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The Nervous System

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Objectives:

• Describe the functions of the nervous system• Identify and describe the functions of various types

of nervous cells• Describe how a nerve impulse is generated and

transmitted• Identify and describe the divisions of the nervous

system• Identify the major structures of the central nervous

system• Describe the major structure of the peripheral

nervous system• List and describe diseases and disorders of the

nervous system

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Functions of the Nervous System

• Sensory input – gathering information– To monitor changes occurring inside and outside

the body– Changes = stimuli

• Integration– To process and interpret sensory input and

decide if action is needed

• Motor output– A response to integrated stimuli– The response activates muscles or glands

(effector organs)

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Functions of the Nervous System

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Divisions of the Nervous System

Structural classifications:

• Central nervous system (CNS)– Brain– Spinal cord

• Peripheral nervous system (PNS)– Nerve outside the brain and spinal cord – Sensory (afferent) and motor neurons

(efferent)

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Divisions of the Peripheral Nervous System

Functional classifications:

• Sensory (afferent) division– Nerve fibers that carry information to the

central nervous system. Receptors of stimulus.

• Motor (efferent) division– Nerve fibers that carry impulses away from

the central nervous system to effector organs.

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Divisions of the Efferent Peripheral Nervous System

• Motor (efferent) division continued– Two subdivisions

• Somatic nervous system = voluntary• Autonomic nervous system = involuntary

• The Autonomic nervous system has two divisions:

– Sympathetic = Becomes active when body is stressed, “fight of flight”

– Parasympathetic = Becomes active when the body is relaxed or at rest.

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Organization of the Nervous System

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Types of Neuroglial Cells and Their Functions

• Microglia– Spider-like

phagocytes– Dispose of debris

• Ependymal cells– Line cavities of the

brain and spinal cord– Circulate

cerebrospinal fluid

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Types of Neuroglial Cells and Their Functions

• Oligodendrocytes– Produce myelin sheath

around nerve fibers in the central nervous system

• Satellite cells– Protect neuron cell bodies

• Schwann cells– Form myelin sheath in the

peripheral nervous system

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Neuroglial Schwann Cells

Schwann cellsSchwann cells – – produce myelin produce myelin sheaths in jelly-roll sheaths in jelly-roll like fashionlike fashion

Nodes of RanvierNodes of Ranvier – – gaps in myelin gaps in myelin sheath along the sheath along the axonaxon

Figure 7.5

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Neurons

Neurons = nerve cells– Cells specialized to transmit

messages

– Major regions of neurons

• Cell body – nucleus and metabolic center of the cell

• Processes – fibers that extend from the cell body (dendrites and axon)

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NeuronsClassification of Neurons as it relates to

their functions:– Sensory (afferent) neurons

• Carry impulses from the sensory receptors– Cutaneous sense organs

» Nerve endings (pain and temperature), Meissner’s corpuscle (touch), Pacinian corpuscle (deep pressure)

– Proprioceptors – detect stretch or tension

– Motor (efferent) neurons• Carry impulses from the central nervous

system

– Interneurons (association neurons)• Found in neural pathways in the central

nervous system• Connect sensory and motor neurons

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Neurons

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Neuron Structure

• Cell body (Soma)– Nucleus– Large nucleolus– Nissl substance –

specialized rough endoplasmic reticulum

– Neurofibrils – intermediate cytoskeleton that maintains cell shape

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Neuron Structure• Extensions outside the cell

body– Dendrites – conduct impulses

toward the cell body– Axons – conduct impulses away

from the cell body

• Axons end in axonal terminals• Axonal terminals contain

vesicles with neurotransmitters (Ach)

• Axonal terminals are separated from the next neuron by a gap– Synaptic cleft – gap between

adjacent neurons– Synapse – junction between

nerves

The black arrows indicate the direction of the impulse movement along the neuron.

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Pop QuizKey Choices

Oligodendrocytes

Satellite Cells

Schwann Cells

Microglia

Ependymal Cells

Neuroglia

Myelin Sheath

1.

2.

3.

4.

5. The type of cells in the diagrams are _____.

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Pop Quiz

1.

2.3.

4.5.

