Ch. 13 - Student Notes

Neuroglia

-"Nerve glue" -Support the neurons of the CNS: --Astrocytes - surround blood vessels, fill the spaces between neurons, and contribute to synaptic function in the CNS; cells that facilitate exchange between neurons and capillaries --Oligodendroglia or oligodendrocytes - form myelin sheaths within the CNS --Microglia - remove debris in the CNS; provide immune defense for nervous system --Ependymal cells - line the CSF-filled cavities of the CNS; cells that create and secrete CSF

Peripheral Nervous System

-31 pairs of spinal nerves --Names correlate with the vertebral level from which they exit --Mixed nerves (contain both sensory and motor neurons) --Arise from the anterior and posterior horn cells of the spinal cord -12 pairs of cranial nerves --Sensory, motor, and mixed

Protective Structures: Vertebral column

-33 vertebrae --7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral, and 4 fused coccygeal -Intervertebral disks --Nucleus pulposus --absorbs shocks, preventing damage to the vertebrae

Blood supply to the brain

-800 to 1000 ml per minute -Carbon dioxide is the primary regulator for CNS blood flow -Internal carotid and vertebral arteries - two systems where the brain derives its arterial supply from -Arterial circle (circle of Willis) - provides an alternative route for blood flow when one of the contributing arteries is obstructed

Protective Structures: Cerebrospinal fluid (CSF) and the ventricular system

-CSF is a clear, colorless fluid similar to blood plasma and interstitial fluid --125 to 150 ml in ventricles --Produced by the choroid plexus (structures that produce CSF, arise from pial membrane) --Reabsorbed through the pressure gradient between arachnoid villi and cerebral venous sinuses -Carbon dioxide is a primary regulator of blood flow in the CNS, it is a potent vasodilator and it ensures an adequate amount of blood supply

Axons

-Carry nerve impulses away from the cell body -Axon hillock --Cone-shaped process where the axon leaves the cell body -Initial segment of the axon --Area of the axon having the lowest threshold for stimulation, so the action potentials begin at this point

3 components of the neuron

-Cell body (soma) --Located mainly in the CNS --Densely packed cell bodies in the CNS are neurons --Densely packed cell bodies in the PNS are ganglia -Dendrites --Receptive portion of the neuron -Axons

Blood-Brain Barrier (BBB)

-Cellular structures that selectively inhibit certain potentially harmful substances in the blood from entering the interstitial spaces of the brain or CSF -Implications for drug therapy because certain types of antibiotics and chemotherapeutic drugs show a greater propensity than others for crossing this barrier

Hindbrain (metencephalon)

-Cerebellum: fine motor control, balance and posture -Pons: a bridge; primarily transmits info from the cerebellum to the brainstem and between the two cerebellar hemispheres

Forebrain: Telencephalon (cerebral hemispheres)

-Cerebrum --Gyri, sulci (grooves between adjacent gyrus), and fissures (deeper grooves) --Gray matter and white matter -Brodmann areas -Lobes --Frontal : primary motor area, limbic system (mediates emotion and long-term memory through connections in the prefrontal cortex; involved in primitive behavioral responses, visceral reaction to emotion, motivation, mood, feeding behaviors, biologic rhythms, and sense of smell), Broca's (speech) --Parietal : somatic sensory input --Occipital : primary visual cortex --Temporal : primary auditory cortex, Wernicke (speech reception and interpretation) -Cerebral nuclei (basal ganglia) -Cerebral cortex and basal ganglia --Cerebral cortex contains an outer layer of cell bodies called gray matter --White matter lies beneath the cerebral cortex (composed of myelinated nerve fibers)

Upper motor neurons

-Completely contained within the CNS -Control fine motor movement and influence/modify spinal reflex arcs -Synapse with interneurons

