Nervous System Notes

The Nervous System

• The body's interface with the external environment.

• Used for:

  • Communication and information intake (speech, hearing, touch, smell).

  • Movement.

  • Thinking.

  • Creating perceptions of the world.

• Operates on a binary system of 0s (no) and 1s (yes).

Vertebrate Nervous System

• Composed of two main divisions:

  • Central Nervous System (CNS).

  • Peripheral Nervous System (PNS).

Central Nervous System (CNS)

• Consists of the brain and spinal cord.

• Functions as the coordinating center for incoming and outgoing information.

Spinal Cord

• Contains grey matter (nerve cell bodies and dendrites).

• Contains white matter (axons).

• Links the brain with the rest of the body.

• Protected by the vertebra.

Brain

• Major center for receiving, processing, storing, and retrieving information.

• Responsible for:

  • Voluntary movements.

  • Consciousness.

  • Behavior.

  • Emotions.

  • Learning.

  • Reasoning.

  • Language.

  • Memory.

• Also responsible for involuntary behaviors like breathing, digestion, heart rate, and blood pressure.

• The right side of the brain controls the left side of the body and vice versa.

Brain Regions and Functions

• Different brain areas have specialized functions:

  • Cerebellum: Motion, balance, learning new things.

  • Medulla (brain stem): Automatic actions (breathing, digestion, etc.).

  • Cerebrum: Sensing, thinking, imagining.

  • Primary motor area: Controls voluntary movements.

  • Primary somatosensory area: Processes sensory information from the body.

  • General sensory and motor association areas.

  • Frontal association area: Planning, personality

  • Parietal lobe

  • Wernicke's area: Understanding language.

  • Frontal lobe.

  • Broca's area: Expressing language.

  • Taste area.

  • Visual association Area.

  • Occipital lobe.

  • Temporal lobe.

  • Smell area.

  • Auditory area: Hearing input.

  • Auditory association area.

  • Facial recognition area.

  • Brain stem.

  • Primary visual cortex: Visual input.

Peripheral Nervous System (PNS)

• Nerves that carry signals between the CNS and the organs.

• Two categories:

  • Somatic Nerves: Voluntary.

  • Autonomic Nerves: Involuntary.

Somatic Nerves

• Voluntary control of body movements.

• Carries information from the CNS to skeletal muscles.

Autonomic Nerves

• Involuntary control.

• Works with the endocrine system to maintain balance (homeostasis).

• Controls digestion, sweating, blood circulation, and breathing.

• Two divisions:

  • Sympathetic Nervous System: Fight or Flight.

    • Used during stress, excitement, or physical activity.

    • Increases heart rate (HR) and blood pressure (BP).

    • Dilates air passages and pupils.

  • Parasympathetic Nervous System: Rest and Digest.

    • Used during low-stress situations.

    • Inhibits the effects of the sympathetic nervous system.

    • Promotes regular body activities (digestion).

Nervous System Divisions Summary

• CNS: Brain and spinal cord

• PNS:

  • Somatic system: Controls skeletal muscles (voluntary).

  • Autonomic system: Controls smooth muscles and glands (involuntary).

    • Sympathetic division: Fight or flight.

    • Parasympathetic division: Rest and digest.

Cells of the Nervous System

• Glial Cells:

  • Non-conducting cells.

  • Provide structural support and metabolism for nerve cells.

• Neurons:

  • Nerve cells that conduct nerve impulses.

Cell Division in the Nervous System

• Glial cells divide throughout life, a source of most brain tumors.

• Neurons divide rapidly early in life, then slow significantly with age.

Types of Neurons

• Afferent Neuron (sensory neuron): Carries impulses from sensory receptors to the CNS.

• Interneuron: Relays impulses between afferent and efferent neurons within the CNS.

• Efferent Neuron (motor neuron): Carries impulses from the CNS to effectors (skeletal muscles or glands).

Neuron Anatomy

• Dendrites: Receive information and carry it to the cell body.

• Cell Body (Soma): Contains the nucleus and other organelles.

• Axon: Carries nerve impulses away from the cell body.

  • May be covered in Myelin Sheath: Fatty protection that insulates the neuron.

• Myelin Sheath: Allows faster impulse transmission.

• Schwann Cells: A type of glial cell that produces the myelin sheath.

• Nodes of Ranvier: Gaps between sections of myelin sheath; impulses jump from node to node.

• Axon Terminals (Synaptic Bulbs): Transmit signals to other neurons or effectors.

• Synapse: The junction between two neurons or a neuron and an effector cell.

Neuron Structure and Function

• Axons are bundled together to form nerve fibers, similar to fiber-optic cables.

Neuron Support System

• Glial cells (Schwann cells) form myelin sheaths around axons.

• Myelin sheaths are electrical insulators.

• Nodes of Ranvier allow electrical signals to jump, increasing speed.

Neural Circuits – Reflex Arc

• A neural circuit through the spinal cord that bypasses brain coordination.

• Example: touching a hot stove.

  1. Pain and thermoreceptors in the finger stimulate an afferent neuron.

  2. Information is sent to the spinal cord (interneurons).

  3. Interneurons also send a signal to the brain.

  4. Signal goes to an efferent neuron (motor neuron) to remove hand.

• The hand is removed before pain is felt because the brain is not directly involved.

Reflex Arc Steps

1. Pain receptor stimulates afferent neuron.

2. Afferent neuron transmits impulses to the spinal cord.

3. Interneurons integrate information.

4. One efferent neuron stimulates flexor muscle contraction.

5. Another efferent neuron sends inhibitory signals to prevent extensor muscle contraction.

6. Biceps muscle contracts, withdrawing hand.

Neural Transmission (Nerve Signaling)

• Nerve impulses are powered by cellular energy.

