Download Nervous System Lecture- Part II

BIOL 2304
Nervous Tissue
Nervous System
Overall function is control and communication, achieved through:
Sensory - receptors monitor changes (stimuli) and gathers information inside and outside the
body
Integration - prrocesses and interprets sensory input, makes decisions
Motor - dictates a response by activating effector organs
Organization of the Nervous System
Central nervous system (CNS)
Brain and spinal cord
The integration and command center
Peripheral nervous system (PNS)
Carries messages to and from the CNS
Paired cranial nerves extending from brain
Paired spinal nerves extending from spinal cord
Peripheral nerves link all regions of the body to the CNS
Ganglia are clusters of neuronal
cell bodies
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Organization of the Nervous System
Nervous Tissue
Cells are densely packed and intertwined
Two main cell types:
1. Neurons
Excitable – transmit electrical signals
2. Glial cells – support cells
Also called neuroglia or simply glia
Non-excitable – do not transmit electrical signals
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The Neuron
Basic structural unit of the nervous system
Human body contains billions of neurons
Neurons conduct electrical impulses along the plasma membrane
Nerve impulses are called action potentials
Other special characteristics
Longevity – can live and function for a lifetime
Do not divide – fetal neurons lose their ability to undergo mitosis; neural stem cells are an
exception
High metabolic rate – require abundant oxygen and glucose, neurons die after 5 minutes
without oxygen
The Cell Body (Soma)
Size of cell body varies from 5–140 µm
Contains Nucleus and Perikaryon (“around nucleus”)
Contains usual organelles plus other structures:
Chromatophilic bodies (Nissl bodies) are clusters of rough ER and free ribosomes
Function to renew cell membrane proteins (channels)
Named chromatophilic because they stain darkly
Neurofibrils – bundles of intermediate filaments, form a network between chromatophilic
bodies
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The Cell Body (Soma)
Most neuronal cell bodies are located within the CNS in cluster called nuclei, protected by bones of the
skull and vertebral column
Clusters of cell bodies outside CNS are called ganglia which lie along nerves in the PNS
Structure of a Typical Large Neuron
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Neuron Processes – Dendrites
Dendrites
Extensively branching from the cell body
Transmit electrical signals toward the cell body
Function as receptive sites for receiving signals from other neurons
Neuron Processes – Axons
Axons
Neuron has only one
Impulse generator and conductor
Transmits impulses away from the cell body
No protein synthesis in axon
Neuron Processes
Consist of neurofilaments, actin microfilaments, and microtubules
Provide strength along length of axon
Aid in the axonal transport of substances to and from the cell body
Axon collaterals - Infrequent branches along length of axon, prior to axon terminal
Multiple branches at end of axon
Terminal branches (telodendria)
End in knobs called axon terminals (also called end bulbs or boutons)
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Supporting Cells
Six types of supporting cells
Four in the CNS
Oligodendrocytes
Microglia
Astrocytes
Ependymal cells
Two in the PNS
Satellite cells
Schwann cells
Provide supportive functions for neurons
Cover nonsynaptic regions of the neurons
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Neuroglia
Neuroglia in the CNS
Neuroglia
Glial cells have branching processes and a central cell body
Outnumber neurons 10 to 1
Make up half the mass of the brain
Can divide throughout life
Neuroglia in the CNS
Astrocytes - the most abundant glial cell type
Sense when neurons release glutamate
Extract blood sugar from capillaries for energy
Take up and release ions in order to control environment around neurons
Involved in synapse formation in developing neural tissue
Produce molecules necessary for neuronal growth
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Neuroglia in the CNS
Microglia – smallest and least abundant glial cell
Phagocytes – the macrophages of the CNS
Engulf invading microorganisms and dead neurons
Derive from blood cells called monocytes
Neuroglia in the CNS
