Session 3:Nervous System Notes

Divisions of the Nervous System

• The nervous system (NS) is divided into two principal parts:

  • Central nervous system (CNS)

    • Brain and spinal cord of the dorsal body cavity

    • Integration and control center that interprets sensory input and dictates motor output

  • Peripheral nervous system (PNS)

    • The portion of the nervous system outside the CNS

    • Consists mainly of nerves that extend from the brain and spinal cord

      • Spinal nerves to and from the spinal cord

      • Cranial nerves to and from the brain

Organization of the Nervous System

• Central Nervous System (CNS)

  • Brain

  • Spinal cord

• Peripheral Nervous System (PNS)

  • Cranial nerves

  • Spinal nerves

  • Ganglia

• Input and Output

  • Input to CNS from periphery (Afferent division)

  • Output from CNS to periphery (Efferent division)

• Afferent Division

  • Sensory stimuli

  • Visceral stimuli (stimuli in the digestive tract)

• Efferent Division

  • Somatic nervous system (motor neurons to skeletal muscles)

  • Autonomic nervous system (to digestive organs, smooth muscle, cardiac muscle, exocrine and some endocrine glands)

    • Sympathetic nervous system

    • Parasympathetic nervous system

    • Enteric nervous system

Functions of the Nervous System

• The nervous system is the master controlling and communicating system of the body.

• Cells communicate via electrical and chemical signals, leading to rapid and specific responses.

• The function of the nervous system is to integrate sensory input and initiate appropriate motor output (action).

Functional Classification of Neurons

• Sensory (afferent) neurons

  • detect changes in the environment called stimuli

  • transmit information to the brain or spinal cord

• Interneurons (association neurons)

  • lie between sensory & motor pathways in the CNS

  • 90% of our neurons are interneurons

  • Responsible for processing and interpretation of sensory input

• Motor (efferent) neurons

  • send signals to muscle & gland cells (effectors) to carry out a response

Divisions of the PNS

• Peripheral nervous system (PNS) has two functional divisions:

  • Sensory (afferent) division

    • Somatic sensory fibers: convey impulses from skin, skeletal muscles, and joints to the CNS

    • Visceral sensory fibers: convey impulses from visceral organs to the CNS

  • Motor (efferent) division

    • Transmits impulses from CNS to effector organs – muscles and glands

    • Two divisions:

      • Somatic nervous system

      • Autonomic nervous system

Nervous Tissue

• Two basic cell types:

  • neurons

  • glial cells

• Neurons (nerve cells)

  • functional unit (over 100 billion)

  • transmit & process information

• Glial cells (neuroglia)

  • supporting cells

Neuroglial Cells

• CNS Neuroglial cells (Supporting cells)

  • Astrocytes: Hold neurons together; Support, supply nutrients, guide new neurons to destination, help establish the blood-brain barrier; control the chemical environment, can degrade neurotransmitters; communication, modify synaptic transmission

  • Microglia: Phagocytize bacteria or neuronal debris

  • Ependymal cells: Line cavities of the brain and spinal cord, provide a permeable barrier between cerebro-spinal fluid and nervous tissue; neural stem cells

  • Oligodendrocytes: Provide myelin sheaths (insulating covering)

• PNS Neuroglial cells (Supporting cells)

  • Satellite cells: Functions similar to astrocytes

  • Schwann cells: Provide myelin sheaths (insulating covering)

Neurons

• Neurons = nerve cells

• Cells specialized to transmit messages

• Important properties:

  • Excitability – highly responsive to stimuli; generate electrical impulses (action potential)

  • Conductivity – ability to transmit an impulse

• Major regions of neurons:

  • Cell body – nucleus and metabolic center of the cell

  • Processes – fibers that extend from the cell body

  • Dendrites – vast numbers; receive signals from receptors or other neurons

  • Axon – single (nerve fibre); generate and conduct electrical signal

Neuron Structure and Function

• Input zone: Dendrites receive incoming neurotransmitter that influences other cells.

• Cell body: Contains the nucleus.

• Trigger zone: Axon hillock initiates action potentials.

• Conducting zone: Axon conducts action potentials in undiminishing fashion, often over long distances (may be from 1mm to more than 1 m long).

• Output zone: Axon terminals release signals from other neurons.

Key Concepts

• Nerve impulse: Communication between neurons

• Resting membrane potential (RMP): Is the voltage (charge) difference across the cell membrane when the cell is at rest. RMP is a product of the distribution of charged particles (ions).

