Chapter 7 - Nervous System

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Basal nuclei

- "Islands" of gray matter buried deep within the white matter of the cerebrum - Regulate voluntary motor activities by modifying instructions sent to skeletal muscles by the primary motor cortex

Action potential initiation and generation (continued)

- A graded potential (localized depolarization) exists where the inside of the membrane is more positive and the outside is less positive - If the stimulus is strong enough and sodium influx great enough, local depolarization activates the neuron to conduct an action potential (nerve impulse)

Motor output

- A response, or effect, activates muscles or glands

Action potential initiation and generation

- A stimulus changes the permeability of the neuron's membrane to sodium ions - Sodium channels now open, and sodium (Na+) diffuses into the neuron - The inward rush of sodium ions changes the polarity at that site and is called depolarization

CNS glial cells: astrocytes

- Abundant, star-shaped cells - Brace and anchor neurons to blood capillaries - Determine permeability and exchanges between blood capillaries and neurons - Protect neurons from harmful substances in blood - Control the chemical environment of the brain

Other specialized areas

- Anterior association area (frontal lobe) - Posterior association area (posterior cortex) - Speech area (for sounding out words)

Brain stem

- Attaches to spinal cord

Autonomic nervous system (involuntary)

- Automatically controls smooth and cardiac muscles and glands - Further divided into the sympathetic and parasympathetic nervous systems

Sensory (afferent) neurons

- Carry impulses from the sensory receptors to the CNS -Receptors include: -- Cutaneous sense organs in skin -- Proprioceptors in muscles and tendons

Interneurons (association neurons)

- Cell bodies located in the CNS - Connect sensory and motor neurons

Major regions of all neurons

- Cell body (nucleus and metabolic center of the cell - Processes (fibers that extend from the cell body)

Cerebral white matter

- Composed of fiber tracts deep to the gray matter -- Corpus callosum connects hemispheres -- Tracts, such as the corpus callosum, are known as commissures -- Association fiber tracts connect areas within a hemisphere -- Projection fiber tracts connect the cerebrum with lower CNS centers

Three-neuron reflex arc

- Consists of five elements: receptor, sensory neuron, interneuron, motor neuron, and effector - Example: flexor (withdrawal) reflex

Processes (fibers)

- Dendrites - Axons

Diencephalon: thalamus

- Encloses the third ventricle - Relay station for sensory impulses passing upward to the cerebral cortex - Transfers impulses to the correct part of the cortex for localization and interpretation

Brain stem: midbrain

- Extends from the mammillary bodies to the pons inferiorly - Cerebral aqueduct (tiny canal) connects the third and fourth ventricles

Diencephalon: epithalamus

- Forms the roof of the third ventricle - Houses the pineal body (an endocrine gland) - Includes the choroid plexus—forms cerebrospinal fluid

Sensory Input

- Gathering information - Sensory receptors monitor changes, called stimuli occurring inside and outside the body

Transmission of the signal at synapses - step 5

- If enough neurotransmitter is released, a graded potential will be generated -- Eventually an action potential (nerve impulse) will occur in the neuron beyond the synapse

Propagation of the action potential

- If enough sodium enters the cell, the action potential (nerve impulse) starts and is propagated over the entire axon - All-or-none response means the nerve impulse either is propagated or is not - Fibers with myelin sheaths conduct nerve impulses more quickly

Cerebral hemispheres are paired (left and right) superior parts of the brain

- Include more than half of the brain mass - The surface is made of ridges (gyri) and grooves (sulci) - Fissures are deeper grooves - Lobes are named for the cranial bones that lie over them

Repolarization (continued)

- Initial conditions of sodium and potassium ions are restored using the sodium-potassium pump - This pump, using ATP, restores the original configuration - Three sodium ions are ejected from the cell while two potassium ions are returned to the cell - Until repolarization is complete, a neuron cannot conduct another nerve impulse

Central Nervous System - Functions

- Integration; command center - Interprets incoming sensory information - Issues outgoing instructions

- Cerebral cortex (continued) - Broca's area (motor speech area)

- Involved in our ability to speak - Usually in left hemisphere

Functional properties of neurons

- Irritability - Conductivity

CNS glial cells: ependymal cells

- Line cavities of the brain and spinal cord - Cilia assist with circulation of cerebrospinal fluid

