The Nervous System: a vast array of cellular communications

Nervous System The real work of your brain goes on in individual cells. An adult brain contains about 100 billion nerve cells, or neurons, with branches that connect at more than 100 trillion pointsnerve cellsbranches Each nerve forms about 1000 synapses each!

Nervous System Central Nervous System – Brain and Spinal Cord (CNS) Peripheral Nervous System – Everything else (PNS)

Nervous System Afferent (Sensory) –Sends information to the brain –Cell bodies in PNS –Types Cutaneous (pain, pressure, touch) Proprioceptors (measure stretch in muscles) Special senses (hearing, taste,sight, smell) Efferent (Motor) –Send info. from the brain to make things happen –Cell bodies in CNS –Types Somatic (conscious control – skeletal muscles) Autonomic (involuntary responses – cardiac and smooth muscle, glands)

Efferent Nervous System Autonomic Branch Sympathetic  Fight or Flight  E – energetic, exercise, excitement, embarassment Parasympathetic  Relaxing  D – digestion, defecation, diuresis Sympathetic and Parasympathetic work opposite each other – what one does, the other does the opposite

Cells of the Nervous System Neurons –Axons away, usually 1, long, branch at end –Dendrites – input to neuron, many, shorter Neuroglia or Glial cells (support cells) –Can divide but can’t transmit impulses –Support, insulate, protect neurons

Support Cells of the CNS Astrocytes – star shaped (about ½ of CNS tissue), anchor neurons to capillaries, barrier between blood and neurons to capture toxins, pick up excess ions and neurotransmitters Microglia – phagocytes Ependymal Cells – ciliated epithelium that line the brain and spinal cord, move CSF around Oligodendrocytes – wrap around nerve fibers producing the myelin sheath (fat covering of nerves to increase nerve conduction speed)

Support Cells of the PNS Schwaan Cells – form the myelin sheath in PNS (have gaps in between sheath called Nodes of Ranvier) Satellite Cells – cushions nerves Ganglia – a bunch of cells bodies located in the PNS

Structure of a Nerve

Nerve Cell Structure

Nerves Irritability – transfer impulses to the nerve cell Conductivity – transfer impulses from the nerve cell to an organ or other nerve cell Reflexes – impulse goes to the spinal cord and back directly without going to the brain –Autonomic – pupils, glands –Somatic - muscles

Neuron Structure At the edges of the cell body are dendrites which receive information via. receptors There is one long axon (can be up to 3 meters) to send signals out by releasing neurotransmitters (usually proteins)

How do neurons send and receive signals? Axon terminals branch out and connect with dendrites of many other nerve cells (synapses) In a synapse, the axon and dendrite don’t touch, there is a gap At each axon terminal, there are vesicles containing a neurotransmitter Once the neurotransmitter is released, it binds to receptors on the dendrite The chemical signal gets transduced to an electrical signal

Some neurotransmitters send excitatory signals (causing the neuron to send a signal to the next one) and some inhibitory When the excitatory signals are greater than the inhibitory ones, the signal is sent to other neurons.

How does the neurotransmitter signal get converted to an electrical signal? Normally, there is more Na+ outside the cell than K+ inside so the normal cell is slightly negative inside compared to the outside (called resting potential) The binding of the neurotransmitters opens gated channels letting the Na+ and K+ freely diffuse, erasing the charge (depolarization) This travels down the dendrite of the receiving cell all the way to its own axon terminals

As Na+ enters the cell at the axon terminal it opens gated channels letting Ca+2 in. Calcium causes the vesicles to fuse with the membrane and release the neurotransmitter into the next synapse. The neurotransmitter binds to the receptors on the dendrites gets converted to an electrical signal and on and on

Overall Process

How does it all work together? Brain receives all of its sensory input (sight, hearing, taste, smell, pain, touch, pressure, proprioception, internal conditions, vestibular (balance)) by the sensory neurons releasing neurotransmitters in the brain to the correct receiving neurons. Neurons for each senses are located in different areas of the brain. The brain integrates this information to understand the whole picture by sending and receiving signals within the brain neurons themselves To respond, the brain then sends out information via neurotransmitters to: glands to secrete hormones, to skeletal muscle to make you move, to smooth muscle to make something squeeze involuntarily

Brain Structure

Brain and Brain Parts Cerebral Hemispheres (Cortex) Dienchephalon –Thalamus – relays sensory input to the cortex –Hypothalamus – endocrine and nervous tissue – major homeostasis center - regulates temperature, hunger, thirst, water balance, emotion, pituitary –Pituitary – endocrine gland – growth hormone, FSH, LH, TSH, etc.) –Pineal Gland – effected by light – controls seasonal breeding Brain Stem – regulates blood pressure and breathing –Midbrain – eye reflexes –Pons & Medulla Oblongata – control heart rate, constriction of blood vessels, digestion, breathing Cerebellum – fine motor control, balance and coordination

Brain – Cerebral Cortex Frontal lobe – helps control skilled muscle movements, mood, planning for the future, setting goals and judging priorities. Occipital lobe – helps process visual information. Temporal lobe – processes hearing, memory and language functions. Parietal lobe – receives and processes information about temperature, taste, touch, and movement coming from the rest of the body. Reading and arithmetic are also processed in this region.