Neurons and Glial Cells

Nervous System Overview

• The nervous system is the body's master controller
• It regulates bodily functions, receives incoming stimuli
• Cells in it communicate via electrical and chemical signals
• Nervous tissue exhibits irritability (ability to be stimulated) and conductivity

Nervous System Functions

• Sensory input involves monitoring stimuli
• Integration processes and interprets sensory input
• Motor output involves responding to stimuli by activating effector organs

Organization of the Nervous System

• Central Nervous System (CNS) includes the brain and spinal cord
• The CNS is the integration and command center
• Peripheral Nervous System (PNS) consists of paired spinal and cranial nerves
• It carries messages to and from the spinal cord and brain
• Sensory (afferent) division transmits impulses from skin, skeletal muscles, and joints to the CNS
• Sensory afferent fibers carry impulses from these areas
• Visceral afferent fibers transmit impulses from visceral organs to the CNS
• Motor (efferent) division transmits impulses from the CNS to effectors
• Somatic nervous system enables voluntary, conscious control of skeletal muscle
• Autonomic nervous system (ANS) includes visceral motor nerve fibers
• The ANS regulates smooth muscle, cardiac muscle, and glands
• Sympathetic nervous system facilitates "fight or flight" responses
• Parasympathetic nervous system promotes "resting" state

Central Nervous System: Brain Regions and Organization

• The four main adult brain regions are cerebral hemispheres, diencephalon, brain stem, and cerebellum

Cerebral Cortex Makeup

• The cerebral cortex is a large area, accounting for 40% of brain mass
• It consists of outer layers of the brain (gray matter) with unmyelinated neurons
• The cortex is arranged into folds to increase surface area and the number of nerves in an area
• Underneath the cortex is white matter with myelinated neurons
• Association nerves and tracts facilitate memory
• The cortex is divided into two contralateral hemispheres that operate somewhat independently
• It is composed of neuron cell bodies, dendrites, glial cells, and blood vessels, but no axons
• The cerebral cortex deals with awareness, sensory perception, voluntary motor initiation, communication, memory storage, and understanding

General Considerations of the Cerebral Cortex

• The outer portion of the cerebrum is the executive "suite" of the brain, and a thin, gray, superficial layer
• It contains motor areas that control voluntary movement and skeletal muscle
• Sensory areas facilitate conscious awareness of sensation
• Association areas integrate diverse information
• Each hemisphere is concerned with the contralateral (opposite) side of the body
• Lateralization of cortical function can occur in only one hemisphere
• Conscious behavior involves the entire cortex in one way or another

Corpus Callosum

• It sends signals faster because of color
• It connects the hemispheres using white myelinated association fibers

Lobes of the Cerebral Cortex

• Each hemisphere is divided into five lobes that share common functions
• Motor areas contain neurons carrying signals to the PNS to control body parts
• Sensory areas contain neurons receiving signals from the body
• Association areas contain neurons for communication and memory
• Nerves in the CNS are divided into tracts
• Tracts are sensory or motor groups of nerves traveling to the same area and transmitting in the same direction

Frontal Lobe Functions

• The frontal lobe is mostly motor and controls personality and intellect, with development longer for men
• Features of it include Broca's area (motor cortex for speech), somatic motor cortex (skeletal muscle movement), premotor association area (learned movements), frontal eye field (eye movements), and olfactory cortex (sense of smell)

Parietal Lobe Functions

• The parietal lobe is mostly sensory and contains the somatosensory cortex (sensory input) and somatosensory association area
• Wernicke's area (shared with the temporal lobe) enables the ability to understand and form speech

Temporal Lobe

• The temporal lobe (ears) functions in auditory processing, memories of sounds and contributes to the sense of smell

Occipital Lobe

• The occipital lobe (eyes) handles all visual input and understanding/memory of visual input

Insula

• Deep to the temporal lobe, the insula includes the vestibular cortex (balance), visceral association area (monitors visceral input), and gustatory cortex (taste and memory of tastes)

