Maintaining Dynamic Equilibrium

The Nervous System

Divisions of the Nervous System

• Central Nervous System (CNS)– Brain– Spinal cord

• Peripheral Nervous System (PNS)– Somatic– Autonomic

The Central Nervous System

• The Brain

• Spinal Cord– Neurons and supportive tissue– Spinal column

CNS Protection

• Skull-enclosing the brain • Vertebrae- enclosing the spin • Meninges- 3 protective membranes that

surrounds the brain and spinal cord. • Cerebrospinal fluid- Fluid that fills the

spaces within the meninges to create a cushion.

• Ventricles- cavities in the brain which are filled with cerebrospinal fluid.

                                 

• Grey Matter- nerve tissue consisting of mainly cell bodies within the brain and spinal cord.

• White Matter- nerve tissues of the brain and spinal cord, consisting of mostly myelinated neurons.

Peripheral Nervous System (PNS)

Peripheral Nervous System(PNS)• Input and output from the CNS.• 2 types of nerves (numerous neurons held together)

– Sensory nerves: From sensory receptors (skin, muscle, some internal organs)

– Motor nerves: Carry orders to muscles,

glands & internal organs

Peripheral Nervous System-PNS

• Somatic NS- Relays information to and form skin and skeletal muscles under your conscious control.

• Autonomic NS- relays information to your internal organs not under your conscious control.– Sympathetic NS- controls organs in times of stress

(fight or flight)

– Parasympathetic NS- Controls organs when body is at rest.

Divisions of the PNS Divided into 2 parts:• Somatic nervous system (i.e. skeletal)

Voluntary actions

• Autonomic nervous system Non-voluntary actions

Regulation of Internal organs Constriction of blood vessels Regulates bladder, stomach and heart

Divided into 2 parts– Sympathetic

Fight or flight Blushing, sweaty palms, heart racing.. .. ..

– Parasympathetic Relaxation Slows things down to conserve energy

Neurons and Reflex Responses

• Neurons are the structural and functional unit of the nervous system

• Nerves are numerous neurons held together by connective tissues.

The Brain

The Cerebrum

• Divided into 2 hemispheres

• Corpus callosum- is a bundle of nerve fibers connecting the left and right hemispheres

Cerebral Cortex

• The surface layer of grey tissue of the cerebrum

• Folded to increase the surface area.

• For Higher forms of thinking

• Can be divided into 4 lobes.

The Four Lobes of the CerebralCortex•Occipital lobe

– Vision•Parietal lobe

– Somatosensory (the perception of sensory stimuli from the skin and internal organs)

– Area associated with taste•Temporal lobe

– Auditory cortex/hearing– Wernicke’s area (sensory speech, spoken language is understood)

•Frontal lobe– Strategic and effortful processing– Goal directed behaviours– Voluntary control– Motor cortex– Broca’s area (motor speech)

Cerebellum

• Motor and muscle coordination

• Contains 50% of brains neurons but only takes up 10% of the space

• Midbrain- a short segment of the brainstem– Involved in sight and hearing

• Pons– contains bundles of axons traveling between the

cerebellum and the rest of the CNS.

• Medulla oblongata– Vital involuntary functions: heart rate, adjusting blood

pressure, breathing, vomiting, hiccupping, swallowing.

• Thalamus- Sensory relay station, governs the flow of information from all parts of the nervous system.

• Hypothalamus- controls autonomic NS and endocrine hormone system, and reestablishes homeostasis.

The Neuron Neurons: the building blocks of the nervous system• Sensory neurons

– send signals from the senses, skin, muscles, and internal organs to the CNS

• Motor neurons– transmit commands from the CNS to the muscles,

glands, and organs

• Interneurons– Located between sensory and motor neurons– Most numerous type of neuron– Function: transmit signals from one part of the body to

another

Reflex Responses

You drive a pin into your foot and immediately recoil from the shock and pain. Provide an explanation of what happens from the time the pin enters the skin to when you finally experience the pain. Your explanation should include any key components involved in the events and detail their role.