Ependymal cells
Line the central cavity of the spinal cord and brain
Bear cilia – help circulate the cerebrospinal fluid
Neuroglia in the CNS
Oligodendrocytes – have few branches , wrap their cell processes around axons in CNS, produce
myelin sheaths
Analogous to Schwann cells of the PNS
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Neuroglia in the PNS
Satellite cells – surround neuron cell bodies within ganglia
Schwann cells (neurolemmocytes) – surround axons in the PNS
Form myelin sheath around axons of the PNS
Structural Classes of Neurons
Unipolar
Possess one short, single process
Dendrite, axon continuous
Afferent neurons
Multipolar
Many dendrites, one axon
Most common class of neuron
Bipolar
One dendrite, one axon
Very rare
Found in some special sensory organs
Neurons Classified by Structure
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Neuron Classification
Functional:
Sensory (afferent) — transmit impulses toward the CNS
Motor (efferent) — carry impulses away from the CNS
Interneurons (association neurons) — shuttle signals through CNS pathways
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Functional Classification of Neurons
Sensory (afferent) neurons
Transmit impulses toward the CNS
Virtually all are unipolar neurons
Cell bodies in ganglia outside the CNS
Short, single process divides into
The central process – runs centrally into the CNS
The peripheral process – extends peripherally to the receptors
Motor (efferent) neurons
Carry impulses away from the CNS to effector organs
Most motor neurons are multipolar
Cell bodies are within the CNS
Form junctions with effector cells
Interneurons (association neurons)
Most are multipolar
Lie between motor and sensory neurons
Confined to the CNS
Neurons Classified by Function
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Myelin Sheaths
Segmented structures composed of the lipoprotein myelin
Surround thicker axons
Form an insulating layer
Prevent leakage of electrical current
Increase the speed of impulse conduction
Nodes of Ranvier – gaps along axon
Thick axons are myelinated
Thin axons are unmyelinated, conduct impulses more slowly
Myelin Sheaths in the PNS
Myalin sheaths formed by Schwann cells (neurolemmacytes)
Develop during fetal period and in the first year of postnatal life
Schwann cells wrap in concentric layers around the axon, cover the axon in a tightly packed coil of
membranes
Neurilemma - material external to myelin layers
Myelin Sheaths in the PNS
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Myelin Sheaths in the CNS
Oligodendrocytes form the myelin sheaths in the CNS
Have multiple processes
Coil around several different axons
Synapses
Site at which neurons communicate
Signals pass across synapse in one direction
Presynaptic neuron - conducts signal toward a synapse
Postsynaptic neuron - transmits electrical activity away from a synapse
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Types of Synaptic Connections
Axodendritic - between axon terminals of one neuron and dendrites of another, most common type of
synapse
Axosomatic - between axons and neuronal cell bodies
Axoaxonic, dendrodendritic, and dendrosomatic -uncommon types of synapses
Synapses
Two types of Synapses:
Electrical synapse
Neurons connected by gap junctions
Electrical activity in pre-synaptic neuron is passed directly to post-synaptic neuron
Chemical synapse (Neurotransmitters)
Neurons connected by physical space
Electrical activity in pre-synaptic neuron produces chemical exocytosis of
neurotransmitters that acts on receptors on post-synaptic neuron
Electrical Synapse – Gap Junctions
Electrical synapse
Neurons connected by gap junctions
Electrical activity in pre-synaptic neuron is passed directly to post-synaptic neuron
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Chemical Synapse – Neurotransmitters
Chemical synapse
Neurons connected by physical space
Electrical activity in pre-synaptic neuron produces chemical exocytosis of neurotransmitters
that acts on receptors on post-synaptic neuron
Features of Chemical Synapse
Axodendritic synapses – representative type
Synaptic vesicles on presynaptic side
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Membrane-bound sacs containing neurotransmitters
Mitochondria abundant in axon terminals
Synaptic cleft - separates the plasma membrane of the two neurons