Diencephalon

• It is the area below the cortex
• It includes the thalamus which makes sure signals go to the right area
• The hypothalamus controls the ANS, regulates hunger/thirst/temperature, controls endocrine function, impacts the limbic system and houses mammillary bodies (smell reflexes)
• The epithalamus contains the pineal gland, which releases melatonin and regulates the sleep cycle

Brain Stem Composition

• It is the part that attaches the brain to the spinal cord
• Includes the midbrain (with corpora quadrigemina), pons (respiration center), and medulla oblongata (houses vitals)

Medulla Oblongata

• Medulla oblongata has the Respiratory rate (inhalation), deglutition (swallowing), Emesis (vomiting reflex) and blood pressure
• It connects to the spinal cord and regulates the vitals
• Importantly NOT temperature

Reticular Formation

• It controls consciousness through grey matter in the brainstem that regulates visceral activity and consciousness.

Cerebellum Functions and Structure

• The cerebellum coordinates skeletal muscle activity and monitors joint position
• It contains both white and gray matter, with white matter called the Arbor Vitae
• Damage can cause ataxia, resulting in drunken and uncoordinated movements

Meninges

• These are coverings around the CNS made up of the dura mater, arachnoid mater, and pia mater
• Meningitis is inflammation of the meninges

Cerebral Spinal Fluid (CSF)

• Ventricles of the brain contain CSF

Brain Protection

• Protection of the brain is provided by the meninges, cerebrospinal fluid, cranium/bones, and blood-brain barrier

Blood Brain Barrier

• It is a selective barrier between the blood vessels of the brain and nerve cells
• It allows glucose, essential amino acids, and electrolytes to pass to teh brain
• It blocks metabolic waste, proteins, some toxins, and most drugs
• The blood brain barrier is ineffective against alcohol, nicotine, and anesthetics
• It is lacking around part of the brainstem and hypothalamus

Common Brain Problems

• CVA (stroke) is a cerebral vascular accident
• TIA is a transient episode of reversible cerebral ischemia
• Ischemia involves a loss of blood flow to tissue
• Concussions are traumatic brain injuries that alter brain function and can progressively worsen
• A coma represents a prolonged state of unconsciousness where the reticular formation is affected
• Alzheimer's disease (AD) is a progressive degenerative brain disease that results in dementia
• Key proteins appear to be misfolded and malfunction
• It initially results in memory loss, short attention span, disorientation, etc
• Later, it leads to eventual language loss, irritability, moodiness, confusion, and hallucinations
• Plaques of beta-amyloid peptides form in the brain, neurofibrillary tangles impede neuron transport - mechanisms, and brain cells die

Spinal Cord Structure

• It is made of grey and white matter
• It is protected by bone, meninges, and CSF
• The spinal dura mater is one layer thick and does not attach to vertebrae
• The epidural space alleviates pain during childbirth, with a cushion of fat and network of veins
• CSF fills the subarachnoid space between arachnoid and pia maters
• Dura and arachnoid membranes extend beyond the conus medullaris and are the site of lumbar puncture or tap

Spinal Cord Structures

• Conus medullaris is the terminal portion of the spinal cord
• The conus medullaris extends to L1
• Caude equina consists of remnants of the spinal cord
• Filum terminale is the last part of the pia mater that attaches to the sacrum

Spinal Nerves

• 31 pairs exit off of the spinal cord through the intervertebral foramen
  • 8 Cervical
  • 12 Thoracic
  • 5 Lumbar
  • 5 Sacral
  • 1 Coccygeal
• Spinal nerves are constructed from nerve fibers that extend from the ventral and dorsal aspect of the cord
• The dorsal root contains a big bulge called a ganglion and carries sensory nerves towards the spinal cord, while the ventral root carries motor nerves away

Cross-Sectional Anatomy of the Spinal Cord

• Spinal cord tracts consist of ventral and dorsal sides
• The ventral side is pyramidal (motor), and the dorsal side is the fasciculus (sensory)
• Cuneatus controls the arms, gracilis controls the legs

