Nervous System Layout and Neurons

Layout of the Nervous System

• The central nervous system (CNS) handles integration.
• The CNS consists of the brain and spinal cord.
• The peripheral nervous system (PNS) manages sensory and motor functions.
• Nerves are bundles of neurons, neuroglia, along with connective tissue and blood vessels outside the CNS.
• Key nerve types include cranial and spinal nerves.

Nerves

• Nerves are composed of neurons, neuroglia, connective tissues, and blood vessels.
• These structures relay signals between the CNS and other parts of the body.
• Cranial and spinal nerves branch out from brain and brain stem.
• Ganglia are masses of neuronal cell bodies.
• Enteric plexuses is an extensive network of nerves in the walls of the gastrointestinal walls.

Functional Classes of Neurons

• Sensory or afferent neurons carry signals towards the CNS.
• Motor or efferent neurons transmit signals away from the CNS.
• Interneurons, or association neurons, are located within the CNS.
• Interneurons analyze sensory input, store information, and initiate appropriate responses based on that information.

Organization of the Nervous System

• The central nervous system consists of the brain and spinal cord.
• The peripheral nervous system comprises all nervous tissue located outside the CNS.
• Somatic nervous system (SNS) handles voluntary movements by relaying sensory information.
• Autonomic nervous system (ANS) oversees visceral functions through sensory and motor neurons.
• Sympathetic deals with fight and flight.
• Parasympathetic is rest and digest.
• The enteric nervous system (ENS) operates as the "brain of the gut."

Neurons

• Neurons are electrically excitable cells.
• Neurons have cellular structures to transmit nervous signals
• Neuroglia support, nourish, and protect neurons.
• There are approximately 25 times more neuroglia than neurons.

Neuronal Excitability

• Neurons can convert stimuli into action potentials.
• A stimulus is any environmental change that triggers an action potential.
• Nervous action potentials are electrical signals that travel along a neuron's surface.
• Neurons use action potentials to communicate and relay information.

Nerve Fibers

• Nerve fibers are processes extending from the cell body.
• Dendrites receive signals, generating graded potentials proportional to the stimulus.
• Graded potentials require a certain level of stimulus to fire short distances.
• Signals are then sent along the axons via action potentials.

Axons

• Axons connect to the cell body at the axon hillock.
• A trigger zone is were the action potenial starts.
• Axoplasm is cytoplasm within the axon.
• There is no ER.
• Axolemma is membrane specialized for impulse conduction.

Distinguishing Features of Axons

• Axon collaterals branch off the main axon structure.
• Axon terminals are the endpoints of axons that connect to target cells, potentially forming a synapse.
• At synapses, information from action potentials is transferred using a neurotransmitter within the synaptic end bulb.

Axons: Myelin Sheaths

• Myelin sheaths are proteins that wrap around axons, insulating signals and boosting transmission speed.
• Nodes of Ranvier are gaps in the myelin sheath along the axon, facilitating faster impulse conduction.

Neuron Structural Classification

• Multipolar neurons are most CNS neurons and all motor neurons.
• These neurons have several dendrites and one axon.
• Bipolar neurons are involved in vision, hearing, equilibrium, and olfaction.
• One main axon and dendrite.

Neuron Structural Classification Continued

• Unipolar are (pseudounipolar) and are sensory receptors.
• The dendrites and axons are fused into one process.
• Purkinje cells are found in the cerebellum.
• Pyramidal cells are found in the cerebral cortex.

Neuroglia

• Neuroglia are cells within the nervous system that are not electrically excitable.
• Neuroglia are more numerous than neurons, composing roughly half the volume of the CNS.
• These cells can multiply and divide.
• One type, glimoas, can develop from them.
• Astrocytes support and maintain the blood-brain barrier.
• Astrocytes are linked to synaptic activity and may play a role in learning and memory.

Neuroglia of the CNS

• Oligodendrocytes form the myelin sheaths surrounding axons.
• Schwann cells work in a similiar way.
• Microglia function as phagocytes, acting as immune cells within the CNS.

Additional CNS Neuroglia

• Ependymal cells produce and assist in circulating cerebrospinal fluid.
• Nucleus are clusters of cells with a specific function.
• Tract are Bundles of axons.

