Synapses

• Eventually, as an action potential passes down the axon, it reaches the end of the axon and all of its branches

• These branches may form junctions with the dendrites of other neurons, muscle cells, or gland cells; such intercellular junctions are called synapses

• The neuron whose axon transmits the action potential to the synapse is called the presynaptic cell, while the receiving cell is called the postsynaptic cell

Synapses

• The synapse is the gap between the synaptic terminals of an axon and its target cell

• There are two basic types of synapses:– Electrical synapses – involve direct cytoplasmic

connections between the two cells formed by gap junctions; the gap junctions allow ion currents to continue; relatively rare in vertebrates

– Chemical synapses – electrical impulses must be converted to a chemical signal that crosses the synapse; more common in vertebrates

Synapses

• Chemical synapses have a synaptic cleft – a narrow space that separates the two cells

• The end of the presynaptic cell contains synaptic vesicles which are packed with neurotransmitters

Chemical synapses

• When an action potential arrives at the end of an axon, it stimulates the opening of voltage-gated calcium (Ca+2) channels; causes a rapid influx of Ca+ into the cell

• Rapid influx of Ca+2 causes the synaptic vesicles to fuse with the plasma membrane, and release neurotransmitters by exocytosis

• Release neurotransmitters diffuse to other side of cleft and binds to receptor proteins in the membrane of the postsynaptic cell (produces graded potentials in postsynaptic membrane)

Chemical synapses/Release of Neurotransmitters

Neurotransmitters

• Neurotransmitters are chemical signals in an otherwise electrical system

• Must be rapidly removed from the synaptic cleft to allow new signals to be transmitted; accomplished by:– Enzymatic digestion– Re-uptake of neurotransmitter molecules by the

neuron– Uptake by glial cells

Neurotransmitters

• No one chemical property defines a neurotransmitter

• Acetylcholine (Ach) is the neurotransmitter that crosses the synapse between a motor neuron and a muscle fiber– This type of synapse is known as a neuromuscular

junction– Ach binds to its receptor proteins in the

postsynaptic membrane, causing ligand-gated ion channels to open

Neuromuscular junction

Release of neurotransmitter

Neurotransmitters• Binding of Ach to receptor produces a

depolarization called an excitatory postsynaptic potential (EPSP) which can open the voltage-gated ion channels for Na+ and K+ that are responsible for action potentials

• Produces a muscle contraction (the postsynaptic cell is a skeletal muscle fiber)

• For the muscle to relax, Ach must be eliminated; an enzyme cleaves Ach into inactive fragments, causes muscle relaxion

Neurotransmitters

• Glycine and GABA are inhibitory neurotransmitters

• These neurotransmitters cause the opening of ligand-gated channels for Cl-, diffuses into neuron, makes the inside of the membrane more negative than it is at rest– Hyperpolarization; called an inhibitory

postsynaptic potential (IPSP)

Excitatory (a) & Inhibatory (b) Neurotransmitters

More Neurotransmitters…• Epinepherine (aka adrenaline) – responsible for

“fight or flight” response– Faster and stronger heartbeat; diversion of blood to

muscles and heart; increased blood glucose• Dopamine – used in some areas of the brain that

control body movement– Important roles in behavior, motor activity,

motivation and reward, sleep, mood, attention and learning

• Seratonin – involved in the regulation of sleep and emotional state– Insufficient activity of neurons that release seratonin

may be one of the causes of clinical depression

More Neurotransmitters…

• Endorphins – block the perception of pain; produced in the brain stem– Opium, Morphine, and Heroin share chemical

similarities with endorphins, such that they bind to receptors normally used by endorphins

Neurotransmitters play a role in drug addiction

• Prolonged exposure to a stimulus that produces a chemically-mediated signal may cause cells to lose the ability to respond to it; habituation

• When receptor proteins are exposed to high levels of neurotransmitter molecules for prolonged periods, the postsynaptic cell often responds by decreasing the number of receptor proteins in its membrane; makes the cell more efficient ( a natural function of the cell)

