The Nervous System

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Nervous System

o Three specific functions that work to maintain homeostasis:

• Sensory input• Integration• Motor output

o Divisions of the Nervous Systemo Nervous Tissue

Nervous System

o Divisions of the Nervous System• Central nervous system (CNS)

Brain Spinal cord

• Peripheral nervous system (PNS) – includes all the cranial and spinal nerves

Afferent (sensory) division Efferent (motor) division

o Nervous Tissue

Fig 8.1

Nervous System

o Nervous Tissue• Neurons (nerve cells) – transmit

impulses• Neuroglia – support and nourish

neurons

Nervous System

• Neuron structure Cell body – contains nucleus and other

organelles Dendrite – receive signals from sensory

receptors or other neurons Axon

Conducts nerve signals away from cell body Nerve – bundle of parallel axons in the PNS Tract – bundle of parallel axons in the CNS May be covered by myelin (lipid coating)

Formed by Schwann cells or neurolemmocytes in PNS

Formed by oligodendrocytes in CNS

Fig 8.2

Nervous System

• Types of neurons Motor neurons

Take nerve impulses from the CNS to muscles, organs, or glands

Multipolar – have many dendrites Sensory neurons

Take nerve impulses from sensory receptors to the CNS

Almost all are unipolar Interneurons

All are in the CNS Typically multipolar Convey nerve impulses between various parts

of the CNS

Nervous System

• Nerve signal conduction Resting potential

Neuron possesses potential energy The cell membrane is polarized

positively charged outside the cell negatively charged inside

Action potential Process of conduction of nerve signals Occurs in the axons Begins with a stimulus Channels in the cell membrane opens and

sodium ions rush into the cell (depolarization) Sodium channels close and the cell repolarizes

Fig 8.3

Nervous System

Conduction of action potentials (APs) In unmyelinated axons

Slow (~1 meter/second) Each section of the axon must be stimulated

In myelinated axons Called saltatory conduction An AP at one node of Ranvier can “jump”

over myelinated portion of axon Much faster (>100 meters/second)

Is an all-or-none event Refractory period

Axon cannot conduct an AP Ensures one-way direction of an impulse

Fig 8.4

Nervous System

• Transmission across a synapse Axon terminal – small swelling at tips of

the branched end of an axon Synapse

Region of close proximity between two neurons Presynaptic membrane – membrane of the first

neuron Postsynaptic membrane – membrane of the

next neuron Synaptic cleft – small gap between the

presynaptic and the postsynaptic neuron Neurotransmitters – molecules that

transmit a nerve impulse across a synapse

Fig 8.5

Nervous System

Graded potentials and synaptic integration

Graded potentials – each small signal from a synapse

Excitatory neurotransmitters produce a graded potential that promotes an AP

Inhibitory neurotransmitters produce a graded potential that inhibits an AP

Integration involves summing up the excitatory and inhibitory signals

Nervous System

Neurotransmitter Molecules At least 50 have been identified Two well-known neurotransmitters:

Acetylcholine (Ach) Norepinephrine (NE)

After a neurotransmitter has initiated a response it is removed from the synaptic cleft Enzymes may inactivate the

neurotransmitter The neurotransmitter may be reabsorbed by

the presynaptic membrane Prevents continuous stimulation (or

inhibition) of postsynaptic membranes

Central Nervous System

o Gray matter – contains cell bodies and unmyelinated fibers

o White matter – contains myelinated axons

o Meninges and Cerebrospinal Fluid• Meninges – protective membranes of the

CNS Dura mater – outer menix composed of tough,

fibrous connective tissue Arachnoid mater – middle menix composed of

spider-web-like connective tissue Pia mater – deepest menix

Fig 8.6

Central Nervous System

• Cerebrospinal fluid (CSF) Produced by ependymal cells Fills the following:

Subarachnoid space – space between the arachnoid mater and the pia mater

Ventricles – hollow, interconnecting cavities of the brain

Central canal – hollow, space of the spinal cord

Central Nervous System

• The spinal cord Extends from the base of the brain to the

lumbar vertebra Protected by vertebral column

Central Nervous System

Structure of the spinal cord Central canal contains CSF Centrally located gray matter

Posterior (dorsal) root – contains sensory fibers

Anterior (ventral) root – contains motor fibers

interneurons White matter

Posterior white matter composed of ascending tracts carrying sensory information to the brain

Anterior white matter composed of descending tracts carrying motor information from the brain

Fig 8.7

Central Nervous System

Functions of the spinal cord Provides communication between the brain and

the peripheral nerves Reflexes

Central Nervous System

• The brain Cerebrum Diencephalon Cerebellum Brain stem Ventricles

Two lateral ventricles (cerebrum) Third ventricle (diencephalon) Fourth ventricle (cerebellum)

Fig 8.8

Central Nervous System

• The Cerebrum Largest portion of the brain Voluntary motor responses Coordinates the activities of the other

parts of the brain Involved in higher thought processes The cerebral hemispheres

Longitudinal fissure divides the left and right Connected by the corpus callosum Gyri (ridges) are separated by sulci (shallow

grooves)

Central Nervous System

Lobes Frontal lobe Parietal lobes Occipital lobe Temporal lobes Insula

Fig 8.9

Central Nervous System

• The cerebral cortex Outer layer of gray matter Accounts for sensation, voluntary

movement, and consciousness Motor and sensory areas

Primary motor area In frontal lobe anterior to central sulcus Voluntary commands to skeletal muscle

