Nervous System and Sense Organs Chapter 49 Table of Contents Section 1 Neurons and Nerve Impulses Section 2 Structure of the Nervous System Section 3 Sensory

Nervous System and Sense OrgansChapter 49

Table of Contents

Section 1 Neurons and Nerve Impulses

Section 2 Structure of the Nervous System

Section 3 Sensory Systems

Section 4 Drugs and the Nervous System

Chapter 49

Objectives

• Describe the structure of a neuron.

• Summarize the electrical and chemical conditions that characterize a resting potential.

• Outline the electrical and chemical changes that occur during an action potential.

• Explain the role of neurotransmitters in transmitting a signal across a synapse.


Section 1 Neurons and Nerve ImpulsesChapter 49

Neuron Structure

• A neuron, or nerve cell, is made up of a cell body, dendrites, and axons.

– A cell body is the nucleus of a neuron and most of its organelles.

– Dendrites extend from the cell body and are covered by a membrane.

– The axon is a long membrane-bound projection that transmits information away from the cell body in the form of electrical signals.

Chapter 49

Structure of a Neuron


Chapter 49

Click below to watch the Visual Concept.

Visual Concept

Parts of a Neuron



Neuron Structure, continued

• The electrical signals that transmit information away from the cell body are called action potentials.

• The end of an axon is called an axon terminal.

• Axons are covered by a lipid layer called a myelin sheath.

– The myelin sheath insulates the neuron which speeds up the transmission of action potentials along the axon.


Neuron Structure, continued

• The signaling activity of the nervous system is made up of electrical activity within neurons and chemical flow between neurons.

• Neurons communicate with each other at special junctions called synapses.

• These synapses do not communicate by touch, but by releasing chemicals, or neurotransmitters, into a gap between the synapses called the synaptic cleft.

– The synaptic cleft is a small gap between the axon terminal and the receiving cell.


Nerve Impulses

• A neuron, like all other cells, has a membrane potential.

– A membrane potential is a difference in the electrical charge across a cell membrane.

• A membrane potential can change with an addition or removal of ions within the cell.

• Ions move in and out of the cell by passing through proteins that act as ion channels. Whether the ion channels are open or closed affects the membrane potential.


Nerve Impulses, continued

• Resting Potential

– A neuron is at rest when it is not sending or receiving a signal.

– When at rest, the neuron is more negatively charged on the inside than on the outside of the cell body.

– The electric potential across the cell membrane when the neuron is at rest is called the resting potential.



• Action Potential

– When a dendrite or cell body is stimulated, the permeability of the neuron’s membrane changes suddenly.

– The membrane potential reverses and the neuron’s cell body becomes more positively charged than the exterior of a cell.

• This reverse in polarity begins an action potential.



• Action Potential, continued

– After the first segment of the neuron is stimulated the segment next to the first will become stimulated.

• The action potential will continue away from the cell body.

– Soon after being stimulated, the interior of the neuron begins to become more and more negative.

• This signals the end of the action potential.



• Action Potential, continued

– A neuron cannot generate another action potential until it has returned to its resting potential.

– The period in which a neuron cannot send a signal is called the refractory period.

– Returning the neuron to its resting potential requires energy.

Conduction of a Nerve Impulse


Chapter 49


Visual Concept

Action Potential



Communication Between Neurons

• Once an action potential reaches the axon terminal, it releases neurotransmitters into the synaptic cleft. These neurotransmitters bind to receptors proteins and open the ion channels of the new neuron cell.

• If enough ion channels are opened, the action potential will continue through the new neuron. If not, the nervous signal will be terminated.

• After the neurotransmitters have opened the ion channels, they will be cleared out of the synaptic cleft by being reabsorbed by the neuron that released them or broken down by enzymes.

Synaptic Transmission


Chapter 49


Visual Concept

Release of Neurotransmitter


Chapter 49

Objectives

• Identify the two main parts of the central nervous system.

• Summarize the functions of the major parts of the brain.

• Describe the roles of the sensory and motor divisions of the peripheral nervous system.

• Distinguish between the somatic and autonomic nervous systems.

Section 2 Structure of the Nervous Systems

Section 2 Structure of the Nervous SystemsChapter 49

Organization of the Nervous System

• The nervous system includes two major divisions.

– The first division is the central nervous system and is made up of the brain and the spinal cord.

• The brain is the control center of the nervous system.

• The spinal cord carries nerve signals between the body and the brain.

Chapter 49

Structure of the Human Brain


Chapter 49


Visual Concept

Parts of a Human Brain



Organization of the Nervous System, continued

• The second division is the peripheral nervous system and consists of neurons that have cell bodies and that are not included in the brain and spinal cord.

