Physiology pt 4

Parts of a Neuron

Dendrite

Propagate action potentials towards cell, signal input

Incoming messages

Axons

propagate signals efferently, away from center/ signal output

Outgoing messages

Schwann cells

Surround the axon, myelinated axons= faster conduction of impulse

Functional Classifications

Most neurons are interneurons, connecting 2 or more neurons

Sensory/ afferent neurons/ first order

Sends info to the brain and spinal cord. Respond to external stimuli and generate and propagate AP

Motor neurons/ efferent neurons

Carry info from brain or spinal cord to effector cells, ex: muscle cells/ gland cells

Soma

Nucleus of cell, DNA here

Afferent signal

Axon hillock

Trigger zone. If signal doesn't reach here, no action potential

Very basic of how it works

Stimulus detected by dendrites--> stimulus goes afferently to spinal cord (white and gray matter)--_ send neurotransmitter with interneurons, stimulate or inhibit motor neurons--> exit spinal cord and goes to muscles

Detect, integrate, activate

Can detect signals and transform it to language of nervous system (action potential)

Ex: retina detects light and changes in it, retina gets hit and you see stars.

Specific types

Types of sensory receptors

Direct generation of action potentials

Indirect

Free nerve endings. stimulus applied, AP in hillock, generated potential, triggers impluse and proagated to nervous system

Taste, cells that detect sugar, neurotransmitter stimulates first order neuron and the rest of the process happens the same way

Specialize cells

Indirect vs direct- the release of neurotransmitter from sensory cells. Both generate receptor potential, both propagate, both selective to type of stimulus

Conduction of action potentials

Unmyelinated

Myelinated

Schwann cells (PNS) and oligodendrocytes (CNS) produce myelin sheaths

Has many layers of neuroglia cell membrane, segmented by nodes of ranvier

Thin layer of neuroglia, some insulation, no segmentation

Structural reasons for myelinated vs unmyelinated

No insulation

Depolarization happening at axon, unmyelinated, electrical signal travels in ECF surrounding axon, signal is not as strong

Insulated

Tight insulation

Distant insulation

Insulate conduction of signal, does a bit of a jump

Some travel into ECF

Slow conduction, has to go along entire length

Electrical currents save some energy, still slow

Force electrical currents to jump over insulation, impulse faster

Conduction of action potentials

Unmyelinated

Myelinated

  1. Na voltage gated channels, they open, (not seen in K, K does not have inactivation gate). Influx of Na, membrane potential changes, electrical signal goes down neuron
  1. Next part of cell membrane depolarizes and previous section repolarizes, signal still being conducted
  1. Local/ graded potentials that reach threshold. Depolarized
  1. Current continues to axon terminal

Axon hillock contain more sodium voltage gated channels of cell membrane compared to rest of neuron

Directionality of signal

Why is action potential in one direction?

The absolute refractory period, previous section is in refractory period so signal cannot go backwards

Saltatory conductions

Conduction jumps from one node of Ranvier to the next, higher speed of conduction

Node of Ranvier is where influx of sodium occurs

More sodium voltage gated channels here

It jumps because the electrical currents are attracted to one another on the node, do not interact with ECF

Speed of conduction

Alpha receptors

Proprioceptors and somatic neuron (remember they are myelinated skeletal muscle) animal detects where limbs are (proprioception)

Fastest

Beta

Preganglionic in autonomic nervous system, still contain myelin

C fibers

Slower than everything else

Post ganglionic sympathetic are unmyelinated

Thicker fiber= less resistance, increase of speed in conduction

Electric current depends on resistance which depends on diameter. Thicker diameter= lower resistance