Synaptic Cleft: Information Transfer
Nerve impulses (AP) reach the axon terminal of the presynaptic neuron and open Ca2+ channels
Neurotransmitter is released into the synaptic cleft via exocytosis
Neurotransmitter diffuses across the synaptic cleft and binds to receptors on the postsynaptic neuron
Postsynaptic membrane permeability changes due to opening of ion channels, causing an excitatory or
inhibitory effect
CNS
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Central Nervous System:
Brain
Spinal cord
The Brain
Performs the most complex neural functions
Intelligence
Consciousness
Memory
Sensory-motor integration
Involved in innervation of the head
Organization of CNS
Centrally located gray matter – neuron cell bodies, interneurons,
unmyelinated fibers
Externally located white matter – myelinated fibers
Additional layer of gray matter external to white matter is the
Cortex
Formed from neuronal cell bodies migrating externally
Located in cerebrum and cerebellum
Basic Parts and Organization of the Brain
Divided into four regions:
Cerebral hemispheres Account for 83% of brain mass
Diencephalon – includes
thalamus and hypothalamus
Brain stem - includes
midbrain, pons, and medulla
Cerebellum – “little brain”
The Cerebral Hemispheres
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Frontal section through forebrain
Cerebral cortex
Cerebral white matter
Deep gray matter of the cerebrum (basal ganglia)
Corpus Callosum – commissural fibers (white matter) which connects the two hemispheres
The Cerebral Hemispheres
Fissures – deep grooves, which separate major regions of the brain
Transverse fissure – separates cerebrum and cerebellum
Longitudinal fissure – separates cerebral hemispheres
Sulci - grooves on the surface of the cerebral hemispheres
Gyri - twisted ridges between sulci
Prominent gyri and sulci are similar in all people
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Lobes, sulci, and fissures of the cerebral hemispheres
The Cerebral Hemispheres
Central sulcus separates frontal and parietal lobes
Bordered by two gyri:
Precentral gyrus
Postcentral gyrus
Parieto-occipital sulcus - separates the occipital from the parietal lobe
Lateral sulcus - separates temporal lobe from parietal and frontal lobes
Deeper sulci divide cerebrum into lobes
The Cerebral Hemispheres
Lobes are named for the skull bones overlying them:
Frontal lobe
Parietal lobe
Temporal lobe
Occipital lobe
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The Cerebral Cortex
Home of our conscious mind
Composed of gray matter - neuronal cell bodies, dendrites, and short axons
Folds in cortex – triples its size
Approximately 40% of brain’s mass
The Cerebral Cortex - Functional Areas
Three general functional areas:
Sensory areas
Association areas
Motor areas
Each of the major senses has a specific brain region called a primary sensory cortex
There are also multimodal association areas to process information
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Functional Areas Of The Cerebral Cortex
Primary Somatosensory Cortex
Located along the postcentral gyrus
Involved with conscious awareness of general somatic senses
Primary Visual Cortex
On medial part of the occipital lobe
Largest of all sensory areas
Receives visual information that originates on the retina
First of a series of areas processing visual input
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Primary Auditory Cortex
Located at superior edge of the temporal lobe
Conscious awareness of sound
Impulses transmitted to primary auditory cortex
Olfactory Cortex
Olfactory nerves transmit impulses to the olfactory cortex
Provides conscious awareness of smells
Lies on the medial aspect of the temporal lobe
Gustatory Cortex
Involved in the conscious awareness of taste stimuli
Located on the “roof” of the lateral sulcus
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Motor Areas – Primary Motor Cortex
Controls motor functions
Located in precentral gyrus
The Diencephalon
Forms the center core of the forebrain, primarily composed of gray matter
Surrounded by the cerebral hemispheres
Composed of three paired structures: thalamus, hypothalamus, epithalamus
Border the third ventricle
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The Thalamus
Makes up 80% of the diencephalon
Contains approximately a dozen major nuclei
