Nervous System and Neurons

• Neurons possess cellular and network properties that are fundamental to the nervous system.

Chapter Overview

• The organization of the nervous system.
• How electrical signals function in neurons.
• Cell-to-cell communication in the nervous system.
• Integration of neural information transfer.

Nervous System Subdivisions

• The central nervous system (CNS) comprises the brain and the spinal cord.
• The peripheral nervous system (PNS) includes sensory (afferent) and efferent neurons.
• Sensory (afferent) neurons transmit information from sensory receptors to the CNS.
• Efferent neurons carry signals from the CNS to effector organs (muscles and glands).
• Somatic sensory neurons are responsible for touch, pain, pressure, vibration, temperature, and proprioception in skin, body wall, and limbs; also hearing, equilibrium, vision, and smell.
• Visceral sensory neurons monitor stretch, pain, temperature, chemical changes, and irritation in viscera; also nausea and hunger and taste.
• Somatic motor neurons innervate skeletal muscles (except pharyngeal arch muscles).
• Branchial motor neurons innervate pharyngeal arch muscles.
• Visceral motor neurons innervate smooth muscle, cardiac muscle, and glands, equivalent to the autonomic nervous system (ANS).
• The flow of information starts with a stimulus, which is detected by a sensor.
• The sensor generates an input signal that is sent to an integrating center.
• The integrating center processes the information and generates an output signal.
• The output signal is sent to a target, which produces a response.

Neuron Structure

• Dendrites receive incoming signals; axons carry outgoing information.
• The pyramidal cell and Purkinje cell are two important types of neurons.
• Key structural elements include dendrites, cell body, nucleus, axon hillock, axon (initial segment), and myelin sheath.
• The synapse is the location where a presynaptic axon terminal communicates with a postsynaptic dendrite.

Neuron Functional Categories

• Sensory neurons (afferent neurons) transmit information from sensory receptors to the CNS.
• Interneurons of the CNS integrate information and connect sensory and motor neurons.
• Efferent neurons (motor neurons) carry signals from the CNS to effector organs.

Neuron Structural Categories

• Pseudounipolar neurons have a single process called the axon.
• Bipolar neurons have two relatively equal fibers extending off the central cell body.
• Anaxonic CNS interneurons have no apparent axon.
• Multipolar CNS interneurons are highly branched but lack long extensions.
• A typical multipolar efferent neuron has five to seven dendrites, each branching four to six times.
• A single long axon may branch several times and end at enlarged axon terminals in multipolar efferent neurons.

Glial Cells

• Glial cells, including satellite cells, Schwann cells, oligodendrocytes, astrocytes, microglia, and ependymal cells, maintain an environment suitable for proper neuron function.
• Satellite cells support cell bodies in the PNS.
• Schwann cells form myelin sheaths in the PNS.
• Oligodendrocytes form myelin sheaths in the CNS.
• Astrocytes help form the blood-brain barrier, secrete neurotrophic factors, and take up K+ and neurotransmitters. Microglia (modified immune cells) act as scavengers.
• Ependymal cells create barriers between compartments and are a source of neural stem cells.

Neural Communication

• Sensory neurons transmit afferent signals from receptors.
• Receptors detect stimuli and initiate afferent transmission.
• Interneurons process and relay signals within the CNS.
• Motor neurons transmit efferent signals to effectors.
• Presynaptic neurons release neurotransmitters that affect postsynaptic neurons.

Excitable Tissues

• Nerve and muscle cells are described as excitable tissues because of their ability to propagate electrical signals rapidly in response to a stimulus.

Electrical Signals in Neurons(Ion Movement)

• Resting membrane potential is determined by the K+ concentration gradient and the cell's resting permeability to K+, Na+, and Cl-.
• Gated channels control ion permeability, including mechanically gated channels, chemically gated channels, and voltage-gated channels.
• Mechanically gated channels respond to physical forces (pressure).
• Chemical gated channels respond to ligands (neurotransmitters).
• Voltage gated channels respond to membrane potential changes.
• Threshold voltage varies from one channel type to another.

