charge. Now the potassium ions begin to move outside to restore the resting membrane potential. The sodium-potassium pump begins to function, pumping out the sodium ions that rushed in and pulling back in the potassium ions that moved outside, thus restoring the original charges. This is called repolarization, as shown in figure on the right, and the inside of the cell again becomes negative. This process continues along the nerve fiber acting like an electrical current, carrying the nerve impulse along the fiber. The nerve impulse is a self-propagating wave of depolarization followed by repolarization moving down the nerve fiber.

Depolarization

reversal of electrical charge. When a nerve impulse begins, the permeability to the sodium (Na1) ions changes; Na1 rushes in, causing a change from a negative (-) to a positive (+) charge inside the nerve membrane. This reversal of electrical charge is called depolarization and creates the cell's action potential. The action potential moves in one direction down the nerve fiber

Nissl bodies/chromatophilic substance

ribosomes attached to the rough ER in a neuron; also called chromatophilic substance

Astrocytes

star-shaped cells that twine around nerve cells to form a supporting network in the brain and spinal cord. star-shaped cells that wrap around nerve cells to form a supporting network in the brain and spinal cord. They attach neurons to their blood vessels, thus helping regulate nutrients and ions that are needed by the nerve cells.

The areas where the terminal branches of an axon are anchored close to, but not touching, the ends of the dendrites of another neuron are known as _______.

synapses

Horns

the areas of gray matter in the spinal cord

reflex arc

the pathway that results in a reflex. It is a basic unit of the nervous system and is the smallest and simplest pathway able to receive a stimulus, enter the CNS (usually the spinal cord) for immediate interpretation and produce a response.

Internuncial or association neurons

transmit the sensory impulse to the appropriate part of the brain or spinal cord for interpretation and processing; also known as association neurons

Two Categories of Efferent Peripheral System

1. The first is the somatic nervous system, which conducts impulses from the brain and spinal cord to skeletal muscle, thereby causing us to respond or react to changes in our external environment. 2. The second is the autonomic nervous system (ANS), which conducts impulses from the brain and spinal cord to smooth muscle tissue (like the smooth muscles of the intestine that push food through the digestive tract), to cardiac muscle tissue of the heart and to glands (like the endocrine glands).

Introduction

1. The nervous system is the body's control center and communication network. 2. It shares in the maintenance of homeostasis with the endocrine system.

Two Major Categories

1.Central Nervous System (CNS) 2.Peripheral Nervous System (PNS)

Cortex

1.smooth-textured area of the kidney extending from the renal capsule to the bases of the renal pyramids; 2. principal portion of the hair

Nerve Cell

A nerve cell is similar to a muscle cell in that there are concentrations of ions on the inside and the outside of the cell membrane. Positively charged sodium (Na1) ions are in greater concentration outside the cell than inside. There is a greater concentration of positively charged potassium (K1) ions inside the cell than outside. This situation is maintained by the cell membrane's sodium-potassium pump. In addition to the potassium ion, the inside of the fiber has negatively charged chloride (Cl2) ions and other negatively charged organic molecules.

Structural Classification of Neurons

Cells that conduct impulses from one part of the body to another are called neurons. They may be classified by both function and structure. The structural classification consists of three types of cells: 1.Multipolar 2.Bipolar 3.Unipolar

Structure of a Neuron

Each nerve cell's body contains a single nucleus. This nucleus is the control center of the cell. In the cytoplasm there are mitochondria, Golgi bodies, lysosomes and a network of threads called neurofibrils that extend into the axon part of the cell, referred to as the fiber of the cell. In the cytoplasm of the cell body there is extensive rough endoplasmic reticulum (ER). In a neuron, the rough ER has ribosomes attached to it. These granular structures are referred to as Nissl (NISS-l) bodies, also called chromatophilic substance, and are where protein synthesis occurs.