Acts as the relay station for incoming sensory information
Every part of brain communicating with cerebral cortex relays signals through thalamic nuclei
Is the “gateway” to the cerebral cortex
The Hypothalamus
Lies between the optic chiasm and the mammillary bodies
Pituitary gland projects inferiorly
Contains approximately a dozen nuclei
Main visceral control center of the body
The Diencephalon – The Hypothalamus
Functions include the following
Control of the ANS
Control of emotional responses
Regulation of body temperature
Regulation of hunger and thirst sensations
Control of behavior
Regulation of sleep-wake cycles
Control of the endocrine system
Formation of memory
The Diencephalon – The Epithalamus
Forms part of the “roof” (top) of the third ventricle
Consists of a tiny group of nuclei
Includes the pineal gland (pineal body)
Secretes the hormone melatonin
Under influence of the hypothalamus
Aids in control of circadian rhythm
The Brain Stem
Several general functions
Produces automatic behaviors necessary for survival
Passageway for all fiber tracts running between the cerebrum and spinal cord
Heavily involved with the innervation of the face and head
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10 of the 12 pairs of cranial nerves attach to it
The Brain Stem
Includes the midbrain, pons, and medulla oblongata
The Brain Stem – The Midbrain
The midbrain processes visual and auditory information and generates involuntary somatic motor
responses
Has reticular activating system - arousal of the whole brain
Has nuclei for cranial nerves II and IV
Has ascending and descending tracts
Lies between the diencephalon and the pons
Cerebral peduncles located on the ventral surface of the brain, contain pyramidal (corticospinal) tracts
Superior cerebellar peduncles - connect midbrain to the cerebellum
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The Brain Stem – The Midbrain
Corpora quadrigemina (quad-ri-gemina)
The largest nuclei
Divided into the superior and inferior colliculi
Superior colliculi – nuclei that act in visual reflexes
Inferior colliculi – nuclei that act in reflexive response to sound
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The Brain Stem – The Pons
A “bridge” between the midbrain and medulla oblongata
Pons contains the nuclei of cranial nerves
V – Trigeminal nerve
VI – Abducens nerve
VII – Facial nerve
Motor tracts coming from the cerebral cortex
Pontine nuclei
Connect portions of the cerebral cortex and cerebellum
Send axons to cerebellum through the middle cerebellar peduncles
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The Brain Stem – The Medulla Oblongata
The core of the medulla contains
Much of the reticular formation
Nuclei then influence autonomic functions
Cardiac center
Vasomotor center
The medullary respiratory center
Centers for hiccupping, sneezing, swallowing, and coughing
Functional Brain Systems: The Reticular Formation
The Brain Stem – The Medulla Oblongata
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Most caudal level of the brain stem
Is continuous with the spinal cord
Choroid plexus lies in the roof of the fourth ventricle
Cranial nerves VIII–XII attach to the medulla
External landmarks of medulla
Pyramids of the medulla lie on its ventral surface
Decussation of the pyramids - crossing over of motor tracts
Inferior cerebellar peduncles - fiber tracts connecting medulla and cerebellum
The Cerebellum
Located dorsal to the pons and medulla
Smooths and coordinates body movements
Helps maintain equilibrium
Consists of two cerebellar hemispheres
Cortex – gray matter
Arbor vitae - internal white matter
Thick tracts connecting the cerebellum to the brain stem are superior, middle, inferior cerebellar
peduncles
Composed of
Cortex – gray matter
Arbor vitae - internal white matter
Thick tracts connecting the cerebellum to the brain stem are
Superior cerebellar peduncles
Middle cerebellar peduncles
Inferior cerebellar peduncles
Fibers to and from the cerebellum are ipsilateral – run to and from the same side of the body
Cerebellum receives information from the cerebral cortex
On equilibrium
On current movements of
Limbs, neck, and trunk
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Ventricles of the Brain
Expansions of the brain’s central cavity
Filled with cerebrospinal fluid (CSF)