– Peripheral neurons send information to and from the central nervous system.


Brain

• The cerebrum is made of two hemispheres, called the cerebral hemispheres.

• Each hemisphere is made up of four lobes, called the frontal, parietal, temporal, and occipital.

• Cerebrum

– The cerebrum is the upper part of the brain that receives sensation and controls movement.


Brain, continued

• Cerebrum, continued

– The folded outer layer of the cerebrum is called the cerebral cortex.

– The cerebral cortex is the portion of the cerebrum that controls higher mental functions, general movement, organ function, perception, and behavioral reactions.

• The many folds of the cerebral cortex allow the brain to have a large surface area and still fit into the skull.


Brain, continued

• Cerebrum, continued

– The cerebral cortex is called gray matter. Beneath the gray matter is white matter.

– White matter is made up of myelinated axons, which link specific regions of the cortex with each other and with other neural centers.

Chapter 49


Visual Concept

Cerebrum



Brain, continued

• Diencephalon

– This section of the brain is made up of two parts, the thalamus and hypothalamus.

• The thalamus directs most incoming sensory signals to the proper region of the cerebral cortex.

• The hypothalamus helps maintain homeostasis and directly controls most of the body’s hormone production.


Brain, continued

• Brian Stem

– This section of the brain is made up of three parts: the midbrain, pons, and medulla oblongata.

• The midbrain relays visual and auditory information.

• The pons relays communications between the cerebral hemispheres and the cerebellum.


Brain, continued

• Brian Stem, continued

– The last section of the brain stem is the medulla oblongata. The medulla oblongata serves as both a relay center and a control center for heart rate, respiration rate, and other homeostatic activities.

– The brain stem also has a network of neurons called the reticular formation. This section of the brain stem helps control respiration and circulation and helps separate signals that are important from those that are not.

Chapter 49


Visual Concept

Brain Stem



Brain, continued

• Cerebellum

– The cerebellum lies below and behind the cerebral hemispheres and helps to coordinate muscle action.

– The cerebellum receives sensory impulses from muscles, tendons, joints, eyes, and ears and other brain centers.

Chapter 49


Visual Concept

Cerebellum



Spinal Cord

• The spinal cord is a column of nervous tissue that starts at the medulla oblongata and runs throughout the vertebral column.

– The spinal cord is composed of white and gray matter.

Chapter 49


Visual Concept

Parts of a Human Spinal Cord



Peripheral Nervous System

• The central nervous system interacts with the peripheral nervous system through nerves.

– Nerves are bundled axons and dendrites of many neurons. Spinal nerves are made up of both a dorsal and ventral root.

– The dorsal roots carry signals into the central nervous system from sensory receptors.

• The sensory receptors are neurons that are specialized to detect stimuli such as light, pressure, and heat.


Peripheral Nervous System, continued

• The peripheral nervous system also contains ventral roots. Ventral roots contain the axons of motor neurons.

– Motor neurons are neurons that carry information from the central nervous system to muscles and glands.

• Interneurons can also be found in both sections of the nervous system. Interneurons can relay information between other neurons.

Peripheral Nervous System



Sensory Division

• The sensory division of the peripheral nervous system contains sensory receptors and the interneurons that connect them to the central nervous system.

– Sensory receptors receive information from the body’s external and internal environments.


Motor Division

• The motor division of the peripheral nervous system allows the body to react to sensory information.

– The motor division of the peripheral nervous system is made up of the somatic nervous system and the autonomic nervous system.


Motor Division, continued

• Somatic Nervous System

– The somatic nervous system contains motor neurons that control the movement of skeletal muscles.

• The somatic system is considered voluntary, but can operate without conscious control.



• Somatic Nervous System, continued

– Somatic system also relays the signals in reflexes.

– Reflexes are involuntary and often self-protective movements. An example is the patellar, or knee-jerk, reflex.

– The patellar reflex is an example of a spinal reflex, which involves only neurons in the body and bypasses the brain.

Knee-Jerk Response


Chapter 49


Visual Concept

Reflex




• Autonomic Nervous System

– The autonomic nervous system controls internal body conditions by regulating smooth muscles in blood vessels and organs.

• The autonomic system is considered involuntary, and can be broken up into two subdivisions.



• Autonomic Nervous System, continued

– The first subdivision of the autonomic nervous system is the sympathetic division.

• The sympathetic division prepares the body when activated by physical or emotional stress.

• For example, pupils dilate and heart rate increases.



• Autonomic Nervous System, continued

– The second subdivision of the autonomic nervous system is the parasympathetic division.

