In-Depth Notes on the Nervous System and Nervous Tissue

Overview of the Nervous System

  • Nervous System: The primary controlling and communication system of the body.
  • Functions: Maintains homeostasis through rapid responses to internal and external changes.
  • Nerve impulses (action potentials) travel to their targets in milliseconds.

Organization of the Nervous System

  • Central Nervous System (CNS)

    • Comprises the Brain and Spinal Cord.
    • Functions as an integrative and control center.

  • Peripheral Nervous System (PNS)

    • Composed of cranial and spinal nerves.
    • Acts as communication lines connecting the CNS to the rest of the body.
Functional Divisions of the PNS

  • Sensory (Afferent) Division

    • Consists of somatic and visceral sensory nerve fibers.
    • Conducts impulses from receptors to the CNS.

  • Motor (Efferent) Division

    • Consists of motor nerve fibers.
    • Conducts impulses from the CNS to effectors (muscles and glands).
    • Further divided into:
    1. Somatic Nervous System (voluntary)
    2. Autonomic Nervous System (ANS) (involuntary)
      • Sympathetic Division: Mobilizes body systems during activity.
      • Parasympathetic Division: Conserves energy and promotes housekeeping functions during rest.

Structure of Neurons

  • Neurons: Individual nerve cells responsible for transmitting impulses.
    • Fun Fact: Over 100 billion neurons in the brain.
Components of Neurons
  • Dendrites: Receptive regions of the neuron.
  • Cell Body (Soma): Contains the nucleus and is the biosynthetic center.
  • Axon: Impulse generating and conducting region; contains:
    • Axon Hillock: Where the axon begins.
    • Myelin Sheath: Insulates the axon, speeding up impulse transmission.
    • Axon Terminals: Secretes neurotransmitters.

Synapses

  • Synapse: A junction between neurons.
    • Presynaptic Neuron: Neuron sending the signal.
    • Postsynaptic Neuron: Neuron receiving the signal.
    • The synaptic cleft separates these two.
Types of Synapses
  • Axo-somatic, Axo-dendritic, and Axo-axonal Synapses.
    • Axo-dendritic: Most common type, between axon and dendrite.

Neurotransmitters (NTs)

  • Neurotransmitter: Chemical molecules that transmit signals across synapses.
    • Can excite (depolarize) or inhibit (hyperpolarize) the postsynaptic neuron.
    • Acetylcholine (ACh): Only NT for somatic motor neurons, causes muscle contraction.

Membrane Potential and Action Potentials (AP)

  • Resting Membrane Potential (Vm): Typically around -70mV.
  • Action Potential: Rapid change in Vm from -70mV to +30mV and back.
    • Phases:
    1. Resting State: All channels are closed.
    2. Depolarization: Na+ channels open, Na+ enters neuron.
    3. Repolarization: K+ channels open, K+ exits neuron.
    4. Hyperpolarization: Briefly dips below resting potential.
  • All-or-None Principle: AP occurs with a sufficient stimulus.

Types of Neurons

  • Sensory Neurons: Afferent; primarily unipolar or bipolar.
  • Motor Neurons: Efferent; primarily multipolar.
  • Interneurons: Integrate signals within the CNS; usually multipolar.

Summation and Integration of Signals

  • Temporal Summation: Rapidfire signals from one neuron add together.
  • Spatial Summation: Simultaneous signals from multiple neurons.
  • EPSP (Excitatory PostSynaptic Potential): Depolarizing potential that brings the neuron closer to firing.
  • IPSP (Inhibitory PostSynaptic Potential): Hyperpolarizing potential that moves the neuron away from firing.

Clinical Implications

  • Multiple Sclerosis (MS): Affects myelin in the CNS; slows down or stops AP.
  • Tetrodotoxin: A potent neurotoxin that blocks opening of Na+ channels; can lead to paralysis.

Key Points to Remember

  • Neurons do not touch; they communicate via the synapse.
  • Ion channels are pivotal in generating and propagating APs.
  • Myelination impacts the speed of impulse transmission (fast in myelinated vs slow in unmyelinated).