Graded Potential

• The cell body receives stimulus.
• The strength of graded potential diminishes over distance due to current leak and cytoplasmic resistance.
• The strength is determined by how much charge enters the cell
• Amplitude increases as more sodium enters, and the higher the amplitude, the further the spread of signal.

Graded Potentials

• Subthreshold and suprathreshold graded potentials in a neuron
• A graded potential will not generate an action potential if it does not go beyond the threshold at the trigger zone.
• Depolarizing graded potentials are excitatory.
• Hyperpolarizing graded potentials are inhibitory.
• Graded potentials lose strength as they travel, but can initiate an action potential.

Action Potentials: Resting Membrane Potential

• The cell is more positive outside than inside.
• Ions move across the membrane during depolarization.
• Graded potentials bring the membrane potential up to threshold, allowing voltage-gated Na+ channels to open and Na+ to enter the cell, while voltage-gated K+ channels begin to open slowly.
• Beyond the threshold potential, sodium gated channels allow the ion to move in, making the inside of the cell more positive.
• Na+ continues to move into the cell until it reaches electrical equilibrium, then it stops.
• K+ moves out of the cell along its gradient and the inside of the cell becomes more and more negative.
• Hyperpolarization occurs when the potential drops below resting and is caused by the continuing movement of K+ out of the cell.
• Leaked Na+ and K+ in the cell increase potential toward resting voltage.
• Returns to its original state where the outside is more positive than the inside, and the membrane potential is -70mv.

Action Potentials: Trigger Zone

• Graded potential enters the trigger zone where the summation brings it to a level above threshold.
• Voltage-gated Na+ channels open and Na+ enters axon-an area of the membrane depolarizes
• Positive charge spreads along adjacent sections of axon by local current flow as the signal moves away. The stimulated area returns to its resting potential.
• Local current flow causes new sections of the membrane to depolarizes New sections create a new set of action potentials that trigger the next area to be depolarized.
• Refractory period prevents backward conduction; loss of is what repolarizes the membrane. When the Na+ close they will not open in response to backward conduction until they have reset to resting position - ensures only one action potential is initiated at time.

Action potentials: Voltage Gated Na+ Channels

• At the resting membrane potential, the activation gate closes the channel.
• Depolarizing stimulus arrives at the channel.
• With the activation gate open, Na+ enters the cell.
• The inactivation gate closes, and Na+ entry stops.
• Caused by K+ leaving cell, the two gates reset to their original positions during repolarization.

Electrical Signals: Ion Movement during Action Potential

• Depolarization triggers rising phase.
• Activation gates open rapidly, where
• Sodium (+) feedback increases depolarization and Na+ enters the cell.
• To stop cycle, slower Na+ channel inactivation gate closes.
• Falling phase occurs w/ slow postassium (K+) opening channels.
• This results w./ repolarization as K+ leaves.

Action Potentials: Voltage and Time

• Action potentials involve changes in membrane potential over time.
• Ion permeability also varies during different phases: resting, rising, falling, and after hyperpolarization.

Action Potentials: Refractory Period

• Action potentials will not fire during an absolute refractory period.
• Includes stages of absolute refractory period action potentials and phases like Na+ to K+ Relative refractory period (Na+ channels resets to original position while K+ channels remain open).
• The high threshold is a condition for excitability.

How Action Potential Travels Down Axon

• Each region of the axon experiences a different phase of the action potential.
• A graded potential above threshold reaches the trigger zone.
• Voltage-gated Na+ channels open and Na+ enters the axon.
• Positive charge flows into adjacent sections of the axon by local current flow.
• Local current flow from the active region causes new sections of the membrane to depolarize.
• The refractory period prevents backward conduction. Loss of K+ from the cytoplasm repolarizes the membrane.

Myelinated Axons

• Saltatory conduction moves signal swiftly by jumping from node to node compensating for smaller diameter.
• Demyelination slows down signal conduction, the current leaks.
• Conduction may not arrive effectively at the subsequent node is often not reached, causing signal reduction.

Speed of Action Potential

• Speed of action potential in neurons is influenced by diameter of axon.
• Larger axons are faster and generate less resistance to ion flow due to the larger diameter are only commonly located found in complexed vertebrate systems.