• The parasympathetic division controls the internal environment during routine conditions.

• For example, pupils constrict and heart rate decreases.

Physiological Effects of the Autonomic Nervous System


Chapter 49


Visual Concept

Comparing the Somatic and Autonomic Nervous Systems


Chapter 49

Objectives

• List the stimuli to which each of the five types of sensory receptors respond.

• Identify the parts of the ear responsible for hearing and for maintaining balance.

• Describe the structure of the eye and roles of rods and cones in vision.

• Discuss how taste and smell are detected.

• Compare the detection of touch, temperature and pain.


Section 3 Sensory SystemsChapter 49

Perception of Stimuli

• In order to detect changes in the environment, organisms use their sense organs.

• Sense organs are organs that receive stimuli and give rise to the senses such as sight, smell, hearing and pain.

• Sense organs are part of the sensory division of the peripheral nervous system.

Chapter 49


Visual Concept

Sense Organs



Perception of Stimuli, continued

• Receptors and Sense Organs

– Sensory receptors can be categorized based on the type of stimuli that they respond to.

• Mechanoreceptors respond to movement, pressure and tension.

• Photoreceptors respond to variations in light.



• Receptors and Sense Organs, continued

– Chemoreceptors are sensory receptors that respond to chemicals.

– Thermoreceptors are sensory receptors that respond to changes in temperature.

– Pain receptors are sensory receptors that respond to tissue damage.

Types of Sensory Receptors




• Receptors and Sense Organs, continued

– Sensory receptors are found in higher concentrations in the sense organs than in other parts of body.

– When stimulated, these sensory receptors turn the stimulus into electrical signals and send those signals to the brain.

– Each signal that is sent to the brain is similar but may be sent to different parts of the brain to be interpreted.


Hearing and Balance

• The ear performs two main functions: detecting sound and maintaining balance.

• Sound vibrations are directed into the ear by the fleshy structure of the outer ear.

• The outer ear is connected to the middle ear by the auditory canal.

– The auditory canal is also the tube that allows air into the ear.


Hearing and Balance, continued

• The middle ear begins with the tympanic membrane.

– The tympanic membrane is also called the eardrum.

• The Eustachian tube controls the air pressure beyond the tympanic membrane.

– The Eustachian tube is an opening to the throat that equalizes the pressure on both sides of the tympanic membrane.



• When sound waves hit the tympanic membrane it begins to vibrate.

• These vibrations cause the three bones of the middle ear (hammer, anvil, and stirrup) to also vibrate.

• These vibrations are passed to the oval window by the stirrup.

– The oval window separates the middle ear from the inner ear.



• Past the oval window is the inner ear. The inner ear includes the cochlea.

• The cochlea is a coiled tube consisting of three fluid-filled chambers that are separated by membranes.

• The middle chamber is called the organ of Corti and is the hearing organ.

– The organ of Corti rests on the bottom membrane in the cochlea.



• The vibrations in the fluid of the cochlea move the bottom membrane, which causes hair cells to bend.

– Hair cells are the mechanoreceptors in the ear and can be easily damaged.

• The bending of the hair cells cause the release of neurotransmitters.

• These neurotransmitters stimulate the neurons in the auditory nerve. This stimulation allows you to hear.

Structure of the Ear


Chapter 49


Visual Concept

Parts of the Human Ear


Chapter 49


Visual Concept

Human Hearing




• The ear also helps maintain balance.

• Balance is maintained by mechanoreceptors in the three fluid-filled semicircular canals of the inner ear.

– When the head moves the hair cells bend.

– The brain interprets the hair cell movement and sends out the proper signals to help the body maintain balance.


Vision

• The eyes detect light and transmit signals to visual processing areas of the brain.

• The retina is where all structures focus light.

– The retina is the light sensitive layer. It receives images and transmits them through the optic nerve to the brain.


Vision, continued

• In order to reach the retina, light must first pass through the cornea, pupil, iris, and lens.

Chapter 49


Visual Concept

Parts of the Human Eye



Vision, continued

• The cornea is a transparent membrane that forms the front portion of the eyeball, where light is first encountered.

• The pupil is the next to receive light and is the opening to the interior of the eye.

• The amount of light that enters the pupil is controlled by the colored circular part of the eye, the iris.

• The lens is a crystalline structure that muscles adjust to focus the light on the retina.


Vision, continued

• The retina interprets light through photoreceptors that translate light energy into electrical signals that can be interpreted by the brain.

• The photoreceptors within the eye can be broken down into two types: rods and cones.

– Rods can detect dim light and play a major role in noncolor and night vision.