PNS (Peripheral Nervous System)

• It is entirely made of the nerves that come off the CNS
• These nerves are made from sensory and/or motor nerve fibers (axons)
• Cranial nerves differ from it in that they inovlve 12 paired nerves extending off the brain to innervate the head and neck, while peripheral nerves include the main divisions of spinal neres and innervate the body

Reflexes

• A reflex is a set pathway from a sensory nerve to the CNS and back to a motor nerve
• This pathway is for a preventative, predictable response to a threatening stimulus

Reflex Arc Components

• Components include a sensory nerve, interneuron integration center, motor nerve, and effector muscle

Autonomic Nervous System (ANS)

• It is the division of the PNS that is involuntary
• It controls blood flow, heart rate, digestion, respiration rate, and hormones
• It is centered within the hypothalamus of the brain
• It includes two antagonist divisions: parasympathetic and sympathetic
• The sympathetic division extends from thoracolumbar regions, and promotes a fight or flight response.
• It uses norepinephrine as the main neurotransmitter
• Enhanced by epinephrine
  • Heart rate, respiration rate and blood flow increase, pupils dilate, and digestive activity decreases
• The parasympathetic division extends from cranial-sacral regions, and promotes a "rest and digest" response
• Main neurotransmitter is ACh
  • Respiration, heart rate and blood flow decrease, pupils constrict, and digestive activity increases

Histology of Nerve Tissue

• Nerve tissue is highly cellular with little extracellular space and tightly packed

Principle Cell Types of the Nervous System

• Neurons (nerve cells) are excitable cells that transmit electrical signals, are irritable and conductive, carry impulses, and are amitotic
• Supporting cells (neuroglia) surround and wrap neurons, act as connective tissue of the nervous system for connection, protection and support

Glial Cell Types

1. Astrocytes: abundant, versatile, and highly branched neuroglia cells

• Cling to neurons, synaptic endings, and cover capillaries
• Functions include supporting/bracing neurons, anchoring them to nutrient supplies, regulating waste/nutrients, guiding migration of young neurons, controlling the chemical environment, reabsorbing neurotransmitters, responding to nerve impulses, and influencing neural function

2. Microglia

   • These are small, ovoid cells with thorny processes that touch and monitor neurons and migrate toward injured neurons
   • In function, they transform to phagocytize microorganisms and neuronal debris (defensive cells in the CNS)

3. Ependymal cells

   • Range in shape from squamous to columnar and may be ciliate
   • They bear cilia to circulate CSF and line the central cavities of the brain and spinal column

4. Oligodendrocytes

   • Branched cells that wrap CNS nerve fibers and create myelin sheath for insulation

5. Schwann Cells (neurolemmocytes)

• They surround peripheral nerve fibers of the PNS
• Create myelin sheath in the, and are vital to regeneration of damaged peripheral nerve fibers

6. Satellite cells
   • Cushion, protect, and surround nerve cell bodies in the PNS, similar to astrocytes in the CNS

Neuron Structure and Function

• Neurons are structural units made of a cell body, axons, and dendrites
• They have a long life (100+ years) with a high metabolic rate and are amitotic
• Plasma membranes function in electrical and cell-to-cell signaling
• Neuron processes (axons and dendrites) are described as tracts in bundles of neuron processes in the CNS and nerves for bundles of neuron processes in the PNS
• Major neuron components include dendrites (receptive areas), cell body (organelle location), axon (transmits electric signal), and axon terminals (attach with neurotransmitters)
• Minor neuron components include soma, nucleus, Nissl bodies, Schwann cells, and node of Ranvier
• Neurons are grouped by the number of processes, including multipolar neurons, bipolar neurons, and unipolar neurons
• Most common are multipolar neurons (3 or more processes)

Neuron Classification

• Functionally classified as carrying sensory (afferent), motor (efferent), or are interneurons