• Action potential: Change in RMP that results in action of the cell

Resting Membrane Potential (RMP)

• = the potential (charge) difference across the plasma membrane of a resting neuron

• Resting membrane potential of a resting neuron is approximately -70 mV

• The actual voltage difference varies from -40 mV to -90 mV

• Potential generated by:

  • Differences in ionic composition of ICF and ECF:

    • There is a higher concentration of potassium (K^+) and anionic proteins (A^-) inside the neuron

    • There is a higher concentration of sodium (Na^+) and chloride (Cl^-) outside the neuron.

  • Differences in plasma membrane permeability

    • Inside is negatively charged

    • Outside is positively charged

Membrane Potential Results From

1. The differences in the concentrations of ions inside and outside of the cell

2. The permeability of the plasma membrane to these ions

Ion Concentrations and the Na+/K+ Pumps

• The differences in the concentrations of ions inside and outside of the cell are established by the Na^+/K^+ Pumps

• The concentrations of Na^+ and K^+ on each side of the membrane are different.

  • Outside cell:

    • K^+ (5 mM)

    • Na^+ (140 mM)

  • Inside cell:

    • K^+ (140 mM)

    • Na^+ (15 mM)

• The Na^+-K^+ pumps maintain the concentration gradients of Na^+ and K^+ across the membrane.

Action Potentials (APs) – Neural Communication

• Principal way neurons send signals

• Means of long-distance neural communication

• Occur only in muscle cells and axons of neurons

• Brief reversal of membrane potential with a change in voltage of ~$100 mV$

• Action potentials (APs) do not decay over distance

• In neurons, also referred to as a nerve impulse

• Involves opening of specific voltage-gated channels

Action Potential Graph

• Depolarization is caused by Na^+ flowing into the cell.

• Repolarization is caused by K^+ flowing out of the cell.

• Hyperpolarization is caused by K^+ continuing to leave the cell.

Action Potential Phases and Ion Channel States

• Resting potential

  • Na^+ voltage-gated channel closed (activation gate closed; inactivation gate open)

  • K^+ voltage-gated channel closed (activation gate closed)

• Rising phase

  • Na^+ channel opens and is activated (activation gate opens; inactivation gate already open)

• Peak

  • Na^+ channel closes and is inactivated (activation gate still open; inactivation gate closes)

• Falling phase

  • K^+ channel opens (activation gate opens)

  • Na^+ channel reset to closed but capable of opening (activation gate closes; inactivation gate opens)

• Return to Resting Potential

  • K^+ channel closes (activation gate closes)

Action Potential Key

• Resting membrane potential: Voltage-gated Na^+ channels are in the resting state and voltage-gated K^+ channels are closed

• Stimulus causes depolarization to threshold

• Voltage-gated Na^+ channel activation gates are open

• Voltage-gated K^+ channels are open; Na^+ channels are inactivating

• Voltage-gated K^+ channels are still open; Na^+ channels are in the resting state

Four Main Phases of AP

• Depolarisation: change in a cell's membrane potential, making it less negative (more positive)

  • A threshold stimulus (> -55mV)

  • Change in resting membrane potential

  • Na^+ channels open → Na^+ rush into the cell → reverse the electrical charge

  • Positive charge of ICF = a neuron fires = generation of an AP

• Repolarization: the process by which a cell membrane returns to its resting potential after depolarization

  • opening of K^+ channels → outflow of K^+

  • closing of Na^+ channels → membrane potential returns to resting state

• Hyperpolarisation (undershoot): cell's membrane potential becomes more negative than its resting potential

  • a polarisation even more negative than the resting membrane potential

  • excessive K^+ efflux

• Restoration:

  • The resting membrane potential is restored

Rules of the Action Potential

• caused by voltage-gated channels

• all-or-none response

• only occur if stimulus exceeds threshold

• all action potentials are identical

• stronger stimulus = more action potentials

Action Potential Propagation (the action potential, travels down the axon of a neuron)

• Action potential causes inside of cell to become positive

• Positive charges are attracted to adjacent negative charges

• Adjacent areas depolarise → action potential

• Action potential propagated down axon – like a “Mexican wave”

Conduction Velocity

• APs occur only in axons, not other cell areas

• AP conduction velocities in axons vary widely

• Rate of AP propagation depends on two factors:

  1. Axon diameter

     • Larger-diameter fibers have less resistance to local current flow, so have faster impulse conduction

  2. Degree of myelination

     • Two types of conduction depending on presence or absence of myelin

       • Continuous conduction (continues)

       • Saltatory conduction (at nodes)

     • Myelinated axons conduct impulses faster

Myelination of Axons

• Myelin sheath

  • Composed of myelin, a whitish, protein-lipid substance which covers some axons; formed by glial cells (Schwann cells in the PNS and oligodendrocytes in CNS)

  • Function of myelin:

    • Protect and electrically insulate axon

    • Increase speed of nerve impulse transmission

  • Myelinated fibers: segmented sheath surrounds most long or large-diameter axons

  • Nodes of Ranvier – gap between the myelin sheath

  • Non-myelinated fibers: do not contain myelin sheath and conduct impulses more slowly

Membrane Potential and Ion Channels

• Membrane potential is the difference in electrical charge between the inside and outside of a cell generated by different concentrations of Na^+, K^+, Cl^−, and protein anions (A^−) inside and outside of the cell.

