EXAM 3

What is the nervous system master of

Controlling and communicating system of the body

How do cells of the nervous system communicate

Via electrical (nerve impulse) and chemical signals

3 overlapping functions of NS

Sensory input

Integration

Motor output

Sensory input

Info is gathered by millions of sensory receptors about internal and external changes

Integration

The nervous system processes and interprets the sensory input

Motor output

The nervous system activates effector organs and produces a response

2 major components of nervous system

Central nervous system (CNS)

Peripheral nervous system (PNS)

Central Nervous System

Consists of the brain and spinal cord (dorsal body cavity), is the integration and control center, interprets sensory input and dictates motor ouput

Peripheral Nervous System

Consists of the portion of nervous system outside of the CNS, mainly nerves that extend from brain and spinal cord

Spinal nerves - to and from spinal cord

Cranial nerves - to and from brain

2 functional divisions

Sensory division (afferent)

Motor division (efferent)

Sensory division (afferent)

Goes to the CNS, somatic sensory fibers convey impulses from skin, skeletal muscles, joints to CNS, visceral sensory fibers convey impulses from visceral organs to CNS

Motor division (efferent)

Goes away from the CNS, transmits impulses from CNS to effector organs,

Somatic nervous system

Conduct impulses from CNS to skeletal muscle, referred to as the voluntary nervous system it allows us to voluntarily contract our muscles

Autonomic nervous system

Consists of visceral motor nerve fibers that regulate smooth muscle, cardiac muscle, and glands, referred to as the involuntary nervous system

Sympathetic (“fight or flight”)

Excites/stimulates

Parasympathetic (“rest and digest”)

Inhibits/relaxes

Neuroglia (glial cells)

Cells that surround and wrap delicate neurons, and support neurons (not just physically)

Neurons (nerve cells)

Excitable cells that carry nervous system signals

Astrocytes

Most abundant, versatile, and highly branched glial cells, processes cling to neurons, synaptic endings, capillaries

Astrocytes functions

Support and brace neurons

Play role in exchanges between capillaries and neurons

Guide migration of young neurons and the formation of synapses between neurons

Control chemical environment around neurons

Microglial cells

Small ovoid cells with thorny processes that touch and monitor neurons, they sense and migrate toward injured neurons, and can become activated to phagocytize microorganisms and neuronal debris

Ependymal cells

Range in shape from squamous to columnar, if they’re ciliated cilia circulate CSF (cerebrospinal fluid), they line central cavities of brain and spinal column where they form permeable barrier between CSF in cavities and other fluid

Oligodendrocytes

Moderately branched cells, processes wrap CNS never fibers, forming insulating myelin sheaths around thicker nerve fibers - up to 60 axons at once

Satellite cells

Surround neuron cell bodies to PNS (function similar to astrocytes of CNS)

Schwann cells (neurolemmocytes)

Surround all nerve fibers in the PNS and form myelin sheaths in thicker nerve fibers (same function as oligodendrocytes of CNS)

Neurons

The main structural and functional units of the nervous system (carry AP’s), all have cell body and 1 or more processes, large highly specialized cells that conduct impulses from one part of the body to another, longevity (100 years or more), most amitotic, high metabolic rate - requiring continuous supply of oxygen

Neuron cell body

Contains spherical nucleus with a large nucleolus, biosynthetic center, synthesizes proteins, membranes, and other chemicals, they have rough ER (chromatophilic substance or Nissl bodies), plasma membrane is also receptive region that receives info from other neurons

Nuclei

Clusters of neuron cell bodies in CNS

Ganglia

Lie along nerves in PNS

Tracts

Bundles of neuron processes in the CNS

Nerves

Bundles of nerve processes in the PNS

Dendrites

Main receptive region (receiving/input), they increase surface area to receive input, short, tapering, diffusely branched processes (100’s of them in single neuron), convey incoming messages toward cell body as graded potentials (short distance signals)

Axon structure

Each neuron has single axon - axon arises from region called axon hillock (cone-shaped area of cell body), can have branches (axon collaterals), contains profuse branching at end (terminus) - can be 10,000 terminal branches!

