Chapter 13 Nervous System. Points to Ponder What are the three types of neurons? What are neuroglia?...

Chapter 13

Nervous System

Points to Ponder• What are the three types of neurons?• What are neuroglia?• What is the structure of a neuron?• What is the myelin sheath? Saltatory conduction? Scwhann

cell? Node of Ranvier?• Explain the resting and action potential as they relate to a

nerve impulse.• How does the nerve impulse traverse the synapse?• What are the two parts of the nervous system?• What 3 things protect the CNS?• What are the 4 parts of the brain and their functions?• What is the reticular activating system and the limbic system?• What are some higher mental functions of the brain?• What are the 2 parts of the peripheral nervous system?• Be able to explain the abuse of several drugs.

The Nervous System • Allows for communication between cells

through sensory input, integration of data and motor output

• Two major divisions– Central Nervous System (CNS):

• brain and spinal cord

– Peripheral Nervous System (PNS): • nerves outside of the CNS

• 2 cell types: – Neurons:

• transmit nerve impulses in nervous system

– Neuroglia: • support and nourish neurons

Functions of the Nervous System

1. Nervous System receives sensory input- Sensory receptors generate nerve impulses

that travel by way of the PNS to the CNS

2. CNS performs integrations- Sums up the input it receives from the body

3. CNS generates motor output- Nerve impulses from the CNS go by way of the

PNS to the muscles and glands

Expanding on neurons• 3 types of neurons:

1.Sensory – takes impulses from sensory receptor to CNS

• Detect changes in the environment2.Interneurons – receive information in the CNS

and send it to a motor neuron• Sum up all the nerve impulses received from sensory

neurons and other interneurons before communication with motor neurons

3.Motor – takes impulses from the CNS to an effector (i.e. gland or muscle fiber)

• Effectors: carry out our responses to the environmental changes

13.1 Overview of the nervous system

Neuron Structure (Ch 4 review)

• Cell body – main cell where organelles and nuclei reside

• Dendrite – many, short extensions that carry impulses to a cell body• Receive signals from sensory receptors or other

neurons• Signals result in nerve impulses that are conducted by

an axon

• Axon (nerve fiber) – single, long extension that carries impulses away from the cell body

The Structure of Neurons

Figure 12–1

The Myelin Sheath• A lipid covering on long axons • Functions:

– Increase the speed of nerve impulse conduction – Nerve insulation – Nerve regeneration in the PNS only

• When severed, myelin sheath remains and serves as a passageway for new fiber growth

• Neuroglia cells involved in Myelin Sheath formation– Schwann cells – in the PNS– Oligodendrocytes – in the CNS

• Nodes of Ranvier – – gaps between myelination on the axons

• Saltatory conduction –– conduction of the nerve impulse from node to node

Gray Vs. White Matter

• Gray matter in CNS:– Contains no myelinated axons

• White matter in CNS:– Contains myelinated axons

The Nerve Impulse: Resting Potential • Voltmeter:

– Allows us to measure the potential difference between two sides of the axonal membrane (plasma membrane of the axon), expressed in term of voltage

• Resting potential – when the axon is not conducting a nerve impulse• More positive ions outside than inside the membrane• There is a negative charge of -65mV inside the axon• More Na+ outside than inside, More K+ inside than outside

• Unequal distribution due to sodium-potassium pump• Active transport of 3Na+ out and 2K+ into the axon

• Membrane is permeable to K+ but not Na+• More positive ions outside the membrane than inside

• Large, negatively charged organic ions in the axoplasm contributes to negative charge inside the membrane

The Nerve Impulse: Action Potential• Action potential – rapid change in the axon membrane that

allows a nerve impulse to occur• If stimulus causes axonal membrane to depolarize to

certain level called threshold, action potential occurs • Steps for Action Potential:1. First, Sodium gates open letting Na+ in

• Depolarization occurs • Interior of axon loses negative charge (+40mV)

2. Secondly, Potassium gates open letting K+ out• Repolarization occurs• Interior of axon regains negative charge (-65mV)

3. Resting potential is restored by moving potassium inside and sodium outside

*Wave of depolarization/repolarization travels down axon*

Propagation of Action Potentials

Two Methods

1. Continuous propagation:– unmyelinated axons

2. Saltatory propagation:– myelinated axons

Propagation of Action Potential

1. Continuous Propagation:- Unmyelinated axons- Whole membrane depolarizes and repolarizes

sequentially hillock to terminal

Continuous Propagation

Figure 12–14 (Step 2)

Propagation of Action Potential2. Saltatory Propagation:

