Nervous System Function and Evolution

Nervous System Function and Evolution

• Animal nervous systems are designed for organisms to perceive and react to their surroundings
• They facilitate movement coordination and control the body's internal operations
• A nervous system comprises interconnected nerve cells or neurons

Types of Nerve Cells

• Sensory neurons receive and relay data concerning the animal's environment or its internal state
• Interneurons handle incoming information and send it to various body parts
• They communicate with motor neurons to produce an appropriate response, for example, a ganglion
• Motor neurons induce movement and regulate internal physiology, such as constricting blood vessels or digestive contractions
• The primary purpose of the nervous system is to maintain a stable internal environment, known as homeostasis

Nervous System Complexity

• Nervous systems vary from simple to complex, aligning with the complexity of the organism
• A brain is a defining trait in vertebrate nervous systems

Neuron Organization

• Neurons consist of dendrites that gather signals, and axons that transmit signals
• Signals can be electrical

Glial Cells

• Glial cells serve as a key support system for neurons, outnumbering the neurons in the human brain
• Glial cells provide nutrition and physical support to neurons
• Astrocytes, a type of glial cell, support blood vessels in the brain and maintain the blood-brain barrier against pathogens and toxins

Electrically Excitable Cells

• Neurons possess electrically excitable membranes that facilitate information transfer
• The number of charged ions differs inside and outside the cell
• A resting neuron has more negative ions inside, resulting in a negative charge relative to the outside
• The membrane potential refers to this charge difference

Neuron Resting Membrane Potential

• Resting potential is negative, determined by potassium ions moving out of the cell
• The resting potential of a neuron is negative and depends on movement of K+ ions out of the cell
• The Na+-K+ pump moves Na+ ions out and K+ ions into the cell using ATP

Action Potential

• Neurons communicate via action potentials
• Voltage-gated Na+ channels open and Na+ ions enter the cell, causing a positive spike in the membrane potential
• K+ channels open more slowly, allowing fewer K+ ions to leave the cell
• As the voltage rises to +40mV, Na+ channels close and voltage-gated K+ channels remain open
• This allows K+ ions to leave the cell, causing the membrane potential to become more negative
• The membrane returns to resting state, as excess K+ ions are returned to the cell, assisted by Na+-K+ pumps
• An overshoot of K+ ions leaving the cell causes hyperpolarization, causing a refractory period where the nerve cannot fire another action potential

Information Coding

• Neurons code information by altering the rate and timing of action potentials
• Higher firing frequency indicates a stronger stimulus

Propagation of Action Potentials

• Neurons transmit action potentials by opening and closing adjacent Na+ and K+ ion channels

Saltatory Conduction

• Myelination insulates axons for faster action potential conduction

Synapses

• Neurons communicate with other neurons or muscle cells by releasing neurotransmitters at a synapse
• Synaptic transmission starts with action potential conduction to the axon terminal
• Axon terminal depolarization opens voltage-gated Ca2+ channels
• Vesicles merge with the presynaptic membrane, releasing neurotransmitters into the synaptic cleft
• Neurotransmitters bind receptors on the postsynaptic cell, causing a change in membrane potential

Neuronal Signals

• Neurotransmitter binding opens ligand-gated Na+ channels, with more Na+ ions entering the cell
• This makes the membrane potential more positive, resulting in an excitatory postsynaptic potential (EPSP)
• Neurotransmitter binding opens ligand-gated Na+ or Cl- channels, with more Na+ ions existing or more Cl- ions entering the cell
• This makes the membrane potential hyperpolarized, resulting in an inhibitory postsynaptic potential (IPSP)
• Neurons integrate information from multiple communicating neurons

Summation of Postsynaptic Potentials

• EPSPs widely spaced in time do not trigger an action potential.
• Multiple EPSPs arriving rapidly at a single synapse can initiate an action potential
• Single EPSPs reaching multiple synapses can initiate an action potential
• An EPSP and an IPSP can cancel each other out, preventing an action potential

Nervous System Organization

• Nervous systems in animals are organized into central and peripheral components
• Nerves serve as communication lines between these components

Peripheral Nervous System (PNS)

• Vertebrate peripheral nervous system is divided into somatic and autonomic components
• The somatic system has sensory neurons responding to external stimuli and motor neurons synapsing with voluntary muscles
• The autonomic system features sensory and motor nerves, generally operating without conscious awareness

Autonomic Nervous System

• Involuntary or autonomic systems control internal body functions
• Parasympathetic division facilitates rest and digest activities, like constricting pupils and slowing heart rate
• Sympathetic division manages fight or flight responses, such as dilating pupils and accelerating heart rate

Reflex Circuits

• Simple reflex circuits allow quick responses to stimuli
• The knee-extension reflex employs a sensory neuron, single synapse, and motor neuron
• Reflex circuits bypass the brain

Sensory Receptor Cells

• Sensory receptor cells detect several stimuli
• Sensory transduction turns a stimulus into an electrical impulse

Sensory Receptors

• Chemoreceptors respond to chemical stimuli, underlying smell(olfaction) and taste(gustation)
• Mechanoreceptors detect physical forces for motion and gravity
• Invertebrates use statocyst organs and vertebrates use vestibular systems of semicircular canals and otolith organs
• Electromagnetic receptors, like opsin proteins, sense light, using light-sensitive pigments
• Invertebrates use eye cups, insects(housefly) use compound eyes, and humans use single-lens eyes

Color Vision

• Color vision uses photoreceptor cells with visual pigments sensitive to different light wavelengths
• Cone cells have color-sensitive pigments versus rod cells being sensitive to light in general

Three Major Brain Regions

• The brain develops in three regions: hindbrain, midbrain, and forebrain
• The forebrain develops into the cerebral cortex in humans
• Midbrain forms part of the brainstem
• Hindbrain includes the pons and medulla, as well as the cerebellum

Limbic System

• Limbic system governs physiological drives, instincts, emotions, and motivation with interactions in the midbrain
• It assists in long term memory with the hippocampus

Cerebral Cortex

• The cerebral cortex is organized with grey and white matter
• The surface of the brain is highly folded, increasing the surface area of the cortex, where neuron cell bodies are located
• Organized into frontal, parietal, occipital, and temporal lobes, as well as the cerebelum, and brainstem

Somatosensory Cortices

• Located behind the frontal lobes, the somatic cortex handles touch and spatial awareness
• Located behind the central sulcus in the frontal lobes, the motor cortex controls skeletal muscle movements

Topographic Maps

• The brain contains topographic maps of what part of the body is controlled by which parts of the brain

Memory

• The hippocampus converts reinforced short-term memories to long-term memories
• Stimulation of synapses in a circuit helps convert it to a long-term memory
• Human brains can process complex information, solve problems, and store memories