• Reflect responses are carried out without assistance form the brain.

Neurons

• Cell Body: has a large centrally located nucleolus. The cytoplasm contains mitochondria, lysosomes and Gogli complex.

• Dendrites: are the primary site for receiving signals from other neurons. The number can range from one to thousands depending on the function.

• Axon is a long cylindrical extension of the cell body. It transmits a pulse or wave of depolarization.

• terminal branches (axon terminal) the end of the axon that branches off and comes in close contact with the dendrites of neighboring neurons

• The Human Body: Nervous System • http://www.youtube.com/watch?v=i-

NgGKSNiNw&feature=fvw

• Schwann cells: insulating cells around the axons of some nerve cells in the PNS

• Schwann cells make up a myelin sheath (fatty layer around the axon). Myelin speeds up nervous transmission

• Node of Ranvier: the gap between Schwann cells around the axon of a nerve cell. The membrane of the axon is exposed and nerve impulses jump from one node of Ranvier to the next.

The All or None Response

• if an axon is stimulated sufficiently (above the threshold), the axon will trigger an impulse down the length of the axon. If not, the impulse is not triggered.

• It is analogous to firing a gun

• Neurons

• http://www.youtube.com/watch?v=sQKma9uMCFk

• Spaces between neurons are synapses. Axon terminals contain synaptic vesicles

which hold neurotransmitters• http://www.youtube.com/watch?v=HkQxyHl3nUo&feature=related

Action Potentials in Neurons

• At Rest (Polarization) → outside of neuron is positively charged compared to inside (sodium ions outside, chloride and potassium ions inside). At rest, the cell membrane is not permeable to sodium ions but very permeable to potassium ions so they diffuse out of the cells. However, a mechanism called the sodium/potassium pump (active transporter) pulls 3 sodium ions to the outside of the cell while pulling 2 potassium ions inside the cell

• resting potential: the difference in charge from the inside to the outside of a cell at rest. It is approximately – 70 mV.

• Depolarization→ when a neuron is stimulated enough, gates for potassium K+ ions channels (transporters) close and gates for sodium ion channels open. Na+ ions move in while K+ ions move out. More positive charges move in than out, and this neutralizes the negative charge inside the cell (actually makes the inside slightly positive). The resulting difference in charge during depolarization is called the action potential. Action potentials occur at cell bodies and dendrites as well.

• depolarization at one point of the axon causes the neighboring Na+ channels to open, and the depolarization continues down the length of the axon

• Repolarization→ after the Na+ channels open, K+ channels reopen to cause K+ to move out. At the same time, Na+ channels close.

• the sodium/potassium pump restores the original concentrations of Na+ and K+ by pumping Na+ out and K+ in.

• This entire process of depolarization and repolarization occurs very quickly. An axon can send many impulses along its length every second if it is sufficiently stimulated.

• Refractory period: the brief time between the triggering of an impulse along an axon and the axon readiness for the next impulse. During that brief time, the axon cannot transmit an impulse. For many neurons, the refractory period is about 0.001 s.

• http://highered.mcgraw-hill.com/olc/dl/120068/bio03.swf• http://outreach.mcb.harvard.edu/animations/

actionpotential.swf• http://www.blackwellpublishing.com/matthews/

channel.html• http://www.blackwellpublishing.com/matthews/

actionp.html• http://ca.dummies.com/how-to/content/understanding-the-

transmission-of-nerve-impulses.html

• As mentioned earlier, myelinated neurons transmit impulses much quicker than unmyelinated ones. This is due to the fact that depolarization only occurs at the nodes of Ranvier. In a sense, the impulse jumps from node to node until it reaches the end of the neuron. Speeds of impulses on myelinated neurons can reach 120 m/s.

• Action Potenital

• http://www.youtube.com/watch?v=SCasruJT-DU