– Cones can distinguish colors and are sensitive to bright light.

Structure of the Eye


Chapter 49


Visual Concept

Comparing Rods and Cones



Taste and Smell• Chemoreceptors responsible for taste are called taste buds

and can be found on the tongue, the throat, and the roof of the mouth.

• Taste buds are found on the tongue between bumps called papillae.


Taste and Smell, continued• Chemoreceptors in the nasal passages that can

detect chemicals in the air are called olfactory receptors.

• They are located in the mucous lining of the epithelium in the nasal passage.

Chapter 49


Visual Concept

Comparing Taste and Smell



Pressure and Temperature

• Mechanoreceptors located throughout the skin make it possible to sense touch, pressure, and tension.

• Two types of receptors monitor skin temperature: hot and cold receptors.

• Pain receptors are located throughout the interior of the body and at the base of the epidermis.

• Sensory input from sensory receptors travels in an orderly way to the brain.

Chapter 49

Objectives

• Define the relationship between addiction and tolerance.

• Explain the physical basis of cocaine addiction.

• Identify six types of psychoactive drugs.

• List the effects of alcohol and tobacco on the body.


Section 4 Drugs and the Nervous SystemChapter 49

Psychoactive Drugs

• Drugs are substances that cause a change in a person’s physical or psychological state.

• A psychoactive drug is a drug that alters the functioning of the central nervous system.

– A psychoactive drug can be manmade or natural.

Psychoactive Drugs of Abuse



Psychoactive Drugs, continued

• Addiction and Tolerance

– The abuse of a psychoactive drug usually leads to dependence.

– Dependence is a state in which a person relies on a drug physically or emotionally in order to function.

– Dependence often leads to addiction. Addiction is a condition in which a person can no longer control his or her drug use.

Chapter 49


Visual Concept

Addiction




• Addiction and Tolerance, continued

– When exposure to a drug is repeated, a person addicted to the drug can develop a tolerance.

– Tolerance is a characteristic of drug addiction in which larger and larger amounts of the drug are needed to achieve the desired sensation.

Chapter 49


Visual Concept

Tolerance




• Addiction and Tolerance, continued

– Once addicted, a user will go into withdrawal if the drug is taken away.

– Withdrawal includes uncomfortable physical and psychological symptoms produced when a physically dependent drug user stops using drugs.

– Symptoms of withdrawal vary from person to person.



• Neural Changes

– An example of how psychoactive drugs affect the brain will be shown using cocaine.

– Cocaine is a stimulant, or a drug that generally increases the activity of the central nervous system.



• Neural Changes

– The effect of psychoactive drugs on the brain can be illustrated with the example of cocaine.

– Cocaine is a stimulant, or a drug that generally increases the activity of the central nervous system.



• Neural Changes, continued

– The excess dopamine overstimulates the postsynaptic neuron, which responds by decreasing the number of receptor proteins on the postsynaptic neuron.

– In the absences of the drug, dopamine release returns to normal, but the postsynaptic cell is understimulated due to a lack of receptors.

Action of Cocaine


Action of Cocaine, continued


Chapter 49


Visual Concept

Neural Changes Caused by Drug Use



Drugs and Abuse

• Alcohol

– Alcohol is a legal drug and can be classified as a depressant.

– A depressant is a drug that decreases the activity of the central nervous system.

– A measurement of how much alcohol is in a person’s blood is called their blood alcohol concentration or BAC.

Effects of Blood Alcohol Concentration


Chapter 49


Visual Concept

Blood Alcohol Concentration and Its Effects



Drugs and Abuse, continued

• Tobacco

– Tobacco, like cocaine, is a highly addictive stimulant. The major drug in tobacco is nicotine.

– Nicotine is a toxic, addictive alkaloid that is derived from tobacco and is one of the major contributors to the harmful effects of smoking.

– Nicotine mimics the action of a neurotransmitter called acetylcholine.



• Tobacco, continued

– Nicotine is not the only harmful substance found in tobacco; burning tobacco produces tars.

• Tars are complex mixtures of chemicals and smoke particles.

Chapter 49


Visual Concept

Withdrawal




• Tobacco, continued

– Long term use of tobacco can lead to cancer, chronic bronchitis, and emphysema.

• Chronic bronchitis is an inflammation of the bronchi and bronchioles.

• Emphysema is a degenerative lung disease in which alveoli lose their elasticity and eventually rupture.

Documents

Nervous System and Sense Organs Chapter 49 Table of Contents Section 1 Neurons and Nerve Impulses Section 2 Structure of the Nervous System Section 3 Sensory