Key Choices

Node of Ranvier

Axonal Terminal

Dendrites

Schwann Cell

Microglia

Soma

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Nerve Impulses• Irritability – ability to respond to stimuli

• Conductivity – ability to transmit an impulse

• The plasma membrane at rest is polarized– Fewer positive ions are inside the cell than

outside the cell– resting potential difference is -70 millivolts

between the outside and inside of the cell– inside of the cell’s axon contains K+ ions

and Na+ ions are found outside of the axon’s membrane

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Neuron Depolarization and Action Potentials

• Depolarization – a stimulus depolarizes the neuron’s membrane

• A deploarized membrane allows sodium (Na+) to flow inside the membrane through special proteins in the membrane called sodium channel proteins.

• The movement of the ions initiates an action potential in the neuron due to the increase in voltage from -70 millivolts up to +30 millivolts within the axon

• The action potential travels down the axon like a wave.

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Neuron Depolarization and Action Potentials

• If the action potential (nerve impulse) starts, it is propagated over the entire axon (all or none response)

• When the axon’s internal charge reaches + 40 millivolts, the Na+ channels close and the K+ channels open; potassium ions rush out of the neuron after sodium ions have entered. The K+ ions move out until a negative charge of -70 millivolts is reestablished in the axon. Then the K+ channel proteins close. This repolarizes the axons membrane. However the Na+ and K + ions are in opposite locations of where they were before the neuron depolarized

• The sodium-potassium pump restores the original configuration by pumping Na+ ions out and K + ions back into the axon.– This action requires ATP

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Action Potentials and Neural Synapse

• The impulse continues to move toward the cell body of the next neuron in the pathway.

• Impulses are able to cross the synapse to another nerve– Neurotransmitter is released from a nerve’s

axon terminal– The dendrite of the next neuron has receptors

that are stimulated by the neurotransmitter– An action potential is started in the dendrite

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Neural Synapse

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Reflex Response ReflexReflex – rapid, predictable, and – rapid, predictable, and

involuntary responses to stimuliinvoluntary responses to stimuli

Reflex arcReflex arc – direct route from a – direct route from a sensory neuron, to an interneuron, to sensory neuron, to an interneuron, to an effectoran effector

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Reflex Response

• Autonomic reflexes – regulate involuntary activity

• Somatic reflexes – all reflexes that stimulate skeletal muscles

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Central Nervous System

• CNS develops from the embryonic neural tube (ectoderm)– The neural tube becomes the brain and

spinal cord– The opening of the neural tube becomes

the ventricles• Four chambers within the brain• Filled with cerebrospinal fluid - nourishes and

cushions the brain

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CNS: The Brain

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CNS: Brain: Cerebrum• Right hemisphere controls

the left side of the body and the left hemisphere controls the right side of the body

• more than half of the brain mass

• surface is made of ridges (gyri) and grooves (sulci)

• Fissures (deep grooves) divide the cerebrum into lobes

• Surface lobes of the cerebrum– Frontal lobe– Parietal lobe– Occipital lobe– Temporal lobe

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CNS: Brain: Cerebrum

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CNS: Brain: Diencephalon

• Sits on top of the brain stem

• Enclosed by the cerebral hemispheres

• Made of three parts– Thalamus– Hypothalamus– Epithalamus

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Diencephalon: Thalamus• relay station for sensory impulses; transfers

impulses to the correct part of the cortex for localization and interpretation

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Diencephalon: Hypothalamus• autonomic nervous system center; regulates body

temperature, controls water balance, and regulates metabolism

• an important part of the limbic system (emotions) and the pituitary gland is attached to the hypothalamus

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Diencephalon: Epithalamus• houses the pineal body (an endocrine gland).

