Nerve Impulse and Transmission

nerve impulse

• communication among neurons and with cells of their control

• originated in response to a stimulus of an electrical, chemical, thermal, or mechanical been received by the neuron cell membrane

• the stimulus elicits a wave of depolarization and repolarization that 

  • spreads along the axolemma

  • away from the site where was received

• results in the transmission of the nerve impulse

mechanisms of transmission

• potential

  • relative electrical charges between two points in a field or circuit

• for the neuron

  • transmembrane potential

    • inside and outside of the cell membrane

• all cells of the body have a transmembrane potential, bit he neurons are unique in being able to alter this potential to produce an impulse

resting membrane potential - resting neuron

• results from the unequal distribution of sodium ions and potassium ions on the outside and inside of the neuron

• active transport of Na to the outside with the transport of K into the neuron

• keeps the concentration of Na low on the inside

• electronegativity is maintained on the inside of the membrane and electropositivity on the outside 

depolarization

• chemical or physical stimulation of a neuron increases the permeability of the membrane for Na at the point of stimulation

• membrane is polarized at 70 millivolts

• high concentration of Na on the outside of the membrane

  • Na rushes inwards

• membrane now becomes positive on the inside and negative on the outside

• the inflow of Na soon stop sand the permeability of the membrane for K increases

repolarization

• the K flows outward because it has a higher concentration inside the neuron than outside

• the outflow of K destabilizes the resting membrane potential at the point of stimulation

• return to the resting membrane potential

• enter absolute refractory period

  • nerve fiber cannot be stimulated again until repolarization is complete

hyperpolarization

• membrane potential becomes more negative

• relative refractory period

propagation of action potential

• AP moves across neurilemma of dendrite, soma, and axon

• myelinated axons 

  • -> nodes of ranvier -> node of ranvier

  • faster because it skips parts of the neurilemma

• non myelinated axons

  • sends signal straight through

  • takes longer

• axon diameter

  • larger means faster signal sending

    • myelin aids in conduction

• local currents generated by an AP flow to adjacent areas of the axonal membrane to depolarize and generate further APs

  • MS

neuron placements

• Converging circuit

• Diverging circuit

• Reverberating circuit

• Parallel circuit

• simple circuits

Converging circuit

• several neurons impinge on one neuron 

• cerebellum converging info from 4 different brain regions

Diverging circuit

• the axon branches of one neuron impinge on two or more neurons, then those impinge on two or more neurons  amplification of signal

• e.g. one neuron stimulating many muscles neurons

Reverberating circuit

• each neuron in a series sends a branch back to the beginning neuron so that a volley of impulses is received at the final neuron

  • rhythmic activities

Parallel circuit

• contains a number of neurons in a series, each neuron supplying a branch to the final neuron

• e.g. reflex arcs

simple circuits

• no more than 2 neurons

  • for their projection to the cerebral cortex

  • olfactory, optic

• 3 neuron circuit classic for conscious sensations

  • 3 required to transmit a nerve impulse from periphery by spinal nerve to the cerebral cortex

reflexes def

• an automatic or unconscious response of an effector organ to an appropriate stimulus

reflex arc

• receptor

• afferent limb

• central connections

• efferent limb

• effector organ

myotatic (stretch) spinal reflex - knee jerk reflex

• Striking the middle patellar ligament: 

  • tendon of insertion for the quadriceps femoris and transmits its action to extend the tibia. 

• Stretches the quadriceps muscle  

  • stimulates muscle spindles (receptors for muscle sense). 

• An impulse’s path

  •  dorsal root of the spinal nerve 

  •  motor neuron in the ventral horn of the gray matter,

• Muscle fibers of the quadriceps muscle, causing it to contract. 

• The purpose of the reflex is to oppose stretch of the muscle.

somatic reflexes

• effector organs are composed of striated muscle

visceral reflexes

• effector organs are either smooth or cardiac muscle, or glands

• regulate visceral functions and are transmitted by the autonomic nervous system

postural reflexes

• maintaining an upright position

• muscle tonus is that state of muscle tension that enables and animal to assume and remain in the erect attitude

• Standing reflex

• Attitudinal reflexes

• Righting reflex

• Hopping reaction

• Crossed extensor 

Standing reflex

• pushing down on the back of a dog causes muscle movements that compensate for and resist the displacement.

Attitudinal reflexes

• displacement of one part of the body is followed by postural changes in other parts (e.g., lifting the head of a horse is followed by postural changes in the rear quarters so that a new attitude is assumed). 

Righting reflex

•  dropping an inverted cat is followed by its landing in the upright position.

Hopping reaction

• pushing a supported dog with three limbs elevated results in a placement correction of the intact leg to act as a rigid pillar.

Crossed extensor

• stepping on something sharp causes leg to contract while other leg stays standing/ extends (postural)

meninges

• covering of the brain and spinal cord

dura mater

• most superficial

• outer

• touch dense regular fibrous CT

• adheres to the skull

  • not present around the brain

• epidural space 

  • between dura mater and bone around spinal cord

arachnoid

• delicate spider web like loose CT around brain

• thin sheet around spinal cord

• subarachnoid space filled with CSF

pia mater

• delicate loose CT on surface of brain and spinal cord

ventricles of the brain - basics

• Cavities or hollowed-out spaces within the substance of the brain 

• each of the four ventricles has a choroid plexus

  • tuft of capillaries that secretes CSF

ventricles of the brain - list

• The lateral ventricles are paired cavities 

  • right and left cerebral hemisphere. 

• Interventricular foramen 

  • (foramen of Monro) 

• Third ventricle 

• The cerebral aqueduct 

  • (mesencephalic aqueduct). 

• The fourth ventricle

  •  is located beneath the cerebellum and above the medulla oblongata. 

  • continuous caudally as the central canal of the spinal cord.

ependymal cells

• glial cell

• unite with the capillaries to form the choroid plexus

  • capillaries in pia mater

flow of CSF

• Lateral ventricles  

• interventricular foramen  

• third ventricle  

• cerebral aqueduct  

• fourth ventricle 

• foramina of Luschka  

• Subarachnoid space and spinal cord

CNS metabolism

• The CNS receives its energy principally from carbohydrates  

  • glucose 

• CNS receives glucose by simple diffusion and insulin is not required. 

• Advantageous for the animal when insulin is lacking or in short supply 

  • enables the CNS function to continue when other systems fail.

• The relatively high rate of metabolism/oxygen consumption  

  • the CNS constitutes only 2% of body mass  

    • consumes approximately 20% of the total oxygen supplied to the body. 

blood-brain barrier

• Many substances in the blood do not readily enter the cells of the CNS

• The capillaries of the CNS have tight junctions between their endothelial cells

  • limit the diffusion of substances from capillaries. 

• Lipid-soluble substances, 

  • readily diffuse

  • oxygen and carbon dioxide

• Transport for most substances is provided for by astrocytes (a glial cell)  

  • selective

• Some areas of the hypothalamus, as well as other portions of the brain that serve as chemoreceptor areas, lack a blood–brain barrier.

blood requirement

• The CNS must have a continuous supply of blood for normal functioning. 

• Hypoxia (deficient oxygen)

• Other tissues can be deprived of a blood supply for extended periods and recover to normal function when the blood supply resumes. 

• Five to 10 minutes of little or no blood to the brain injures higher brain cells (in the cerebrum) so that no recovery occurs.