NEURONS Structural and functional units of the nervous system. React to physical and chemical changes in their surroundings. Transmit information in the form of electrochemical changes called nerve impules.
WHITE MATTER VS GRAY MATTER Myelinated axons appear white and masses of these axons form the white matter in the CNS. Unmyelinated axons and neuron cell bodies form gray matter in the CNS.
TYPES OF NEURONS Sensory Neurons (afferent)- carry nerve impulses from peripheral body part into the brain or spinal cord. Interneurons (association) – lie within the brain or spinal cord and link other neurons. Transmit impulses from one part of the brian or spinal cord to another. Motor Neurons (efferent) – carry nerve impulses out of the brain or spinal cord to effectors. Muscles contract, glands to release secretions.
NEUROGLIAL CELLS Provide physical support, insulation, and nutrients for neurons. Before birth, neuroglial cells release and relay signals that guide the differentiation of neurons. Oligodendrocytes – provide myelin sheath around axons in CNS Astrocytes – found between neurons and blood vessels – provide nutrients and ions Microglial cells – scattered throughout CNS and are the “garbage trucks” phagocytizing bacterial cells and cellular debris
CNS VS PNS CNS – brain and spinal cord. PNS – composed of peripheral nerves that connect the CNS to the other body parts. Together the CNS and PNS provide 3 functions: sensory, integrative, and motor. Motor Division of the PNS is somatic sending info to the skeletal muscles and autonomic sending info to the smooth and cardiac muscles.
SENSORY FUNCTION OF NS Sensory receptors are a the ends of peripheral neurons. Gather info by detecting changes inside and outside the body. Sensory receptors convert environmental info into nerve impulses. Nerve impulses are transmitted over P nerves to the CNS.
INTEGRATIVE FUNCTION Signals are brought together creating sensations, adding to memory, help produce thoughts that translate sensations in perceptions. As a result of integration we make conscious or subconscious decisions. The we use MOTOR functions to act on them.
MOTOR FUNCTIONS Use P neurons to carry impulses from the DNS to responsive structures called effectors. Effectors are OUTSIDE the NS and include muscles and glands.
NERVE IMPULSES Travel along complex nerve pathways. Synapse is the junction between any two communicating neurons. Neurons at a synapse ARE NOT in direct physical contact, they are separated by a gap. The gap is called a synaptic cleft. Communication must cross the gap.
SYNAPTIC TRANSMISSION One way process carried out by neurotransmitters. Distal ends of axons have one or more synaptic knobs with can sacs called synaptic vesicles. Nerve impulse reaches synaptic knob some synaptic vesicles release neurotransmitters. Neurotransmitter diffuses across the synaptic cleft and reacts with specific receptors on the postsynaptic neuron membrane.
ACTION OF NEUROTRANSMITTER Postsynaptic neuron is either excited (turned on) OR inhibited (turned off).
CELL MEMBRANE POTENTIAL The surface of a cell membrane is usually electrically charged = polarized (in respect to the inside of the cell) Polarized because of unequal distribution of positive and negative ions between sides of the membrane. Important to the conduction of muscle and nerve impulses!!
NERVE IMPULSE Is formed by a change in the neuron membrane polarization and return to the resting state.
DISTRIBUTION OF IONS Potassium ions pass through cell membrane more easily than sodium ions. Pumps in cell membrane work to pump sodium ions out of cell and potassium ions into cell to create a concentration gradient. Sodium is pumped out of cell, potassium is pumped into cell.
RESTING POTENTIAL A resting cell membrane is more permeable to potassium ions than to sodium ions. Potassium ions diffuse out more quickly than sodium ions. Outside of cell membrane gains a slight surplus of positive charges and inside is left with slight negative charge. Difference in charge between 2 regions is called potential difference.
RESTING POTENTIAL -70MV The difference in electrical charge between the inside and the outside of an undisturbed nerve cell membrane.
POTENTIAL CHANGES Changes (stimuli) affect the resting potential in a particular region of a nerve cell membrane. If the membrane’s resting potential decreases (inside of membrane becomes less negative when compared to the outside) it is DEPOLARIZED. The greater the stimulus the greater the depolarization.
THRESHOLD POTENTIAL -55MV If neurons are depolarized sufficiently, the membrane reaches a level called the threshold potential. At the TP permeability suddenly changes at the trigger zone. Channels open and allow sodium ions to diffuse freely INWARD. Membrane loses it negative electrical charge and becomes depolarized.
HANG IN THERE, I KNOW THIS STUFF ISN’TEASY TO UNDERSTAND. Membrane channels open that allow potassium ions to pass through and as these positive ions diffuse OUTWARD. The inside of the membrane becomes negatively charged once more. Membrane potential may briefly come overly negative (hyperpolarization). Membrane quickly returns to resting potential (repolarization) Remains in this state until stimulated again.
RAPID SEQUENCE OF DEPOLARIZATION ANDREPOLARIZATION IS THE ACTION POTENTIAL If threshold is reached, an action potential results. Action Potential is the basis for the nerve impulse. Only a fraction of sodium and potassium ions move through the membrane during an AP. Many AP can occur and RP’s be reestablished.
NERVE IMPULSES AP occurs in one region of a nerve cell membrane and causes a bioelectric current to flow to adjacent portions of the membrane. Local current stimulates the adjacent membrane to its threshold level and triggers another AP. This stimulates the net adjacent region. A wave of Ap’s moves down the axon to the end. This propagation of AP’s along a nerve axon constitutes the nerve impulse.
EVENTS LEADING TO THE CONDUCTION OF ANERVE IMPULSE (TABLE 9.1. PG224) 1. Neuron membrane maintains RP 2. Threshold stimulus is reached 3. Sodium channels in trigger zone open 4. Sodium ions diffuse inward, depolarization 5. Potassium channels open 6. Potassium ions diffuse outward, repolarization 7. AP causes current, stimulates adjacent portions 8. Wave of AP’s travel length of axon as nerve impulse.
IMPULSE CONDUCTION Speed of nerve impulse conduction is proportional to the diameter of the axon – the greater the diameter the faster the impulse. Unmyelinated axons have smaller diameter and conduct impulse over its entire surface. Myelinated axons have larger diameter (think about thickness of Schwann cells) and conduct impulses faster.
SALTATORY IMPULSE CONDUCTION Myelin insulates and prevents almost all ion flow through the membrane it encloses. Nodes of Ranvier between Schwann cells interrupt the myelin sheath. AP’s occur at the nodes (exposed axon has sodium and potassium channels) A nerve impulse traveling along myelinated axon appears to jump from node to node (saltatory) Many times faster than conduction on an unmyelinated axon.
ALL-OR-NONE RESPONSE Nerve impulse conduction is all or none If a neuron responds at all it responds completely A nerve impulse is conducted whenever a stimulus of threshold intensity or above is applied to an axon. Higher intensity = more rapid impulses per second. ALL impulses carried on an axon are of the same strength.
REFRACTORY PERIOD For a very short time after a nerve impulse, a threshold stimulus will not trigger another impulse on an axon. This limits the frequency of impulses in a neuron. Also ensures impulse proceeds in only one direction -> down the axon. Frequency of 700 impulses per second possible but 100 per second is more common