3.E.2 Nervous System Animals have nervous systems that detect external and internal signals, transmit and integrate information, and produce responses.

The Nervous System can be divided into the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). The CNS includes the brain and spinal cord. The PNS consists of all nerves outside of the brain and spinal cord.

The neuron is the functional unit of the nervous system.

A typical neuron has a cell body, axon, dendrites and an axon terminal.

The direction of the neural impulse is from the dendrites (input) to the axon terminal (output).

Dendrites receive and integrate signals from other neurons and conduct the signal towards the cell body.

Axons conduct signals away from the cell body Axons conduct signals away from the cell body. Axons can be as long as many meters in large vertebrates. A “nerve” is a bundle of axons.

Many axons have a myelin sheath that acts as an electrical insulator Many axons have a myelin sheath that acts as an electrical insulator. Myelinated axons appear white: the “white matter” of the brain. Unmyelinated axons appear gray: the “gray matter” of the brain.

Glial cells called Schwann cells form the myelin sheath in the PNS Glial cells called Schwann cells form the myelin sheath in the PNS. Swann cells are separated by gaps of unsheathed axon called Nodes of Ranvier.

Two types of glial cells, oligodendrocytes and astrocytes, form the myelin sheath in the CNS. Astrocytes form the "blood-brain barrier”, separates brain cells from the blood and protects the brain from many common bacterial infections. 

The structure of the neuron allows for the detection, generation, transmission and integration of signal information.

Neurons are excitable; their membranes can depolarize and produce an action potential, or electrical impulse.

Action potentials propagate impulses along neurons.

Membranes of neurons are polarized by the establishment of electrical potentials across the membranes.

At rest, the membrane potential (Vm) of a neural membrane is about -70 mV. This is due to an unbalanced electric charge distribution.

Sodium is maintained outside the cell, potassium inside the cell Sodium is maintained outside the cell, potassium inside the cell. The cell membrane is impermeable to anions.

This voltage difference across the membrane produces a kind of electromotive force.

The cell membrane has protein channels, called leak channels that allow Na+ or K+ to diffuse down their concentration gradients. 

The sodium-potassium pump, powered by ATP, counters the effect of the leak channels, working to maintain the resting membrane potential at -70 mV.

An action potential is a rapid change in membrane potential in response to a stimulus.

An action potential:

A stimulus from a sensory cell or another neuron causes the target cell to depolarize toward the threshold potential. 

2. If the stimulus exceeds threshold (-55 mV), voltage-gated sodium channels open.

3. When the voltage-gated sodium channels open, sodium floods into the cell. The membrane depolarizes (the inside of the cell becomes more positive).

4. At the peak action potential, the voltage-gated sodium channels close and potassium channels open, allowing K+ to diffuse out of the cell down its electrochemical gradient. The membrane repolarizes (the inside of the cell becomes more negative.)

4. The membrane hyperpolarizes (becomes more negative than resting potential). During this period of recovery, called the refractory period, the nerve cell cannot be stimulated again. The sodium-potassium pump returns the membrane potential to -70 mV.

Diagram showing the opening and closing of voltage-gated ion channels:

The action potential propagates as a wave down the axon The action potential propagates as a wave down the axon. An action potential at one place on the axon depolarizes the adjacent sections of its membrane. If sufficiently strong, this depolarization provokes a similar action potential at the neighboring membrane patches. 

Transmission of information between neurons occurs across synapses.

Chemical messengers called neurotransmitters transmit the neural message across synapses. Function Acetylcholine Neuromuscular junctions Epinephrine (adrenalin) Stimulates sympathetic nervous system Norepinephrine Dopamine CNS neurotransmitter involved with reward –motivated behavior Glutamate The most common neurotransmitter in the brain Serotonin CNS neurotransmitter involved with mood, memory and learning GABA Inhibitory neurotransmitter of the CNS

The response produced by transmission of a signal can be stimulatory or inhibitory.

Different regions of the vertebrate brain have different functions.

The human brain is divided into left and right cerebral hemispheres.

The forebrain consists of the cerebrum, thalamus, and hypothalamus The forebrain consists of the cerebrum, thalamus, and hypothalamus. The brainstem is composed of both the midbrain and the hindbrain.

The cerebellum plays an important role in motor control.

The cerebrum is a major part of the brain, controlling emotions, hearing, vision, personality and much more. It controls all voluntary actions. 

Example Neurohormones The hypothalamus produces neurohormones, which are hormones produced and released by neuroendocrine cells into the blood. Example Neurohormones Oxytocin Dopamine Epinephrine Norepinephrine Vasopressin

Learning Objectives: LO 3.43 The student is able to construct an explanation, based on scientific theories and models, about how nervous systems detect external and internal signals, transmit and integrate information, and produce responses. [See SP 6.2, 7.1] LO 3.44 The student is able to describe how nervous systems detect external and internal signals. [See SP 1.2] LO 3.45 The student is able to describe how nervous systems transmit information. [See SP 1.2] LO 3.46 The student is able to describe how the vertebrate brain integrates information to produce a response. [See SP 1.2]

LO 3.47 The student is able to create a visual representation of complex nervous systems to describe/explain how these systems detect external and internal signals, transmit and integrate information, and produce responses. [See SP 1.1]   LO 3.48 The student is able to create a visual representation to describe how nervous systems detect external and internal signals. [See SP 1.1] LO 3.49 The student is able to create a visual representation to describe how nervous systems transmit information. [See SP 1.1] LO 3.50 The student is able to create a visual representation to describe how the vertebrate brain integrates information to produce a response. [See SP 1.1]