Speed of Action Potential

• Resistance of axon membrane to ion leakage out of the cell affects the speed of action potential.
• Myelinated axons are faster because the myelin sheath insulates the membrane allowing potential action to occur by stopping conduction due to slowing down by ion channels.

Coding Stimulus Intensity

• Weak stimulus releases little neurotransmitter.
• Strong stimulus causes more action potentials and releases more neurotransmitter.
• All action potentials are identical.
• The neurotransmitter released is directly proportional to frequency as the long as a sufficient supply is available.
• The strength of a stimulus is indicated by the frequency of action potentials.

Cell Communication: Chemical Synapse

• Chemical synapses use neurotransmitters; electrical synapses pass electrical signals.
• Chemical synapses are most common.
• Electrical synapses are found in the CNS and other cells that use electrical signals (heart).

Cell to Cell: Synapse

• Synapses are the primary site communication between two neurons
• 1. Action potentials are what depolarize the terminal of azons
• 2. The depolarization opens voltage gated Ca2+ channels allow the calcium to inter the cell
• 3. The calcium triggers the exocytosis of synthetic vesicles

4. The neurotransmitter diffuse across synpatic cleft to the synaptic cleft. the neurotransmitter diffused across the synaptic cleft until it encounters a bind with a neurotransmitter.

• Neurotransmitter Bynes
• Neurotrasmitter initiate signal for synthetic
• Ca2 enter cells for second response
• Exocitris classic versus kiss and run
• Releasing the contents

Cell-to-Cell: Acetylcholine Synapse

• Acetylcholine (ACh) is made from choline and acetyl CoA.
• ACh is rapidly broken down in the synaptic cleft by the enzyme acetylcholinesterase.
• Choline is transported back into the axon terminal and is used to make more ACh.

Long Term Potentiation

• Long-term potentiation is a mechanism used in learning and memory using Glutaminergic Receptors.
• Glutamate is released.
• Net Sodium entry is what Depolarization to be the post synaptic system. Magnesium ejects the receptor the
• Ca + open channel. Ca+ ions enter to be in the cytoplasm.
• Cell becomes more sensitive to growmate to perform signal from system so far.

Inactivation of Neurotransmitters

• Neurotransmitters are what return to Axel terminals for reuse or transported into glial cells.
• Enzymes inactivate neurotransmitters
• Neurotransmitters can diffuse out of the synaptic cleft.

Cell Composition: Function of Neural System for structure

• The seven classes of neurotransmitters by their structures:
  • acetylcholine (ACh) is a neurotransmitter that functions through colic receptors.
  • amines are neurotransporters that act on single base, they function through domine norepinephrine, epinephrine, serotonin systems.
• Aminiac function system transmitters work through cluminate aspirate gamma Amon by tearing Glycine.
• Perins: made from addining.
• Gasses: act as nerotransmitter with half life of 2 to 30 seconds
• Hep ties, neorhormones, nor transmitters.

CELL-TO-CELL: AMINE

• They originate from single amino acid.
• Made out of the
• Made out of Tyrisime - that are functions done by transmitter/heorhormones/ secretes from neurons
• Epinephrine transmitter neurons
• Neurotranmistters :
• 1. Seronine transmitter that is made out of tripitan
• 2. Histamine transmitters

Cell to cell: Amino Acids

• Glutamate : primary excitatory and is CNS
• Aspartate: primary excitatory to the brain
• Gama-amminobutyric is on the inhibitory and brain
• Glycline inhibatory on the spinal cord.

Cell-to-Cell: Neurocrines:

• Peptides-involved in and pain relieve pathways such as sustance P and opiod Peptides
• Purines- binf purinergic receptors AMP and ATP
• Cases- produced inside the function and echanisms nor underston NO and CA
• Lipids: bind cannacoid receptors in and imune cell system Eico

Cell to Cell: Receptors:

• Cholinergic Receptors

• Nicotinic on skeletal muscles.

• Murovalinent caion channels

• Muscarinic in CNS and PNS. Includes proteins G linked

• Adrengic Receptors

• Alpha and B two classes

• Initiate by linked G protiens

Integration: Neurons

• If the cell is not damaged what will be the more likely outcome is the the body will survive
• Axon healing is similar to that of a growth cone of a developing axon.