- Myelinated axons- Depolarization only on exposed membrane at nodes- Myelin insulates covered membrane from ion flow- Action potential jumps from node to node

- Faster and requires less energy to reset

Saltatory Propagation

Figure 12–15 (Steps 1, 2)

Saltatory Propagation

Figure 12–15 (Steps 3, 4)

The synapse

• A small gap between the sending neuron (presynaptic membrane) and the receiving neuron (postsynaptic membrane)

• Transmission is accomplished across this gap by a neurotransmitter (e.g. ACh, dopamine and serotonin)

• Neurotransmitters are stored in synaptic vesicles in the axon terminals

13.1 Overview of the nervous system

Transmission across the synapse 1. Nerve impulse reaches the axon terminal

- Close to dendrite or cell body of another neuron2. Calcium ions enter the axon terminal

- stimulate synaptic vesicles to fuse with presynaptic membrane

3. Neurotransmitters are released- diffuse across the synapse and bind with the

postsynaptic membrane via specific receptors that inhibit or excite the neuron- Excitation:

- neurotransmitters cause Na+ gates to open, and Na+

diffuses into the receiving neuron- Inhibition:

- neurotransmitters cause K+ to enter the receiving neuron

Transmission across the synapse

4. Neurotransmitter removal from the cleft, either

1. Enzymes that rapidly inactivate the neurotransmitter

2. Sending membrane rapidly reabsorbs the neurotransmitter for

- repackaging in synaptic vesicles

- molecular breakdown

Neurotransmitter Molecules

• Acetylcholine – act at neuromuscular junctions excites skeletal muscle– Inhibits cardiac muscle– excites or inhibits smooth muscles and glands

• Norepinephrine – excites smooth muscle– Important to dreaming, waking, and mood

• Serotonin– Involved in thermoregulation, sleeping, emotions, and

perception

• Decreased levels of norepinephrine and serotonin is linked to depression

Drugs

1. Block the release of neurotransmitters

2. Mimic the action of a neurotransmitter

3. Block the receptor

4. Interfere with the removal of a neurotransmitter from a synaptic cleft

Neuromodulators

• Block the release of a neurotransmitter or modify a neuron’s response to a neurotransmitter

1. Caffeine- Interferes with effects of inhibitory

neurotransmitters in the brain

2. Substance P- Released by sensory neurons during pain

3. Endorphins- Block release of substance P, natural painkiller

Synaptic Integration

• Integration is the summation of the inhibitory and excitatory signals received by a postsynaptic neuron

• This occurs because a neuron receives many signals

The nervous divisions

• 2 divisions:– Central nervous system (CNS):

• Brain and spinal cord– Peripheral nervous system (PNS):

• Nerves and ganglia (cell bodies)

The central nervous system

• Consists of the brain and spinal cord• Both are protected by:

• Bones – skull and vertebral column• Meninges – 3 protective membranes wrap around CNS• Cerebral spinal fluid (CSF) – space between meninges is

filled with this fluid that cushions and protects the CNS• Also contained in the ventricles of the brain

• Both made up of 2 types of nervous tissue:• Gray matter – contains cell bodies and nonmyelinated

fibers• White matter – contains myelinated axons

Ventricles

• Interconnecting chambers that produce and serve as a reservoir for CSF

A. Lateral Ventricle (2)- Left and Right Cerebral hemisphere

B. Third Ventricle (1)- Diencephalon (Hypothalamus and thalamus)

C. Fourth Ventricle (1)- Brain stem and cerebellum- Connects to central canal of spinal cord

The CNS: Spinal cord• Extends from the base of the brain through

foramen magnum and along the length of the vertebral canal formed by the most vertebrae

• Functions– Provide communication between the brain and

the body– Center for reflex arcs– Act as “gate” control flow of pain messages from

peripheral nerves to brain• Pain message may pass to the brain to be perceived • Pain message may be blocked from reaching the brain

The CNS: Spinal cord• Nerves project from the cord between the vertebrae • Central canal and meninges contains CSF • Gray matter is in the center is a butterfly shape

– Portions of sensory neurons, motor neurons, and interneurons are found here

– Dorsal root of spinal nerves contains sensory fibers entering the gray matter

– Ventral root of a spinal nerve contains motor fibers exiting the gray matter

• White matter surrounds the gray matter– Ascending tracts info to brain– Descending tracts info from brain to motor neurons

