The Nervous SystemChapter 48-49

What you need to know!• The anatomy of a neuron.• The mechanisms of impulse transmission in a neuron.• The process that leads to release of neurotransmitter,

and what happens at the synapse.• The components of a reflex arc and how they work.• The organization and function of the major parts of

the nervous system.• One function for each major brain region.• The location and function of several types of sensory

receptors.• How skeletal muscle contracts.• Cellular events that lead to muscle contratction

The Neuron• Functional unit of the nervous system• Dendrites: cell extensions that receive

incoming messages from other cells

• Axon: transmit messages to other cells• Covered in myelin sheath (Schwann cells or oligodendrocytes)

to speed up transmission• Synapse: is the junction between axons and dendrites• Neurotransmitters (NT): chemical messengers (drugs)

released from the synaptic terminals of the axon which bind to receptors of the receiver (another neuron, muscle cell, or gland)• Acetylcholine, dopamine, serotonin, GABA, etc.

Nervous System• Central nervous system (CNS) = brain & spinal

cord• Peripheral nervous system (PNS) = everything

else including motor and sensory neurons• Nerves are clusters of neuronsTypes of neurons:• Sensory receptors: receive information from the

environment• Sensory neurons: send signals from receptors to

CNS• Interneurons: brain and parts of the spinal cord• Motor neurons: transmit signals to effectors

(muscles, glands)

Action Potential (nerve impulse)• Membrane

potential: difference in electrical charge across the cell membrane

• Resting potential: neuron at rest (-70mV)

Action Potential (AP)• All or none

response to stimulus

• If membrane potential reaches threshold (-55mV) AP will fire

1. Resting state2. Depolarization3. Depolarization

>= threshold AP

4. Repolarization5. Undershoot

Axon• APs propagate down the axon• It jumps from one node of Ranvier to the next

(saltatory conduction)• The signal is revitalized at every node• At the synapse the signal continues via electrical

or chemical (NT) stimulus• Stimulus can be inhibitory or excitatory

Reflex Arc• Simplest response

to stimulus• Receptor sensory

neuron interneuron (spinal cord) motor neuron effector (reaction)

• The brain finds out later

Central Nervous System• Brain and spinal cord• Cells bathed in cerebrospinal fluid (CSF) for

nutrients and cleansing• Grey matter = neuron cell bodies and

unmyelinated axons• White matter = myelinated axons• Neural glial cells (glia) support neurons:• Astrocytes provide nutrients for neurons• Oligodendrocytes make myelin sheath in the CNS

(Schwann cells in the PNS)

Peripheral Nervous System• Divided into two subdivisions:1. Somatic nervous system: muscles you control2. Autonomic nervous system: automatic muscle

and organ control subdivided into two more categories

1. Sympathetic division: fight or flight mechanism2. Parasympathetic division: rest and digest

Brain• Compartmentalized:• Brainstem = medulla oblongata, pons,

and midbrain• Primitive brain = primitive functions like

homeostasis and breathing• Cerebellum: coordinated motor movement• Thalamus: main center for all sensory and

motor information• Hypothalamus: regulates homeostasis

and basic survival behaviors• Cerebrum: two hemispheres with grey

over white matter• Cerebral cortex: Higher order thinking occurs

here (largest in mammals) and voluntary movement

• Corpus callosum: white matter enabling communications of the two hemispheres

Sensory ReceptorsPerception: brain recognition of stimulusReception: receptor detects a stimulus• Mechanoreceptors: stimulated by physical stimuli• Thermoreceptors: stimulated by temperature• Chemoreceptors: chemical stimulation (taste and

smell)• Electromagnetic receptors (photoreceptors):

detect various forms of energy (light, electricity, or magnetism)

• Pain receptors: detect too much heat or chemicals released from dying cells

Movement