Nervous System: Nervous Tissue and Brain

Chapter 10

Lisa Ochs RN, BSN 2008

Structure of the Nervous System

• Central Nervous System (CNS)– Brain– Spinal cord

• Peripheral Nervous System (PNS)– Outside of the CNS– Includes nerves that connect the CNS with the rest

of the body

Figure 10-1 Nervous system: central nervous system and peripheral nervous system.

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Function of the Nervous System

• Sensory– Gather information from inside and outside the

body and transmit back to the CNS• Integrative

– Processing and interpreting information received from the sensory nerves

• Motor– Causing an action or movement in response to the

information received

Figure 10-2 Three functions of the nervous system: A, Sensory function. B, Integrative function. C, Motor function.

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Types of Cells

• Neuroglia– Also called glial cells; mostly found in the CNS– Support, protect, insulate and “care for” neurons– Do not conduct nerve impulses – Include astrocytes (most abundant), ependymal

cells (secrete CSF), Schwann cells, oligodendrocytes

Figure 10-3 Neuroglia, or glia; astrocytes and ependymal cells.

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Types of Cells

• Neurons– Transmits information in the form of nerve

impulse– Various shapes, sizes and lengths– Do not replicate (nonmitotic); cannot replace

themselves when injured

Types of Cells

• Parts of a neuron– Dendrites

• Tree like structures that receive information

– Cell body• Contains the nucleus of the cell

– Axon• Long extension that transmits information away from

the cell body• Includes myelin sheath, nodes of Ranvier, & axon

terminals

Figure 10-4 Structure of a neuron: A, Dendrites, cell body, axon, and axon terminals. B, Structure surrounding the axon, showing the myelin sheath, the nodes of Ranvier, and the neurilemma.

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• Types of neurons• Sensory

– Carries information from the periphery to the CNS– Also called afferent

• Motor– Carries information from the CNS to the periphery– Also called efferent

– Remember S.A.M.E.

Types of Cells

Nerve Impulse

• An electrical signal that conveys information along a neuron

• Also called an action potential• Inside the cell, normal resting state is negative

(polarized)• The cell changes to a positive state

(depolarization) when stimulated• Returns to resting state of negative

(repolarization)

Figure 10-5 Nerve impulse (action potential): A, Polarization, B, depolarization, and C, repolarization.

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Movement of Nerve Impulse

• The nerve impulse travels along the length of the axon in a wave like manner

• Each impulse depolarizes the next section of membrane

• Each nerve impulse fires in an “all or nothing” manner; this ensures that the nerve impulse does not weaken as it travels along the axon

Speed of Nerve Impulse

• Most axons are wrapped in myelin sheaths; the spaces between the myelin are the nodes of Ranvier; nerve impulses cannot travel through myelin

• The nerve impulse jumps across the myelin to the bare nodes

• This is called saltatory conduction• Myelinated fibers are considered fast-

conducting fibers

Figure 10-8 Jumping from node to node. A, A myelinated axon and the nodes of Ranvier. B, The nerve impulse jumps from node to node toward the axon terminal. C, The jumping of the nerve impulse resembles the jumping of a kangaroo.

Elsevier items and derived items © 2007, 2003, 2000 by Saunders, an imprint of Elsevier Inc.

Structure of Synapse• Synapse is the place where two neurons meet• Includes:

– Synaptic cleft (space between the neuron & dendrite)

– Neurotransmitters (most common is ACh and norepinepherine)

– Inactivators (stop the activity of neurotransmitters)

– Receptors (on the dendrite where the neurotransmitters attach)

Events at Synapse

• The nerve impulse of the first (presynaptic) neuron causes the release of neurotransmitter into the synaptic cleft.

• The neurotransmitter diffuses across the synaptic cleft and binds to the receptors on the second (postsynaptic) membrane.

• The activation of the receptors stimulates a nerve impulse in the second neuron.

Figure 10-9 The synapse. Steps in the transfer of information across the synapse.

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The Brain

Brain Structure

• Control center of the body- emotions, actions, memory, sleep/ wake, etc.

• 2% of total body weight; requires 20% of body’s oxygen supply

• Primary source of energy is glucose• 4 major areas: cerebrum, diencephalon, brain

stem and cerebellum

Figure 10-10 Four major areas of the brain: cerebrum, diencephalon, brain stem, and cerebellum.

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Cerebrum

• Largest portion of the brain• Right and left hemispheres joined by the

corpus callosum (allows the separate sides to communicate)

• Each hemisphere has four lobes– Frontal– Parietal– Temporal– Occipital (sound familiar?)

Lobes & Function

• Frontal– Motor, personality, behavior, emotion, intellect

• Parietal– Somatosensory (skin/ muscle, taste, speech, reading)

• Temporal– Hearing, smell, memory, some speech

• Occipital– Vision and vision functions (reading, distance, 3D)

• Decussation: crossing of fibers from one side of the brain to the other

• Frontal lobe includes Broca’s area (left hemisphere); responsible for motor speech; patients with a stroke here suffer from expressive aphasia

• Temporal & parietal lobes include Wernicke’s area; broad area that involves translating thoughts into words; damage here can result in severe language deficits (Wernicke’s aphasia)

Specific Functions

• Gyri (sing. gyrus)– “bumps” or elevations on the surface

• Sulci (sing. sulcus)– “grooves”

• The extensive folding of the cerebral tissue increases its surface area; the greater the number of folds and grooves, the more intelligent the species

Markings on the Cerebrum

• There are time-keeping neurons in our brains. Specifically in the prefrontal cortex and striatum of the cerebrum. Discovered recently in the brains of monkeys by researchers at MIT, these time-keeping neurons fire consistently at certain rhythms . . . – thus helping our brains to figure out when things are

happening. This helps us with rhythmic activities, of course, but also with any number of tasks and memories that rely on knowing what came first, in what order, and so on.