Neuroglia of the PNS

• Schwann cells form the myelin sheath around axons in the PNS and can enclose multiple unmyelinated axons.
• Satellite cells support and regulate interstitial fluid around cell bodies in ganglia.
• A cluster of cell bodies in the PNS is called a ganglia.

Myelination of Neurons

• Myelin sheath is a lipid and protein covering produced by glial cells, surrounding axons to insulate them.
• Myelinated axons conduct impulses more rapidly than unmyelinated.
• Nodes of Ranvier are gaps in the myelin sheath, that are less frequent in central nervous system.

Gray vs White Matter

• White matter is comprised of myelinated axons, responsible for fast communication over distances.
• Gray matter consists of unmyelinated axons, dendrites, and cell bodies.

Overview of the Nervous Signal

• Sensations lead to graded potentials in sensory receptors.
• Action potentials then travel to the CNS, triggering neurotransmitter release to interneurons.
• Neurotransmitters cause graded potentials on interneuron dendrites.
• The interneurons fire action potentials.

Brain and Nervous Signals

• Brain cells stimulate upper motor neurons, initiating graded potentials and triggering action potentials.
• Upper motor neurons in turn stimulate lower motor neurons, resulting in muscle activation by efferent nerves.

Graded vs Action Potentials

• Action potentials facilitate both short and long distance communication.
• Graded potentials are restricted to short-distance signaling.
• Generating action potentials or graded potentials relies on the a resting membrane potential.
• Production depends on the presence of specific ion channels.

How Action Potentials Work

• An exact balance of charges leads to no membrane potential.
• A few positive ions must cross to generate potential.

Electrical Potentials

• Small changes in the resting membrane potential are called graded potentials.
• Graded Potentials trigger (sometimes inhibit) action potentials.
• Hyperpolarizing graded potentials are inhibitory.
• Depolarizing graded potentials are excitatory.

Neuron Resting Membrane Potential

• The distribution of charges across a neuron defines its resting membrane potential.
• This potential generates a small electrical charge in the neuron.

Factors Contributing to Resting Membrane Potential

• Neuron membranes are characterized by a positive charge outside and a negative charge inside.
• The selective membrane permeability to Na+ and K+ ions.
• There is an unequal distribution of ions.
• Appropriate charge is referred to as Na+/K+ pumps

Neuron Ion Channels

• Leakage channels alternate between open and closed states.
• K+ channels are generally more abundant than Na+ channels.
• As K+ gradually exits through leakage channels, the interior of the intracellular membrane develops a negative charge
• Most of the neuron's resting potential depends on this K+ permeability.

Graded Potentials

• Graded potentials occur in response to stimuli like mechanical forces and ligand binding, altering ion flow in channels.
• Depolarizing graded potentials may occur from pressure changes, such as mechanically gated channels.
• Depolarizing, ligand-gated channel examples include the neurotransmitter acetylcholine.
• Hyperpolarizing graded potentials result from the neurotransmitter glycine, which leads to the the influx of Cl- ions.

Voltage-Gated Ion Channels

• Voltage-gated channels are important for action potentials and open/close in response to direct changes in membrane potential.

Graded Potentials: Stimulus Strength

• The amplitude of a graded potential depends on the intensity of the stimulus
• A greater amplitude indicates a stronger response.
• Resting membrane is the starting point.

Graded Potentials: Summation

• Graded potentials can be added together.
• Stimulus 1 and Stimulus 2 contribute to reach resting membrane.
• Together they form an larger amplitude.

Properties of Action Potentials

• Action potentials are designed for long-distance communication.
• Action potentials occur on muscle fibers or axons of neurons.
• Action potenitals function to communicate with other cells.
• Neurotransmitter at the synaptic end bulb
• Voltage-gated channels open along the axon in succession from axon hillock to the synaptic end bulb.
• The effect is all-or-nothing.

Stages of Action Potential

• Depolarizing Phase: Begins with the opening of voltage-gated Na+ channels, which causes sodium ions flow into the cell.
• Rapid Na+ influx leads to membrane depolarization until it reaches the threshold.
• Then causes Repolarization to start.
• In that phase voltage gated K+ channels open and Na inactivation starts.
• The stimulus must causes depolarization to reach the threshold.
• Resting and refracting stages happen inbetween.