Midbrain - Mesencephalon

-Corpora quadrigemina (tectum) --Superior and inferior colliculi -Tegmentum --Red nucleus and substantia nigra and basis pedunculi *superior colliculi involved with voluntary and involuntary visual motor movements (ability of the eyes to track moving objects in the visual field); inferior colliculi accomplish similar motor activities but involve movements affecting the auditory system (positioning the head to improve hearing) *red nucleus receives ascending sensory info from the cerebellum and projects a minor motor pathway, the rubrospinal tract, to the cervical spinal cord; the last portion of the basal ganglia is the substantial nigra, which synthesizes dopamine; the basis pedunculi are made up of efferent fibers of the corticospinal, corticobulbar, and corticopontocerebellar tracts

Peripheral nervous system (PNS)

-Cranial nerves -Spinal nerves -Pathways: --Afferent (ascending)- carry impulses toward the CNS --Efferent (descending)- innervate skeletal muscles or effector organs by transmitting motor impulses away from the CNS

Aging and the Nervous System

-Decrease in the number of neurons -Decreased brain weight and size -Lipofuscin and intracellular neurofibrillary tangles -Slowing of tendon responses

Protective Structures: Cranium

-Eight bones -Encloses and protects the brain and its associated structures (does not expand to meet demands; holds 3 things - brain (all the mater), blood, CSF)

Peripheral Nervous System cont.

-Fascicles (bundle of fibers) --Bundles of myelinated axons in the PNS -Rami -Plexuses (networks of nerve fibers) --Brachial plexus (C5 - C8 and T1) --Lumbar plexus (L1 - L4) --Sacral plexus (L5 - S5) -Dermatomes (specific areas of cutaneous innervation at these spinal cord segments)

Central Nervous System

-Forebrain: Cerebral hemispheres -Midbrain : Corpora quadrigemina, tegmentum, and cerebral peduncles -Hindbrain: Cerebellum, pons, and medulla -Midbrain, medulla, and pons comprise the brainstem, which connects the hemispheres of the brain, cerebellum, and spinal cord

Parasympathetic Nervous System

-Functions to conserve and restore energy --"Rest-and-tranquility" response -Receives innervation from cell bodies located in the cranial nerve nuclei and sacral region of the spinal cord --Craniosacral division -Preganglionic neurons travel to ganglia close to the organs they innervate

Motor Pathways

-Lateral corticospinal - leave the tract to go to specific interneurons or motor neurons in the anterior horn; these tracts are involved in precise motor movements -Corticobulbar - synapse on motor cranial nuclei within the brainstem that control muscles of the face, head, and neck -Reticulospinal - modulates motor movement by inhibiting and exciting spinal activity -Vestibulospinal - arises from a vestibular nucleus in the pons and causes the extensor muscles of the body to rapidly contract, most dramatically witnessed when a person starts to fall backward -Rubrospinal - important for muscle movement and fine muscle control in upper extremities

Autonomic Nervous System

-Located in both the CNS and PNS -Coordinates and maintains a steady state among the visceral (internal) organs -Neurons --Preganglionic (myelinated) --Postganglionic (unmyelinated)

Function of Autonomic Nervous System

-Maintains a steady state in internal environment -Two opposing systems --Sympathetic responds to stress by mobilizing energy and prepares for defense --Parasympathetic conserves energy and resources

Hindbrain: Myelencephalon

-Medulla oblongata (usually called this) --reflex activities, such as HR, respiration, BP, coughing, sneezing, swallowing, vomiting, are controlled in this area --contralateral control

Sympathetic Nervous System

-Mobilizes energy stores in times of need --Fight or flight response -Receives innervation from cell bodies located from the first thoracic (T1) through the second lumbar (L2) regions of the spinal cord -Thoracolumbar division --Sympathetic (paravertebral) ganglia

Axons cont.