Hmmmm…

Researchers speculate that damage to these neurons, or damage to the mechanisms that read the timing pattern, may contribute to disorders (such as Parkinson Disease) that involve ill-timed movements and other functions.

Hmmmm…

From http://theapprofessor.blogspot.com/, retrieved December 30, 2009

In their paper, researchers failed to speculate whether this is why A&P students know exactly when to start slamming their books shut moments before a class is scheduled to end.

Hmmmm…

Diencephalon

• 2nd main area of the brain, beneath the cerebrum and above the brain stem

• Includes the thalamus and the hypothalamus

• Thalamus– Relay station for sensory fibers traveling from

lower brain & spinal cord to sensory areas of cerebrum

• Hypothalamus– Regulates many processes in the body- body

temperature, water balance, metabolism– “thermostat” of the body

Diencephalon

Brain Stem

• Connects the spinal cord with higher brain structures

• Includes midbrain, pons and medulla oblongata

• Midbrain– Relays sensory and motor information; reflex centers for

vision and hearing

• Pons– “bridge”; relays information; very important in regulating

breathing rate and rhythm

• Medulla Oblongata– Relays information; very important in regulating heart

rate, blood pressure and respiration; called the “vital center”; also contains vomiting center

Brain Stem

Cerebellum

• Located under the occipital lobe and the base of the skull; “little brain”

• Primarily concerned with coordination of voluntary muscle activity; balance and coordination

• Persons with damage to the cerebellum may appear intoxicated

Other Structures

• Not confined to specific lobes• Limbic system

– “emotional brain”; responsible for various emotional states and behaviors

• Reticular formation– Concerned with the sleep-wake cycle and

consciousness– Sensitive to certain drugs and alcohol

• Memory areas– Many lobes involved in recalling thoughts and

images– Short term (seconds to hours)– Long term (years to decades)

Other Structures

Protection of the CNS

• BONE• First layer of protection for the brain and

spinal cord• Cranium and vertebral column

Types of skull

fractures

• MENINGES• Three layers of membranes that surround the brain and spinal cord

• Dura mater (“tough mother”)– Tough connective tissue

• Arachnoid (“spider-like”)– Weblike membrane

• Pia mater (“soft mother”)– Delicate, thin; lies directly over the brain and spinal

cord

Protection of the CNS

Figure 10-14 The three layers of meninges are the dura mater, arachnoid, and pia mater.

Elsevier items and derived items © 2007, 2003, 2000 by Saunders, an imprint of Elsevier Inc.

• CEREBROSPINAL FLUID• Third layer of protection; formed from the

blood within the brain; clear fluid that resembles plasma

• Composed of water, glucose, protein and ions (esp. Na+ and Cl-)

• Acts as a cushion and shock absorber, also delivers nutrients and removes waste.

Protection of the CNS

• CEREBROSPINAL FLUID• Formed in the choroid plexus of the ventricles

in the brain• Circulates around and through the brain and

spinal cord; continuously produced and drained

• Rate of production must equal rate of drainage, otherwise increased ICP will result

Protection of the CNS

• BLOOD BRAIN BARRIER• An arrangement of cells (glial astrocytes)

associated with blood vessels that supply the brain and spinal cord

• Act as gate keepers- allows only certain substances to cross. Most harmful substances cannot cross this barrier

• Pharmacology can be difficult- most antibiotics do not cross blood-brain barrier

Protection of the CNS

NCLEX Question

• After a brain stem infarction, a nurse would observe for which condition?

1. Aphasia2. Bradypnea3. Contralateral hemiplegia4. Numbness and tingling in the face

Rationale

• 2. Bradypnea, or slowed respiratory rate, would result from damage to the brain stem. The brain stem contains the control center for vital functions such as breathing, heart rate and blood pressure.

NCLEX Question

• The nurse is caring for a client with a cerebral injury that has impaired his speech and hearing. The client most likely experienced damage to the:

1. Frontal lobe2. Parietal lobe3. Occipital lobe4. Temporal lobe

Rationale

• 4. The temporal lobe contains the primary control centers for speech and hearing

NCLEX Question

• A client is experiencing problems with balance and fine gross motor function. Which area of the brain has been affected?

1. Cerebellum2. Brain stem3. Diencephalon4. Cerebrum

Rationale

• 1. The cerebellum contains the control center for voluntary muscle activity and balance.

Disorders of the CNS

Bell’s Palsy

Types of hematomas

Coronal section of brain illustrating large cerebral hemorrhage that has compressed and displaced the cerebral ventricles.

Normal CT- Brain

Intracerebral Bleed

Subdural HematomaWith midline shift

Intracerebral Bleed43 year old male

Transcranial gunshot wound

Knife vs. cranium