Structures & Processes of the Nervous System Miss Richardson SBI4U

The Human Nervous System Components: Central nervous system (CNS) Brain Spinal cord Peripheral nervous system (PNS) Somatic nervous system (conscious control) Autonomic nervous system (no conscious control) Messages are communicated through three types of nerve cells (neurons) Sensory (afferent)neurons Interneurons Motor (efferent) neurons

The Human Nervous System

The Reflex Arc A simple reflex is an immediate response to a stimulus that doesn’t require the brain – only the spinal cord Sensory receptors pick up the stimulus – the sensation of the tap on the knee. This information travels along the afferent  (sensory) neuron to the spinal cord. The information is transmitted from the afferent neuron in the spine to an interneuron in the spine The interneuron relays the information to an efferent (motor) neuron in the spine. The efferent neuron sends the information to the muscles (the effector) in the thigh. These muscles contract and the knee jerks.

Neurons Neurons are responsible for the reflex response All neurons contain: Cell body Dendrites Axon

Neurons

Neurons The dendrites receive information and send it out to the cell body The cell body is where the nucleus is found Determines if information will be passed on Axons project out from the body Carry the nerve impulse from the cell body to the axon terminal and synapse When nerve impulse reaches the axon terminal, the messages are relayed to the effector using chemicals called neurotransmitters

Resting Membrane Potential If a neuron is not sending a signal or impulse it is said to be at rest While at rest, potassium ions (K+) are found mainly inside the membrane, and sodium ions (Na+) are found mainly outside the membrane Resting potential - difference in electric charge across the membrane of a neuron, measured in millivolts (-70mV)

Membrane permeability While at rest the neuronal membrane is more permeable to K+ than to Na+ Therefore K+ diffuses out of the nerve cell (down its concentration gradient) faster than Na+ diffuses in. Na+ K+

The Nerve Impulse Neurons communicate with each other by sending nerve impulses down the length of the axon and across a gap (called a synapse) to another neuron. As a nerve cell is stimulated there is a rapid influx of Na+ A nerve impulse occurs as there is a rapid reversal of the electrical potential across the neurons membrane due to this influx  ACTION POTENTIAL

Terminology Depolarization: Diffusion of Na+ ions into the nerve cell resulting in an ACTION POTENTIAL Repolarization: Na+/K+ pump restores the original resting potential of the neuron

The Action Potential At rest, the outside of the neuron is more positive than the inside Nerve stimulation causes membrane to become more permeable to Na+ Depolarization: Na+ rushes in making the inside more permeable and resulting in an ACTION POTENTIAL Repolarization: K+ flows out of the membrane restoring the resting potential (outside more +ve) Na+/K+ pump uses ATP to pump Na+ out and K+ into neuron restoring ion concentrations

Action Potential

The Action Potential Action potentials are often represented in a graph

“All or Nothing” Neurons either fire maximally or not at all, this is referred to as the “all or none” response Increasing neuronal stimulation beyond a critical level will not result in an increased response Neurons response to increased stimulation by increasing the frequency of firing, not the intensity at which they fire. The Threshold level is the minimum strength of stimulation required to produce an action potential

Communication When an AP reaches the end of the axon the impulse must be transmitted to another adjacent neuron. This communication occurs between neurons across spaces called synapses. The message is not transmitted electrically as it was down the axon, but rather by the release of chemical messengers.

Synapse