- Cerebral cortex (continued) - Primary motor area

- Located anterior to the central sulcus in the frontal lobe - Allows us to consciously move skeletal muscles - Motor neurons form pyramidal (corticospinal) tract, which descends to spinal cord - Motor homunculus is a spatial map

Diencephalon: hypothalamus

- Makes up the floor of the diencephalon - Important autonomic nervous system center -- Regulates body temperature -- Regulates water balance -- Regulates metabolism - Houses the limbic center for emotions - Regulates the nearby pituitary gland - Houses mammillary bodies for olfaction (smell)

Repolarization

- Membrane permeability changes again—becoming impermeable to sodium ions and permeable to potassium ions - Potassium ions rapidly diffuse out of the neuron, repolarizing the membrane - Repolarization involves restoring the inside of the membrane to a negative charge and the outer surface to a positive charge

Protection of CNS

- Meninges - Cerebrospinal fluid (CSF) - Blood-brain barrier

Peripheral Nervous System (PNS)

- Nerves extending from the brain and spinal cord

Integration

- Nervous system processes and interprets sensory input and decides whether action is needed

Axons - conduct impulses AWAY from the cell body

- Neurons have only one axon arising from the cell body at the axon hillock - End in axon terminals, which contain vesicles with neurotransmitters - Axon terminals are separated from the next neuron by a gap

Cell body is the metabolic center of the neuron

- Nucleus with large nucleolus - Nissl bodies -- Rough endoplasmic reticulum - Neurofibrils -- Intermediate filaments that maintain cell shape

Cerebral cortex

- Primary somatic sensory area -- Located in parietal lobe posterior to central sulcus -- Receives impulses from the body's sensory receptors -- Pain, temperature, light touch (except for special senses) - Sensory homunculus is a spatial map - Left side of the primary somatic sensory area receives impulses from right side (and vice versa)

Somatic reflexes

- Reflexes that stimulate the skeletal muscles - Involuntary, although skeletal muscle is normally under voluntary control -- Example: pulling your hand away from a hot object

Autonomic reflexes

- Regulate the activity of smooth muscles, the heart, and glands -- Example: regulation of smooth muscles, heart and blood pressure, glands, digestive system

Supporting cells (called neuroglia, or glial cells, or glia)

- Resemble neurons - Unable to conduct nerve impulses - Never lose the ability to divide

Myelin Sheaths

- Schwann cells - Neurilemma - Nodes of Ranvier - Oligodendrocytes

PNS glial cells

- Schwann cells - Satellite cells

Motor (efferent) 2 subdivisions

- Somatic nervous system - Autonomic nervous system

CNS glial cells: microglia

- Spiderlike phagocytes - Monitor health of nearby neurons - Dispose of debris

Nervous system classifications are based on:

- Structures (structural classification) - Activities (functional classification)

Neuroglia functions

- Support - Insulate - Protect neurons

Nervous tissue is made up of two principal cell types

- Supporting cells - Neurons

Transmission of the signal at synapses - step 6

- The electrical changes prompted by neurotransmitter binding are brief - The neurotransmitter is quickly removed from the synapse either by reuptake or by enzymatic activity - Transmission of an impulse is electrochemical -- Transmission down neuron is electrical -- Transmission to next neuron is chemical

Brain stem: medulla oblongata

- The most inferior part of the brain stem that merges into the spinal cord - Includes important fiber tracts

Electrical conditions of resting neuron's membrane

- The plasma membrane at rest is inactive (polarized) - Fewer positive ions are inside the neuron's plasma membrane than outside -- K+ is the major positive ion inside the cell -- Na+ is the major positive ion outside the cell - As long as the inside of the membrane is more negative (fewer positive ions) than the outside, the cell remains inactive

Brain stems: pons

- The rounded structure protruding just below the midbrain - Mostly composed of fiber tracts - Includes nuclei involved in the control of breathing

Cerebellum

- Two hemispheres with convoluted surfaces - Outer cortex of gray matter and inner region of white matter

Cerebral areas involved in special senses

- Visual area (occipital lobe) - Auditory area (temporal lobe) - Olfactory area (temporal lobe)

Myelin

- White, fatty material covering axons - Protects and insulates fibers - Speeds nerve impulse transmission

CNS glial cells: oligodendrocytes

- Wrap around nerve fibers in the central nervous system - Produce myelin sheaths

Unipolar Neurons

- have a short singe process leaving the cell body - sensory neurons found in PNS ganglia - conduct impulses both toward and away from the cell body

Multipolar neurons

- many extensions from the cell body -- all motor and interneurons are multipolar -- most common structural type