Reflex Arcs = Single Reflex

• Spinal cord is the center for reflex arcs

• Rapid, automatic nerve responses triggered by specific stimuli

• Used to maintain homeostasis

• Simple reflex:– Sensory perception in, motor response out

Figure 13–14

5 Steps in a Neural Reflex

Reflex Arcs for Internal Organs

• Blood Pressure, if low:– Detected by carotid arteries and aorta– Generate nerve impulses that pass through

sensory fibers to the cord – Travels ascending tract to cardiovascular

center in the brain– Nerve impulse passes down a descending

tract to spinal cord– Motor impulses cause blood vessels to

constrict to rise blood pressure

The CNS: Brain

4 major parts:1. Cerebrum

2. Diencephalon

3. Cerebellum

4. Brain stem

13.2 The central nervous system

The brain: Cerebrum• Last center to receive sensory input and carry

out integration before commanding voluntary motor responses

• Consists of:– Cerebral hemisphere– Cerebral cortex– Primary motor and sensory areas of the cortex– Association areas– Processing centers– Central white matter


1. The brain: Cerebrum – the lobes• Cerebrum – largest portion of the brain• Longitudinal fissure (deep grooves called sulci)

– divides the left and right cerebral hemispheres• Corpus callosum:

– connects the two hemispheres via a bridge of tracts• Sulci divide cerebrum into 4 lobes/hemispheres:

• Frontal lobe: • primary motor area and conscious thought

• Temporal lobe: • primary auditory, smell and speech area

• Parietal lobe: • primary somatosensory and taste area

• Occipital lobe:• primary visual area

The brain: Cerebrum – the cerebral hemispheres

1. The brain: Cerebrum – the cerebral cortex

• Cerebral cortex – thin, outer layer of gray matter– Accounts for sensation, voluntary movement, and all the

thought processes we associate with consciousness

1. Primary motor area – voluntary control of skeletal muscle- In the frontal lobe before the central sulcus

2. Primary somatosensory area – sensory information from skeletal muscle and skin arrive here

- Dorsal to the central sulcus in the parietal lobe- Primary taste area taste sensation- Primary visual area occipital lobe receives info from eyes- Primary auditory area temporal lobe receives info from ears- Primary olfactory area temporal lobe receives info for smell

1. Cerebrum – the cerebral cortexPrimary Motor area and Primary

Somatosensory area


3. Association areas – integration occurs here- Premotor area organizes motor functions for skilled

motor activities- Primary motor area sends signals to the cerebellum,

which integrates them- Somatosensory association area processes and

analyzes sensory information from the skin and muscles

- Visual association area associates new visual information with previously received visual info

- Auditory association area associates new auditory information with previously received auditory info


4. Processing centers – perform higher level analytical functions including Wernicke’s and Broca’s areas both involved in speech– Prefrontal areas receive info from other association areas

and uses this info to reason and plan our actions– Wernicke’s area (dorsal part of left hemisphere)

• helps us understand both the written and spoken word • sends the info to the Broca’s area

– Broca’s area (portion of primary motor area)• adds grammatical refinements • directs the primary motor area to stimulate the appropriate muscles

for speaking and writing


5. Central White Matter (mylinated axons)– Develops as a child grows

• makes children more capable of speech

– Descending tracts: • primary motor lower brain centers

– Ascending tracts: • Lower brain centers primary somatosensory area

– Tracts cross over in the medulla• Left controls right, right controls left

– Tracts take info between sensory, motor, and association areas within the brain

– Corpus collosum: • tract that joins the left and right hemispheres

2. The brain: Diencephalon• Hypothalamus – helps maintain homeostasis

(hunger, sleep, thirst, body temperature and water balance) and controls pituitary gland• Link between the nervous and endocrine systems

• Thalamus – 2 masses of gray matter that receive all sensory input except smell; involved in memory and emotions• Visual, auditory, and somatosensory info arrives at the

thalamus via the cranial nerves and tracts from the spinal cord

• Integrates this info and sends it to the appropriate portions of the cerebrum

• Involved in arousal of the cerebrum

• Pineal gland – secretes melatonin that controls our daily rhythms

2. The brain: Diencephalon

3. The brain: Cerebellum

• White matter = arbor vitae– primary composition of cerebellum

• Gray matter = thin layer overlying white matter• Receives and integrates sensory input from

the eyes, ears, joints and muscles about the current position of the body

• Functions to:• Maintains posture• Coordinates voluntary movement• Allows learning of new motor skills (i.e. playing the

piano or hitting a baseball)


