Animal Nervous Systems

Animal Nervous Systems

• Sponges are the only multicellular animals that do not possess a nervous system.

Nerve Cells and a Nervous System

• Neurons/nerve cells are the basic functional unit of the nervous system.
• Neurons connect to form nerve nets, nerves, ganglia, and brains.
• Arrangement depends on the organism.

Types of Neurons

• All neurons share 3 main components:
  • Dendrites
  • Cell body
  • Axon
• Sensory neurons send and receive information about an animal’s environment / internal physiological state.
  • Respond to physical things like temperature, light, and touch.
  • Respond to chemical signals such as odor and taste.
• Interneurons process information received by sensory neurons and transmit it to different body regions.
• Motor neurons produce a motor response based on information received from sensory and interneurons.

Neuron Structure

• Stimuli are received by the dendrites and cell body.
• Synaptic stimuli is summed at the axon hillock, where an action potential is triggered if the sum of the arriving signals is high enough.
• Action potentials are conducted to the axon terminal, where they cause the release of neurotransmitters stored in vesicles.
• Neurotransmitters bind to receptors on the postsynaptic cell membrane, creating a new signal in the postsynaptic neuron.
• In all neurons: Dendrites, cell body, and axon are present.

Electrical Signals

• Action potentials are a brief electrical signal transmitted from the cell body along one or more axon branches.
• Neurotransmitters are chemical signals and action potentials are electrical signals.
• Neurons have electrically excitable plasma membranes that can transmit information.
• When a cell does not receive a signal, there are more negative ions on the inside of the cell; there is an overall negative charge relative to the outside.
  • A charge difference is known as the membrane potential.
• The negative voltage across the membrane at rest is referred to as the cell’s resting membrane potential.
  • Created by Na+/K+ pump AND the leak of K+ out of the cell at rest via sodium and potassium movement.
• When a nerve cell is excited, its membrane becomes less negative/less polarized - called depolarization.
• After depolarization:
  • If the change reaches a threshold potential, an action potential is produced.

Information Coding

• Neurons code information by changing the rate and timing of action potentials.
• "Strength" is not a factor, as action potentials are "all-or-none"
• A higher firing frequency codes for a more intense stimulus.
  • E.g. brighter light, louder noise, stronger signal transmitted by the nerve cell to other cells it contacts.

Glial Cells and Propagation

• Glial cells do not transmit electrical signals - they provide support, insulation and nutrition to the neurons.
• Action potentials propagate from cell body toward the axon terminals in one direction.
  • They cannot propagate backward.
• Saltatory propagation increases the speed of conduction.

Synapses: Communication Between Neurons

• Synaptic transmission starts with action potential conduction to the axon terminal.
• Depolarization of the axon terminal opens voltage gated Ca2+ channels
• Vesicles respond by fusing with the presynaptic membrane, releasing neurotransmitters into the synaptic cleft.
• Neurotransmitters bind with receptors on the postsynaptic cell (ligand gated ion channels), opening the channels to allow in ions and causing a change in membrane potential.
• Neurotransmitters are actively re-absorbed into the presynaptic terminal and stored in vesicles until the next action potential arrives.
• This is where the electrical signal becomes a chemical signal.
• Excitatory synapses transmit relevant information between nerve cells
• Excitatory Post-Synaptic Potential = EPSP
• Inhibitory synapses filter out unimportant information
• Inhibitory Post-Synaptic Potential = IPSP
• EPSPs and IPSPs can be summed over time (temporal summation) and space (spatial summation).
• Summation of EPSPs and IPSPs are the fundamental forms of information processing by the nervous system
  • They provide mechanisms for determining whether a particular sensory stimulus is responded to or ignored.

Nervous System Organization

• The nervous system is separated into the central nervous system, and the peripheral nervous system.
• Central Nervous System
  • Brain
  • Spinal Cord
  • Organizes/coordinates complex responses to sensory information.
• Brain:
  • The cerebral cortex governs most advanced cognitive functions, and has mutliple lobes with different functions:
    • Frontal: decision-making and planning
    • Parietal: body awareness
    • Temporal: processing sound
    • Occipital: processing vision
• Gray matter in the cerebral cortex
  • Appears gray because of the densely packed cell bodies of neurons.
  • Found in the superficial regions of the cortex.
  • Highly folded to increase surface area which allows for an increased number of neurons.
• White matter in the cerebral cortex
  • Appears white due to the myelin produced by glial cells.
  • Contains axons (covered in myelin).
  • Allows for communications between the grey matter and other regions of the brain.
  • The thalamus relays sensory information from below to the cortex.
  • The hypothalamus interacts with the autonomic portion of the PNS and the endocrine system.
  • The cerebellum coordinates actions for complex motor tasks (catching a ball, writing), integrating sensory and motor information.
• Peripheral Nervous System:
  • Includes cranial nerves and spinal nerves
  • Contains an autonomic and somatic division
  • The somatic nervous system includes sensory neurons that respond to external stimuli and motor neurons that synapse with voluntary muscles.
  • The autonomic nervous system includes both sensory and motor nerves, which act without our conscious awareness (involuntary).
    • The autonomic nervous system controls internal functions of the body like heart rate, blood flow, digestion, excretion, and temperature.
    • Contains sympathetic (fight-or-flight) and parasympathetic (rest and digest) divisions

Sensory Receptors and Transduction

• Animals sense their environment through physical properties and sensory receptor cells that detect features in the environment
  • Housed in sensory organs: eyes, ears, nose, mouth.
• Physical stimuli converted to an action potential = Sensory Transduction.
  • Chemoreceptors
  • Mechanoreceptors
  • Photoreceptors
• Detect chemical stimuli in the environment

Senses

• Activation of a receptor leads to signaling which opens an ion channel and causes depolarization of the cell.
  • Important for taste and smell.
• Mechanoreceptors
  • Detect physical forces
  • Mechanical forces directly open ion channels and cause depolarization of cell
  • Important for hearing and balance.
• Photoreceptors
  • Detect light
  • Respond to light energy by closing Na+ channels, causing cell membrane to become hyperpolarized rather than depolarized
  • Important for vision.
• Olfactory receptors respond to chemical odors (chemoreceptors) in the nasal passages.
• Taste cells are chemosensory receptors located in taste buds, and they send information to the brain through sensory neurons.
• The perceived flavor of food depends on both the sense of taste and the sense of smell.
  • Sensory receptors of olfaction / taste buds = chemoreceptors.
• The light-sensitive protein Opsin is what turns light into electrical signals.
• Rods detect light and change it into different shades of white, grey, and black; they are very numerous/sensitive & good in low light
• Cones detect blue, green, or red light; mixing the colors enables a full range of vision.
• Single-lens eyes are for vertebrates
  • Give advantages as they can focus light rays on a particular region of photoreceptors, and improve image quality and sensitivity.
• Flatworms have eyecups to detect direction and intensity of light used to determine the direction of movement
  • Move directly from light to avoid predators.
• Compound eyes:
  • Consist of a large number of individual light-focusing lenses.
  • The more lenses, the higher the resolution.