Motor or Efferent Neuron

From the association or internuncial neurons, the impulse is passed to the final nerve cell, the motor or efferent neuron. The motor neuron is of the multipolar type. This neuron brings about the reaction to the original stimulus. It is usually muscular (like pulling away from a source of heat or pain) but it can also be glandular (like salivating after smelling freshly baked cookies

Internuncial or Association Neurons

From the sensory neuron, the impulse may pass through a number of internuncial or association neurons. These are found in the brain and the spinal cord and are of the multipolar type. They transmit the sensory impulse to the appropriate part of the brain or spinal cord for interpretation and processing

Ranvier

Gaps in the myelin sheath are called nodes of Ranvier also called neurofibral nodes. These gaps allow ions to flow freely from the extracellular fluids to the axons, assisting in developing action potentials for nerve transmission.

Functional Classification of Neurons

Nerve cells pick up various changes in the environment (stimuli such as changes in temperature or pressure) from receptors. Receptors are the peripheral nerve endings of sensory nerves that respond to stimuli. There are many different types of receptors. Our skin has an enormous number of such receptors. These receptors change the energy of a stimulus, like heat, into nerve impulses. The first nerve cell receiving this impulse directly from a receptor is called a sensory or afferent neuron. These neurons are of the unipolar type. The receptors are in contact with only one end of the sensory neuron (the peripheral process in the skin), thus ensuring a one-way transmission of the impulse. The central process of the sensory neuron goes to the spinal cord

Classification of Nerve Cells

Nervous tissue consists of groupings of nerve cells or neurons that transmit information called nerve impulses in the form of electrochemical changes. A nerve is a bundle of nerve cells or fibers. Nervous tissue is also composed of cells that perform support and protection. These cells are called neuroglia or glial (GLEE-al) cells (neuroglia means nerve glue). Over 60% of all brain cells are neuroglia cells.

Endorphins

Neurotransmitters

Transmission of nerve impulses across synapses is brought about by the secretion of very low concentrations of chemicals called _____, which move across the gap

Neurotransmitters

More About Autonomic nervous system (ANS)

The ANS is considered to be involuntary. The organs affected by this system receive nerve fibers from two divisions of the ANS: 1. the sympathetic division, which stimulates or speeds up activity and thus involves energy expenditure and uses norepinephrine as a neurotransmitter, and 2. the parasympathetic division, which stimulates or speeds up the body's vegetative activities such as digestion, urination and defecation and restores or slows down other activities. It uses acetylcholine as a neurotransmitter at nerve endings.

Types of neurotransmitter

The autonomic nervous system uses adrenaline (also called epinephrine) as the transmitting agent. Many kinds of neurotransmitters are found in the nervous system. Some neurons produce only one type, others produce two or three. The best known neurotransmitters are acetylcholine and norepinephrine. Some others are serotonin, dopamine, and endorphins.

Large Peripheral Axons

The large peripheral axons are enclosed in fatty myelin sheaths produced by the Schwann cells. These are a type of neuroglial cell that wrap tightly in layers around the axon, producing fatty sheets of lipoprotein. The portions of the Schwann cell that contain most of the cytoplasm of the cell and the nucleus remain outside of the myelin sheath and make up a portion called the neurilemma. Narrow gaps in the sheath are the nodes of Ranvier

5 Components

The reflex arc has five components: 1.a sensory receptor in the skin 2.a sensory or afferent neuron 3.association or internuncial neurons within the spinal cord 4.a motor or efferent neuron 5.an effector organ like a muscle.

Neuroglial Cells

There are different kinds of neuroglial cells, and, unlike neurons, they do not conduct impulses. The table on the right lists the types of neuroglia

Peripheral Nervous System

This second category consists of all the nerves that connect the brain and spinal cord with sensory receptors, muscles and glands The PNS can be divided into two subcategories: 1. the afferent peripheral system, which consists of afferent or sensory neurons that convey information from receptors in the periphery of the body to the brain and spinal cord 2. the efferent peripheral system, which consists of efferent or motor neurons that convey information from the brain and spinal cord to muscles and glands.

Resting Potential

Thus, the nerve fiber has an electrical distribution as well, such that the outside is positively charged while the inside is negatively charged. This condition is known as the membrane or resting potential. Na1 and K1 ions tend to diffuse across the membrane but the cell maintains the resting potential through the channels of the sodium-potassium pump that actively extrudes Na1 and accumulates K1 ions.