4. The brain: The brain stem1. Midbrain –

- Relay station between cerebrum and spinal cord or cerebellum

- Reflex centers for visual, auditory, and tactile responses

2. Pons – - a bridge between cerebellum and the CNS- regulate breathing rate with the medulla oblongata - reflex center for head movements in response to visual

and auditory stimuli3. Medulla oblongata –

- reflex centers for regulating breathing, heartbeat and blood pressure- contains tracts that ascend or descend between the spinal cord and higher brain centers

4. Reticular formation


4. The brain: The brain stem• Reticular formation – major component of

the reticular activating system (RAS) that regulates alertness• Receives sensory signals and sends them up to

higher centers, and motor signals which it sends to the spinal cord

• RAS arouses the cerebrum via the thalamus• Can filter out unnecessary sensory stimuli

• Example: study with the TV on

• To inactivate RAS• remove of visual and auditory stimuli

• Injury to RAS coma

The reticular activating system

The limbic system• Located between cerebrum and diencephalon• Joins primitive emotions (i.e. fear, pleasure) with

higher functions such as reasoning• Can cause strong emotional reactions to situations but

conscious thought can override and direct our behavior

• Functions:1. Establishes emotional states and drives2. Links conscious functions of cerebrum to

autonomic functions of brainstem3. Facilitates memory storage and retrieval

The limbic system

• Includes:• Amygdala – has emotional overtones

• Creates sensation of fear, triggers the fight-or-flight reaction

– Frontal cortex can override the limbic system and cause us to rethink the situation

• Hippocampus – important to learning and memory

• Info gateway during learning process• Link of hippocampus to Alzheimers

The limbic system

13.3 The limbic system and higher mental functions

Higher mental functions• Learning –

– what happens when we recall and use past memories• Memory –

– ability to hold a thought or to recall past events• Short-term memory –

– retention of information for only a few minutes• Long-term memory – retention of information for more than

a few minutes and include the following:• Episodic memory – persons and events• Semantic memory – number and words• Hippocampus serves as a bridge between the sensory

association areas, where memories are stored, and the prefrontal area, where memories are utilized

• Long-term potentiation occurs after synapses have been used intensively for a short period of time, they release more neurotransmitters than before

• Causes memory storage


Higher mental functions

• Skill memory – performing skilled motor activities (i.e. riding a bike)– When a skill is first learned

• more areas of the cerebral cortex are involved – Skill memory involved all the motor areas of the cerebrum

below the level of consciousness• Language – depends on semantic memory

– Any disruption can contribute to an inability to comprehend our environment and use speech correctly

– Damage to • Wernicke’s area inability to comprehend speech• Broca’s area inability to speak and write

What parts of the brain are active in reading and speaking?


The peripheral nervous system (PNS)

• Includes cranial (12 pr) and spinal nerves (31 pr) and ganglia outside the CNS- Spinal nerves conduct impulses to and from the

spinal cord- Cranial nerves conduct impulses to and from the

brain

• Divided into 2 systems:- Somatic- Autonomic

13.4 The peripheral nervous system

The peripheral nervous system (PNS)

• Cell body and dendrites in CNS or ganglia

• Axons or neurons project from the CNS and form the spinal cord

• Nerves = axons (long part of neurons)

The PNS

• Cranial Nerves– Sensory nerves, motor nerves, or mixed nerves– Controls the head, neck, and facial regions – Example is the Vagus Nerve (X)

• Controls the pharynx, larynx, and internal organs

• Arise from medulla oblongata and communicates with the hypothalamus to control internal organs

The PNS• Spinal Nerves:

– Dorsal root of spinal nerve • contains sensory fibers that conduct impulses toward the

spinal cord from sensory receptors

– Cell body of sensory neuron is in a dorsal root ganglion– Ganglion

• collection of cell bodies outside of the CNS

– Ventral root of spinal nerve• Contains motor fibers that conduct impulses away from the

cord to effectors

– All spinal nerves are mixed nerves– Each spinal nerve serves the particular region of the body

The peripheral nervous system

The PNS: Somatic division• Serves the skin, skeletal muscles and tendons

• Includes sensory receptors, sensory nerves, and motor nerves

• Automatic responses are called reflexes

Features of the Autonomic System

• Function automatically and usually in an involuntary manner

• They innervate all internal organs

• They utilize two neurons and one ganglion for each impulse– Preganglionic nerve fiber ganglion

postganglionic nerve fiber contact with organs

• Reflex actions of the ANS– Regulate blood pressure and breathing rate

The PNS: Autonomic division

• Regulates the activity of involuntary muscles (cardiac and smooth) and glands

• Divided into 2 divisions:– Sympathetic: neurotransmitter Norepinephrine

• coordinates the body for the “fight or flight” response• speeds up metabolism, heart rate and breathing while

down regulating other functions

– Parasympathetic: neurotransmitter acetylcholine• brings a relaxed state • slows down metabolism, heart rate and breathing and

returns other functions to normal

13.4 The peripheral nervous system

Degenerative brain disorders

• Alzheimer disease – Usually seen in people after 65 yrs. old– Starts with memory loss– APOE4: 65% of AD persons have this gene– Two histological causes:

• Abnormal neurons with plaques of beta amyloid – Sticky B-amyloid forms when snipped by secretases– Accumulation results in inflammation and neuronal death

• Neurofibrillary tangles in axons that extend around the nucleus

– Protein tau losses it shape and grabs onto other tau molecules resulting in tangles

Degenerative brain disorders

• Parkinson disease– Usually begins between the ages of 50-60– Characterized by loss of motor control– Due to degeneration of dopamine-releasing

(inhibitory effect) neurons in the brain• Dopamine is an inhibitory neurotransmitter• Without dopamine, excessive excitatory signals form the

motor cortex and brain result in symptoms of Parkinsons disease

• Treatment– I-dopa, chemical that can be changed into dopamine

Drugs and drug abuse

• Drugs have two general effects on the nervous system: – affect the limbic system – Promote or decrease the action of a certain

neurotransmitter (stimulants or depressants)• Most drug abusers take drugs that affect dopamine

– Dopamine involved in reward circuit, regulates mood– Drugs artificially affect the reward circuit to the point

they ignore basic physical needs in favor of the drug• Drug abusers show physiological and psychological

effect• Once a person is physically dependent

– They usually need more of the drug for the same effect because their body has become tolerant

Drug abuse: Alcohol• Alcohol

– depressant directly absorbed from the stomach and small intestine

– Increases action of GABA and increases the release of beta-endorphins in the hypothalamus

• Most socially accepted form of drug use• About 80% of college-aged people drink• Effects on the body

– Denatures proteins, causes damage to tissues such as the brain and liver

– Chronic consumption can damage the frontal lobe, decrease brain size, and increase the size of the ventricles

• High blood alcohol levels can lead to– poor judgment, loss of coordination or even coma and

death

13.5 Drug abuse

Drug abuse: Nicotine and Cocaine• Nicotine – stimulant derived from tobacco plant

– Causes neurons to release dopamine – Mimics acetylcholine in PNS

• increases skeletal muscle activity, heart rate, blood pressure, and digestive tract motility

– Adversely affects a developing embryo or fetus– Psychological and physiological dependency– “immunize” the brain against nicotine, prevent passage

through BBB• Cocaine – stimulant derived from a plant

– Interferes with the re-uptake of dopamine at synapses– Results in a rush sensation (5-30 minutes) and an

increased sex drive– Results in hyperactivity and little desire for food and sleep– Extreme physical dependence with this drug– Continued use body makes less dopamine to

compensate for the excess at synapses • result is withdrawal symptoms

Drug abuse: methamphetamine

• Powder form is called speed • Crystal form is called meth or ice• Stimulatory effect mimics cocaine• Reverses the effects of fatigue and is a mood

elevator• High agitation is common after the rush and can

lead to violent behavior• Causes psychological dependency and

hallucinations• “Ecstasy” is the street name for a drug

– has the same effects as meth without the hallucinations

13.5 Drug abuse

Drug abuse: Heroin• Depressant from the sap of the opium poppy plant• Leads to a feeling of euphoria and no pain

– it is delivered to the brain and is converted into morphine– Depresses breathing, activates the reward circuit, and

blocks pain pathway

• Side effects – nausea, vomiting and depression of the respiratory and

circulatory systems

• Can lead to– HIV, hepatitis and other infections due to shared needles

• Extreme dependency

13.5 Drug abuse

Drug abuse and its use: Marijuana• Psychoactive drug derived from a hemp plant

called Cannabis• Binds to receptors located in the hippocampus,

cerebellum, basal ganglia, and cerebral cortex– Brain areas important for memory, orientation,

balance, motor coordination, and perception• Causes

– Mild euphoria and brain damage– Alterations to vision and judgment as well as impaired

motor coordination with slurred speech• Heavy users may experience

– depression, anxiety, hallucinations, paranoia and psychotic symptoms

• Banned in the US in 1937– recently has been legalized in a few states for medical

use in seriously ill patients

Chapter 13 Nervous System. Points to Ponder What are the three types of neurons? What are neuroglia?...

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Transcript of Chapter 13 Nervous System. Points to Ponder What are the three types of neurons? What are neuroglia?...