Nerve fibers

Long axons

Axon terminals (terminal boutons)

Distal endings

Axon

Conducting region (sending/output), generates AP’s and transmits them along axolemma to axon terminal, AP reaches the axon terminals, it causes neurotransmitters to be released into extracellular space, either excite or inhibit the cells that receive the messages. Contain the usual organelles, except they lack rough ER and Golgi apparatus

Axolemma

Neuron cell membrane

Axon function

Rely on the cell body to renew proteins and membranes

Efficient transport mechanisms

Quickly decay if cut or damaged

Axon terminal

Secretory region of the neuron

Myelin sheath

Many nerve fibers, long or large in diameter, covered by whitish, fatty segmented myelin sheath composed of myelin, 1 cell forms 1 segment of myelin sheath

Myelinated fibers

Conduct nerve impulses more quickly

Nonmyelinated fibers

Conduct nerve impulses more slowly

Myelin sheath’s function

Protect and electrically insulate axon - increases speed of nerve impulses

Myelination in PNS

Myelin sheath itself is concentric layers of a single Schwann cell’s plasma membrane around axon; plasma membranes of Schwann cells have less protein than most cells - good electrical insulators, adjacent Schwann cells do not touch one another, gaps in the sheath called Nodes of Ranvier (myelin sheath gaps )

Myelination in CNS

Formed by multiple, flat processes of oligodendrocytes, not by whole cells (like Schwann cells do), myelin sheath gap is not present, Regions of brain and spinal cord with dense collections of myelinated fibers are called white matter

White matter

Dense collections of myelinated fibers, composed of fiber tracts

Gray matter

Contains nerve cell bodies and nonmyelinated fibers

Structural classification of Neuron

Multipolar

Bipolar

Unipolar

Multipolar

3 or more processes: 1 axon and 2+ dendrites, most common type (99% of body’s
neurons); motor neurons and interneurons

Bipolar

2 processes: 1 axon and 1 dendrite, very rare; smell and vision sensory
neurons

Unipolar

1 short process that divides in a T-like fashion; both branches function as axons

Distal (peripheral) process

Associated with sensory receptors (the “dendrites end”)

Proximal (central) process

Enters CNS (the axon terminals end”)

Functional classification

Sensory (afferent)

Motor (efferent)

Interneurons (association neurons)

Sensory (afferent)

Transmit impulses from sensory receptors toward CNS, almost all are unipolar and cell bodies are sensory ganglia located in the PNS

Motor (efferent)

Carry impulses from CNS to effectors, multipolar and most cell bodies in CNS (except some autonomic neurons)

Interneurons (association neurons)

Lie between motor and sensory neurons, shuttle signals through CNS pathways, facilitate communication between different regions within the CNS and is 99% of body's neurons

Action potential

Neuron is adequately stimulated; an electrical impulse is generated and conducted along
the length of its axon this response is. Always the same voltage regardless of the stimulus.

Potential

Any voltage difference that exists across a plasma membrane (inside a cell vs. outside the cell)

2 types of ion channels that affect electrical potentials

Leakage channels

Gated channels

Leakage channels

Always open (non-gated)

Gated channels

Part of protein changes shape to open/ close channel

Ligand gated (chemically gated)

Channels opened by specific neurotransmitter

Voltage gated

Channels open and close in response to changes in membrane potential (voltage change)

Resting membrane potential

voltage difference (= a potential) exists across the membrane of a resting cell (–70 mV); this means inside the plasma membrane is negatively charged, relative to outside it and the membrane is said to be polarized

Resting membrane potential is generated by

Differences in ionic makeup of intracellular fluid (ICF) and extracellular fluid (ECF)

Differential permeability of the plasma membrane to K⁺ ions and Na⁺ ions

At rest, ECF has higher concentration of Na⁺ than ICF; balanced chiefly by negatively charged chloride ions (Cl^-)

At rest, ICF has higher concentration of K⁺ than ECF; balanced chiefly by negatively charged proteins

K⁺ ions play the most important role in generating the membrane potential – depending on when the cell holds them in, and when it lets them out

