Presentation on theme: "CHAPTER 48 NERVOUS SYSTEM 25122411311004070101. A Simple Nerve Circuit – the Reflex Arc - Sensory neuron - bings info from sense organs/receptors to spinal."— Presentation transcript:
A Simple Nerve Circuit – the Reflex Arc - Sensory neuron - bings info from sense organs/receptors to spinal cord -> synapses on motor neurons that take information to the muscles/glands to contract.
If the right side of your cerebral cortex was rolled out flat, it would be the size of an extra large pizza. Same with the left side. That makes two extra large pizzas. Food for thought! Your brain is electric. It generates 10 to 12 watts of electricity - enough to power a flashlight.
Nervous systems perform the three functions - sensory input, integration, and motor output Motor output is the conduction of signals from integration centers to effector cells. Effector cells carry out the body’s response to a stimulus
The central nervous system (CNS) is responsible for integration. Peripheral nervous system (PNS) – 12 cranial nerves + 33 spinal nerves
Interesting Facts: Average number of neurons in the human brain: 100 billion Average number of neurons in an octopus brain: 300 billion Rate of neuron growth during development of a fetus (in the womb): 250,000 neurons/minute Diameter of a neuron: 4 to 100 microns Longest axon of a neuron: around 15 feet (Giraffe primary afferent axon from toe to neck) Velocity of a signal transmitted through a neuron: 1.2 to 250 miles/hour
The neuron is the structural and functional unit of the nervous system. Nerve impulses are conducted along a neuron. Dentrite (processes bring info to cell body) cell body (integrates) axon hillock axon (conducts info) -> synapse (transmits) Some axons are insulated by a myelin sheath. Neuron Structure and Synapses
Axon endings are called synaptic terminals. They contain neurotransmitters which conduct a signal across a synapse. A synapse is the junction between a presynaptic and postsynaptic neuron. Synapses can be electrical or chemical. Ions carry information in electrical synapses. In chemical synapses, a neurotransmitter is released by the presynaptic neuron at the junction when the axon depolarization (message) reaches the synapse. This neurotransmitter diffuses across a space (cleft) to the postsynaptic dendrite/cell body and binds to receptors. These receptors can cause the next neuron to fire a wave of depolarization.
A membrane potential is a localized electrical gradient across membrane. Every cell has a voltage, or membrane potential, across its plasma membrane An unstimulated cell usually have a resting potential of -70mV.
Membrane potential (-70mV) is created and maintained by Na- K pump - uses ATP to maintain a higher Na+ [ion] outside the neuron and a higher [K+] ion concentration inside. Also inside of neuron has large negative anions. Membrane is not permeable to large anions, but there are specific channels for Na+ and K+ ions.
How a Cell Maintains a Membrane Potential. Anions - (Proteins, amino acids, sulfate, and phosphate; Cl – ) Cations - (K + ; Na + )
Ungated ion channels allow ions to diffuse across the plasma membrane. This diffusion does not achieve an equilibrium since sodium-potassium pump transports these ions against their concentration gradients. Fig. 48.7
So, how does ion concentration gradient lead to membrane potential? E ion = 62mV (log [ion] outside) (log [ion] inside) E na = 62mV (log [150mM]) = +62mV (log [15mM]) E K = 62mV (log [5mM]) = -92mV (log [150mM]) Resting potential of neuron = -70mV. If Na + ions alone existed and moved across the membrane thru channels If K + ions alone existed
Gated ion channels open or close in response to stimuli. The subsequent diffusion of ions leads to a change in the membrane potential. Changes in the membrane potential of a neuron give rise to nerve impulses
Graded Potentials: Hyperpolarization and Depolarization Graded potentials are changes in membrane potential Hyperpolarization. Gated K + channels open K + diffuses out of the cell the membrane potential becomes more negative. (Approaches -92mV) Fig. 48.8a
Depolarization. Gated Na + channels open Na + diffuses into the cell the membrane potential becomes less negative. (Approaches +62mV) Fig. 48.8b
The Action Potential: All or Nothing Depolarization. If graded potentials sum to -55mV a threshold potential is achieved. This triggers an action potential. Axons only. Fig. 48.8c
In the resting state Na and K ion channels are closed. Graded potentials collected in the cell body and dendrites arrive at axon hillock. Voltage-gated Na + channels have two gates. Closed activation gates open rapidly in response to depolarization. Open inactivation gates close slowly in response to depolarization. Closed voltage-gated K + channels open slowly in response to depolarization.