Sensory Neurons

• Carry Sensory (afferent) impulses toward the CNS, and can be myelinated or unmyelinated.
• Almost all are unipolar and contain specialized dendritic receptors

Motor Neurons

• Motor (efferent) neurons carry impulses away from the CNS
• Stimulate changes in effector (muscle or gland), are multipolar, very large, and myelinated

Interneurons

• Interneurons (association neurons) shuttle signals through CNS pathways, lie between motor and sensory, and are mostly within the brain and spinal cord
• Act as relay points with 99% of the body's neurons

Neurophysiology

• Neurons are highly irritable and easily stimulated
• Action potentials represent electrical impulses carried along the length of axons

Role of Membrane Ion Channels

• Large proteins serve as selective membrane ion channels
• Leakage (non-gated) channels are always open to allow free flow of ions
• Gated channels open/close based on protein shape changes
• Ligand-gated channels open with a neurotransmitter
• Voltage channels open/close based on membrane potential.
• Mechanical channels open/close based on deformation of receptors

Resting Membrane Potential

• Potential difference across the resting membrane is slightly negative
• Generated by differential permeability of the plasma membrane and differences in ionic makeup

Changes in Membrane Potential

• Changes in membrane potential are due to alterations to ion concentration and permeability
• Changes produce Graded potentials(short distance operation) and action potentials (long distance operation)
• Changes in membrane potential is used as signals to receive, integrate, and send information

Depolarization

• It is a reversal of the membrane potential where the cell membrane becomes less negative (Na+ in)

Hyperpolarization

• It involves an increase in membrane potential, where the inside of cell is more negative than resting membrane potential (K+ out or Cl- in)
• It Reduces the probability of producing a nerve impulse

Graded Potentials

• These are short-lived, localized changes in the membrane potential
• Magnitude changes based on stimulus strength
• These can be either Depolarization (less) or hyperpolarization (more)
• Graded potentials occur at dendrites and require a stimulus that opens gated ion channels

Action Potential

• When stimulated, a nerve undergoes a process of being excited and creating an impulse called an action potential
• Action potentials are the result of muscle cells and axons and are for long distance communication
• Action potentials do not decay like graded potential signals

Events Causing Changes in Membrane Potential

• Depolarization occurs as it becomes more negative (Na+ in)
• Repolarization occurs as membrane returns to resting potential (K+ out)
• Hyperpolarization occurs as the inside of the membrane becomes more negative than the resting potential

Depolarization Process

• It starts with stimulation, which can be touch, pressure, hot, or cold
• Nerves can be signaled by another nerve at the synapse, becoming highly permeable to Na
• If it is too small, not every stimulation will result in depolarization,
• The stimulus must cross a level called the threshold This can happen all at once or in successive attempts

Excitatory Postsynaptic Potential (EPSP) and Inhibitory Postsynaptic Potential (IPSP)

• Excitatory Postsynaptic Potential-excitatory post-synaptic potential
• Inhibitory Postsynaptic Potential-inhibitory post-synaptic potential
• Summation is the adding of signals together resulting in threshold

Action Potential Travels

• Action potentials are also called the all-or-nothing action
• As the membrane depolarizes, the action potential will travel down the nere to the axon terminals
• The action potential ALWAYS travels from cell body to the axon terminal–Toward the synapse
• When the action potential reaches the axon terminals, it releases neurotransmitters

Repolarization Process

• As action potential peaks, the membrane potential has to be restored to its resting state so the muscles can rest before they are used again
  • Sodium gates will close, Potassium gates will open, the Sodium/Potassium pump will expel Sodium and bring Potassium back

Refractory Period

• This is the amount of time it takes to repolarize
• Absolute refractory period means a neuron does not perform another action potential at this time
  • From the opening of the Sodium channels until the resetting of the channels, important because it is the time

Relative Refractory Period

• Occurs during at that time and ensures that each action potential is all-or-none, enforcing one way transmission of nerve impulses and is triggered by an exceptionally strong stimulus