Bipolar neurons

- one axon and one dendrite - located in special sense organs, such as nose/eye - rare in adults

Brain stem: reticuluar formation

- plays a role in awake/sleep cycles and consciousness

Oligodendrocytes

- produce myelin sheaths around axons of the CNS - Lack a neurilemma

Reflexes

- rapid, predictable, and involuntary responses to stimuli - occur over neural pathways called reflex arcs

Two-neuron reflex arcs

- simplest type - patellar (knee-jerk) reflex

Diencephalon

- sits on top of the brain stem - enclosed by the cerebral hemispheres

Two types of reflexes

- somatic reflexes - autonomic reflexes

Central Nervous System - Organs

-Brain -Spinal Cord

Three main regions of cerebral hemisphere

1. Cortex is superficial gray matter 2. White matter 3. Basal nuclei are deep pockets of gray matter

5 elements of a reflex arc

1. Sensory receptor—reacts to a stimulus 2. Sensory neuron—carries message to the integration center 3. Integration center(CNS)—processes information and directs motor output 4. Motorneuron—carries message to an effector 5. Effectororgan—is the muscle or gland to be stimulated

Diencephalon is made up of 3 structures

1. Thalamus 2. Hypothalamus 3. Epithalamus

Parts of the brain stem

1. midbrain 2. pons 3. Medulla oblongata

Irritability

Ability to respond to a stimulus and convert it to a nerve impulse

Conductivity

Ability to transmit the impulse to other neurons, muscles or glands

Structural Classification

Based on number of processes extending from the cell body

Central nervous system

Brain and spinal cord

Functional anatomy of the brain

Brain regions -- cerebral hemispheres -- diencephalon -- brain stem -- cerebellum

Tracts

Bundles of nerve fibers in the CNS

Nerves

Bundles of nerve fibers in the PNS

Transmission of the signal at synapses - step 2

Calcium, in turn, causes the tiny vesicles containing the neurotransmitter chemical to fuse with the axonal membrane

Motor (efferent) neurons

Carry impulses from the central nervous system to viscera and or muscles and glands

Cranial Nerves

Carry impulses to and from the brain

Spinal Nerves

Carry impulses to and from the spinal cord

Neuron = nerve cells

Cells specialized to transmit messages (nerve impulses)

Nuclei

Cluster of cell bodies in the central nervous system

Ganglia

Collections of cell bodies outside of the central nervous system in the PNS

Somatic nervous system (voluntary)

Consciously (voluntarily) controls skeletal muscles

Peripheral nervous system

Cranial and spinal nerves

Visceral sensory (afferent)

Fibers carry information from visceral organs

Somatic sensory (afferent)

Fibers that carry information from the skin, skeletal muscles and joints

Schwann cells

Form the myelin sheath around verve fibers in the PNS

Synapse

Functional junction between nerves where a nerve impulse is transmitted

Meninges

Have 3 layers: -Dura mater - Arachnoid layer - Pia mater

Saltatory Conduction

Myelin sheath separated by the node of ranvier

Motor (efferent) division

Nerve fibers that carry impulses away from the central nervous system organs to effector organs (muscles and glands)

Support cells in the CNS are grouped together as...

Neuroglia

Dendrites - conduct impulses TOWARD the cell body

Neurons may have hundreds of dendrites

Sensory (afferent) division

Never fibers that carry information to the central nervous system

Satellite Cells

Protects and cushion neuron cells bodies

Peripheral Nervous System - Function

Serve as communication lines among sensory organs, the brain and spinal cord and glands or muscles.

Transmission of the signal at synapses - step 3

The entry of calcium into the axon terminal causes porelike openings to form, releasing the neurotransmitter into the synaptic cleft

Transmission of the signal at synapses - step 4

The neurotransmitter molecules diffuse across the synaptic cleft and bind to receptors on the membrane of the next neuron

Transmission of the signal at synapses - Step 1

When the action potential reaches the axon terminal, the electrical charge opens calcium channels

White matter

collections of myelinated fibers (tracts)

Synaptic Cleft

gap between one axon terminal to the next neuron

Nodes of Ranvier

gaps in myelin sheath along the axon

Gray matter

mostly unmyelinated fibers and cell bodies

Neurilemma

part of the Schwann cell external to the myelin sheath

Schwann Cells

wrap axons in a jelly roll-like fashion (PNS) to form the myelin sheath


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