  • Concentrations of K^+ and A^− higher inside the cell and concentrations of Na^+ and Cl^− higher outside the cell.

  • The resting membrane potential of the neurons is between -45 to -90 millivolts (in average about -70 mV).

• Na^+ - K^+ pump maintains the concentration gradient; it constantly expels three Na^+ ions from the cell, and moves two K^+ ions back into the cell.

• Ion channels: The membrane pores made of protein permeable selectively to specific types of ions.

  • Types:

    • I. Passive, or leakage, channels – always open;

    • II. Gated channels:

      • a) Chemically gated channels – open with binding of a specific neurotransmitter;

      • b) Voltage-gated channels – open and close in response to change in membrane potential;

      • c) Mechanically gated channels – open and close in response to physical deformation of receptors.

Phases of the Action Potential

1. Resting state: all Na^+ and K^+ gated channels are closed, membrane potential at resting level.

2. Depolarization: Na^+ channels are open, Na^+ flows into the cell; the inside of the membrane becomes less negative than the restingpotential.

3. Repolarization: Na^+ channels inactivating, K^+ channels are open, K^+ ions move out of the cell; the membrane returns to its resting membranepotential.

4. Hyperpolarization: some K^+ channels remain open, some K^+ ions move out of the cell; the inside of the membrane becomes more negative than the restingpotential.
   Depolarization of membrane by 15 to 20 mV to induce the action potentialfiring.

Rate of Impulse Propagation

• is determined by:

  • 1) Axon diameter – the larger the diameter, the faster the impulse;

  • 2) Presence of a myelin sheath – myelination dramatically increases impulse speed.

• Saltatory conduction occurs in myelinated axons

• Action potentials are triggered only at the nodes of Ranvier (gaps between Schwann cells) and jump between the nodes.

Synapses

• Definition: A junction that mediates information transfer from one neuron to another neuron and to an effector cell

  • Presynaptic neuron – conducts impulses toward the synapse

  • Postsynaptic neuron – transmits impulses away from the synapse

Types of Synapses

• Electrical synapse

• Chemical Synapse

Chemical Synapses - Structure

• Most common type of synapse

• Specialized for release and reception of chemical neurotransmitters

• Typically composed of two parts:

  • Axon terminal of presynaptic neuron: contains synaptic vesicles

    • filled with neurotransmitter

  • Receptor region on postsynaptic neuron’s membrane: receives neurotransmitter; usually on dendrite or cell body

• Two parts separated by fluid-filled synaptic cleft

• Electrical impulse changed to ‘chemical’ across synapse, then back into ‘electrical’

Chemical Synapses - Transmission

1. Action potential arrives at axon terminal.

2. Voltage-gated Ca^{2+} channels open and Ca^{2+} enters the axon terminal.

3. Ca^{2+} entry causes synaptic vesicles to release neurotransmitter by exocytosis

4. Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane.

5. Binding of neurotransmitter opens ion channels, resulting in graded potentials.

6. Neurotransmitter effects are terminated by reuptake through transport proteins, enzymatic degradation, or diffusion away from the synapse.

Synapse - Definitions

Transmission at the Chemical Synapse

1. Nerve impulses reach the axonal terminal of the presynaptic neuron and open Ca^{2+} channels

2. Neurotransmitter is released into the synaptic cleft via exocytosis

3. Neurotransmitter crosses the synaptic cleft, binds to receptors on the postsynaptic neuron

4. Postsynaptic membrane permeability changes, causing an excitatory or inhibitory effect. Transmission is slow.

• Removal of neurotransmitters occurs when they: are degraded by enzymes; are reabsorbed by astrocytes or the presynaptic terminals (reuptake); diffuse away from the synaptic cleft.

Synapse: Neurotransmitters

• Synthesized in neurons, stored in secretory vesicles in pre-synaptic axon terminals

• 50 or more neurotransmitters have been identified

• Most neurons make two or more neurotransmitters

• Excitatory receptors depolarize membranes (e.g., glutamate)

• Inhibitory receptors hyperpolarize membranes (e.g. GABA)

• Some neurotransmitters have both excitatory and inhibitory effects

  • Determined by the receptor type on the postsynaptic neuron

  • eg: acetylcholine which is Excitatory at neuromuscular junctions with skeletal muscle and Inhibitory in cardiac muscle

Neuroglia of the CNS main types

major events that occur in an action potential