-Myelin --Segmented layer of lipid material --Insulating substance --Formed and maintained by the Schwann cell in the PNS -Nodes of Ranvier --Regular interruptions of the myelin sheath

Reticular formation

-Network of connected nuclei that regulate vital reflexes, such as cardiovascular and respiratory function -Together with the cerebral cortex is referred to as the reticular activating system (RAS) --the RAS consists of nuclei in the brainstem reticular formation plus fibers that conduct sensory info to the nuclei and fibers that conduct from the nuke to widespread areas of the cerebral cortex; functioning of the RAS is essential for consciousness

Nerve Impulse

-Neurons generate and conduct electrical and chemical impulses by selectively changing the electrical potential of their plasma membranes and influencing other nearby neurons by the release of neurotransmitters -All-or-none response

Lower motor neurons

-Neurons having direct influence on muscles -Cell bodies originate in the gray matter of the spinal cord, but their axons extend into the PNS

Synapses cont.

-Neurotransmitters (ex. norepinephrine, acetylcholine, dopamine, histamine, serotonin ) --More than 30 substances --Excitatory (excitatory postsynaptic potential) --Inhibitory (inhibitory postsynaptic potential) -Synaptic boutons -Synaptic cleft - the space between the neurons (reflux of calcium to transmit signals to vesicles) *when an impulse originates in a presynaptic neuron, the impulse reaches the vesicles, where chemicals (neurotransmitters) are stored in the synaptic button *once released from the vesicles, the neurotransmitters diffuse across the synaptic cleft and bind to specific neurotransmitter (protein) receptor sites on the plasma membrane of the postsynaptic neuron, relaying the impulse

Sensory Pathways

-Posterior column - carries fine-touch sensation, two-point discrimination, and proprioceptive info -Anterior spinothalamic tract -Lateral spinothalamic tract *both anterior and lateral are responsible for vague touch sensation and for pain and temperature perception; referred to as protopathic

Nerve Injury and Regeneration cont.

-Proximal to the injury --Swelling and dispersal of the Nissl substance --Cell increases in metabolic activity, protein synthesis, and mitochondrial activity --New terminal sprouts project from the proximal segment -Process limited to myelinated axons --Generally only in the PNS -Depends on location, type of injury, inflammatory response, and scar tissue formation

Reflex Arc

-Receptor -Afferent (sensory) neuron - *goes through dorsal root -Efferent neuron - *goes through ventral root -Effector *interneurons are usually present and provide a link between sensory and motor neurons

Synapses

-Region between adjacent neurons -Impulses are transmitted across the synapse by chemical and electrical conduction -Presynaptic neurons --Proximal to the synapse --relay impulses toward the synapse -Postsynaptic neurons --Distal to the synapse --relay impulses away from the synapse

Axons cont. (again)

-Saltatory conduction: The flow of ions between segments of myelin rather than along the entire length of the axon -Divergence: Ability of branching axons to influence many neurons Convergence: Branches of numerous neurons converging on one neuron

Functional Classification of Neurons

-Sensory neurons: Transmit impulses from sensory receptors to the CNS -Associated neurons (interneurons): Transmit impulses from neuron to neuron -Motor neurons: Transmit impulses from the CNS to an effector organ

Autonomic Nervous System cont.

-Sympathetic --Thoracolumbar division (T1- L2) --Responds to stress by mobilizing energy stores and prepares the body to defend itself -Parasympathetic --Craniosacral division --Conserves energy and the body's resources -Both systems function, more or less, at the same time

Neurotransmitters and Neuroreceptors of the ANS

-Sympathetic preganglionic fibers --Acetylcholine and cholinergic receptors -Sympathetic postganglionic fibers --Norepinephrine and adrenergic receptors -Parasympathetic preganglionic and postganglionic fibers --Acetylcholine and cholinergic receptors

Blood supply from the brain

-The cerebral veins, classified as superficial and deep veins, drain into venous plexuses and dural sinuses and eventually join the internal jugular veins at the base of the skull -Head injury: avoid turning head

Blood supply to the spinal cord

-Vertebral arteries --Anterior spinal arteries --Posterior spinal arteries (paired) --both descend alongside the spinal cord -Aorta