Neurotransmitters play a role in drug addiction

• Drugs produce artificial neurotransmitter effects, such that long-term use requires more of the drug in order to obtain the same effect

• Cocaine affects neurons in the brain’s “pleasure pathway” (limbic system)– Binds dopamine transporters on presynaptic

membranes that normally remove dopamine from the synaptic cleft, preventing the reuptake of dopamine; dopamine ‘survives’ longer in the synapse and fires pleasure pathways repeatedly

Cocaine addiction

• Prolonged exposure to cocaine reduces the number of dopamine receptors; the cocaine user now needs the drug to maintain normal levels of limbic activity

• The cocaine user is now addicted• When a cocaine addict stops using cocaine, a

‘crushing’ depressive state ensues; feelings of pleasure are essentially impossible; user wants more drug to end depression

• Drugs are bad, mmmKay?

Effects of Cocaine

Receptor proteinDrug

moleculeSynapse

1. Reuptake of neuro- transmitter by transporter at a normal synapse.

2. Drug molecules block transporter and cause overstimulation of the postsynaptic membrane.

3. Neuron adjusts to overstimulation by decreasing the number of receptors.

4. Decreased number of receptors make the synapse less sensitive when the drug is removed.

Neurotransmitter

Transporter protein

Transporterprotein

Dopamine

CocaineReceptorprotein

The Central Nervous System

• All vertebrate brains have 3 basic divisions:• Hindbrain (rhombencephalon)• Midbrain (mesencephalon)• Forebrain (prosencephalon)• Through evolution, the relative sizes of

different brain regions have changed– Forebrain became dominant feature

The Central Nervous System

Primitive fish brain

The Central Nervous System

• The forebrain is composed of two elements:– Thalamus (integration and relay center) and

hypothalamus (basic drives and emotions; controls pituitary gland)

– Cerebrum (aka “end brain”) – devoted to associative activity

• The increase in brain size in mammals reflects the great enlargement of the cerebrum

• The cerebral cortex is the outer layer of the cerebellum; contains ~10% of all brain neurons

The Cerebral Cortex

• Highly convoluted surface (increases surface area of the brain)

• Divided into three regions, each with a specific function– Motor– Sensory– Associative

The Spinal Cord

• The spinal cord is a cable of neurons extending from the brain down through the backbone

• The spinal cord is enclosed and protected by the vertebral column and the meninges; layers of membrane

• Serves as the body’s “information highway”; relays messages between the body and brain

• Also functions in reflexes, the sudden, involuntary movement of muscles

The Knee-Jerk Reflex

The Peripheral Nervous System

• The peripheral nervous system (PNS) collects information and carries out responses

• The PNS consists of nerves and ganglia• The PNS has somatic and autonomic systems– Somatic associated with voluntary control of body

movements through the action of skeletal muscles, and with reception of external stimuli

– Autonomic consists of motor neurons that control smooth muscle, cardiac muscles and glands; monitors visceral organs and blood vessels

The Autonomic System

• The Autonomic Nervous System (ANS) is further divided into the sympathetic and parasympathetic nervous systems

• Sympathetic nervous system prepares the body for situations requiring alertness or strength, or situations that arouse anger, fear, excitement of embarassment– Stimulates cardiac muscles to increase the heart

rate, causes dilation of the bronchioles of the lungs , and cause dilation of blood vessels

The Autonomic System

• The parasympathetic system is active during periods of digestion and rest; stimulates the production of digestive enzymes and the processes of digestion, urination, and defecation; reduces blood pressure and heart and respiratory rates, and conserves energy through relaxation and rest

• The SNS and PNS act in opposition; one system stimulates an organ, while the other inhibits

Constrict

Secrete saliva

Dilate

Stop secretion

Dilate bronchioles

Speed up heartbeat

Increase secretion

Empty colon

Increase motility

Empty bladder

Slow down heartbeat

Constrict bronchioles

Sympatheticganglionchain

StomachSecrete adrenaline

Decrease secretion

Decrease motility

Retain colon contentsDelay emptying

Adrenal gland

Bladder

Small intestine

Large intestine

Spinal cord

ParasympatheticSympathetic