Threshold Stimulus Strength

• Action potentials require the membrane potential to reach a defined threshold, or the AP will not fire.
• If action potential occurs with high frequency, can get to suprathreshold stimulus.

Nerve Action Potentials

• Across the membrane of a nerve are a series of Na+ ion channels that help perpetuate nervous signals.
• The gate if is closed until perturbed.
• This is known as Depolarization
• There is channel inactivation as even more Na+ causes effects.
• Gates require Recovery and Repolarization
• Then the membrane will recover and repolarize

Signal Propagation

• One ion channel activation causes influx of NA+
• The influx changes local levels.
• This send the signal down the whole nerve.
• It is all or nothing.

Neurotoxins

• Neurotoxins and local anesthetics often disrupt nerve signals by blocking the Na+ voltage gates, interfering with polarization.

Resting state

• Nerve action potentials begin with a resting membrane.
• Voltage gated Na+ and K+ channels are closed.
• A small buildup of negative charges
• The membrane at the inside and with equal positive charges outside

Depolarizing the Axon

• . Voltage-gated Na+ channel allows rapid influx, further increasing membrane potential.

Repolarizing the Axon

• Na⁺ channel inactivation gates close and K+ channels open.
• The membrane starts to become repolarizing.
• K ions come out.

After-hyperpolarization

• K⁺, Na⁺ will close leaving high negative charge.
• Membrane potential is reset by the sodium potassium.
• The can is be only one refractory period .

Action potential propagation

• Depolarizing and repolarizing occur in a step by step nature along unmyelinated axon, which is called continuous conduction.
• Saltatory conduction utilizes myelination for faster "leaping."

Saltatory Conduction

• Saltatory conduction utilized myelinated nerves.
• Most gate are found in these nodes of Ranvier.
• Action potential appears as leap.
• This minimizes the amount of ATP needed.

Action Potential Speed Factors

• Axon diameter affects speed; larger diameter, faster the impulse moves.
• Amount of myelination insulates neurons.
• Temperature also plays a crucial role.

Signal Transmission at Synapses

• Synapses are specialized junctions between neurons or between a neuron and an effector cell.
• Electrical synapses use gap junctions for direct, synchronized communication.
• Chemical synapses involve neurotransmitters for one-way signal transmission to postsynaptic neurons.

Synapses and Neurotransmission

• Neurotransmitters released move over synapsis.
• Then bind to post synaptic receptors.
• Cause communication and potentials.

Neurotransmitters

• Neurotransmitters can cause:
• Excitatory post synaptic potentials (EPSP) if sodium enters the cell,
• Inhibitory post synaptic potentials (IPSP) of anions like chloride enter, or cations potassium.
• Must be removed after.

Synaptic Signalling

• Synaptic neurotransmitters must be removed for effects to cease.
• This happens via diffusion, enzymatic degradation and uptake by cells.
• lonotropic & Metabotropic Receptors do these taks.

Synaptic Integration Process

• If multiple presynaptic neurons converge onto single postsynaptic neuron
• a large quantity of neurotransmitters are released at once.
• This referred to as Spatial summation
• This could be temporal summation
• summation may be spatial or temporal
• The combined action may generate nerve potential.

Temporal Summation

• A single presynaptic neuron rapidly sends impulses.
• The stimuli build up to reach a action potential..

Neural Pathways

• Reverberating circuits include long axons.
• This includes Permitting effects with longer duration than single impulses.

CNS Repair

• Neurogenesis is limited in the CNS to Inhibitory influences and glia cells.There is also rapid scar tissue formation.
• Injury repair can happen if axon is cut.

Nerve Damage

• Chromatolysis occurs where Nissl bodies break down.
• Wallerian degeneration destroys distal myelin sheath.
• And regeneration happens.

Nervous System Disorders

• Multiple sclerosis causes degenerative destruction of myelin sheaths by autoimmune reaction.
• Epilepsy leads to recurrent seizures, despite normal intelligence.
• Improper discharge of nerves causes seizures.
• Neuropathy is another disorder.

Explore the layout of nervous system including central nervous system (CNS) and peripheral nervous system (PNS). Learn about nerves, neurons, neuroglia and their functional classes. Understand how sensory, motor, and interneurons transmit signals within the body.