SM 1.501 The Cell: Brain and Nervous System

Introduction to the Nervous System

• This series of lectures will explore the brain and nervous system, focusing on the human nervous system but with comparisons to other organisms.
• The aim is to understand how different nervous systems equip organisms with specific behaviors.
• While much is known, there's still a significant amount that remains unknown about the brain and nervous system's capabilities.

Contact Information

• Email address provided for questions and clarifications.

Why a Nervous System is Needed

• Multicellular organisms require communication between different areas for controlling functions like movement and digestion.
• Nervous systems regulate responses to environmental conditions and sensory stimuli.
• Homeostasis: Higher organisms regulate internal physiology to match external conditions.
• The nervous system facilitates communication and harmony between different parts of an organism to maintain optimal performance for survival and thriving.

Basic Unit: The Neuron

• Neurons are nerve cells; the human brain contains approximately 100 billion neurons.
• Neurons are excitable cells that produce and transmit electrical impulses.
• These impulses, called action potentials, are transient changes in voltage across the neuron's membrane.
• Action potentials sweep down the neuron membrane, enabling communication across the nervous system.
• Analogy: Action potentials are like a Mexican wave moving along neurons.

Neuron Anatomy

• Soma (Cell Body): Contains the nucleus and is the site of protein synthesis.
  • Integrates incoming signals.
• Dendrites: Numerous branches extending from the soma.
  • Receive incoming inputs/impulses from other neurons.
• Axon: Single projection that carries the impulse away from the soma to other neurons.
  • Branches at the bottom into terminals with terminal boutons.
• Terminal Boutons: Form connections with dendrites of adjacent neurons.
  • These connections, or synapses, allow for the conduction of the impulse from one nerve to another.
• Myelin Sheath: Insulating fatty coating that makes electrical impulse conduction more efficient.

Synapses and Signal Transmission

• Synapses are connections between neurons where axons connect to dendrites.
• Impulse transmission involves a brief change from an electrical impulse to a chemical signal, then back to an electrical impulse.
• Transmission is unidirectional (nerve one to nerve two only).
• Nerve one (presynaptic cell) occurs before the synapse, while nerve two (postsynaptic cell) occurs after the synapse.
• Single axons make many connections through branches and terminal boutons, while dendrites receive inputs from multiple axons.
• The soma integrates incoming impulses before sending a signal down the axon.

Number of Connections

• The brain's 100 billion neurons form approximately 100,000 synapses each.
• The soma plays a crucial role in integrating these numerous incoming inputs prior to firing an impulse down the axon.

Synaptic Communication Process

• The presynaptic cell contains synaptic vesicles filled with neurotransmitters.
• The postsynaptic cell membrane has receptors that bind to neurotransmitters, causing changes in the postsynaptic cell.
• Action potential moves down the axon to the presynaptic terminal.
• This stimulates synaptic vesicles to fuse with the membrane, releasing neurotransmitters into the synaptic cleft.
• Neurotransmitters bind to receptors on the postsynaptic cell, re-establishing the impulse.
• This triggers the action potential in the postsynaptic cell to continue communication onward.
• Electrical impulse triggers a chemical signal, which is then converted back to an electrical impulse.

Resetting the Synapse

• Synaptic transmission needs to be rapid and quickly reset for the next signal.
• Three mechanisms reset the synapse:
  • Transport Pumps: Blue pumps in glial and neuronal cells pump neurotransmitters out of the synapse into these cells.
    • Drugs like sertraline/Prozac inhibit these pumps, prolonging serotonin presence in the synapse to alleviate depression.
  • Enzymes: Membrane-bound enzymes break down neurotransmitters, biochemically inactivating them.
  • Bloodstream: Neurotransmitters wash out into the bloodstream due to the highly vascular nature of the brain.
• These mechanisms ensure rapid on/off signaling, critical for the dynamic nature of the nervous system.

Divergent and Convergent Pathways

• Synapse arrangement influences function.
• Divergent Pathway: Synapses expand signals throughout the nervous system.
  • One impulse expands to recruit extra parts of the nervous system
  • Example: Smelling a familiar perfume/aftershave triggers memories, autonomic responses, and emotional responses. A single input leading to a complex experience.
• Convergent Pathway: Signals channel down into a small number of nerve cells.
  • Multiple inputs channel down into a single input.
  • Example: Playing the piano involves visual, auditory, and tactile inputs refined into specific motor responses.
  • More presynaptic neurons than postsynaptic neurons.
  • In divergent pathways, there are more postsynaptic neurons than presynaptic neurons.

Types of Neurons

• Neurons have different jobs and can be categorized based on their function.
• Afferent Neurons: Incoming neurons that carry sensory information from the periphery to the central nervous system.
  • Convert sensory information into action potentials.
  • Specific neurons convert visual, auditory, and other sensory information.
  • Cell bodies are typically kept outside the central nervous system, in collections called ganglia (dorsal root ganglia).
• Efferent Neurons: Outgoing neurons that carry commands from the nervous system to effector organs (e.g., skeletal muscle, glands).
• Interneurons: Relay neurons or circuit neurons that form connections between afferent and efferent neurons.
  • Amplify complexity by creating circuits and networks, storing information.
  • Possess highly branched axons for communication with many other neurons.

Specializations of Neurons

• Neurons have unique features regardless of overall category.
• Retinal Neuron: Low dendrites and axonal branching, specialized for vision and communication within dedicated visual centers.
• Cerebellar Neuron: Many dendrites, direct single output, associated with intricate movement patterns. Receives extensive input and produces a strong, direct signal.
• Cerebral Cortex Neuron: Highly branched dendrites at different levels, very long axon. Communicates with different levels within the cerebral cortex and distant tissues/organs.

Simple Reflexes

• Reflexes protect and rapidly remove from danger.
• Withdrawal Reflex: Involves three neurons: afferent, interneuron, and efferent.
  • Operates at the level of the spinal cord (spinal reflex).
  • Example: Touching an open flame.
    • Afferent neuron detects heat and converts it into an action potential.
    • Signal travels to the spinal cord where it synapses with an interneuron.
    • Interneuron synapses with an efferent neuron.
    • Efferent neuron signals muscle contraction, removing the finger from the flame.

Neuronal Networks

• Building on simple reflexes creates complex neuronal networks with varied cell numbers and interconnectivity.
• Nerve Net: A loose connection of neurons.
• The basic nerve nets are built up into more interconnected systems with integration, more cell numbers, subdivisions, and components that drive specific functions.
• Neurons are grouped together in ganglia (singular: ganglion), the brain is an example of two large ganglia.
• Brain (headquarters) and spinal cord (thickened cluster of nerve fibres branching throughout organism).

Examples in Different Organisms

• Sea Anemone: Nerve nets drive contraction and relaxation.
• Earthworm: Segmented nervous system with ganglia in each segment and an anterior ganglion for executive function which allows the worm to move in a coordinated and segmented way.
  • Displays complex behaviors like surfacing in response to rainwater tapping. Rain water is detected as a sensory input followed by a motor response.
• Squid: Advanced nervous system used in neuroscience research (squid giant axon).
  • Visual ganglia, specialized ganglia for complex motor functions.
  • Integration between sensory and motor responses.
  • Complex behaviors, sensory perception, intricate motor coordination.

Human Nervous System

• Most complex: Central nervous system (brain and spinal cord encased in bone) and peripheral nervous system (communication between periphery and center).
• Massive amounts of nerve cells, processing centers, subdivisions. The structure will be unpacked in future lectures.