The Nervous System

2 division of the Nervous System

• Central Nervous System (CNS) - consists of the brain and the spinal cord; located in the dorsal body cavity surrounded by meninges.

• Peripheral Nervous System (PNS) – consists of the all neural structures outside of the CNS including the cranial nerves, spinal nerves and sensory receptors

Figure 13.2

Figure 13.1

Composition of Nervous Tissue

• The Nervous System is composed mainly of Nervous Tissue; connective tissue and blood vessels are also present.

• Nervous tissue is composed of 2 types of cells: Neurons and Supporting Cells

• Neurons = nerve cells are conducting cells

• Supporting cells are non-conducting cells

Structure of a neuron• 3 regions of a neuron: cell body + 2 types of processes• Cell body = Soma = Perikaryon

– Contains the nucleus and all other cytoplasmic organelles EXCEPT CENTRIOLES hence, neurons are generally AMITOTIC

– Contains well-developed rough ER called Nissl Body or Chromatophilic

substance

_ Contains intermediate filaments called neurofibrils

_ BIOSYNTHETIC region a neuron

• Dendrites– Tapering processes that act as the RECEPTIVE regions of a neuron

– Receive and convey electrical signals toward the cell body

• Axon– A single process extending from the cell body – each neuron has only one axon

– Generates and transmits action potentials=CONDUCTING region of a neuron

– Branches at the end to form terminal branches which end in bulbous ends called axon terminals=synaptic knobs=boutons

Classification of Neurons• 2 types:• 3 Structural Classification of neurons:

– Multipolar neuron has at least 3 processes – one axon and at least 2 dendrites; most abundant neuron in the human body

– Bipolar neuron has 2 processes – one axon and one dendrites– Unipolar neuron has one short process from the cell body and it

bifurcates into a central process and a peripheral process

• 3 Functional classification of neurons:– Motor or Efferent neuron transmits impulses AWAY from the

CNS to effector organs = glands, organs– Sensory or Afferent neuron transmits impulses from sensory

receptors TOWARD the CNS– Association neurons or interneuron located in the CNS

between the sensory neurons and the motor neurons– Most of the neurons (99%) in the body are associated neurons

Definitions

• Tract = a bundle of axons in the CNS

• Nerve = a bundle of axons in the PNS

• Nucleus – a cluster of neuron cell bodies in the CNS

• Ganglion = a cluster of neuron cell bodies in the PNS

Figure 13.3

Structure of a Nerve ( or a Tract)

• The plasma membrane of an axon is called an axolemma

• Each axon is wrapped in a delicate connective tissue membrane called ENDONEURIUM

• A bundle of endoneurium-covered axons is called a fascicle

• Each fascicle is covered by the coarse connective tissue membrane called the PERINEURIUM

• A bundle of perineurium-covered fascicles form the nerve or a tract which is covered in a tough connective tissue membrane called the EPINEURIUM

6 Types of Supporting cells

• Supporting cells = Neuroglia

• 4 Supporting cells are located in the CNS– Astrocytes– Microglia– Ependymal– Oligodendrocytes

• 2 Supporting cells are located in the PNS– Schwann cells– Satellite cells

4 Types of supporting Cells in the CNS• Astrocytes

– Most abundant– Numerous extensions that wrap around neurons– Involved in forming the BLOOD-BRAIN BARRIER, a selective

barrier that regulate the chemicals environment of the brain– Regulate brain function

• Microglia– Since the specific immune system does not have access to the

CNS; the microglia act as macrophages to engulf/destroy pathogens and cell debris.