Forebrain: Diencephalon

-controls vital functions and visceral activities and is closely associated with those of the limbic system -Epithalamus -Thalamus - major integrating center for afferent impulses to the cerebral cortex; also serves as relay center for info from the basal ganglia and cerebellum to the appropriate motor area -Hypothalamus - functions to maintain constant internal environment, implement behavioral patterns (interacts with endocrine system) -Subthalamus - important basal ganglia center for motor activities

Which of the following neurons transmit impulses from neuron to neuron? A. Interneurons B. Motor neurons C. Sensory neurons D. Unipolar neurons

A. Interneurons, or associational neurons, transmit impulses from neuron to neuron. Motor neurons transmit impulses away from the central nervous system (CNS). Sensory neurons carry impulses from the peripheral sensory receptors to the CNS. Unipolar neurons have one process that branches shortly after leaving the cell body.

Structural Classification of Neurons

Based on the number of processes extending from the cell body -Unipolar - have one process that branches shortly after leaving the cell body, ex. in retina -Pseudounipolar - also have one process; dendritic portion of each of these extends away from the CNS and the axon portion projects into the CNS, typical of sensory neurons in both cranial and spinal nerves -Bipolar - have two distinct processes arising from the cell body; this type of neuron connects the rod and cone cells of the retina -Multipolar - most common and have multiple processes capable of extensive branching, ex. a motor neuron

Central Nervous System (CNS)

Brain and spinal cord

Which of the following motor pathways is responsible for rapidly contracting extensor muscles? A. Corticobulbar B. Reticulospinal C. Vestibulospinal D. Lateral corticospinal

C. The vestibulospinal tract arises from the pons and causes the extensor muscles to contract rapidly. The lateral corticospinal and corticobulbar tracts are similar to each other and function as lower motor neurons involved in precise motor control. The reticulospinal tract modulates motor movement by inhibiting and exciting spinal activity.

Gray and White matter

Gray matter -Posterior/dorsal horn (posed primarily of interneurons and axons from sensory neurons whose cell bodies lie in the dorsal root ganglion); substantial gelatinous (involved in pain transmission) -Lateral horn (contains cell bodies involved in ANS) -Anterior/ventral horn (contains nerve cell bodies for efferent pathways that leave the spinal cord by way of spinal nerves) White matter -Spinal tracks (named to dentate their beginning and ending points, ex. spinothalamic tract)

Spinal Cord

Lies within the vertebral canal and is protected by the vertebral column -Connects the brain and the body -Conducts somatic and autonomic reflexes -Provides motor pattern control centers -Modulates sensory and motor function

Neuron

Primary excitable cell of the nervous system that transmits information -Variable size and structure throughout the nervous system -Cellular constituents: --Microtubules (transport substances within the cell) --Neurofibrils (very thin supportive fibers that extend throughout the neuron) --Nissl substances (endoplasmic reticulum and ribosomes) --Microfilaments (thought to be involved in transport of cellular products) -Detect environmental changes and initiate body responses to maintain a dynamic steady state -Fuel source is glucose (long-lived, irreplaceable, and huge appetites)

Protective Structures: Meninges

Protective membranes surrounding the brain and spinal cord -Dura mater - composed of 2 layers, with the venous sinuses formed between them -Arachnoid - a spongy, weblike structure that loosely follows the contours of the cerebral structure -Pia mater - delicate; adheres to the contours of the brain and spinal cord; provides support for blood vessels serving brain tissue

Schwann & Satellite Cells

Schwann -form the myelin sheath around axons and direct axonal regrowth and functional recovery in the PNS Satellite -surround and support cell bodies (in this case neurons)

PNS cont.

Somatic nervous system -Motor and sensory pathways regulating voluntary motor control of skeletal muscle Autonomic nervous system -Motor and sensory pathways regulating the body's internal environment through involuntary control of organ systems --Sympathetic --Parasympathetic

Nerve Injury and Regeneration

Wallerian degeneration—degeneration of a nerve fiber that has been separated by injury or disease; occurs in the distal axon -Distal to the injury --Swelling appears --Neurofilaments hypertrophy --Myelin sheath shrinks and disintegrates --Axon portion degenerates and disappears