What makes something less negative

Adding positives somewhere

What makes something more negative

Removing positives

Sodium-potassium pump

stabilizes resting membrane potential by maintaining the concentration gradients for Na⁺ and K⁺

How much potassium leaves the cell for every 1+ sodium

25 potassium leave

Local changes in the membrane potential can go 2 ways

Graded potential

Action potential

Graded potential

Incoming short-distance signals; may lead to them becoming…AP’s (if they’re strong enough)

Action potentials

Long-distance signals of axons

Depolarization (less negative)

Decrease in membrane potential (toward zero and above), inside of membrane becomes less negative than resting membrane potential, increases probability of producing an AP

Hyperpolarization (more negative)

An increase in membrane potential (away from zero), inside of cell more negative than resting membrane potential, reduces probability of producing an AP

When do AP’s peak

At +30mV

Activation gate

Closed at rest; open with depolarization allowing Na⁺ to enter cell

Inactivation gate

Open at rest; block channel once it is open to prevent more Na⁺ from entering cell

Voltage gated K+ channels have single voltage sensitive gate

Closed at rest, opens slowly during repolarization

Resting potential

All gated Na⁺ and K⁺ channels are closed, only leakage channels for Na⁺ and K⁺ are open

Depolarization

As local currents depolarize the axon membrane voltage-gated Na⁺ channels open and Na⁺ rushes into cell

Na⁺ influx causes more depolarization which opens more Na⁺ channels à ICF less negative

At threshold (~–55 mV) positive feedback causes opening of all local Na⁺ channels à a reversal of membrane polarity to +30mV

Repolarization

The inactivation gates of the Na+ channels begin to close

Membrane permeability to Na⁺ declines to resting levels

Voltage-gated K⁺ channels open and K⁺ exits the cell along its electrochemical gradient and internal negativity is restored

Hyperpolarization

Some K⁺ channels remain open, allowing K⁺ to leave cell; inside the membrane is now more negative than resting state - slight dip below resting voltage

At the same time Na⁺ channels begin to reset to their original position by moving their inactivation gates and closing their activation gates

Action potential phenomenon

An all or none

For axon to fire

Depolarization must reach threshold when it arrives at the axon hillock, reaching threshold is dependent upon current flowing through the membrane – which relies on the strength of the stimulus: strong stimuli depolarize the membrane quickly, weak stimuli must be applied for longer periods of time to provide the crucial amount of electrical current

Propagation

Allows AP to serve as a signaling device

Refractory period

Brief period of Na+ channel inactivation, to prevent bidirectional propagation of the AP forcing it to go in only 1 direction

What happens after Na+ channels become inactivated

As the potential becomes more positive, and they cannot open again until they are "reset" by hyperpolarization at the end of an action potential

Absolute refractory period

Time from opening of Na⁺ channels until resetting of the channels, ensures that each AP is an all-or-none event and enforces one-way transmission of nerve impulses

Relative refractory period

Follows ARP, most Na⁺ channels have returned to their resting state, some K⁺ channels still open; repolarization is occurring

Axon diameter

Larger fibers have less resistance to local current flow so faster impulse conduction

Degree of myelination

Continuous conduction and Saltatory conduction

Continuous conduction

In nonmyelinated axons - slower

Saltatory conduction

In myelinated axons - 30x faster

Where are voltage-gated Na+ channels located

At myelin sheath gaps, APs generated only at gaps; Electrical signal appears to jump rapidly from gap to gap

Strong Stimuli

Cause action potentials to occur more frequently

Longer or stronger stimuli

Cause more neurotransmitters to be released

Presynaptic neuron

The neuron conducting impulses toward synapse - sends the information

Postsynaptic neuron

The neuron conducting electrical signal away from synapse - receives the information

Electrical synapse

Neurons electrically coupled (joined by gap junctions that connect adjacent neurons), communication via the flow of ions is very rapid, may be unidirectional or bidirectional.

Key feature is they provide a means of synchronizing the activity of all the interconnected neurons, more abundant in regions of the brain involved with stereotypical movements