Nervous System: Organization and Communication

Nervous System Organization

• The central nervous system (CNS) comprises the brain and spinal cord and functions in control and integration, making and sending decisions.
• The peripheral nervous system (PNS) consists of cranial and spinal nerves, facilitating communication by connecting the CNS to receptors and glands.

Cell Types

• Neurons communicate by conducting electrical signals.
• Neuroglia, the majority of nerve tissue cells, support neurons.

Neural Communication: Neuron Structure

• A neuron is the basic cell of a nervous system
• Its structure includes dendrites for receiving, a cell body as an integrator, an axonal hillock (trigger zone), and an axon.
• The axon has "excitable membrane" that fires action potential
• The axon includes axonal transport mechanisms and an axon terminal that releases neurotransmitters.
• The Axon releases neurotransmitters to act on the next neuron

Myelination and Saltatory Conduction

• Many axons are myelinated.
• Myelin is a fatty sheet wrapped around the axon, increasing speed through saltatory conduction.

Functional Types of Neurons

• Based on the direction impulses are conducted:
• Afferent neurons go in and have sensory receptors and a cell body in the middle.
• Efferent neurons go out and have a Somatic (voluntary; Skeletal muscle) and Autonomic Nervous System (involuntary)
• Interneurons reside entirely in the CNS for association and processing circuits.
• Efferent autonomic nerve pathways involve a two-neuron chain between the CNS and the effector organ.

Neuroglia

• Neuroglia are non-conducting cells found in both the PNS and CNS.
• Types include Schwann cells (PNS), oligodendrocytes (CNS), astrocytes (blood-brain barrier), microglia (immune function), and ependymal cells (CSF).

Regeneration of a Cut Neuron

• When an axon in the PNS is cut, the severed part degenerates.
• A regeneration tube, formed by Schwann cells guides future growth, but very slowly.
• The CNS cant regrow nerves but is adaptable and plastic.

Electrical Activity and Resting Membrane Potential

• Nerve and muscle are excitable tissues capable of rapid changes in their resting membrane potentials and converting them into electrical signals.
• The inside of a cell is negative relative to the outside, approximately -70 mV for many neurons.
• The cell is relatively permeable to K+ (-90 mV) but relatively impermeable to Na+ (+100 mV).
• At resting membrane potential, neither K+ nor Na+ are in equilibrium.

Electrical Activity Definitions

• Polarization is any membrane potential other than 0 mV.
• Depolarization involves becoming less negative than resting membrane potential.
• Repolarization means returning to the resting potential after depolarization.
• Hyperpolarization refers to becoming more negative than the resting membrane potential.

Ion Gating in Axons

• Gating means the opening and closing of ion channels.
• Leaky channels are always open and contribute to resting potential.
• Ligand-gated channels open in response to synaptic potentials (graded potentials).
• Voltage-gated channels open at a certain voltage threshold).
• K+ has two types of channels: leaky (always open) and voltage-gated (open when a particular membrane potential is reached, closed at resting potential).
• Na+ has only voltage-gated channels that are closed at rest and open when a specific membrane potential (~55 mV) is reached.

Neural Communication Signals

• When a nerve cell 'fires', an electrical signal moves down the neuron.
• Two types of electrical signals exist: graded potential and action potential.
• Electrical signals are produced by changing ion concentrations.
• Changes are brought about by a triggering event, altering ion concentrations by altering ion permeability.

Graded Potentials

• Magnitude varies with stimulus strength.
• They are decremental graded potentials.
• They are either Depolarizing or Hyperpolarizing
• Produced by some specific change in the environment acting on a specialized region.

Action Potentials Characteristics

• It is a brief, rapid, large (100mV) change in membrane potential.
• It operates on an all or none principle.
• Potential must reach threshold
• Not decremental
• Begins at axon hillock

Action Potential Sequence

• Depolarization occurs when voltage-sensitive Na+ channels open as the membrane depolarizes, reaching the threshold that induces more Na+ channels to open.
• At the peak, Na+ channels close and become inactivated, while voltage-gated K+ channels open.
• Repolarization happens as K+ leaves the neuron.
• After hyperpolarization, K+ still slowly leaves.
• The process returns to the resting membrane potential.

Refractory Period

• Absolute → can't fire more
• Relative → must be big

Action Potential Propagation

• Action potentials open voltage-gated Na+ channels.
• Depolarization stimulates the voltage gated Na+ channels in the adjacent section of the axon and triggers as AP at the segment
• Action potentials are produced continuously along the plasma membrane of unmyelinated axons.

Myelinated Axons: Saltatory Conduction

• Action potential "jumps" from one node to the next, where depolarization takes place, increasing AP conduction speed.

Synapses

• The synapse is a communication junction between a neuron and another neuron, a muscle, or a gland cell.
• Chemical synapses involve the release of neurotransmitters for integration.
• Electrical synapses involve gap junctions for speed.
• Synapses Stimulates physiological change (usually change in membrane potential) in the recipient cell.

Anatomy of a Synapse

• The Presynaptic neuron sends the signal and has a synaptic cleft.
• The Synaptic cleft a narrow space between cells.
• The Postsynaptic cell receives the signal.

Crossing the Synapse

• An action potential triggers the opening of Ca2+ channels.
• Ca2+ rushes in and induces exocytosis of synaptic vesicles containing neurotransmitters.
• Neurotransmitters bind to receptors on the postsynaptic cell membrane, opening ligand-gated ion channels.
• This induces a synaptic potential (electrical signal) in the postsynaptic cell.

Chemical Synapse Outcomes

• Two possible types of synaptic potential: excitatory postsynaptic potential (EPSP) and inhibitory postsynaptic potential (IPSP).
• EPSPs depolarize the postsynaptic cell membrane.
• IPSPs hyperpolarize the postsynaptic cell membrane.
• If an EPSP is a strong enough depolarization to reach the threshold, an action potential forms in the postsynaptic cell.

Neurotransmitters

• Inotropic receptors are neurotransmitters that act through ion channels

G-Protein Coupled Channels

• The neurotransmitter receptor is separate from the protein that serves as the ion channel.

Acetylcholinesterase (AChE)

• Found in the synaptic cleft.
• breaks down acetylcholine, making cell less excitable

Synaptic Integration: Summation

• Spatial summation is summation over space across both space & time
• Temporal summation is summation over time