Lined with ependymal cells (glial cells)
Continuous with each other
Continuous with the central canal of the spinal cord
Ventricles of the Brain
Lateral ventricles – located in cerebral hemispheres
Horseshoe-shaped from bending of the cerebral hemispheres
Third ventricle – lies in diencephalon
Connected with lateral ventricles by interventricular foramen
Cerebral aqueduct – connects 3rd and 4th ventricles
Fourth ventricle – lies in hindbrain
Connects to the central canal of the spinal cord
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Ventricles of the Brain
Protection of the Brain
The brain is protected from injury by
The skull
Meninges
Cerebrospinal fluid
Blood-brain barrier
Cerebrospinal Fluid (CSF)
Formed in choroid plexuses in the brain ventricles
Choroid plexus is
Located in all four ventricles
Composed of ependymal cells and capillaries
Arises from blood - 500 mL/day
Cerebrospinal Fluid
Fills the hollow cavities of the brain and spinal cord
Functions:
Provides a liquid cushion for the spinal cord and brain
Nourishes brain and spinal cord
Removes wastes
Carries chemical signals between parts of the CNS
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Cerebrospinal Fluid (CSF)
Blood Brain Barrier
Extensive impermeable capillaries & sinuses
Perivascular feet of astrocytes cover and wrap around capillaries and promote tight junction formation
Protects brain from hormones & circulating chemicals
Prevents most blood-borne toxins from entering the brain
Not an absolute barrier
Nutrients such as oxygen pass through
Allows alcohol, nicotine, and anesthetics through
Many glucose transporters
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Meninges
Functions of meninges:
Cover and protect the CNS
Enclose and protect the vessels that supply the CNS
Contain the cerebrospinal fluid between pia and arachnoid maters
Meninges
Dura Mater
Strongest of the meninges
Composed of two layers: periosteal layer & meningeal layer
Arachnoid Mater
Located beneath the dura mater
Arachnoid villi - Project through the dura mater, allow CSF to pass into the dural blood sinuses
Pia Mater
Thin, delicate connective tissue, clings tightly to the surface of the brain
Follows all convolutions of the cortex
The Spinal Cord
Functions of the spinal cord:
Spinal nerves attach to it
Provides two-way conduction pathway
Major center for reflexes
Location of the spinal cord:
Runs through the vertebral canal
Extends from the foramen magnum to the level of the
vertebra L1 or L2
Cervical and lumbar enlargements - where nerves for upper and
lower limbs arise
Conus medullaris - the inferior end of the spinal cord
Cauda equina - collection of spinal nerve roots
Filum terminale - long filament of connective tissue, attaches to
the coccyx inferiorly
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Spinal Cord Segments
Segments that indicate the region of the spinal cord from which spinal nerves emerge
Designated by the spinal nerve that issues from it
T1 is the region where the first thoracic nerve emerges
The Spinal Cord
Two deep grooves run the length of the cord
Posterior median sulcus
Anterior median fissure
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White Matter of the Spinal Cord
White columns:
Dorsal (posterior) funiculus
Ventral (anterior) funiculus
Lateral funiculus
Composed of myelinated axons
Allow communication between spinal cord and brain
White Matter of the Spinal Cord
Fibers classified by type:
Ascending fibers - afferent (sensory)
Descending fibers – efferent (motor)
Commissural fibers
Major Fiber Tracts in White Matter of the Spinal Cord
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Gray Matter of the Spinal Cord
Shaped like the letter “H”
Gray commissure – contains the central canal
Dorsal horns consist of interneurons
Ventral horns and lateral horns contain cell bodies of motor neurons
Organization of the Gray Matter of the Spinal Cord
Divided according to somatic and visceral regions
SS – somatic sensory
VS – visceral sensory
VM – visceral motor
SM – somatic motor
Review of Protection of the Spinal Cord
Protected by vertebrae, meninges, and CSF
Meninges:
Dura mater – a single layer surrounding spinal cord
Arachnoid mater – lies deep to the dura mater
Pia mater – innermost layer, delicate layer of connective tissue
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