The Nervous System Chapters 39 & 40

Overview Three overlapping functions: sensory input, integration, and motor output Sensory input – the conduction of signals from sensory receptors to integration centers of the nervous system Integration – information from sensory receptors is interpreted and associated with appropriate responses from the body Motor output – the conduction of signals from the processing center to effector cells (muscle cells or gland cells) that carry out the response

Parts Central nervous system (CNS) Brain and spinal cord Responsible for integration Peripheral nervous system (PNS) Nerves which carry sensory input to the CNS and motor output away from the CNS

Neurons Specialized for transmitting chemical and electrical signals from one location to another Neurons are made up of: Cell body: contains cytoplasm, nucleus, organelles Dendrites: carry information to the cell body; short, numerous and very branched Axons: conduct impulses away from the cell body; long and singular

Types of Neurons Sensory Convey information about external and internal environments from sensory receptors to the CNS Interneurons Integrate sensory input and motor output; Located within the CNS Motor Convey impulses from the CNS to effector cells

Supporting Cells Glial cells Structurally reinforce, protect, insulate, and assist neurons Do not conduct impulses Outnumber neurons to 1

The Nature of Neural Signals Signal transmission along the length of a neuron depends of voltages created by ionic changes across the plasma membranes There is a difference in ion concentrations between the cell’s contents and the extracellular fluid – this is the membrane potential All cells have this membrane potential Only neurons and muscle cells can change their membrane potentials in response to stimuli

Action Potential An all-or-none change in the membrane potential Cells which can do this (nerve and muscle) are called excitable cells The membrane potential of an excitable cell at rest (unexcited state) is called a resting potential The presence of gated ion channels in these cells permits them to change the plasma membrane’s permeability and alter the membrane potential in response to stimuli An action potential is the rapid change in membrane potential of an excitable cell, caused by stimulus-triggered selective opening and closing of voltage-gated ion channels.

Refractory period A refractory period occurs after the changes in membrane potential during which time the neuron is insensitive to stimuli. The action potential is all-or-none: the neuron either fires or doesn’t fire The nervous system distinguishes between strong and weak stimuli based on the frequency of action potentials generated

Action potentials travel along an axon A neuron is stimulated at its dendrites or cell body, and the action potential travels along the axon to the other end of the neuron Chemical or electrical communication between cells occurs at synapses

Synapses Gaps between neurons Electrical synapses: Allows impulses to travel quickly and without loss of signal strength Not very common Chemical synapses: A neurotransmitter is released from the axon of one neuron into the gap This in turn stimulates the dendrites of the next neuron The neurotransmitter chemical is quickly degraded by enzymes and the components recycled Some neurotransmitters:  Acetylcholine, epinephrine, dopamine

Organization of the CNS Provides the basis for complex behavior in vertebrates Spinal cord Carries information to and from the brain Can integrate simple responses to some stimuli (reflexes)

CNS… The Brain: Carries out complex integration for homeostasis, perception, movement, intellect, and emotions White matter – the inner region Gray matter – the outer region (opposite orientation in the spinal cord)

The Brain Brain stem Conducts data and control automatic activities essential for life Cerebellum Controls movement and balance Thalamus and hypothalmus Regulates homeostasis Cerebrum The ‘thinking’ part of the brain

CNS… In both: Meninges – protective layers of connective tissue Cerebrospinal fluid – fills the ventricles of the brain and the central canal of the spinal cord; acts as a shock absorber and circulates hormones, nutrients, and white blood cells

The PNS Sensory division Brings information from the sensory receptors to the CNS Motor division Carries signals from the CNS to effector cells Two separate divisions: Somatic nervous system – carry signals to skeletal muscles; voluntary Autonomic nervous system – controls smooth and cardiac muscles; involuntary