Neurotransmitters

Transmission across synapses is brought about by the secretions of very low concentrations of chemicals called neurotransmitters that move across the gap. As the nerve impulse travels down the fiber, it causes vesicles in the axon endings of a presynaptic neuron to release the chemical neurotransmitter. Most of the synapses in our bodies use acetylcholine as the neurotransmitter. The acetylcholine allows the impulse to travel across the synaptic cleft to the postsynaptic neuron. However, it does not remain there because an enzyme in the cleft, acetylcholinesterase, immediately begins to break down the acetylcholine after it performs its function.

Tract

a bundle of fibers inside the central nervous system. Tracts can run long distances up and down the spinal cord. Tracts are also found in the brain and connect parts of the brain with each other and parts of the brain with the spinal cord. Ascending tracts conduct impulses up the cord and are concerned with sensation. Descending tracts conduct impulses down the cord and are concerned with motor functions. Tracts are made of myelinated fibers and therefore are classified as white matter

All-or-none law

a contraction or nervous transmission either occurs or does not occur. On any nerve fiber, the impulse will never vary in strength. If the stimulus or change in the environment is barely great enough to cause the fiber to carry the impulse, the impulse will be the same strength as one excited by a stronger stimulus. This is known as the all-or-none law, which states that if a nerve fiber carries any impulse, it will carry a full strength impulse

Myelin sheath

a fatty sheath surrounding some axons

Dopamine

a neurotransmitter

Star-shaped cells that twine around neurons for support in the brain and spinal cord and connect neurons to blood vessels are known as

astrocytes

Bipolar neuron

consist of one dendrite and one axon. They function as receptor cells in special sense organs. Only two (bi) processes come off the cell body. They are found in only three areas of the body: the retina of the eye, the inner ear and the olfactory area of the nose.

Peripheral nervous system (PNS)

consists of all the nerves that connect the brain and spinal cord with sensory receptors, muscles and glands

Central nervous system(CNS)

consists of the brain and spinal cord

The gray matter on the brain's surface is known as the _____.

cortex

Reversal of electrical charge is _____.

depolarization

Gray matter

gray areas of the nervous system. consisting of nerve cell bodies and dendrites. It also can consist of bundles of unmyelinated axons and their neuroglia. The gray matter on the surface of the brain is called the cortex.

Norepinephrine

hormone produced by the adrenal medulla; noradrenalin

Serotonin

hormone secreted by the pineal gland that acts as a neurotransmitter and vasoconstrictor

An area of gray matter in the spinal cord is known as a _______.

horn

Unipolar neurons

neurons that have only one process extending from the cell body; most sensory neurons are unipolar. This single process then branches into a central branch that functions as an axon and a peripheral branch that functions as a dendrite. Most sensory neurons are unipolar neurons. The branch functioning as an axon enters the brain or spinal cord; the branch functioning as a dendrite connects to a peripheral part of the body.

Multipolar neurons

neurons that have several dendrites and one axon. Most neurons in the brain and spinal cord are this type. The neuron studied in Chapter 5 is this type. Recall that the part of the neuron with the nucleus is called the cell body. The smaller extensions of the cell body are the dendrites, and the single long extension is called the axon

Acetylcholine

neurotransmitter substance

The _____ stimulates the body's vegetative activities and slows down other activities

parasympathetic division

Parasympathetic division

part of the autonomic nervous system that operates under normal nonstressful conditions

Sympathetic division

part of the autonomic nervous system that prepares the body for stressful situations that require energy expenditure

Anterior or ventral gray horn

part of the spinal cord

The second major category of the nervous system consisting of all the nerves that connect the brain and spinal cord with sensory receptors, muscles, and glands is the

peripheral nervous system

Oligodendroglia

provide support by forming semirigid connective-like tissue rows between neurons in the brain and spinal cord. look like small astrocytes. They also provide support by forming semi rigid connective-like tissue rows between neurons in the brain and spinal cord. They produce the fatty myelin (MY-eh-lin) sheath on the neurons of the brain and spinal cord of the CNS