Hyperpolarization Details

• Some Potassium channels open, allowing excessive Potassium to exit
• An excess of open Potassium channels and Sodium efflux causes the membrane potential to be very negative

Propagation of Action Potential

• Once initiated, action potentials are self-propagating
• In non-myelinated axons each successive segment of membrane depolarizes and repolarizes
• Propagation differs in myelinated axons

Conduction Velocity

• The velocities of neurons vary
• Rate of action potential is reliant on axon size and diameter
• Thicker axons have a faster rate and are larger in diameter

Conduction Velocity and Degree of Myelination

• Continuous conduction in unmyelinated axons is low grade
• Myelin sheaths speed up an action potential by insulating and preventing leakage of charge

Saltatory Conduction and Oligodendrocytes

• It occurs only in myelinated axons and is about 30 times faster
• Rapid method by which nerve impulses move down a myelinated axon with excitation occurring only at the Nodes of Ranvier
• Oligodendrocytes form myelin sheaths in the CNS, schwaan cells in the PNS

Multiple Sclerosis (MS)

• It an autoimmune disease affecting young adults
• In MS, the myelin sheaths in the CNS are destroyed because the immune system attacks myelin and hardness into lesions called scleroses
• It creates visual disturbances, weakness, loss of muscular control, speech disturbances, and urinary incontinence
• Treatments modify the immune system, prevention involves higher blood levels of vitamin D

Nerve Fiber Classification

• Nerves are classified via diameter and degree of myelination
• Group A Fibers*
• Large diameter, myelinated somatic sensory and motor fibers of skin, skeletal muscles, and joints
• Transmit at 150 m/s

Group B Fibers

• Intermediate diameter with light myelination
• Transmit at 15 m/s

Group C Fibers

• Smallest diameter; unmyelinated ANS fibers
• Transmit at 1 m/s

Synapses

• Synapses are connections between nerve cells that allow information to flow from neuron to neuron
• Synapses help to relay signals

Chemical Synapses

• Specialized for release and reception of chemical neurotransmitters
• Composed of an Axon terminal of presynaptic neuron (contains synaptic vesicles filled with neurotransmitter (exocytosis)) and neurotransmitter receptor region on postsynaptic neuron's membrane (usually on the dendrite or soma)
• Electrical synapses electrically couple neurons by bridging gap junctions to connect the cytoplasm of adjacent neurons

Common Synapses

• Occur between axon terminals of one neuron and dendrites/soma of others
• Include axodendritic (axon to dendrite), axosomatic (axon to soma), axoaxonic (axon to Axon), dendrodendritic (dendrite to dendrite) or somatodendritic (dendrite to soma) connections

Important Neurotransmitters

• Acetycholine works in the S, is exitatory in parasympathetic nervous system,
• Norephineprine - works in thefight or flight and is inhibitory in parasympathetic
• Dopamine - mostly in the CNs and excritory in hypothalamus
• Serotonin - mostly in the CNS and is inhibitory

Postsynaptic Potentials

• Neurotransmitter receptors cause graded potentials that vary in strength based on the amount of neurotransmitter released and time that the neurotransmitter stays in a position.
• EPSP (excitatory) facilitates the process to signal chemically gated channels, trigger the opoening of voltage gates and stimulate the action potentail
• IPSP (inhibitatory) reduces teh production to open actin and create hyperpolarization

Summation

• A single EPSP cannot induce an action potentail
• Occurs by the adding of graded potentials to reach a threshold

Processing

• Processing is how the way the nerves recognize information along a path
• Serial processing is a linear pathway
• The action Potentials has two types
• Serial - very direct way to receognize the information
• The action Potentials has two types

This quiz covers the structure and function of neurons and glial cells in the nervous system. It explores topics such as myelin sheath formation, astrocyte functions, ependymal cells, interneurons, neuron classification, voltage-gated ion channels, resting membrane potential, nerve regeneration, action potentials, and synaptic transmission.