Primary somatosensory area Posterior to central sulcus in parietal lobes Receives sensory information from the skin

and skeletal muscles

Fig 8.10

Central Nervous System

Primary taste area Located in insula Taste sensations

Primary visual area Located in the occipital lobe Receives information from our eyes

Primary auditory area Located in the temporal lobe Receives information from our ears

Central Nervous System

Association areas Where integration occurs Where memories are stored

Processing centers Prefrontal area Uses information from other association areas Reasoning and planning actions Motor speech area (Broca’s area) Wernicke’s area (general interpretive area)

Central Nervous System

Central white matter Tracts communicate information between the

different sensory, motor, and association areas Corpus callosum join the two cerebral hemispheres

Basal nuclei Masses of gray matter Integrate motor commands

Limbic system Inferior to the cerebral cortex Contains neural pathways that connect portions of

the cerebral cortex and the temporal lobes with the thalamus and the hypothalamus

Causes emotion Involved in memory and learning Hippocampus involved in processing short-term

memory to become long-term memory

Central Nervous System

• The diencephalon Hypothalamus

Forms the floor of the third ventricle Integrating center involved in homeostasis Regulates hunger, sleep, thirst, body

temperature, and water balance Link between nervous and endocrine systems

Thalamus Located in sides and roof of the third ventricle Functions as a sensory relay center Involved in arousal of the cerebrum Participates in memory and emotions

Pineal gland regulates body’s daily rhythms

Central Nervous System

• The cerebellum Separated from brain stem by the fourth

ventricle Two hemispheres composed primarily of

white matter Receives sensory input from the eyes,

ears, joints, and muscles Receives motor output from the cerebral

cortex Maintains posture and balance and

ensures smooth, coordinated voluntary movement

Assists in learning of new motor skills

Central Nervous System

• The brain stem Midbrain

Relay station between cerebrum and the spinal cord or cerebellum

Reflex centers for visual, auditory, and tactile responses

Pons Contains bundles of axons traveling between the

cerebellum and the rest of the CNS Helps regulate breathing rate

Medulla oblongata Vital reflex centers Reflex centers for vomiting, coughing, sneezing,

hiccupping, and swallowing Reticular formation

Involved in maintaining muscle tone Assists in regulating respiration, heart rate, and

blood pressure Helps rouse a sleeping person

Peripheral Nervous System

o Lies outside of CNSo Composed of nerves and gangliao Subdivisions

• Afferent (sensory) Somatic sensory

Serves the skin, skeletal muscles, joints, and tendons Special senses

Visceral sensory supplies the internal organs• Efferent (motor)

Somatic motor system carries commands from CNS to the skeletal muscles

Autonomic motor system regulates cardiac and smooth muscle and glands

Peripheral Nervous System

o Cranial nerves• 12 pairs• Sensory nerves contain only sensory fibers• Motor nerves contain only motor fibers• Mixed nerves contain both sensory and

motor fibers• Mostly involved with the head, neck, and

facial regions of the body• The vagus nerve (cranial nerve X) has

sensory and motor branches to the face and most of the internal organs

Fig 8.11a

Peripheral Nervous System

o Spinal nerves• 31 pairs• Designated according to their location

in relation to the vertebrae• Mixed nerves

Sensory fibers enter at the posterior root Motor fibers exit at the anterior root

• The cell body of a sensory neuron is in a posterior-root ganglion

Fig 8.11b

Peripheral Nervous System

o Somatic Motor Nervous System and Reflexes

• Voluntary actions that originate in the motor cortex

• Reflexes are automatic involuntary responses

Cranial reflexes involve the brain Spinal reflex involves only the spinal cord

Peripheral Nervous System

o Autonomic Motor Nervous System and Visceral Reflexes

• Two divisions of ANS Function automatically and involuntarily Innervate all internal organs Utilize two motor neurons and one

ganglion for each impulse

• Visceral reflexes are important to the maintenance of homeostasis

Peripheral Nervous System

• Sympathetic division of ANS (“Fight or Flight”) Preganglionic fibers arise from the thoracic-lumbar

portion of the spinal cord Preganglionic fibers are short and postganglionic fibers

are long Accelerates heartbeat and dilates the bronchi Inhibits the digestive tract Neurotransmitter released by the postganglionic neuron

is primarily norepineprhine (NE)

TA 8.3

Peripheral Nervous System

• Parasympathetic division of ANS (“Rest and Digest”) Includes a few cranial nerves and preganglionic fibers

that arise from the sacral portion of the spinal cord (craniosacral portion of ANS)

Preganglionic fiber is long and postganglionic fiber is short

Promotes digestion of food, slows heart rate, and decreases the strength of cardiac contraction

Acetylcholine (Ach) is the neurotransmitter of the parasympathetic division

TA 8.4

Table 8.4 not available in this printed version

Fig 8.14

Effects of Aging

o Brain mass decreaseso Learning, memory, and reasoning

declineo Neurotransmitter production

decreaseso Although structural changes occur,

mental impairment is not an automatic consequence of getting older

Homeostasis

o Detects, interprets, and responds to changes in the internal and external environment

o With the endocrine system, it coordinates and regulates the functioning of other systems

o The hypothalamus and medulla oblongata control vital functions

o Sympathetic division of the ANS works to keep us from danger