Parts of the Nervous system

• Central nervous system (CNS).

  • Brain & Spinal cord.

• Peripheral nervous system.

  • Somatic: Skeletal & Sensory.

  • Autonomic: Sympathetic & Parasympathetic

The Autonomic Nervous System

• Parts of the nervous system include:

  • Part A | Central nervous system (CNS):

    • Brain & Spinal cord.

  • Part B | Peripheral nervous system:

    • Somatic: Skeletal & Sensory.

    • Autonomic: Sympathetic & Parasympathetic.

• The sequence for activation of the fight-flight response

  • Sympathetic portion:

    • Gets ready for an emergency.

    • Becomes ready to be attacked.

    • Triggers multiple physiological responses.

  • Parasympathetic portion:

    • Returns the body to a relaxed state.

The Structure and Function of Neurons

• The function of the neuron:

  • Neurons receive and transmit signals/information

• The Structure of the Neuron:

  • The neuron is composed of two branches that extend from the call body; those branches include the following.

    • Axon

    • Dendrites

Axons

• Axons are part of the neuron that carries information away from the cell body and toward other cells.

• There is only one axon per cell, and they are typically long, but it is not unusual for them to be extremely long.

• There isn’t as much branching compared to dendrites.

• Axons are also myelinated.

Dendrites

• Dendrites are part of the neuron that receives information and brings the received data to the cell body.

• Dendrites are typically short.

• Typically have numerous branches as well as dendrite spines which allow for more surface area to receive data from other neurons.

• Has no myelination.

Factors That Affect the Speed of Action Potential

• The diameter of an axon:

  • Neurons that are larger in diameter fire faster.

• Myelin sheath/myelination:

  • The Myelin sheath increases firing.

• Myelin sheath:

  • A layer of fat cells that encases and insulates many axons.

  • Speeds up nerve impulses.

• The interaction between the axon and the myelination/myelin sheath:

  • Nodes of Ranvier

    • Gaps of space between the myelin (bare portions.)

  • Saltatory Conduction

    • The action potential sequence jumps from node to node and speeds up AP.

• Due to the myelination of neurons, action potentials may only occur at the nodes of Ranvier.

• Depolarization cannot occur in myelinated regions.

  • Depolarization can only occur at the nodes of Ranvier.

• Saltatory conduction

  • Is when action potential jumps from node to node traveling down an axon.

  • It serves as a means of increasing the rate of propagation of an action potential.

  • It increases the speed of impulse transmission.

  • Conserves energy for the axon as depolarization only occurs at the node.

    • This causes 100x less movement of ions and allows the re-establishment of Na+ and K+ concentration across the membranes.

Multiple Sclerosis: Myelin Sheath Degenerates

• The degeneration of the myelin sheath.

• Causes weakness of limbs and difficulty walking. Standing, fatigue as well as tingling pain.

• Classified as an autoimmune disease.

• It is more common to occur in women than in men.

Reflex Arc Neurons

• Sensory/Afferent:

  • Bring sensory information into the spinal cord.

• Motor/Efferent:

  • Carry motor commands away from the spinal cord.

• Interneuron/Association:

  • Neurons between the sensory and motor neurons.

• Example of the reflex arc in action:

  • Sensory information is carried into the spinal cord via sensory neurons.

  • The sensory information then travels across the interneurons to the motor neurons.

  • The motor neurons carry the sensory information out to pull a hand away.

• Reflex Arc:

  • An involuntary response that occurs at the level of the spinal cord

    • A reflex is pulling a hand away without the need of your brain, like when touching something scolding hot.

• Electrochemical activity terms:

  • Graded potential (Dendrites)

    • Graded:

      • The more intense the stimulation, the more electrochemical activity (more Na+) moves in.

    • Decremental:

      • As the electrochemical activity moves down the dendrite, less Na+ moves in (decreases).

Action Potential & Activity in the Axon

• Action Potential (Axon) Nerve Impulse

  • All-or-None Effect:

    • Once the electrochemical activity reaches the threshold at the axon hillock, the axon fires with the same intensity.

  • Non-decremental:

    • The amount of electrochemical activity (Na+ moving in) does not decrease as it moves down the axon.

The Synapse

• The Synapse

  • A gap/space between the neurons.

  • A functional contact between neurons.

    • Most neurons do not touch each other.

  • Neurotransmitters carry a message across a gap.

Parts of the Synapse

• Pre-synaptic neuron

  • A neuron, before the synapse sends a message(s).

• Post-synaptic neuron

  • Neurons after the synapse receive a message(s).

• Telodendria/axonal branching/terminal branches

  • The branching at the end of the axon.

• Synaptic knob/ Synaptic button/ End bulb/ Terminal bouton

  • The enlarged area at the end of the branching.

• Pre-synaptic membrane

  • Membrane before the synapse.

• Post-synaptic membrane

  • The membrane after the synapse (Has receptor sites.)

• Synaptic vesicles

  • Fluid-filled structures (Hold and release transmitters .)

Steps at the Synapse

• Action potential propagates down the axon (domino effect.)

  • Vesicles merge with the presynaptic membrane.

    • Released neurotransmitters into the cleft (vesicles don’t move across.

      • Transmitters travel; across the cleft & plug into the receptor sites.

        • Excite or inhibit the next neuron.

Inactivation & Deactivation of transmitters

• There are two ways to deactivate a transmitter after it has plugged into receptor sites on the postsynaptic membrane and excited or inhibited the next neuron, which is:

  • Reuptake into the presynaptic knob.

  • Enzymes.

Antidepressants & Reuptake

• Depression can occur for a multitude of reasons; one reason is too little serotonin; treatment for this is SSTIs or antidepressants.

  • Antidepressants selectively block the reuptake of serotonin, forcing it to stay out in the cleft longer.

Neurotransmitters

• A chemical substance transmits information across the synaptic gap to the next neuron.

• Stored in the synaptic vesicles.

• Neurotransmitters

  • Dopamine

    • Pleasure transmitter: all drugs that are pleasurable increase DA in addition to other transmitters

    • Cocaine & Amphetamines: directly increase DA.

    • Schizophrenia: too much DA.

    • Antipsychotic drugs: block DA.

    • Parkinson’s: too little DA, Treatment: L-Dopa.

    • L-Dopa:precursor to DA. Gets converted to DA.

    • Tourette’s: imbalance in DA.

  • Serotonin

    • Involved in many disorders, including depression.

    • SSRIs: Increase SE.

    • Sleep.

    • Tryptophan is a precursor to SE: Efficacy of
      tryptophan as sleep aid gets overstated.

  • Endorphins

    • Endogenous opiates/opioids: similar to heroin,
      morphine, opium, etc.

    • Pleasure transmitter: Such as a runner’s high.

    • Analgesic: pain relief.

Acetylcholine (ACh)

• Memory: Alzheimer’s Acetylcholine pathways
  are degenerate.

• The transmitter between the motor neurons & muscle.

• Drugs that block or interfere with ACh can cause paralysis and even death.

  • Examples: Curare, Botulism

    • Botox is botulism: medical & cosmetic uses.

Norepinephrine

• Similar to adrenaline.

• Sympathetic nervous system.

• Connected with mood disorders/depression.

• Treatment: Antidepressants; some increase NE.

Gaba

• Main inhibitory transmitter.

• Depressant drugs such as tranquilizers,
  barbiturates, and alcohol increase GABA.

  • Withdrawal from these drugs can cause seizures.