Topic 6.5 - Nerves

Overview of the Nervous System Three major functions: Sensory input – sensory receptors receive signal – peripheral nervous system (nerves, eyes, ears, etc.) Integration – signal is interpreted and response started – central nervous system (brain and spinal cord) Motor output – response to stimulus – peripheral nervous system (nerves, muscle or gland cells) 6.5.1

Overview of the Nervous System

Overview of the Nervous System

Neurons Function - conduct messages to help communication between parts of nervous system. Neurons are helped by numerous supporting cells, which provide structural support, protection, and insulation of neurons. 6.5.2

Neuron Structure Cell body – large central part of neuron Contains nucleus and other organelles – Dendrites – receive and move signal from tips to cell body (into neuron) Axons – carry signals away from cell body to tips (out of neuron) 6.5.2

Neuron Structure Schwann cells – supporting cells that form insulating myelin sheath layer. Increases speed of signal Nodes of Ranvier – spaces in between the Schwann cells Synaptic terminal – end of axon where neurotransmitters are released into synapse 6.5.2

Neuron Structure 6.5.2

Types of Neurons Sensory neurons – communicate information about external and internal environments to central nervous system (input) Interneurons – link sensory response to motor output. Motor neurons – communicate response from central nervous system to effector cells (motor output) All combined, these neurons create a reflex arc, which integrates a stimulus and response. 6.5.3

A Reflex Arc

Membrane Potential Membrane potential – the difference in electrical charge across the plasma membrane. The inside of the cell is negative with respect to the outside. Neurons have a resting membrane potential of -70mV

Membrane Potential Inside the cell: Outside the cell: Cations: potassium (K+) and few sodium (Na+) Anions: proteins, sulfate, phosphate (collectively A-) and few chloride (Cl-) Outside the cell: Cations: Sodium (Na+) and few potassium (K+) Anions: chloride (Cl-)

Membrane Potential

Membrane Potential – How it’s Created The plasma membrane is more permeable (more membrane channels) to K+ than to Na+. Therefore, large amounts of K+ are transferred out of the cell (down the concentration gradient) Small amounts of Na+ are transferred into the cell (down the concentration gradient) The movement of K+ and Na+ across the membrane generate a net negative membrane potential (-70mV) A sodium-potassium pump is used to move K+ back into the cell and Na+ back out of the cell to maintain the constant concentration gradients.

Membrane Potential

Changes in Membrane Potential Neurons are excitable cells – a stimulus can change the neuron’s membrane potential Resting potential – membrane potential of unexcited neuron (-70mV) Neurons become “excited,” when a stimulus opens a gated ion channel and increases the movement of K+ or Na+ across the plasma membrane 6.5.4

Changes in Membrane Potential Hyperpolarization: A stimulus opens a K+ ion channel and efflux of K+ out of the cell increases Membrane potential becomes more negative

Hyperpolarization

Changes in Membrane Potential Depolarization: A stimulus opens a Na+ ion channel and influx of Na+ into the cell increases Membrane potential becomes more positive 6.5.4

Depolarization 6.5.4

Action Potential When depolarization reaches a certain point, the threshold potential is achieved. When threshold potential is reached, an action potential is triggered. Action potential is a nerve impulse. Action potentials consist of a rapid depolarization, a rapid repolarization, and undershoot (hyperpolarization) 6.5.4

Action Potential 6.5.2

Action Potential Caused by voltage-gated channels Open and close in response to changes in membrane potential K+ channels – one gate; closed at resting potential; opens slowly during depolarization Na+ channels – two gates: Activation gate – closed at resting potential; opens rapidly during depolarization Inactivation gate – open at resting potential; closes slowly during depolarization 6.5.5

Steps in Action Potential Depolarization: Na+ activation gates open and Na+ enters cell. Repolarization: Na+ inactivation gate closes (prevents Na+ influx) and K+ gate opens and K+ exits cell. Undershoot: K+ gates remain open and K+ continues to leave cell Resting state: All gates closed, Na+/K+ pump (active transport) moves Na+ out and K+ in to restore resting potential. 6.5.5

Steps in the Action Potential

Steps in Action Potential

Propagation of the Action Potential Action potentials are all-or-none events There is no BIG action potential or small action potential The nervous system determines the strength of a stimulus by the frequency of action potentials Action potentials do not travel along the axons of neurons, but are continually regenerated.

Synapses Synapse – junction between two neurons Transmitting cell – presynaptic cell Receiving cell – postsynaptic cell Neurons are separated by a gap called the synaptic cleft. Messages are transmitted across the synaptic cleft by chemical neurotransmitters. 6.5.6

Steps in Synaptic Transmission A nerve impulse reaches end of presynaptic neuron. Presynaptic membrane depolarizes, opening voltage-gated Ca2+ channels. Ca2+ ions diffuse into presynaptic neuron Influx of Ca2+ causes neurotransmitter vesicles to fuse to presynaptic membrane and release neurotransmitters into the synaptic cleft (exocytosis) 6.5.6

Steps in Synaptic Transmission Neurotransmitter diffuses across synaptic cleft and bind to receptors on postsynaptic membrane. Receptors open gated ion channels in postsynaptic membrane. Specific receptors open specific ion channels May open Na+, K+, or Cl- channels Different ions have different responses (excitatory or inhibitory) 6.5.6

Steps in Synaptic Transmission Enzymes quickly degrade neurotransmitter, ending its activity. E.g. acetylcholine is degraded by cholinesterase. Ca2+ is pumped out of presynaptic cell back into synaptic membrane. 6.5.6

Chemical Synapse