• Ependymal cells– Ciliated columnar cells that line the ventricles – cavities in the

brain that contain cerebrospinal fluid (CSF) – Currents created by beating of cilia circulate the CSF

• Oligodendrocytes– Their extensions myelinate axons of neurons in the CNS

2 Types of Supporting Cells in the PNS

• Schwann cells = neurolemmocytes– Myelinate axons of neurons in the PNS

• Satellite cells– Surround cell bodies of neurons and control

their chemical environment

Myelination of axons • Myelination of axons in the PNS by Schwann cells:• Each Schwann cell wraps around a segment of an axon

( external to the axolemma)• Schwann cell squeezes around the segment of axon

wrapping concentric rings of its plasma membrane called MYELIN SHEATH around the axon

• The cytoplasm and the nucleus of the Schwann cell squeezed outside the myelin sheath is called the NEURILEMMA

• The spaces between adjacent myelin sheaths are called NODES OF RANVIER

• Myelination of axons in the CNS by oligodendrocytes

• The axons in the CNS are myelinated by extensions from the oligodendrocytes hence, neurilemma is absent

Severed axons in the PNS can regenerate but severed axons in the CNS cannot

• Severed axons in the PNS regenerate because– When the axon is severed in the PNS, cells of the immune

system clean up the damaged area of cell debris, a process known as debridement, which sets the stage for regeneration

– The neurilemma of the Schwann cell forms a REGENERATION TUBE that guides regeneration of the severed axon

• Severed axons in the CNS fails to regenerate because:

– The microglia poorly clean up area of damage – debridement is not complete

– No neurilemma to guide growth of severed axon– Presence of growth-inhibiting proteins in the CNS inhibit

regeneration of a severed axon

Figure 13.4

Neurophysiology – Generation of Action Potential

Resting membrane potential RMP) is -70mV

. Depolarization phase –entry of sodium ions = sodium influx, makes membrane potential less and less negative

Threshold Potential - action potential develops = an all-or-none phenomenon

Upshoot or spike due to an explosive entry of Sodium ions = a positive membrane potential reached

• Repolarization phase – sodium channels close and potassium channels open and potassium ions rush out = potassium efflux, and reversal of membrane potential toward a negative membrane potential

• Hyperpolarization phase – more potassiom ions leave the cell driving the membrane potential below the RMP

Characteristics of Action Potentials

• All-or-none phenomenon – an action potential will be generated if depolarization reaches a threshold potential

• Self propagating – once generated by the axon, it is propagated down the axon to the axonal terminals; a propagated or transmitted action potential is called a IMPULSE

• Since all action potentials appear the same have the same shape and amplitude irrespective of stimulus strength. Thus, the difference between a stronger stimulus that causes the generation of an action potential and a weaker stimulus that causes the generation of an action potential is that the stronger stimulus causes the impulse to be generated at a higher frequency than the weaker stimulus

2 Refractory periods during an action potential

• Absolute Refractory Period – the depolarization phase of the action potential when sodium channels are opened, another action potential can not be generated

• Relative Refractory Period – the repolarization phase of the action potential when the sodium channels are closed ( potassium channels are open), an exceptionally strong stimulus can cause sodium channels leading to depolarization and the generation of another action potential

Factors affecting the rate of impulse transmission = Conduction Velocity

• Diameter of the axon – larger axons transmit impulses at a faster rate than smaller axons because the larger axon have larger diameter and therefore presents with less resistance impulse transmission; the resistance in the smaller axons is higher which impedes impulse transmission

• Degree of myelination – myelinated axons transmit impulses at a faster rate than unmyelinated axons.

Myelinated axons use SALTATORY conduction where action potentials are generated only at the nodes of Ranvier hence, the impulse “jumps from node to node down the axon

Unmyelinated axons use CONTINUOUS conduction where action potentials developed stepwise across the entire axolemma

3 Types of Nerve Fibers

• Based on their diameter and degree of myelination

• Group A fibers – have the largest diameter and heavily myelinated; transmit impulse at the rate of 150 m/s (=300 miles per hour)

• Group B fibers – intermediate diameter and lightly myelinated;

transmit impulses at a rate of 15 m/s

• Group C fibers – smallest diameters and unmyelinated; transmit impulses at a rate of 1 m/s