• includes the choroid plexus which forms cerebrospinal fluid

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CNS: Brain: Brain Stem

• Attaches to the spinal cord

• Parts of the brain stem– Midbrain– Pons– Medulla oblongata

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Brainstem: Midbrain • composed of tracts of nerve fibers

– two parts: cerebral peduncles and corpora quadrigemina

– function - reflex centers for vision and hearing

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Brainstem: Pons• mostly composed of fiber tracts• contains nuclei involved in the control of breathing

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Brainstem: Medulla oblongata• lowest part of the brain stem • merges into the spinal cord• contains important control centers such as:

heart rate control, blood pressure regulation, breathing, swallowing, and vomiting

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CNS: Brain: Cerebellum• composed of two hemispheres with convoluted

surfaces• provides involuntary coordination of body

movements

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Meninges• specialized membranes

below the bone surrounding the brain and spinal cord

• Dura mater (outer most layer) “tough mother”– Double-layered external

covering• Periosteum – attached

to surface of the skull• Meningeal layer – outer

covering of the brain– Folds inward in several

areas• Arachnoid layer

– Middle layer– Web-like

• Pia mater “gentle mother”– Internal layer– Clings to the surface of the

brain

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CSF: Cerebrospinal Fluid

• Similar to blood plasma composition

• Forms a watery cushion to protect the brain

• Circulated in arachnoid space, ventricles, and central canal of the spinal cord

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Location and Circulation of CSF

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Spinal Cord Structure

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Nerve Anatomy

• Nerve = bundle of neuron fibers

• Neuron fibers are bundled by connective tissue

• Endoneurium surrounds each fiber

• Groups of fibers are bound into fascicles by perineurium

• Fascicles are bound together by epineurium

• Mixed nerves – both sensory and motor fibers

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Quiz Key Choices

Hypothalamus

Cerebellum

Diencephalon

Cerebrum

Thalamus

Pons

Brain Stem

Medulla Oblongata

1.

2.

3.

4.

5. Which part of the brain helps you to vomit when you are sick?

6. Which part of the brain helps you to regulate body temperature?

7. Which part of the brain helps you to see and smell?

8. Which area of the brain includes the pons and medulla oblongata?

9. The hypothalamus is part of which area of the brain?

10. Which areas of the brain are divided into left and right hemispheres?

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Diseases and Disorders of the Nervous System

• Cerebrovascular Accident: CVA

• Commonly called a stroke• The result of a ruptured blood

vessel supplying (cerebral hemorrhage) a region of the brain or a vessel is obstructed by a clot.

• Brain tissue supplied with oxygen from that blood source dies, swelling occurs in the brain due to leaking of blood from vessels.

• Loss of some functions or death may result

• This is due often to elevated blood pressure or hypertension.

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Diseases and Disorders of the Nervous System

Epilepsy: • This disease is due to

random, mis-firing of neurons within the brain affecting sensory and motor regions of the brain.

• Ranging in effects from sleep-like state of consciousness (narcolepsy), muscle paralysis and spasms (Petit mal and Grand mal seizures). Still not understood why this disease occurs. However in some cases it can result from brain trauma or injury.

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Diseases and Disorders of the Nervous System

• Concussion– Slight brain injury– No permanent brain damage

• Contusion– Nervous tissue destruction

occurs– Nervous tissue does not

regenerate

• Cerebral edema– Swelling from the inflammatory

response or injury– May compress and kill brain

tissue– May be caused by infectious

agents such as viruses (encephalitis) or bacteria which cross the blood brain barrier or infect the meninges or CSF surrounding the brain (meningitis)

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Diseases and Disorders of the Nervous System

• Alzheimer’s Disease• Progressive degenerative

brain disease• Mostly seen in the elderly,

but may begin in middle age• Structural changes in the

brain include abnormal protein deposits and twisted fibers within neurons called plaques

• Can only be truly diagnosed by autopsy and study of brain tissue at death

• Victims experience memory loss, irritability, confusion and ultimately, hallucinations and death

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Diseases and Disorders of the Nervous System

Parkinson’s Disease• Normally effects those in 50’s & 60’s• Dopamine releasing neurons degenerate

and this causes the dopamine targets to become overactive

• Symptoms include persistent tremors at rest, head nodding and pill-rolling movement of fingers, stiff facial expressions and forward bent walk

• Michael J. Fox has this disease

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Diseases and Disorders of the Nervous System

• Hydrocephalus– “Water on the brain” – Occurs mostly in babies– CSF builds up in ventricles

• Multiple Sclerosis– An autoimmune disease– Body’s antibodies attack the myelin sheath of

the neuron, making impulses much slower