• The electrical impulse is just as strong at the end as at the beginning.

• Studies on nerve impulses often use squids (giant axons).

• Sensor → (sensory neurons at arrow) CNS → (motor neuron’s at arrow) effector (

Neural Transmission Details

• Occurs when a neuron is activated, sending out an electrical impulse.

• Activation happens via pressure, heat, light, or chemical signals from other cells.

• Neuron firing is "all-or-nothing"; it either fires or it doesn’t.

• All firings produce the same movement of charge, regardless of stimulus strength.

The Signal

• Fast signals are sent via electrical conduction down a neuron, then jump to the next neuron. (via Myellin Sheet)

Nerve Signal Transmission - Action Potential

• Potential Difference: Voltage difference across a nerve cell membrane due to unequal ion distribution.

Stage 1: Resting Potential (Pre-stimulus)

• Inside of the cell is more negatively charged compared to the outside (approximately -70 \, mV).

• Charges cannot freely move across the membrane.

Membrane Channels

• Ions can only move across the membrane through membrane channels.

• Sodium/Potassium pump maintains ion concentrations.

• 3 \, Na^+ pumped out, 2 \, K^+ pumped in.

• Potassium and Sodium ion channels are closed when the membrane is at the resting potential.

• Typical ion concentrations:

  • Inside: [K^+] = 150 \, mM, [Na^+] = 15 \, mM

  • Outside: [K^+] = 5 \, mM, [Na^+] = 150 \, mM

Stage 2: Depolarization

• Signal causes depolarization (abrupt change in charge difference).

• Known as action potential.

• Stimulus causes positive ions to move into the cell, increasing voltage from resting state (-70 mV) to -50 to -55 \, mV.

  • this is due to positive ions moving into the cell, making the inside of the cell less negative and approaching the threshold level needed for an action potential to be generated.

• This is the threshold potential.

• Above this, the cell fully depolarizes as Na^+ channels open and rush in, making the inside positive.

• Occurs in less than 1 ms.

Action Potential

• Potential difference becomes positive as Na^+ ions move in when a nerve is excited, leading to a rapid change in membrane potential and the propagation of the nerve impulse along the axon.

Repolarization (Refractory Period)

• Restores the original polarity of the nerve membrane.

• Na^+ channels close.

• K^+ channels open, sending positive charges out.

• Bringing back to resting stage

• Sodium-Potassium pump pumps Na^+ out and moves K^+ in.

Sodium-Potassium Pump

• Carrier protein in the cell membrane.

• Transports 3 \, Na^+ out and 2 \, K^+ in.

• Requires ATP.

Refractory Period Details

• Recovery period before another action potential.

• Depolarization must be complete; nerve must repolarize.

• Re-establishes the -70 \, mV potential.

• Approximately 1-10 \, ms.

• Prevents signal from going backwards.

Synaptic Transmission

• Synapses: Regions between neurons or between neurons and effectors.

• Neurotransmitters: Chemicals released from vesicles into synapses.

• Presynaptic Neuron: Neuron carrying impulse to the synapse.

• Synaptic Cleft: Space between presynaptic neuron and another neuron/effector.

• Postsynaptic Neuron: Neuron carrying impulse away from the synapse.

Neurotransmitters

• Chemicals which allow — Transmit signals across synapses or to an effector.

• Made in the nerve cell body.

• Acetylcholine: Stimulates muscles, including gastrointestinal muscles. Curare causes paralysis by blocking acetylcholine receptor sites.

Neurotransmitters and Feelings

• The brain changes how we feel by triggering connections, releasing neurotransmitters.

• Certain connections/neurotransmitters make us feel good; blocking them makes us feel bad.

• Pain occurs when nerve cells send messages via neurotransmitters.

• Blocking these messages prevents pain.

Common Neurotransmitters and Their Roles

• Serotonin: Anxiety, obsessions and compulsions.

• Dopamine: Attention, motivation, pleasure, reward.

• Epinephrine: Alertness, energy, mood.

Synaptic Transmission Process

• Action potential reaching the axon terminal opens Ca^{2+} gates, causing influx.

• Neurotransmitters released into synaptic cleft, bind to receptors on postsynaptic cell.

• Binding excites (opens Na^+ channels, causing depolarization) or inhibits the next neuron.

Synaptic Transmission Detailed Steps

1. Action potential reaches axon terminal of presynaptic neuron.

2. Ca^{2+} enters axon terminal.

3. Neurotransmitter is released by exocytosis.

4. Neurotransmitter binds to postsynaptic receptor.

5. Ion channels open in postsynaptic neuron.

Neurotransmitter Lifecycle

1. Synthesis of neurotransmitter and vesicle formation.

2. Transport of neurotransmitter down the axon.

3. Action potential travels down the axon.

4. Ca^{2+} entry triggers neurotransmitter release.

5. Neurotransmitter binds to receptor, exciting or inhibiting the postsynaptic neuron.

6. Separation of neurotransmitter molecules from receptors.

7. Reuptake of neurotransmitter for recycling.

8. Vesicles without neurotransmitter transported back to cell body.

Alcohol and Inhibitory Synapses

• Alcohol enhances the effects of the inhibitory neurotransmitter GABA.

• Inhibiting brain sections involved in alertness/thought causes sluggishness.

• Alcohol weakens the excitatory neurotransmitter glutamine, increasing sluggishness.