Fig. 48.9 Step 1: Resting State- Na and K voltage gated channels closed - potential in axon is -70mV (RMP)
Step 2: Threshold is crossed by graded potentials that arrive at the hillock - the minimum increase in membrane potential that triggers an action potential (Na channels open) = Na moves in = Membrane potential becomes a little positive and crosses threshold potential = crosses –50mV Fig. 48.9
Step 3: Depolarization phase of the action potential – Na channels open; K channels closed; Na ions move into axon making it more positive on the inside - axon potential becomes +40mV (not quite E Na ) Fig. 48.9
Step 4: Repolarizing phase of the action potential (Na channels close; K channels open). K moves out = inside becomes negative again. Fig. 48.9
Step 5: Undershoot – hyperpolarization – Na channel closed; K channels open - so axon overshoots -70mV and becomes a little more negative than Resting potential. Slowly RMP is reestablished to = -70mV. Fig. 48.9
During the undershoot both the Na + channel’s gates are closed. At this time the neuron cannot depolarize in response to another stimulus: refractory period.
The action potential is repeatedly regenerated along the length of the axon. An action potential achieved at one region of the membrane is sufficient to depolarize a neighboring region above threshold. Thus triggering a new action potential. The refractory period assures that impulse conduction is unidirectional. Nerve impulses propagate themselves along an axon
Vertebrate nervous systems have central and peripheral components Central nervous system (CNS). Brain and spinal cord. Both contain fluid-filled spaces which contain cerebrospinal fluid (CSF). The central canal of the spinal cord is continuous with the ventricles of the brain. Peripheral nervous system. Everything outside the CNS.
White matter is composed of bundles of myelinated axons Gray matter consists of unmyelinated axons, nuclei (cell bodies), and dendrites.
The divisions of the peripheral nervous system interact in maintaining homeostasis
A closer look at the (often antagonistic) divisions of the autonomic nervous system (ANS) - controls smooth, cardiac muscles, internal organs, glands- involuntary. Fig. 48.18 Flight or fight response Rest or digest response
Embryonic development of the vertebrate brain reflects its evolution from three anterior bulges of the neural tube Fig. 48.19
The Brainstem. The “lower brain.” Consists of the medulla oblongata, pons, and midbrain. Derived from the embryonic hindbrain and midbrain. Functions in homeostasis, coordination of movement, conduction of impulses to higher brain centers. Evolutionary older structures of the vertebrate brain regulate essential autonomic and integrative functions
The Medulla and Pons. Medulla oblongata. Contains nuclei that control visceral (autonomic homeostatic) functions. Breathing. Heart and blood vessel activity. Swallowing. Vomiting. Digestion. Relays information to and from higher brain centers.
Pons. Contains nuclei involved in the regulation of visceral activities such as breathing. Relays information to and from higher brain centers.
The Midbrain. Contains nuclei involved in the integration of sensory information. Superior colliculi are involved in the regulation of visual reflexes. Inferior colliculi are involved in the regulation of auditory reflexes. Relays information to and from higher brain centers.
The Reticular System, Arousal, and Sleep (midbrain). The reticular activating system (RAS) of the reticular formation. Regulates sleep and arousal. Acts as a sensory filter. Fig. 48.21
Sleep and wakefulness produces patterns of electrical activity in the brain that can be recorded as an electroencephalogram (EEG). Most dreaming occurs during REM (rapid eye movement) sleep. Fig. 48.22b-d
The Cerebellum. Develops from part of the metencephalon. Functions to error-check and coordinate motor activities, and perceptual and cognitive factors. Relays sensory information about joints, muscles, sight, and sound to the cerebrum. Coordinates motor commands issued by the cerebrum.
The thalamus and hypothalamus. The epithalamus, thalamus, and hypothalamus are derived from the embryonic diencephalon.
Epithalamus. Includes a choroid plexus and the pineal gland.
Thalamus. Relays all sensory information to the cerebrum. Contains one nucleus for each type of sensory information. Relays motor information from the cerebrum. Receives input from the cerebrum. Receives input from brain centers involved in the regulation of emotion and arousal.
Hypothalamus. Regulates autonomic activity. Contains nuclei involved in thermoregulation, hunger, thirst, sexual and mating behavior, etc. Regulates the pituitary gland.
The Hypothalamus and Circadian Rhythms. The biological clock is the internal timekeeper. The clock’s rhythm usually does not exactly match environmental events. Experiments in which humans have been deprived of external cues have shown that biological clock has a period of about 25 hours. In mammals, the hypothalamic suprachiasmatic nuclei (SCN) function as a biological clock. Produce proteins in response to light/dark cycles. This, and other biological clocks, may be responsive to hormonal release, hunger, and various external stimuli.