Nervous System: Functions and Divisions

• Nervous System (NS) is the body's primary control and communication system.
• The nervous system has three main functions: sensory input, integration, and motor output.

Functions of the Nervous System

• Sensory input involves using sensory receptors to monitor changes inside and outside the body.
• Integration processes and interprets sensory input to decide appropriate actions.
• Motor output involves activating effector muscles to produce a response.

Central Nervous System (CNS)

• The CNS consists of the brain and spinal cord.
• It serves as the integration and command center.
• The CNS interprets sensory input and dictates motor responses based on experience, reflexes, and current conditions.

Peripheral Nervous System (PNS)

• The PNS includes nerves extending from the brain and spinal cord, located outside the CNS.

Sensory (Afferent) Division of PNS

• Sensory receptors send impulses to the CNS.
• Somatic afferents originate from the skin, skeletal muscles, and joints.
• Visceral afferents come from visceral organs like the heart.

Motor (Efferent) Division of PNS

• The motor division sends impulses from the CNS to effector organs like muscles and glands.
• Voluntary NS/Somatic NS impulses travel from the CNS to skeletal muscles.
• Involuntary NS/Autonomic NS impulses travel from the CNS to smooth and cardiac muscles, and glands.
• Within the involuntary system, the sympathetic division stimulates, while the parasympathetic division inhibits.

Neurons

• The cell body contains the nucleus and most organelles.
• Dendrites receive impulses from presynaptic neurons.
• The axon generates impulses at the axon hillock and transmits them to other cells.
• Axons divide into synaptic/axonal terminals at the presynaptic end.

Structural Classification of Neurons

• Neurons can be classified based on the number of processes extending from the cell body.

Functional Classification of Neurons

• Sensory neurons transmit impulses from sensory receptors to the CNS.
• Motor neurons transmit impulses from the CNS to effector organs.
• Interneurons are located between sensory and motor neurons.

Glial Cells

• Glial cells support neurons.

Reflexes

• Reflexes are rapid, automatic responses to stimuli.
• Reflexes occur independently of the brain via reflex arcs with five components.

Components of a Reflex Arc

• The five essential components are the receptor, sensory neuron, integration center, motor neuron, and effector.

Membrane Potential

• Ions are unequally distributed across a neuron.
• The cell interior is negatively charged; surrounding fluid is positively charged.
• Separated electrical charges create potential energy as potential difference/voltage.
• Membrane potential is the attraction of opposite charges across the plasma membrane.
• Resting potential is the potential of a neuron not sending a signal, typically -60 to -80 mV.
• Negative values indicate the cytoplasmic side of the membrane has a negative charge compared to the outside.
• Potassium ions flow down the concentration gradient through K+ channels while sodium is also present.
• The sodium-potassium pump maintains the sodium and potassium gradients.

Ion Export and Voltage in Resting Potential

• Sodium ions are exported and potassium ions imported, resulting in the net export of positive ions.
• The export of sodium ions leads to very few millivolts.
• Few sodium ion channels exist, and the neuron exhibits great potassium permeability.
• The end result of the combination of ion channels is -60 to -80 mV.
• A chemical and electrical balance is reached due to the continuous flow of potassium.

Impulse Generation

• Membrane potential changes in response to signals through gated ion channels.
• Gated ion channels change the membrane's permeability to ions, altering the membrane potential.
• Hyperpolarization increases the membrane potential.
• The inside of the membrane becomes more negative during hyperpolarization.
• Hyperpolarization results from the outflow of positive potassium ions or the inflow of negative ions.
• Depolarization decreases the membrane potential.
• The inside of the membrane becomes less negative, closer to zero, during depolarization.
• Depolarization involves the inflow of positive ions, specifically sodium ions.

Graded Potential

• Graded potential is a shift in membrane potential.
• Magnitude varies with the strength of the stimulus.
• Signals from the graded potential do not flow along the axon.
• It supports the generation of nerve signals.

Action Potential (Nerve Impulse)

• Action potential happens when the threshold of -55 mV is reached.
• Sodium channels open, causing an inflow of sodium ions.
• Increased depolarization results in more channels opening.
• Once initiated, stimulus strength does not impact the magnitude of the response.
• The resting state of voltage-gated channels is closed.
• Sodium has a closed activation and open inactivation gate (must both be open to pass).
• Potassium only has an activation gate, closed.

Stages of Nerve Signalling

• A stimulus causes sodium channels to open, resulting in sodium ions rushing into the cell.
• Reaching a threshold leads to more channels opening in chain reaction due to positive feedback.
• The 1 ms time frame of the rising phase has increased potassium permeability due to open activation gates.
• Decreased sodium permeability occurs due to closed inactivation gates.
• Potassium permeability lasts longer than needed, and channels remain open.
• Sodium channels begin to reset.
• Sodium-potassium pump restores ion concentrations.

Conduction of Action Potentials

• Depolarization occurs with the inflow of sodium ions on the membrane.
• Depolarization then spreads, triggering an action potential at the next site of the membrane.
• The original region experiences repolarization following the outflow of potassium ions.
• The process of depolarization-repolarization repeats, propagating the action potential down the axon to the synaptic terminals.

Factors Affecting Action Potential Conduction Velocity

• Both axon diameter and degree of myelination affect action potential conduction velocity.
• Impulses travel faster in larger axon diameters due to less resistance to the flow of local currents.
• Action potentials generate immediately adjacent to each other in unmyelinated axons.
• Unmyelinated axons lead to slow conduction.
• Action potential propagation is increased by myelinated axons, and acts as insulators.
• Myelinated axons lead to a quicker response called saltatory conduction.
• Current only passes at nodes of Ranvier, where the axon lacks insulation and the nodes have concentrated sodium channels.

Synapses

• Electrical synapses contain gap junctions.
• The gap junctions allow electrical current to flow through, but they are less common than the alternative.
• Chemical synapses involve the release of a chemical neurotransmitter by the presynaptic neuron.

Steps Towards Chemical Synapse Transmission

1. An action potential arrives, which depolarizes the presynaptic membrane. Depolarization leads to voltage-gated channels opening, and then an inflow of calcium ions.
2. Greater calcium concentrations lead to synaptic vesicles merging with the presynaptic membrane, then neurotransmitters are released within the synaptic cleft.
3. The neurotransmitters diffuse across the cleft and bind to ligand-gated ion channels in the postsynaptic membrane.
4. The channel has been opened by binding, so ions may cross to cause a graded potential (permeable to sodium = depolarization.)